WO2010012028A1

WO2010012028A1 - Covered pond-type photobioreactor for large-scale, intensive cultivation of microalgae

Info

Publication number: WO2010012028A1
Application number: PCT/AU2009/000959
Authority: WO
Inventors: Robert W. Vowles
Original assignee: BIOSEQ Pty Ltd
Current assignee: BIOSEQ Pty Ltd
Priority date: 2008-07-29
Filing date: 2009-07-28
Publication date: 2010-02-04
Anticipated expiration: 2011-01-29
Also published as: AU2009276284A1

Abstract

A system for the large-scale, intensive cultivation of algae employing pondage containers in the form of wide, flat, transparent or translucent polymer film tubes. Said pondage tubes are laid in long, parallel runs upon a graded, flat, level surface and large flows of shallow water are continuously circulated through them by paddle wheel-type circulators in closed housings. Said pondage tubes have an upper chamber used for biological activity and a lower chamber employed as a water delivery conduit or storage vessel during periods of darkness, water being transferred as required from the lower to the upper chamber at a controlled rate. The upper film surfaces are maintained clear of the water surface by gas pressure and entry of solar irradiance is optionally modulated. Large pondages are sub-divided into smaller units, each with its own water circulation and management means and the aquatic environment within said pondages is continuously monitored and adjusted.

Description

COVERED POND-TYPE PHOTOBIOREACTOR FOR LARGE-SCALE, INTENSIVE CULTIVATION OF MICROALGAE

This invention relates generally to ponds for the intensive cultivation of microalgae in the production of any of the variety of products yielded by different algal species. More specifically, it relates to the intensive cultivation of microalgae in closed, race track, pond- type photobioreactors. hi the large-scale, commercial cultivation of algae, pond-type photobioreactors have been limited to static (unmixed) bodies of water, such as estuaries, shallow lakes, ponds, sewage treatment lagoons and the like. Where they have been used, circulating race track-type pondages have generally been open and limited in area to only a few thousand square metres. Covered pondages for use in aquaculture are well known in the art and a typical example is that taught by Marinkovich in US 5,216,976. In this example, a plurality of discrete, covered ponds approximately 30 metres in length are arranged in parallel in rows and serviced via manifolds extending along the rows with individual conduits to each pond. The ponds are covered by dual layers clear plastic film material. Serfling et al teach the use of pondages covered by a light transmitting, heat retaining cover made from polyethylene film, fibreglass, glass or the like. Dewey, in US 2,732,663, teaches a system for photosynthesis which includes the use of long, thin-walled, highly flexible, translucent tubes laid in continuous, parallel runs joined end to end with U-tubes and laid on a graded, flat, level surface or in shallow trenches. The tubes are nominated to have circumferences ranging from 200 millimetes to approximately 6 metres and possibly thousands of feet in length. The area above the working water body in the tubes is inflated to a low pressure with a suitable gas to prevent collapse. Tubes are optionally laid upon a double-skinned metal floor through which coolant is able to be circulated, effectively in heat-exchange relationship with water in the tubes. In US 6,827,036, Connolly teaches the use of inflated, elongate tubes of transparent material to define a water course for aquacultural purposes. Tubes are nominated to ideally have a diameter of 6 metres and to have a length of at least 100 metres or longer. Tubes are laid upon a graded, flat, level surface or in shallow trenches and intermediate support mounds may be provided between parallel tube runs. Tubes are made double-skinned or covered by a layer of film material to provide an insulating air space and are arranged in parallel runs connected as required by suitable conduits. In US 4,091,800, Fletcher teaches the use of flat, flexible tubes running along the floor of a long solar pond to achieve stratification of water. The principal defect of the covered pondage arrangements cited is that they are unable to provide the control necessary for reliable, large-scale, intensive cultivation of algae under the very wide range of operational conditions normally encountered in the desert or semi-desert environments in which this activity is best undertaken. The object of the present invention is to provide a system of covered pondage which permits the close management of conditions in large volumes of water necessary for the reliable large-scale, intensive cultivation of algae.

According to the present invention, a system is provided for the large-scale, intensive cultivation of algae in which pondages take the form of large flows of relatively shallow water continuously circulated through wide, flat, transparent or translucent polymer film tubes. Said pondage tubes are preferably laid in long, parallel runs with alternate adjacent ends joined and the circuit closed by a circulation duct. Said tubes are laid upon a graded, flat, level surface, with low, narrow, earth berms provided between said runs to provide locational stability. A low gas pressure is maintained within said tubes to keep their upper film surfaces clear of the operating water surface. Provision is optionally made to recover gases from said tubes for processing and use in other applications. Provision is also optionally made to restrict the entry of sunlight into said tubes to control solar radiation levels and heating. To provide a suitable degree of biological security, said pondages, which optionally extend over square kilometres, are sub-divided into discrete, racetrack-type circuits, each containing a multi-megalitre water volume and each with its own water circulation and management means. The aquatic environment within said pondages is continuously monitored and provision is made for adjustment, as required, of water level, water temperature, dissolved carbon dioxide levels, nutrient levels, salinity and the like. Water is continuously withdrawn from said pondages, carbonated and returned to multiple points to maintain optimal levels of dissolved carbon dioxide. Water is also continuously withdrawn for harvesting of algae, the rate of withdrawal being dependent upon the algal species under cultivation. Provision is made to impound any water leakage and said polymer film tubes are adapted to collect rainwater. Provision is made for access to the exterior of said polymer film tubes for the purposes of servicing and repair. The various aspects of the present invention will be more readily understood by reference to the following description of preferred embodiments given in relation to the accompanying drawings in which:

Figure 1 is a plan view of a typical arrangement of a discrete, racetrack-type pondage circuit;

Figure 2 is a partial transverse cross-sectional view of the edges of said polymer film tubes abutting a separating earth berm, some components being shown slightly separated for illustrative clarity; Figure 3 is a partial transverse cross-sectional view of an alternative embodiment of the arrangement depicted in Figure 2, some components being shown slightly separated for illustrative clarity;

Figure 4 is a plan view of a rigid 180 degree bend used to join the ends of parallel runs of said polymer film tubes; Figure 5 is a plan view of a flexible 180 degree bend used to join the ends of parallel runs of said polymer film tubes;

Figure 6 is a plan view of a guide vane and supporting arrangement for use with the bend depicted in Figure 5;

Figure 7 is an end view of a reinforced opening into the rigid bend of Figure 4 or of a straight section for joining lengths of said polymer film tube;

Figure 8 is a partial transverse cross-sectional view of the edge detail of a said discrete, racetrack-type pondage circuit;

Figure 9 is a partial transverse cross-sectional view of a provision to discharge water and solutes into a said pondage circuit at multiple points; Figure 10 is a transverse cross-sectional view of an arrangement to provide vehicular access across a run of a said pondage circuit;

Figure 11 is a transverse cross-sectional view of a take-off or delivery well within a run of a said pondage circuit;

Figure 12 is a plan view of an alternative provision to discharge water and solutes into a said pondage circuit at multiple points;

Figure 13 is a fragmentary longitudinal cross-sectional view of the arrangement depicted in Figure 12;

Figure 14 is a transverse cross-sectional view of double-chamber pondage with the lower chamber in an expanded state; Figure 15 is a transverse cross-sectional view of the double-chamber pondage of Figure 14 with the lower chamber contracted;

Figure 16 is a transverse cross-sectional view of a pipe used as a divider between adjacent pondage runs; Figure 17 is a transverse cross-sectional view of a pipe used as a divider between adjacent pondage runs and as rainwater collection means;

Figure 18a is a transverse cross-sectional view of a pipe used as a divider between adjacent pondage runs and as rainwater collection means with water ingress apertures covered by the upper polymer film panel of a pondage tube;

Figure 18b is the pipe of Figure 18a with water ingress apertures uncovered;

Figure 19 is a longitudinal cross-sectional view of the end part of the double-chamber pondage run of Figure 15 showing a bend incorporating optional gas take-off means and water take-off or delivery means;

Figure 20 is a fragmentary plan view of a diffusion membrane of the pondage run of Figures 14 and 15;

Figure 21 is a transverse cross-sectional view of a pondage tube of Figures 14 and 15 showing its method of assembly; Figure 22 is a transverse cross-sectional view of means to terminate the pondage tube of Figures 14 and 15;

Figure 23 is a longitudinal cross-sectional view of water and gas take-up and return means incorporated into a pondage run;

Figure 24a is a transverse cross-sectional view of an electrostatically deployed pondage shading unit in its relaxed state;

Figure 24b is a transverse cross-sectional view of the pondage shading unit of Figure 24a in its deployed state;

Figure 25a is a transverse cross-sectional view of a pneumatically deployed pondage reflector unit in its relaxed state; Figure 25b is a transverse cross-sectional view of the pondage reflector unit of Figure 25a in its deployed state;

Figure 26 is a transverse cross-sectional view of an alternative embodiment of pneumatically deployed pondage reflector unit in its relaxed state;

Figure 27a is a transverse cross-sectional view of another alternative embodiment of pneumatically deployed pondage reflector unit in its relaxed state;

Figure 27b is a transverse cross-sectional view of the pondage reflector unit of Figure 27a in its deployed state;

Figure 28 is a transverse cross-sectional view of a plurality of the pondage reflector units of Figure 27b deployed on a pondage tube;

Figure 29 is a transverse cross-sectional view of means of terminating the pondage reflector unit of Figure 27a;

Figure 30 is a fragmentary, longitudinal cross-sectional view of the end part of the diffusion membrane of the pondage run of Figures 14 and 15;

Figure 31 is a schematic plan view of a bridging vehicle providing bridging of said polymer film tubes;

Figure 32 is a schematic plan view of a service vehicle adapted to operate on said polymer film tubes without interrupting said water flow; Figure 33 is a partial face view of means to promote mixing in said pondage water flow;

Figure 34 is a fragmentary transverse cross-sectional view of a provision to generate turbulent mixing in said pondage water;

Figure 35a is a fragmentary transverse cross-sectional view of doubled, partially metallised film layers of said polymer film tubes;

Figure 35b is the view of Figure 35a with said layers separated; Figure 36 is a transverse cross-sectional view of the gas take-off and water take-off or delivery means of Figure 23;

Figure 37a is a fragmentary transverse cross-sectional view of the positioning of side and upper polymer film panels of a pondage run prior to joining;

Figure 37b is a partially exploded, fragmentary, transverse cross-sectional view of the polymer film panels of Figure 37a after joining.

It should be noted that all components in the figures are not depicted at the same scale. Where used throughout, the word, 'algae', should be taken to mean, 'microalgae'. With reference to Figure 1, a system is provided for the large-scale, intensive cultivation of algae in which pondages 1 take the form of large flows of relatively shallow water (not shown) continuously circulated through wide, flat tubes 2 fabricated from a transparent or translucent, thin, flexible, polymer film material. Said polymer film material is optionally used in single or multiple layers and ranges in thickness from 0.1 millimetre to 3.0 millimetres or greater. Flexible, polymer sheet film materials for use in greenhouse applications and the like are well known in the art and are optionally modified to provide improved tensile strength, resistance to deterioration due to ultra-violet radiation, flexibility, screening of ultra-violet or infra-red light wavelengths, resistance to fogging, high reflectivity from the application of metallised coatings, and ease of joining and fabrication. In a preferred embodiment, said pondage tubes are made from two pieces joined at their longitudinal edges. The upper part of said pondage tubes is optionally made transparent or incorporating passive or active light-excluding means. Said passive light excluding means include strips or patterns of light reflective material applied to the upper surface, hi the preferred embodiment, said light reflective material takes the form of an aluminised coating. Said active light excluding means are described herein in relation to Figures 24a to 28 (inclusive) and Figures 35a and 35b. The lower part of said pondage tubes is preferably made from a light-excluding material to suppress plant regrowth below said pondage tubes and from a thicker material to better resist puncture damage from asperities in the supporting surface. In the preferred embodiment, the material of said upper parts of said pondage tubes incorporates a suitable fabric reinforcement to better resist wind damage.

Said pondage tubes are preferably laid in long, parallel runs with alternate adjacent ends joined by 180 degree bends 4. A closed circuit is created by circulation duct 5 joining the ends of the initial and final said tube runs via 90 degree bends 6. One or more paddle wheel-type water circulators (not shown) are located within gas-tight enclosure 7, preferably situated in and sealingly connected to said circulation duct. Said paddle wheel- type water circulators are well known in the art and are optionally located at multiple points throughout said pondage. In the preferred embodiment, said paddle wheel-type water circulators have a diameter in the range 1 to 3 metres, a width in the range 0.5 to 1.5 times the width of said pondage runs, and 15 to 30 straight paddle blades. Said water circulators turn in part-cylindrical pits from the flat and curved surfaces of which said blades have minimal clearance, the speed of rotation of said circulators preferably being variable. Pondage tubes typically have a width of approximately five metres, but greater or lesser widths are optionally employed; pondage water depths are typically approximately 300 millimetres, but greater or lesser depths are employed as operational conditions dictate; pondage water circulation velocities are typically approximately 300 millimetres per second, but greater or lesser velocities are employed as operational conditions dictate. Said pondage tubes are laid upon a graded, flat, level surface, with low, narrow, earth berms 3 preferably provided between said runs to provide locational stability. A low gas pressure is continuously maintained within said tubes to keep the surfaces of their upper polymer film panels clear of the operating water surface to prevent fouling. Provision is optionally made to recover gases from said pondage tubes for processing and use in other applications. Such gases include oxygen produced by photosynthesis and carbon dioxide outgasing from said carbonated water flows. Where said gases are not recovered, said pondage tubes are pressurised by airflows from small centrifugal blowers (not shown) and suitable valves and vents (not shown) are provided in said tubes to vent said air and gas to atmosphere. Where said gases are recovered, reference signals from sensitive pressure sensors (not shown) located at various points throughout said pondage tubes are transmitted to a microprocessor-based control unit (not shown) which controls the operation of extractor fans (not shown) to extract said gases while maintaining gas pressures within said tubes within a predetermined range. Where the possibility exists of disturbance of said pondage tubes by high winds, said pondage tube gas pressures or water depths are optionally increased to provide increased stability. To provide a suitable degree of biological security, said pondages, which optionally extend over areas of square kilometres, are sub-divided into discrete, racetrack-type circuits, each containing a multi-megalitre water volume and each having its own water circulation and management means. For example, a pondage area of approximately 2.5 square kilometres is required to absorb the carbon dioxide from flue gases of a natural gas-fired, 100 megawatt power station. One square kilometre of pondage having a tube length of 200 kilometres, a water depth of 300 millimetres and pondage tube width of 5 metres has a pondage water volume of approximately 300 megalitres. In preferred embodiments, this area is subdivided into between 6 and 15 discrete racetrack-type pondages of the arrangement described having approximate lengths of from 33.3 to 13.3 kilometres and a water volume of from 50 to 20 megalitres. Discrete pondages larger in area and with greater water volumes are employed where conditions permit. The aquatic environment within said pondages is continuously monitored by suitable sensors (not shown) and provision is made for adjustment, as required, of parameters such as water level, water temperature, dissolved carbon dioxide level, nutrient level, salinity and the like, hi the preferred embodiment, reference signals from said sensors are transmitted to a microprocessor-based control unit (not shown) which controls valves, pumps and the like to maintain said parameters within a predetermined range. Water is continuously withdrawn from said pondages via suitable conduits 9, carbonated in separate carbonation units (not shown) and returned to multiple points via suitable conduits 8 to maintain optimal levels of dissolved carbon dioxide. Transversely-located conduits (typical positions indicated in broken line as 10, 12) pass beneath and normal to said pondage tube runs and are connected to appropriate plant via mains 11, 13. Said transversely located conduits take flow from or discharge flow into said pondage via means of any of the types (as appropriate) depicted at Figures 9, 11, 12, 13, 14, 15, 20, 23, 28. Water is also continuously withdrawn from said pondage circuits via suitable take-off conduits (not shown) for harvesting of algae in separate harvesting means (not shown), the rate of withdrawal being dependent upon the algal species under cultivation and its rate of growth. Following harvesting of said algae, water is returned to said pondage circuits via suitable delivery conduits (not shown). Suitable earth berms and channels (as depicted in Figure 8) are provided surrounding the total said pondage to impound adventitious water leakage.

In their simplest form, said pondage tubes take the form of two panels of thin, flexible, polymer film material securely joined along the edges.

With reference to Figure 2, in a first preferred embodiment, pondage tubes 2 are made with laterally extending aprons 14 fused or bonded to their lower surfaces in a zone 16 along each edge. When two said pondage tubes are located on a graded, flat, level supporting surface 19 at either side of an intermediate earth berm 3, said aprons are made to overlap over said earth berm in medial zone 15 and are secured together by suitable means. Drainage panels 20 are fused or bonded to the upper surfaces of said pondage tubes in a zone 23 along each edge. Said drainage panels are provided with a plurality of small drainage slits or apertures - the upper drainage apertures (not shown) - along the medial zone between two said pondage tubes. A plurality of small drainage slits or holes - the lower drainage apertures (not shown) - is provided in said pondage tubes along a region below the lowest expected operating level 18 of pondage water 17. The gas pressure inside said pondage tubes is normally insufficient to displace water outwardly through said drainage slits or holes, hi the preferred embodiment, said drainage slits are between 10 and 30 millimetres in length and separated by a distance of 5 to 25 millimetres, but other cut shapes (for example, + or star shapes) and combinations of dimensions and separation distance are optionally employed. In the preferred embodiment, said drainage holes are arranged in 2, 3 or 4 parallel rows, are 2.5 to 6.0 millimetres in diameter and separated by a distance of 5 to 15 millimetres, but other combinations of row numbers, hole diameters and separation distances are optionally employed. During periods of rain, water gravitates to said medial zone and passes inwardly through said upper drainage apertures to accumulate in spaces 24 between the upper edges of said pondage tubes and the upper part of said apron-covered earth berm. A small head of accumulated water is sufficient to separate the abutting surfaces of said pondage tubes and said aprons, thereby permitting an inflow of water to said pondage tubes via said lower drainage apertures. During dry weather, evaporative losses from said arrangement are minimal.

With reference to Figure 3, in a second preferred embodiment, pondage tubes 2 are made with laterally extending aprons 14 fused or bonded to their lower surfaces in a zone 16 along each edge. When two said pondage tubes are located on a graded, flat, level supporting surface 19 at either side of an intermediate earth berm 3, said aprons are made to overlap over earth berm in medial zone 15 and are secured together by suitable means. Drainage panels 21 are located between the sides of said pondage tubes and the sides of said apron-covered earth berm. Said drainage panels are made corrugated and foraminous (not shown) over their whole surfaces and are positioned with their upper edges exposed while extending below the expected operating level of pondage water 17. hi the preferred embodiment, said corrugations have a wavelength of between 2.5 and 4.0 millimetres and an amplitude of between 3.0 and 6.0 millimetres, said foramina being round or other shape with an approximate diameter of 2.5 to 6.0 millimetres at a separation of between 2.5 and 8.0 millimetres. Other combinations of corrugation wavelength and amplitude or foramina size and spacing are optionally employed. A plurality of small drainage slits or holes is provided in said pondage tubes in a zone (general position indicated as 22) along a region below the lowest expected operating level of pondage water 17. The gas pressure inside said pondage tubes is normally insufficient to displace water outwardly through said drainage slits or holes, hi the preferred embodiment, said drainage slits are between 10 and 30 millimetres in length and separated by 5 to 25 millimetres, but other cut shapes (for example, + or star shapes) and other combinations of dimensions and separation distance are optionally employed. In the preferred embodiment, said drainage holes are arranged in 2, 3 or 4 parallel rows, are 2.5 to 6.0 millimetres in diameter and separated by a distance of 5 to 15 millimetres, but other combinations of row numbers, hole diameters and separation distances are optionally employed. During periods of rain, water gravitates to accumulate in spaces 25 between the upper edges of said pondage tubes and the upper part of said apron-covered earth berm. Said accumulated water drains down through said corrugations and foramina of said drainage panels and between the surfaces of said pondage tubes and said aprons to enter said pondage tubes via said drainage apertures. During dry weather, evaporative losses from said arrangement are minimal.

With reference to Figure 4, alternate adjacent ends of said long, parallel runs of pondage tubes 2 are joined by rigid 180 degree bends 4. In the preferred embodiment, said bends are moulded or formed from a suitable, more or less rigid material in two complementary shells 26 which are sealingly joined by fixing together edge flanges 27 of the said parts with suitable fastenings (not shown). The lower said complementary shell is provided with integral guide vanes (positions indicated in broken line as 28), the height of which approximates the depth of pondage water flowing through said bend. Said guide vanes are shaped to be more or less semi-circular in planform and act to ensure that water flows through said bends with no tendency towards pooling or stagnation. The open ends of pondage tubes 2 are positioned over attachment mouldings 29 of said bends, a face view of which is depicted in Figure 7. A suitable sealant is preferably provided between said pondage tube ends and said attachment mouldings and said pondage tube ends are preferably secured to said opening mouldings with suitable clamping bands 30. Where used, rigid 90 degree bends or rigid straight couplings take substantially the same form as said rigid 180 degree bends.

With reference to Figure 5, in an alternative embodiment, said alternate adjacent ends of said long, parallel runs of pondage tubes 2 are joined by 180 degree bends 4. In this embodiment, said bends are fabricated from a suitable thin, flexible, sheet polymer material of the type used to make said pondage tubes. Said bends comprise radially segmented panels 31 which are joined by fusion or bonding at strong, radial seams 32, the more or less semi-circular sheets so formed being assembled in pairs by their abutting edges being formed into strong seams 33 by fusion or bonding. The open ends of pondage tubes 2 are joined to the openings of said bends by their being formed into strong seams by fusion or bonding. During joining of said pondage tube ends to said openings of said bends, said parts to be joined are preferably overlapped and stretched to provide a flat seam. In one embodiment, the parts to be joined are coated in a suitable adhesive and pressure applied to said stretched seam by a suitable press until said adhesive has cured or set. One or more sheets of soluble paper are preferably placed between the inner surfaces of said pondage tubes and said bends in the area adjacent the seam to prevent any accidental adhesion during said bonding process. A separate assembly of guide vanes of the type depicted in Figure 6 are employed with the type of soft, fabricated bend described.

With reference to Figure 6, in the type of soft, fabricated bend depicted in Figure 5, a separate guide vane assembly is placed between each said pair of more or less semicircular sheets (as described in relation to Figure 5) prior to their being joined at the edges. Said guide vane assemblies comprise radially arranged frame members 43 joined at gusset 45. Said frame members are optionally slotted or provided with clips to accommodate the edges of guide vanes 44. The height of said guide vanes more or less approximates the depth of pondage water flowing through said bend. Said guide vanes are shaped to be more or less semi-circular in planform and act to ensure that water flows through said bends with no tendency towards pooling or stagnation. A platform (position indicated in broken line as 46) is optionally provided above said guide vanes to accommodate suitable weighting employed to maintain said guide vane assembly in place in said bend.

With reference to Figure 7, attachment mouldings 29 of said bends described in relation to Figure 4 are made thick enough to be more or less rigid and are created when two said complementary shells (as described in relation to Figure 4) are joined at flanges 27. In the preferred embodiment, bracing webs 35 are provided within said attachment mouldings to support them against the crushing forces of a clamping band (depicted as 30 in Figure 4) employed to sealingly attach the open ends of said pondage tubes to said attachment mouldings. In the preferred embodiment, said bracing webs are arranged in zig-zag fashion, as depicted, the apices of which are bonded to the inner surfaces of both said shells. Other arrangements of said bracing webs are optionally employed. With reference to Figure 8, in the preferred embodiment, a pondage complex is surrounded by a substantial earth berm 47 capable of impounding any adventitious water leakage, cyclonic rain or the like. Said earth berm is preferably surrounded by a substantial ditch 36 to capture any leakage through said earth berm. Surface 37 immediately outside of said ditch preferably accommodates a perimeter road and fencing (not shown). With reference to figure 9, transverse conduit 38 runs beneath and more or less normal to the alignment of runs of pondage tubes 2. Standpipe 39 is sealingly connected to said transverse conduit and passes up through earth berm 3. Flexible hose 40 is sealingly connected to the open end of said standpipe by clamping band 42 and to said pondage tube by flange 41 fused or bonded into place. In the preferred embodiment, said flexible hose descends at an angle at least 30 degrees from the vertical, said arrangement accommodating movement of the surface of pondage tube without impedence of flow from said standpipe. In an alternative embodiment (not shown), a remotely operable valve is provided in said standpipe. With reference to Figure 10, in order to provide vehicular access to parts of a pondage complex, a roadway 49 is provided by connecting pondage tubes 2 to a subsurface siphon 50. Said siphon optionally takes the form of a single siphon duct of elongated cross-sectional shape or of multiple siphon ducts debouching into a single short duct 29 of elongated cross-sectional shape formed at each end. In either case, the ends of said single siphon duct or said single short ducts have substantially the same form as the attachment mouldings described in relation to Figure 7. The open ends of said pondage tubes are sealingly attached to said single short ducts with clamping bands 48. To minimise impediment to flow, the total cross-sectional area of said siphon ducts is preferably made between 10 an 50 per cent greater than the cross-sectional area of the water flow 17.

With reference to Figure 11, take-off or delivery well 51 is provided in a said pondage run, said well being separated from flow-through duct 52 by foraminous screen 54 supported level with the floor of said pondage run. Said well is connected to transverse conduit 55 by short duct 56 and coupling 57. In the preferred embodiment, said well and flow-through duct are constructed as a unit in the manner described in relation to Figure 4 and the ends 29 of said flow-through duct take the form of attachment mouldings as described in relation to Figure 7. The open ends of said pondage tubes are sealingly attached to said attachment mouldings with clamping bands 53. Said foraminous screen acts to minimise the ingestion of water containing algae, which congregate at the upper levels of water 17 where solar irradiance is greatest. In an alternative embodiment (not shown), the water level in said pondage tube for a suitable distance upstream of said flow- through duct and within said flow-through duct above said foraminous screen is made deeper to provide slower, more tranquil flow without mixing, thereby encouraging stratification of said algae in the upper water layers. In another alternative embodiment

(not shown), a plurality of flow straightening fences is provided in said water flow upstream of and within said flow-through duct for the same purpose. Said well optionally has a width ranging from 25 per cent to 100 per cent of that of a said pondage tube, hi an alternative embodiment (not shown), said well is bisected by a transversely arranged partition, permitting an outflow to be drawn off from the upstream end via a first said transverse conduit and an inflow to be discharged at the downstream end via a second said transverse conduit.

With reference to Figures 12 and 13, water and solutes or heated or cooled water are discharged into pondage tubes via diffusers 61. Said diffusers optionally take the form of multiple smaller ducts secured to spigots 59 of transverse conduit 58 by clamping bands 60 or one or more larger ducts secured to said transverse conduit in a similar way. In the preferred embodiment, said diffusers are made from a flexible hose material made either foraminous or with an open weave, both arrangements permitting a controlled outflow throughout the length of a said diffuser. Said diffusers optionally range in length from one metre to 20 metres and effectively act to provide a perfusional inflow to a said pondage tube which prevents shocking of algae by the introduction of volumes of water differing substantially in character from that of the biological environment prevailing in said pondage tube. Said spigots are sealed to the floor of said pondage tube by their being sealingly fixed to apron 62 which is, in turn, sealingly fixed to said pondage tube floor by fusion or bonding when said pondage tube is laid in place on graded, flat, level supporting surface 19.

With reference to Figures 14 and 15, in situations in which algae may be subject to injury of a mechanical nature as a result of pumping or injury resulting from rapid temperature or pH change during the carbonation process, the pondage water is preferably divided into two streams - a biological activity stream 17 in which said algae grow and a carbonation stream 92 (depicted as vertical hatching) which is substantially free of algae. In this embodiment, said biological activity stream is separated from said carbonation stream by more or less medially arranged diffusion membrane 93. Pondage 1 is thereby divided into an upper chamber containing said biological activity stream and a gas body which maintains the upper polymer film panel 94 of said pondage tubes above the surface of said biological activity stream, and a lower chamber containing said carbonation stream (indicated by vertical hatching) and a plurality of longitudinally arranged, inflatable displacement tubes 95. Said displacement tubes are fixed along opposite extremities of a vertical, diametral plane of each (diametral in relation to their inflated form) to the under surface of said diffusion membrane and the upper surface of lower polymer film panel 96 of said pondage tubes. In the preferred embodiment, the fully inflated diameters of said displacement tubes are all identical and fall in the range 0.5 to 1.5 times the normal daytime working water depth in said biological activity stream. In alternative embodiments, the fully inflated diameters of said displacement tubes are all identical and fall in the range 0.5 to 3.0 times the normal daytime working water depth in said biological activity stream. In another alternative embodiment, the fully inflated diameters of said displacement tubes are not identical and are made larger in one zone of said lower chamber and smaller in another zone. When fully deflated, said displacement tubes are able to be extended to a height of approximately 1.5 times their inflated diameters, in the manner depicted in Figure 14. Also in the preferred embodiment, said displacement tubes are located on centres (that is, the centres of their fully inflated forms) of 1.0 to 4.0 times their fully inflated diameters. Additional spacing is optionally provided between the edges of a said pondage run and the adjacent said displacement tubes. In a first typical embodiment, the normal daytime working water depth in said biological activity stream is 300 millimetres and the fully inflated diameters of said displacement tubes is 250 millimetres. During night hours, through progressive deflation of said displacement tubes, said working water depth in said biological activity stream is reduced to 160 millimetres and the depth of water in said carbonation stream is increased to 390 millimetres. In a second typical embodiment, the normal daytime working water depth in said biological activity stream is

300 millimetres and the fully inflated diameters of said displacement tubes is 200 millimetres. During night hours, through progressive deflation of said displacement tubes, said working water depth in said biological activity stream is reduced to 190 millimetres and the depth of water in said carbonation stream is increased to 310 millimetres. Other dimensional combinations are used as required.

With additional reference to Figure 20, said diffusion membrane takes the form of a sheet of suitable polymer film material of suitable thickness provided over the greater part of its area with short slits 97 in staggered, parallel arrays. In the preferred embodiment, said slits are arranged in longitudinal rows with a slit in one row adjacent substantially un- slit areas in adjacent rows. In alternative embodiments, said slits are arranged in oblique rows, transverse rows or herringbone arrangement. Typically, the length of said slits falls in the range 10 to 30 millimetres, the separation (end-to-end) distance of said slits falls in the range 10 to 50 millimetres, and the separation (lateral) distance between rows of said slits falls in the range 5 to 50 millimetres. In alternative embodiments (not shown), irregular arrangements of slit length, end-to-end separation and lateral separation are optionally employed in different parts of a pondage. For example, in a first alternative embodiment, the configuration of said slits is optionally adjusted in zones along the side edges of said diffusion membrane to permit a greater water flow through said membrane in that zone and thereby to ensure that no areas of stagnation develop within the side edge zones of said pondage lower chamber In a second alternative embodiment, in order to provide a more even flow of water through said diffusion membrane, said slits are made smaller in zones closer to the point of discharge of water into said lower chamber and larger in zones more remote from said points of discharge. In other alternative embodiments (such as those described in relation to Figure 19), suitable apertures are optionally used in place of said slits. In the preferred embodiment, said polymer film material is similar to those detailed in relation to Figure 1 with a thickness in the range 0.1 to 1.0 millimetres. In alternative embodiments, a thickness of up to 2.5 millimetres is optionally employed. In the preferred embodiment, said diffusion membrane is made free of said slits in narrow, longitudinally arranged zones in which said inflatable displacement tubes are fixed to it. The edges of said pondage tubes are supported by suitable supporting means 98 interposed between pondage runs.

In operation, water is drawn off from said pondage upper chamber via wells as described herein in relation to Figures 19 and 23, pumped to suitable means in which its temperature is optionally adjusted and carbon dioxide and other compounds are optionally dissolved in it, and returned to said pondage lower chamber. Persons expert in the art will readily understand how the arrangement described in relation to Figure 11 may be adapted by the addition of a second flow-through duct (as described in relation to Figure 22) to direct a flow of water into said pondage lower chamber. During periods of darkness, inflatable displacement tubes 95 are deflated and water entering said pondage lower chamber simply causes diffusion membrane 93 to be displaced upwardly as depicted in Figure 14. Said inflowing water thus accumulates in said lower chamber and, as no pressure differential exists across said diffusion membrane, slits 97 of said membrane remain closed and no cross-flow of water occurs. Care is taken to ensure that upward displacement of said diffusion membrane does not exceed one half of the circumference of said displacement tubes. When daylight returns, said displacement tubes are slowly inflated until they achieve a more or less round cross-sectional shape, as depicted in Figure 15. The resultant drawing down of said diffusion membrane and increasing cross-sectional area of said displacement tubes acting to displace water from said lower chamber across said diffusion membrane to mix with the water in said biological activity stream in said upper chamber. In the preferred embodiment, the rate of inflation of said displacement tubes is controlled with reference to pH sensors in said biological activity stream. Said control of the rate of inflation of said displacement tubes is optionally effected by progressively inflating all said tubes simultaneously or by inflating some tubes only. Said discharge of water into said pondage lower chamber is maintained via the longitudinal passages effectively created between said inflated displacement tubes and as said diffusion membrane is restrained from upward displacement by said displacement tubes, a slight pressure differential is created across it. Said pressure differential has the effect of slightly stretching said diffusion membrane such that slits 97 begin to open and permit a cross- flow. With typical water volume flows and areas of said diffusion membrane, the rate of water flow across said membrane is normally less than one litre per minute per square metre of said membrane area, which flow mixes with the flow of water within said upper chamber. As the water flow velocity in said upper chamber is normally of the order of 300 millimetres per second, said water flowing across said diffusion membrane, whether hotter, colder or of lower pH, is rapidly mixed into said flow without shock or other injury to said algae. When darkness again returns, said displacement tubes are again progressively deflated to permit said diffusion membrane to rise, thereby eliminating said pressure differential across said diffusion membrane, interrupting the flow of water across said membrane and allowing water to again accumulate in said lower chamber. The use of pondage comprising said upper and lower chambers provides the advantage of a greater working water volume with the concomitant advantage of greater thermal stability. During inflation of said displacement tubes, lateral stress upon said diffusion membrane is accommodated by stretching of said membrane and drawing in of the sides of said pondage. Longitudinal stress of said diffusion membrane is easily accommodated by a very small degree of stretch over the long pondage lengths normally employed. The increased water pressure in said lower chamber caused by the superincumbent load of said biological activity stream in said upper chamber acts to improve in a minor way the solubility of carbon dioxide in said carbonated water in said lower chamber. Any carbon dioxide outgasing from said carbonated water tends to diffuse through said diffusion membrane in well dispersed, small bubbles which are taken up in the rapid flow of said biological activity stream and rapidly redissolved. hi an alternative embodiment (not shown), said diffusion membrane is replaced with an impervious panel of polymer film material. Water is taken off from said biological activity stream in the manner described, processed in the manner described and, as appropriate, directed to said lower chamber for storage or returned to said upper chamber and injected into said biological activity stream via means similar to those described in relation to Figures 12 and 13. Where said inflowing water is stored in said lower chamber, it is transferred, as appropriate, to said upper chamber and injected into said biological activity stream via means similar to those described in relation to Figures 12 and 13.

With reference to Figure 16, said pondage runs are separated in a lateral sense by longitudinally arranged pipes 99 laid upon supporting surface 19. In this embodiment, material from said supporting surface is heaped against said pipes and shaped to provide smoothly curving transition surfaces 100 between said supporting surface to said pipes.

With reference to Figure 17, said pondage runs are separated by elongated pipes 101. In the preferred embodiment, said pipes are made with a ratio of length to width in the range 2:1 to 8:1 and are set in said supporting surface with their major axes orientated more or less vertically, hi the preferred embodiment, the cross-sectional shape of said elongated pipes is regular and more or less elliptical or, as depicted in the figure, made with more or less flat sides and semi-circular ends. In an alternative embodiment (not shown), said elongated pipes are made with an irregular cross-sectional shape and broader at the top or at the bottom, hi the preferred embodiment, said elongated pipes are made with curving wings 102 fixed to their lower ends, said wings curving downwardly and outwardly such that their outer ends contact said supporting surface. Said wings provide smoothly curving transition surfaces between said supporting surface and said pipes, hi the preferred embodiment, said elongated pipes are provided in their upper zones with suitable apertures, transversely arranged slots or the like which permit ingress to their interiors of rainwater running off the upper polymer film panel 94 of said pondage tubes (as depicted in Figures 14 and 15). Also in the preferred embodiment, in order to permit proper drainage of said rainwater from said elongated pipes, said pipes have their lower edges set into said supporting surface to a greater or lesser degree to provide a suitable fall. Said elongated pipes are carried across the tops of drainage wells 104 covered by cover plates 107 provided with suitable apertures 105. The bottom edge of said pipes immediately above said apertures (depicted in broken line as 106) is cut away to permit drainage of said rainwater into said wells. Water collecting in said wells is pumped away via suitable conduits to storage or disposal.

With additional reference to Figures 18a and 18b, with normal water levels maintained in said biological activity stream and normal gas pressures maintained above the surface of said water, said apertures of said elongated pipes are located such that they are covered by upper polymer film panels 94, urged against said apertures by the pressure of water 17. With said gas pressure within said upper chamber suitably increased, said polymer film panels are over-inflated such that their sides are drawn inwardly, thereby exposing said apertures and permitting the ingress of rainwater into the interiors of said elongated pipes, hi this way, said apertures may be kept covered during dry periods, thereby minimising the possibility of colonisation of water accumulated in said pipes by aquatic insects, such as mosquitoes.

With reference to Figures 14, 15 and 19, at the end of each said pondage run, said pondage upper chamber, as defined by upper polymer film panel 94 and diffusion membrane 93, terminates in bend 125 which transfers flow through a directional change of 180° to enter the start of the next said pondage run. Integral guide vanes 128 guide the water flow through said bend and ensure that no areas of stagnation develop within said bend. Said bend is generally of the same character and arrangement as that depicted in Figure 4, with the optional incorporation of gas collection shroud 127 and/or water takeoff or return well 114. Where said gas collection shroud is incorporated, the upper or roof panel 126 of said bend is made foraminous to permit the passage of gas therethrough. Outlet conduit 129 is connected to said gas collection shroud. Where said takeoff or return well is incorporated, the lower or floor panel 124 of said bend is made foraminous to permit the passage of water therethrough. The walls 120 of said takeoff or return well extend downwardly into the sub-surface area 131 where they intercept takeoff or return conduit 121. The cross-sectional area of said well is selected to ensure that vertical flow velocities are substantially below the flow velocities maintained in said biological activity stream. Said pondage lower chamber, as defined by said diffusion membrane and lower polymer film panel 96, terminates in closure plug 132. Said closure plug is provided with curved inner deflection face 130 which directs flow upwardly and ensures no areas of stagnation develop within the end zone of said pondage lower chamber. Displacement tubes 95 attached to said diffusion membrane and said lower polymer film panel are drawn together at their ends 110 and sealingly clamped to a rigid part of inflation/deflation hoses 111 by clamps 112. Said inflation/deflation hoses pass out through said curved deflection face to join manifold 113 and are made with flexible zones (not shown) to permit movement of said ends of said displacement tubes. With additional reference to Figure 22, the entry to and exit from said bend take the form of attachment mouldings 182 comprising upper panel 115, lower panel 116 and ends 122. Said upper panel is made arcuate, said lower panel is made flat and the two are braced from each other by zig-zag-type bracing panels 119. The means of attaching said ends of said pondage lower runs to closure plugs 132 take the form of attachment mouldings 183 comprising upper panel 117, lower panel 118 and ends 122. Said upper panel is made flat and complementary to said lower panel of said bend entry. Said lower panel is made arcuate and with the same width and degree of curvature as said upper panel of said bend entry. Said closure plug upper and lower panels are braced from each other by zig-zag-type bracing panels 119. In an alternative embodiment (not shown), said upper and lower panels of said bend entry and closure plug are braced from each other by vertical bracing panels. Regardless of the type, said bracing panels are made with minimal cross- sectional area and provide minimal impediment to flow through said bends. Upper polymer film panel 94 and diffusion membrane 93 pass neatly around attachment moulding 182 with no wrinkling or pleating. Diffusion membrane 93 and lower polymer film panel 96 pass neatly around attachment moulding 183 with no wrinkling or pleating. When said bend entry and said closure plug are assembled together with their said complementary flat panels in abutment, said diffusion membrane is captured between their adjacent flat faces. Moulded filler pieces 123 made from a suitably resilient material pass along the ends of said bend entry and said closure plug and, by filling the recess between said ends, create a smoothly curving surface covering both ends of said assemblage. Said smoothly curving surfaces allow said upper and lower polymer films to be sealingly clamped into place by clamping band 119 which passes completely around both said attachment mouldings. In the preferred embodiment, the external surfaces of said attachment mouldings and said filler pieces are coated with a suitable sealing material to ensure the sealing attachment to them of said upper polymer film panel 94, diffusion membrane 93 and lower polymer film panel 96. In a first preferred embodiment, said sealing material takes the form of a room temperature-vulcanising silicone rubber, hi a second preferred embodiment, said sealing material takes the form of a thin, soft polymer foam material impregnated with a suitable non-setting sealant. In the preferred embodiment, said bend, gas collection shroud, well and closure plug are made by spraying a thermo-setting resin reinforced with chopped strand glass fibre into a suitable mould, hi alternative embodiments, said components are fabricated from suitable metal alloy sheet material, rotationally moulded from suitable thermo-plastic resins or fabricated from suitable thermo-plastic sheet material, hi all cases, sub-components are joined by suitable welding or bonding processes.

With further reference to Figure 22, in an alternative embodiment (not shown), a method of joining sections of pondage run is provided comprising attachment mouldings 182 and 183, each supported by internal zig-zag-type bracing panels 119 and fixed in correct juxtaposition to a flat joining flange. The ends of said upper polymer film panel 94, diffusion membrane 93 and lower polymer film panel 96 are sealingly attached to said attachment mouldings in the manner described. Abutting ends of a said pondage run are joined by sealingly fixing their said joining flanges together using a plurality of suitable fastenings. Each said joining flange is made with an aperture conforming to the shape of the combined openings of said attachment mouldings and, when a pair of said joining flanges are joined in the manner described, provide an uninterrupted flow of water from one section of pondage run to the other. In another alternative embodiment (not shown), pondage runs are terminated in the manner described and are joined to fabricated system components, such as said bends, by fixing them in the manner described to complementary joining flanges incorporated into said system components. In another alternative embodiment (not shown), said attachment mouldings are made without said internal bracing and their flat abutting surfaces are fastened together using suitable fastenings to capture said diffusion membrane between them. Said fastenings are positioned and tightened via suitably located apertures in one of said upper or lower arcuate panels, said apertures being subsequently closed using suitable plugs. Said upper and lower polymer film panels are then positioned over said attachment mouldings in the manner described and sealingly attached using a single clamping band which passes totally around both said attachment mouldings. In this embodiment, said filler pieces and sealing means are also employed as described in relation to Figure 22.

With further reference to Figure 19 and Figure 30, in the preferred embodiment, the zone 109 of diffusion membrane 93 adjacent curved inner deflection face 130 is made with a plurality of apertures 133 of suitable diameter which are covered by hinged flaps 134 fixed to the upper surface of said membrane, hi the preferred embodiment, said apertures are made with diameters in the range 2.5 to 7.5 millimetres and are arranged in transverse rows facilitating the coverage of each row by a single, elongated, transversely-arranged, hinged strip. Said strips or flaps are fixed to said diffusion membrane at their upstream ends and are unseated to permit flow through said apertures whenever the water pressure in said lower chamber exceeds that in said upper chamber. Said apertures act to ensure that no areas of stagnation develop within the end zone of said pondage lower chamber but prevent any backflow from said upper chamber into said lower chamber.

With reference to Figure 21, upper polymer film panel 94, diffusion membrane 93, lower polymer film panel 96 and displacement tubes 95 are assembled by a suitable machine (not shown) on a continuous basis to create a said pondage channel. In this embodiment, said diffusion membrane, said displacement tubes in flat form and said lower polymer film panel are fed from rolls (not shown) and guided by rollers (not shown) into the relative positions depicted in the figure. Said flattened displacement tubes are provided internally with a continuous strip of a suitable separation material 154 which prevents adhesion of their inner surfaces one to another during welding in the assembly process. In the preferred embodiment, said separation material takes the form of a suitable paper or other film material coated with graphite, talcum, silicone or the like. Suitable welding heads (not shown) weld a narrow zone 155 along the mid upper surfaces of said inflation tubes to said diffusion membrane and a narrow zone 156 along the mid lower surfaces of said inflation tubes to said lower polymer film panel. Said welding of said upper and lower surfaces of said inflation tubes is optionally performed sequentially or simultaneously. Sequentially or simultaneously, said upper polymer film panel is fed from rolls (not shown), guided by rollers (not shown) into the relative position depicted in the figure, and welded by suitable welding heads (not shown) to said diffusion membrane and said lower polymer film panel along narrow edge zones 153. Where means to control solar irradiance (as described in relation to Figures 24a to 28 (inclusive) and 35a and 35b) are incorporated into said upper polymer film panel, these are welded to said upper polymer film panel prior to its assembly to said diffusion membrane and said lower polymer film panel. The joining together of polymer films on a continuous basis is well known in the art and methods which may be adapted to the present invention include direct thermal welding, ultrasonic welding, induction welding, laser welding, radio frequency (RF) welding and bonding with adhesives which are optionally radiation-cured. In order to generate the required depth of said upper and lower pondage chambers, said upper and lower polymer film panels are made suitably wider than said diffusion membrane, said additional width being accommodated in temporarily induced pleating during welding or bonding of said upper and lower polymer films to said diffusion membrane. In the preferred embodiment, said pleating is induced by suitably positioned rollers, shoes or the like (not shown). Also in the preferred embodiment, slitting and apertures (as depicted in Figures 20 and 30) are deleted from narrow, longitudinally arranged zones of said diffusion membrane in which said welding or bonding is to be performed. Also in the preferred embodiment, at least one outer surface of each said welding zone is coated with a tell-tale material the colour of which is changed by the heat of the welding process. Machine vision is employed to detect any area in which the colour change is absent or inconsistent with that of proper welding. Where a bonding process using a suitable adhesive is employed in place of welding, said adhesive preferably being coloured and machine vision being employed to detect any area in which said adhesive is absent or inconsistent with that of proper bonding. Said pondage channel assemblies are made to length, with said displacement tubes made shorter than said upper and lower polymer film panels and said diffusion membrane and with short lengths at their open ends left free to permit their being drawn together for connection to said inflation/deflation hoses. In an alternative embodiment (not shown) each said strip of separation material is made with a width to match the interior width of its said flattened displacement tube and carries temporarily fixed to each side on its centreline a strip of suitable width of a suitable thermoplastic reinforcement material. When said diffusion membrane and said lower polymer film panel are welded to a said displacement tube, said strips of thermoplastic reinforcement material are simultaneously welded to the interior surfaces of said displacement tube. The incorporation of said strips of thermoplastic reinforcement material serves to strengthen the welded joint between said components. In another alternative embodiment (not shown), a similar strip of thermoplastic reinforcement material is also positioned on said diffusion membrane and said lower polymer film panel prior to their welding to said displacement tubes. In this embodiment, a welded joint thus comprises the thickness of said diffusion membrane or said lower polymer film panel, the thickness of said displacement tube and the thicknesses of two said strips of thermoplastic reinforcement material. Similarly, the incorporation of said additional strips of thermoplastic reinforcement material serves to strengthen the welded joint between said components. With reference to Figures 23 and 36, said ends of said upper chamber, as defined by polymer film panel 94 and diffusion membrane 93, and said ends of said lower chamber, as defined by diffusion membrane 93 and polymer film panel 96 are sealingly clamped to attachment mouldings 144, 148 of a take-off or delivery well 135. Water is taken off from or returned to biological activity stream 17 in flow-through duct 148 via a plurality of narrow, vertical ducts 136 passing vertically through said lower chamber to said take-off or delivery well, the openings of said vertical ducts being covered by foraminous plate 140. Said carbonation stream in said lower chamber passes above said well via horizontally arranged ducts 137 which are increased in depth (typical lower extent indicated as 146) to provide an increase in cross-sectional area and, thereby, to reduce impediment to flow. In the preferred embodiment, the width and depth of said horizontally arranged ducts are selected to provide, in total, the same or substantially the same cross- sectional area as the uninterrupted said lower chamber. Said attachment mouldings are braced by zig-zag bracing generally of the type depicted in Figure 22, said bracing having been deleted for illustrative clarity. The approximate inward extent of said bracing within said attachment mouldings is delineated in broken line as 151. Said horizontally arranged ducts pass from wall 138 to wall 139 across the full width of said take-up and delivery well and debouch at each end into a collector 145 of similar depth which contracts towards its open end to create attachment moulding 144. Said collectors are made separately from flow-through duct 148 and are fixed to the sides of said take-off or delivery well such that a narrow separation is provided between their upper surfaces and the lower surfaces of said flow-through duct. During the sealing attachment of the ends of said upper and lower chambers to said inlets and outlets, said diffusion membrane is captured in said narrow separation. As clamping band 143 is tightened to effect said sealing attachment (generally as described in relation to Figure 19), a small elastic distortion of said well sides allows said collectors to be drawn into intimate contact with said flow-through duct. Gas collection shroud 141 is optionally incorporated into the upper or roof panel 152 of flow- through duct 148, the area of said upper roof panel between them either being removed or made foraminous (as depicted in Figure 23) to permit the passage of gas therethrough. Outlet conduit 142 is connected to said gas collection shroud. Where water is taken off for the purpose of harvesting said algae, a foraminous plate (position indicated in broken line as 147) is optionally installed at approximately mid depth of biological activity stream 17 and the upper, open ends of said narrow, vertical ducts 136 are extended to abut the edges of suitable apertures provided in said foraminous plate. In this embodiment, algae concentrated in the upper layers of said biological activity stream as a response to solar irradiance are selectively collected and water having a lower concentration of algae continues in uninterrupted flow beneath said foraminous plate. The walls 138, 139 of said takeoff and delivery well extend downwardly into the sub-surface area 131 where they intercept takeoff or delivery conduit 149. The cross-sectional area of said well is selected to ensure that vertical flow velocities are substantially below the flow velocities maintained in said biological activity stream, hi this embodiment, said displacement tubes in said lower chamber are terminated short of said take-up and delivery well and the closed, adjacent ends of said tubes are joined by inflation/deflation hoses 111 passing through horizontally arranged ducts 137. Said inflation/deflation hoses are made with flexible zones (not shown) to permit movement of said ends of said displacement tubes.

With reference to Figures 24a and 24b, in many hot, sunny climatic situations, pondage channels will receive solar irradiation at levels injurious to algae or, alternatively, water in said biological activity stream will become heated to a temperature injurious to algae. To mitigate these effects, upper polymer film panel 94 is provided on its exterior surface with a large plurality of small, flexible leaves 157 coated with a suitable reflective material. The darker line in the figures is intended to denote areas having a metallised coating. Each said leaf has at its fixed end a short flexible part 158 attached to said polymer film panel at point 159. In the preferred embodiment, longitudinally arranged, narrow, metallised conductors (not shown) are provided on the exterior surface of said upper polymer film panel, said conductors passing beneath points 159 and being in electrically conductive communication with said leaves. Said conductors are joined by a bus strip (not shown) at the end of a said pondage run to which a suitable source of static electricity (not shown) may be connected. In their relaxed condition, said leaves fall to the surface of said upper polymer film panel (as depicted in Figure 24a) and act to substantially reflect away solar irradiance. Connection of said static source to said conductors via said bus strip causes all said leaves to receive the same static charge. Mutual repulsion then causes said leaves to be deployed to their erect position (as depicted in Figure 24b), thereby substantially eliminating their reflective function. In the preferred embodiment, said leaves are made from a light, strong, flexible material, such as metallised Mylar with a thickness in the range 0.025 to 0.1 millimetre. In the preferred embodiment, said leaves project from said upper polymer film panel 94 by a distance in the range 10 millimetres to 75 millimetres. Also in the preferred embodiment, said leaves have a width in the range 10 millimetres to continuous lengths. Electronic generators of static electricity suitable for employment in this embodiment are well known in the art. Obviously, in their relaxed positions, said leaves act to reduce radiation heat loss from said biological activity stream.

With reference to Figures 25a and 25b, excessive solar irradiation and excessive heating of water in said biological activity stream are prevented by the provision on the exterior surface of upper polymer film panel 94 of a plurality of closely-spaced, longitudinally arranged, pneumatically deployable reflector units. In this embodiment, in inflated form, said reflector units take the form of tubes of substantially circular cross- sectional shape, approximately one third of which 160 is coated with a suitable reflective material (depicted in darker line) and two thirds of which 161 is transparent or translucent. hi the preferred embodiment, in their inflated form, said reflector units have an approximate diameter in the range 50 millimetres to 300 millimetres, hi this embodiment, each said transparent or translucent part is provided with a suitable inwardly directed crease at its mid width to promote its inward folding and, at its junction with said reflectively coated part, with a suitable outwardly directed crease to promote inward folding of said transparent or translucent part. The edges 162, 163 of said reflector units are sealingly fixed to said exterior surface of said upper polymer film panel and the ends of said reflector units are folded down and sealingly fixed to said exterior surface of said upper polymer film panel. At least one said sealingly fixed end of each said reflector unit incorporates a suitable inflation/deflation fitting (not shown) by means of which said reflector units may be inflated or deflated using a suitable gaseous medium. When said gaseous inflation medium is evacuated from said reflector units, they assume the collapsed position depicted in Figure 25a in which solar irradiance is substantially reflected. Said reflector units are inflated to deploy them to the position depicted in Figure 25b in which their reflective function is substantially eliminated. Obviously, said reflector units may be partially inflated to a greater or lesser degree to more precisely regulate the amount of solar irradiance entering said biological activity stream. With reference to Figure 26, entry of solar irradiance into said biological activity stream is controlled by a plurality of closely-spaced, longitudinally arranged, pneumatically deployable reflector units fixed to the exterior surface of upper polymer film panel 94. In this embodiment, in their inflated form, said reflector units take the form of tubes of substantially circular cross-sectional shape having an approximate diameter in the range 50 millimetres to 300 millimetres. In collapsed form, each said reflector unit comprises two laterally disposed, light obstructing parts 164 of more or less equal width coated with a suitable reflective material (depicted in darker line) and a centrally located, transparent or translucent part divided into two lateral panels 181 positioned more or less parallel to said light obstructing parts and a lower panel 165 joining said lateral panels positioned more or less parallel to said upper polymer film panel 94. Said light obstructing parts are made to constitute approximately 40 per cent of the surface area of said reflector units. Said lateral parts are provided with two suitable inwardly directed creases at their inner ends to promote their inward folding and the positioning of said lower panel. Said lateral panels are provided at their junctions with said reflectively coated parts with suitable outwardly directed creases to promote inward folding of said lateral panels. The edges 162, 163 of said reflector units are sealingly fixed to said exterior surface of said upper polymer film panel and the ends of said reflector units are folded down and sealingly fixed to said exterior surface of said upper polymer film panel. At least one said sealingly fixed end of each said reflector unit incorporates a suitable inflation/deflation fitting (not shown) by means of which said reflector units may be inflated or deflated using a suitable gaseous medium. When said gaseous inflation medium is evacuated from said reflector units, they assume the collapsed position depicted in the figure in which solar irradiance is substantially reflected. When said reflector units are inflated (not shown) solar irradiance is admitted to said biological activity stream.

With reference to Figures 27a and 27b, entry of solar irradiance into said biological activity stream is controlled by a plurality of closely-spaced, longitudinally arranged, pneumatically deployable reflector units fixed to the exterior surface of upper polymer film panel 94. hi their inflated form, said reflector units have an approximate diameter in the range 50 millimetres to 300 millimetres. In this embodiment, said reflector units take a form similar to that depicted in Figure 26, excepting that said light obstructing parts 169, 170 are made to constitute only approximately 25 to 35 per cent of the surface area of said reflector units. Each said transparent or translucent part is provided with a suitable outwardly directed crease at its mid width to promote outward folding of a central part 166 and two inwardly directed creases at the inner ends of said central part to promote the inward folding of two lateral panels 167, 168. Each said lateral panel is provided at its junction with said reflectively coated parts with suitable outwardly directed creases to promote inward folding of said lateral panels. When said gaseous inflation medium is evacuated from said reflector units, they assume the collapsed position depicted in figure

27a in which solar irradiance is substantially reflected. When- said reflector units are inflated (as depicted in Figure 27b) solar irradiance is admitted to said biological activity stream.

With reference to Figure 28, entry of solar irradiance into said biological activity stream is controlled by a plurality of closely-spaced, longitudinally arranged, pneumatically deployable reflector units 171 fixed to the exterior surface of upper polymer film panel 94. Said reflector units preferably take any of the forms or any combinations of the forms described in the foregoing. It will be appreciated from the figure that, with said reflector units fully deployed (inflated), little obstruction of solar irradiance occurs.

In preferred embodiments, said reflector units are made from a suitable polymer film with the thickness in the range 0.025 to 0.5 millimetre, hi installing any of said reflector units on said upper polymer film panels, due consideration is given to the direction from which the most intensive solar irradiance is to be received and said reflector units are optionally aligned accordingly. Where multiple rows of said reflector units are installed on said upper polymer film panels in closely-spaced arrangement, the edges 162, 163 of each said reflector unit are fixed in such a way as to fully overlap the edges of adjacent said units, hi this way, the least obstruction is offered to the entry of solar irradiance to said biological activity stream. When said reflector units installed on said upper polymer film panels in closely-spaced arrangement are fully inflated, said reflectively coated parts are more or less in full abutment and parallel, in which position they provide the least obstruction to the entry of solar irradiance to said biological activity stream. In alternative embodiments (not shown), said reflectively coated parts are not fully coated, but are coated in a pattern permitting the passage of a predetermined proportion of incident solar irradiance. hi a first preferred embodiment, said reflector units are inflated to a greater or lesser degree to more precisely regulate the amount of solar irradiance entering said biological activity stream. In a second preferred embodiment, parts of the complement of said reflector units on a pondage upper polymer film panel are inflated or deflated to regulate the amount of solar irradiance entering said biological activity stream. Obviously, in their collapsed conditions, said reflector units also act to reduce radiation heat loss from said biological activity stream. In regulating the entry of solar irradiance into said pondage and regulating radiation heat loss from said pondage, said reflector units substantially eliminate the need to heat or cool said pondage water, hi the preferred embodiment, said reflector units are made with diameters (in their freely inflated forms) falling in the approximate range 50 millimetres to 300 millimetres with the areas constituted by said reflectively coated parts falling in the range 25 to 60 per cent. The pressure required to inflate said reflector units is low, in the preferred embodiment, falling in the range 15 to 70 KPa. With reference to Figures 27a and 29, at least one end of each said reflector unit is provided with suitable means for the inflation and deflation of said reflector units using a suitable gaseous medium, hi this embodiment, suitably shaped shoe 174 made from a suitable flexible, polymer material is sealingly bonded to said upper polymer film panel. Said shoe incorporates a suitable inflation/deflation fitting 175. In the example depicted in Figure 29, the reflector unit depicted in Figure 27a is shown with the ends of its reflectively coated parts 169, 170 and transparent or translucent parts 166, 167, 168 folded sequentially across and sealingly bonded to said shoe and to each other. In this embodiment, the dimensions of said shoe and the lengths of said two reflectively coated parts and three said transparent or translucent parts are made to provide a precise coverage of said shoe and of each successive layer in said bonded assembly. In an alternative embodiment (not shown) said shoe is made from a suitable hard, rigid material and an arcuate-shaped keeper of a suitable rigid material is fixed to said shoe over said bonded assembly. This embodiment ensures that said ends of said reflectively coated parts and said transparent or translucent parts remain in place under inflation-induced forces, hi all cases, the material of said reflector units is preferably made thicker in the region of said creases, the effect being to promote folding of said reflector units into their collapsed state when deflated or evacuated. With reference to Figures 37a and 37b, upper polymer film panel 94 of said pondage runs is subject to the greatest level of ultra-violet radiation and is thus the component most susceptible to deterioration. As an economy measure, said upper polymer film panel is thus made replaceable. In this embodiment, said pondage runs are made with side panels 176, the upper edges of which are carried up and over continuous beads 177 and folded under and inwardly for a suitable distance. The edges of said upper polymer film panel are similarly carried over said beads and folded under and inwardly in the manner depicted in Figure 37a. To join said side panels to said upper polymer film panels, stapling strips 178, 179 made from a suitable stiff polymer material are placed above and below said superimposed and folded-under parts and secured tightly together using suitable staples 180, thereby capturing said superimposed and folded-under parts between them, hi the preferred embodiment, said edge parts of said upper polymer film panels are extended to form aprons 177 which extend downwardly to cover the upper parts of said side panels, hi this embodiment, said aprons are preferably coated with a suitable light obscuring material which protects said upper parts of said side panels from deterioration as a result of exposure to ultra-violet radiation. In an alternative embodiment (not shown), all surfaces of said polymer film panels apart from said replaceable upper panel are coated with a suitable light obscuring material which protects said panels from deterioration as a result of exposure to ultra-violet radiation. A said side panels is separated from an edge of said upper polymer film panel by peeling upper stapling strip 180 upwardly and away from lower stapling strip 179. This has the effect of opening said staples and withdrawing them cleanly through said joined components. Those skilled in the art will appreciate that the processes of this embodiment, joining said side panels to said upper polymer film panels and separating them, are readily adapted to being performed by mechanised means.

With reference to Figure 31, bridging vehicle 90 is provided to permit access to said pondage tubes for maintenance and repair purposes. Said vehicle comprises main deck 75, tail ramp 76, inner breaking ramp 77, outer breaking ramp 78 and elevated control station 82. In the preferred embodiment, said vehicle is driven by hydraulic motors through two or four wheels, a battery or internal combustion engine-powered hydraulic system being accommodated beneath said main deck. Steering of said vehicle is optionally through two or four-wheels, the latter arrangement being preferred where said vehicle is required to negotiate tight bends. Said tail ramp pivots upwardly or downwardly along line 79. Said inner breaking ramp pivots upwardly and downwardly at line 80 and said outer breaking ramp pivots upwardly or downwardly at line 81. During transit of said vehicle, said tail ramp is preferably pivoted upwardly, said inner breaking ramp pivoted upwardly and said outer breaking ramp pivoted downwardly. To gain entry to a said pondage tube, said vehicle is positioned such that said inner and outer breaking ramps can be extended across said 180 degree bend or said circulation duct (both as depicted in Figure 1) at the end of a said pondage tube run. Said tail ramp is lowered to the ground and said inner and outer breaking ramps lowered such that the outer end of said outer breaking ramp is in contact with the floor of said pondage tube. A padded bar is preferably provided at said outer end of said outer breaking ramp to minimise the possibility of damage to said pondage tube. As the width of said outer breaking ramp is substantially less than the width of said pondage tube, flow through said pondage tube is only partially interrupted. The width of said main deck and said ramps is such that the service vehicle depicted at Figure 32 may be driven over them.

With reference to Figure 32, service vehicle 91 is provided to permit travel along said pondage tubes. Said vehicle comprises body part 84, parallelogram supporting linkage 85, work platform 86 and cleated walkway 87. The power unit is accommodated within said body part and all controls are brought to a control station (not shown). Said parallelogram linkage is employed to raise or lower said work platform while maintaining it level. Said cleated walkway provides access from said body part to said work platform. Said work platform is sufficiently wide to permit working upon all parts of a said pondage tube. Steering of said vehicle is optionally through two or four-wheels, the latter arrangement being preferred where said vehicle is required to negotiate tight bends. The tyres 88 of said wheels are wide and treadless and operate at low pressure. Grit removal brushes (not shown) are employed to maintain said tyres free of foreign material which might puncture said pondage tubes, hi operation, said vehicle gains access to a run of said pondage tube by driving across the main deck and ramps of the bridging vehicle described in relation to Figure 31 and is then able to drive along the length of said pondage tube run without damage to said pondage tube or incurring any significant diminution of water flow. Said low pressure tyres are able to negotiate said pondage tubes without damage to features such as the diffusers described in relation to Figures 12 and 13 or the flat duct described in relation to Figure 15. hi an alternative embodiment (not shown), said service vehicle is provided with smooth caterpillar tracks of rubber or other soft, polymer material.

Where wells such as that described in relation to Figure 11 are provided in said pondage tubes, ladder-type bridges are preferably employed to carry said service vehicle over said wells.

With reference to Figure 33, baseplate 63 supports a spaced plurality of vertically arranged fins 64 having a length more or less equal to the operating water depth within said pondage tubes. Said fins are slit at their mid points 65 for approximately one third to one half of their width and their upper halves 67 are deflected in one sense and their lower halves 66 deflected in the opposite sense. Said baseplate is preferably made heavy and sits upon the floor of a pondage tube 2 normal to its water flow axis. Said deflected parts act to induce vortices in water passing said fins, thereby ensuring thorough mixing throughout said water operating depth. With reference to Figure 34, flat duct 69 sits upon the floor of a pondage tube 2 normal to its water flow axis. Said duct is sealed to said pondage tube floor by apron 71 fused to said duct being sealingly fixed to said pondage tube floor by fusion or bonding when said pondage tube is laid in place on graded, flat, level supporting surface 19. The upper surface 70 of said duct is made foraminous and gas supplied to said duct via conduit 68 is emitted from said upper surface, thereby generating a dense cloud of bubbles which promote thorough mixing throughout the depth of said biological activity stream. In the preferred embodiment, said gas is carbon dioxide which is either dissolved in said pondage water or collects within the upper parts of said pondage tubes. Said duct optionally has a transverse width ranging from 25 per cent to 100 per cent of that of a pondage tube and a longitudinal width of from 100 millimetres to one metre. Said foramina are provided in said upper surface of said duct in multiple rows and range in size from 0.25 millimetre to 3.0 millimetres at a spacing ranging from 1.5 millimetres to 10 millimetres. In an alternative embodiment, said gas emitted from said duct is replaced by jets of water which act to promote thorough mixing throughout the depth of said biological activity stream. Said mixing prevents stratification in said biological activity stream and promotes the outgasing of oxygen generated in said biological activity stream by photosynthesis.

With reference to Figures 35a and 35b, the upper surface of pondage tube 2 is made with two film layers having strips 72 of suitable width which are metallised or otherwise made light reflective. Said light reflective strips are arranged alternately such that, when said film layers are abutting, incident light beams 74 are reflected and light transmission through said pondage tube is thereby substantially impeded. Where said film layers are separated by distance 73 by light gas pressure between them, some of said incident light beams are able to pass through said film layers. Said screening effect is greatest where said incident light beams are normal to the surface of said film layers and decreases as the angle between said incident rays and said surface of said film layers is reduced. Said light reflective strips are optionally arranged such that a predetermined proportion of incident light is transmitted when said film layers are in abutment. In an alternative embodiment (not shown), said two film layers are made with interlinking flexible strips which restrict the separation of said layers to a predetermined maximum distance.

Where said pondage tubes are to be laid upon a graded, flat, level surface, said surface is carefully inspected for material which might damage said tubes. Where asperities are found to exist and cannot readily be removed, said surface is covered with a layer of suitable protective material (not shown), such as sand, sawdust, newspaper, geotextile or the like. Said pondage tubes are optionally laid with their edges in abutment and without said low earth berms between them, hi alternative embodiments (not shown), said earth berms are replaced with dividers of a suitable material, including pre-cast concrete sections of generally pyramidal cross-sectional shape, pipes, vertical flat panels of wood, concrete or the like, or sheets of metal, hi an alternative embodiment (not shown), the upper surfaces of a single-layer film of said pondage tubes are covered with a pattern of light reflective material to reduce the amount of light entering said tubes, hi an alternative embodiment (not shown), the upper surfaces of a single-layer film of said pondage tubes are coated with a temporary layer of light reflective material to reduce the amount of light entering said tubes during hot weather conditions. Infra-red filtering layers are well known in the art and may comprise silver, gold, copper, aluminium, or alloys of such metals. Carbon dioxide accumulating within the upper parts of said pondage tubes is reabsorbed into said biological activity stream in zones from which carbon dioxide is depleted. Wherever practical, all flows through said conduits and ducts are regulated using remotely- controlled valve and pump means. Where heating of said pondage water is required in Winter, solar ponds are optionally used as heat collectors and water from said pondage is passed is passed in heat-exchange relationship with water from said solar ponds. Where heating of pondage water is excessive during hot weather conditions or where it is desired to reduce solar irradiance to said pondage water, balls or spheres of light, buoyant, light- reflective material are optionally floated on the surface of said pondage water. In an alternative embodiment (not shown) access ports are provided in said pondage tubes, said ports being closed by flaps sealingly secured in place by polymer slide fasteners which are well known in the art. hi another alternative embodiment (not shown) for the same purpose, flexible entry chutes are fixed to the upper surface of said pondage tubes by fusion or bonding and are closed by folding and clamping. In another alternative embodiment (not shown) for the same purpose, straight sections of rigid construction are inserted into a said pondage tube run, a removable entry hatch being provided in said straight section, hi an alternative embodiment (not shown), where it is required to minimise outgasing of carbon dioxide from carbonated water pumped to remote points of said pondage, said water is discharged into the base of a well of sufficient diameter and depth such that the pressure upon said water is slowly reduced as it rises with minimal turbulence to pondage level. During start-up of a pondage operation, a minimum viable algae colony must be maintained, hi order to achieve this, water volumes must be increased progressively, the maximum tolerable increases being determined by the particular algal species under cultivation. To accommodate this requirement, an initial algal colony of commercial size is cultivated to maturity in a suitable tank or small pondage and then transferred in a controlled manner to two full-length runs of said pondage tubes joined by a short circulation duct (as described in relation to Figure 1). When the enlarged colony has reached maturity, additional pairs of said pondage tube runs are progressively brought into operation and said circulation duct is extended as required. The process or algal colony development and pondage area increase is continued until the planned total area of pondage has been brought into operation.

Where said pondage runs are joined to fabricated components, care is taken to ensure that edges at the entry to said components are feathered to prevent any trapping and accumulation of biological material at that point. hi the preferred embodiment, where water is to be taken off from said biological activity stream at night, a zone of said pondage of suitable length immediately upstream of said take-off point is intensely illuminated to promote stratification of said algae, hi this way, water taken off from said biological activity stream downwardly at low velocity will have a minimal load of entrained algae, hi all cases, in order to encourage stratification of said algae, said zone upstream from a said water take-off point is preferably made to be deeper and tranquil with no turbulence or mixing. Wherever possible, flow guides or straighteners are employed for the same purpose. hi an alternative embodiment (not shown), strong metal stanchions are installed at suitable intervals between said pondage runs and suitable protective netting is fixed to them. Said netting is made from a suitable UV-stabilised polymer material and typically has a mesh size in the range 100 to 600 millimetres. Said netting is optionally maintained in contact with said pondage upper polymer film panel or said polymer film panel is raised by over-inflation into contact with said netting. Said netting is employed to minimise the possibility of wind damage to said upper polymer film panel. In another alternative embodiment (not shown) for the same purpose, said upper polymer film panel is strengthened by the incorporation into it of suitable straps, mesh, fibres or other embedded reinforcement material.

The use of closed pondage eliminates evaporative and percolative losses, obviates need for pond sealing, allows smaller volumes of water to be used over time, ensures a higher level of carbon dioxide utilisation, allows oxygen produced by photosynthesis to be captured for other use, allows better control of the biological environment, minimises the possibility of colonisation by wild algal species, and eliminates ingress of pollutants. The use of double-chambered pondage provides greater thermal stability, minimises thermal and pH and shock to algae and permits carbonation during periods of darkness.

Claims

1. Apparatus for the large-scale, intensive cultivation of microalgae employing pondage containers in the form of wide, substantially flat, transparent or translucent polymer film tubes, said tubes being laid in a plurality of long, parallel runs upon a graded, flat, level surface, the extreme ends of a plurality of said tube runs being connected to permit a continuous circulation of shallow water through said tube runs; said pondage tubes being divided by a selectively- permeable membrane into an upper biological activity chamber and a lower chamber employed as a conduit for the delivery of carbonated water or vessel for the storage of carbonated water during periods of darkness; the volume of said lower chamber being controllable by the deflation or inflation of longitudinally- arranged tubular elements fixed to said membrane and the lower film surface of said lower chamber such that water is either stored in said lower chamber or transferred from it to said upper chamber at a controlled rate; the upper film surfaces of said upper chamber being maintained clear of the water surface by light gas pressure and the entry of solar irradiance to said upper chamber being optionally regulated by the deployment of light-reflecting elements; adjacent ends of said pondage upper chambers being joined by suitable flow-transferring 180° bends, flow impelling means being provided to maintain said water flow through said pondage upper chambers and said bends and mixing means being provided to generate a mixing effect within said body of flowing water, said pondage lower chambers being closed at the ends of each said pondage run; means being provided to draw off water from said upper chamber, to supply water to said lower chamber or to draw off oxygen generated by biological activity within said upper chamber; the possible adverse effects of mass extinctions of microalgae being minimised by sub-dividing large areas of pondage into discrete, smaller, closed-circuit flow units, each sub-division being biologically separate from others and having its own water circulation and management means and continuous monitoring and adjustment of its aquatic environment.

2. Apparatus according to Claim 1 in which said polymer film material of said pondage tubes and said membrane is optionally used in single or multiple layers which range in thickness from 0.1 millimetres to 3.0 millimetres or greater.

3. Apparatus according to Claim 1 in which said polymer film material of said pondage tubes is modified to improve tensile strength, to endow resistance to deterioration due to ultra-violet radiation, to improve flexibility, to provide screening of ultra-violet or infra-red wavelengths, to provide resistance to fogging or to provide light reflectivity.

4. Apparatus according to Claim 3 in which light reflectivity is improved by applying strips or patterns of light-reflecting material to the upper film surface of said polymer film pondage tubes.

5. Apparatus according to Claim 4 in which said light-reflecting material takes the form of an aluminised coating.

6. Apparatus according to Claim 1 in which said lower film panel of said polymer film pondage tubes is made from a thicker, light-excluding material to better suppress plant regrowth and resist puncturing by asperities in the supporting surface.

7. Apparatus according to Claim 1 in which said upper film panel of said polymer film pondage tubes incorporates a reinforcement material in the form of suitable straps, mesh, fibres or other embedded material to better resist wind damage.

8. Apparatus according to Claim 1 in which water is circulated by means of one or more paddle wheel-type circulators provided in each said pondage sub-division.

9. Apparatus according to Claim 8 in which said paddle wheel-type water circulators are accommodated in part-cylindrical pits and enclosed within gas- tight enclosures sealingly connected to said polymer film pondage tubes.

10. Apparatus according to Claim 8 in which said paddle wheel-type water circulators have a diameter in the range 1 to 3 metres, have a length of between

0.5 and 1.5 times the width of said polymer film pondage tubes and are provided with between 15 and 30 paddle blades.

11. Apparatus according to Claim 1 in which said polymer film pondage tubes are typically approximately 5 metres wide, said water typically has an approximate depth of 300 millimetres and the flow velocity of said water is typically approximately 300 millimetres per second.

12. Apparatus according to Claim 11 in which said polymer film pondage tube width, water depth and water flow velocity are greater or less.

13. Apparatus according to Claim 1 in which said parallel runs of polymer pondage tube are separated and given locational stability by low, narrow earth berms; precast concrete sections of generally pyramidal cross-sectional shape; vertical flat panels of wood, concrete or the like or sheets of metal; regularly-shaped pipes of round or elliptical cross-sectional shape, flat pipes with parallel sides in the ratio of length to width in the range 2:1 to 8:1, pipes with irregular cross-sectional shape and broader at the top or the bottom; and pipes supported by heaped-up supporting surface material or downwardly-curving supporting wings fixed to their lower surfaces.

14. Apparatus according to Claim 13 in which said pipes are provided in their upper zones with suitable apertures, transversely-arranged slots or the like to permit ingress to their interiors of rainwater draining from the upper panels of said polymer film pondage tubes.

15. Apparatus according to Claim 14 in which said drainage apertures in an upper zone of said pipes can be covered or uncovered by increasing or decreasing the gas pressure within said polymer film pondage tubes.

16. Apparatus according to Claim 14 in which said pipes are provided with a suitable fall to carry said rainwater to sub-surface drainage wells from whence it is conveyed by suitable conduits to storage or disposal.

17. Apparatus according to Claim 1 in which said upper chambers of said polymer pondage tubes are optionally pressurised with air by small centrifugal blowers, and suitable valves and vents are provided in said pondage tubes to vent air and gas to atmosphere.

18. Apparatus according to Claim 1 in which the depth of said water or gas pressure within said polymer pondage tubes are increased to provide increased stability to reduce disturbance of said pondage tubes by high winds.

19. Apparatus according to Claim 1 in which said pondage sub-divisions are made with polymer pondage tubes having lengths in the range 13.3 to 33.3 kilometres giving water volumes in the range 20 to 50 megalitres.

20. Apparatus according to Claim 1 in which sensors are incorporated into said pondage to measure water level, water temperature, dissolved carbon dioxide level, nutrient levels, salinity, pH and the like, signals being transmitted from said sensors to a microprocessor-based control unit which acts to maintain physical parameters within a predetermined range.

21. Apparatus according to Claim 1 in which said pondage tubes are made with a single chamber formed by joining two panels of thin, flexible polymer film material along their edges.

22. Apparatus according to Claim 1 in which laterally-extending aprons are fused to the lower surfaces of said polymer pondage tubes, said aprons being brought up the sides of said pondage tubes to cover lower drainage apertures provided along the sides of said pondage tubes and drainage panels are fused to the upper panels of adjacent said pondage tubes form a rainwater collection zone between adjacent runs of said pondage tubes; rainwater collecting on said upper panels draining onto said drainage panels and passing downwardly through upper drainage apertures provided in said drainage panels to accumulate between said aprons and the sides of said pondage tubes, the head of pressure so created being sufficient to uncover said lower drainage apertures permitting said rainwater to enter said pondage tubes via said lower drainage apertures.

23. Apparatus according to Claim 1 in which laterally-extending aprons are fused to the lower surfaces of said polymer pondage tubes, said aprons being brought up the sides of said pondage tubes to cover porous drainage panels extending along the sides of said pondage tubes, rainwater collecting on the upper panels of said pondage tubes draining to their sides and passing downwardly through said drainage panels to enter said pondage tubes via drainage apertures provided at their lower edges.

24. Apparatus according to Claim 1 in which said 180° bends are made from a more or less rigid material and are sealingly connected to adjacent ends of said tube runs, said bends incorporating integral guide vanes extending downwardly through the depth of said water flow and preventing pooling or stagnation of water within said bends.

25. Apparatus according to Claim 1 in which said 180° bends are fabricated from radially-segmented panels of a thin, flexible, sheet polymer material and fused or bonded to adjacent ends of said tube runs, said bends accommodating separate guide vane assemblies to prevent pooling or stagnation of water within said bends.

26. Apparatus according to Claim 24 in which the openings of said 180° bends are reinforced internally by a plurality of bracing webs which support said openings against the crushing forces of clamping bands passing around said ends to clamp said ends of said tube runs to them.

27. Apparatus according to Claim 1 in which water is supplied to said pondage tubes via standpipes which pass up between adjacent said pondage tube runs from subsurface conduits, said standpipes being connected to said pondage tubes by flexible hoses.

28. Apparatus according to Claim 1 in which water flow from a said pondage tube is conducted beneath a roadway by means of a siphon.

29. Apparatus according to Claim 1 in which water flow is drawn off from or supplied to a said pondage tube via a sub-surface well provided in said pondage tube, said well being separated from said pondage tube by a foraminous screen and the dimensions of said well being such as to ensure a very low take-off water flow velocity, thereby acting to minimise the ingestion of microalgae from a layer stratified in the upper levels of said water flow.

30. Apparatus according to Claim 29 in which said well is divided throughout its depth by a transversely-arranged partition, permitting a flow of water to be taken off via the part upstream of said partition and supplied via the part downstream of said partition.

31. Apparatus according to Claim 1 in which a flow of water is supplied to a said pondage tube via a transversely-arranged, sub-surface main to which is connected a plurality of parallel diffusers passing up through the lower film layer of said pondage tube said diffusers being flexible and made foraminous or formed from an open weave-type material, having a length of between one and 20 metres and acting to provide a perfusional water inflow that minimises shock effects to microalgae.

32. Apparatus according to Claim 1 in which the end of a said pondage upper chamber is sealingly connected to the opening of a said 180° bend of more or less rigid construction, said bend incorporating integral guide vanes extending downwardly through the depth of said water flow to preventing pooling or stagnation, the upper panel of said bend being made foraminous and covered by a gas collection shroud and the floor of said bend being made foraminous and forming the roof of a sub-surface water take-off or return well connected to which is a sub-surface water take-off or return conduit; the end of said pondage lower chamber being sealingly closed by a closure plug of more or less rigid construction through which passes a transversely-arranged manifold connected by a plurality of flexible hoses to the closed ends of said tubular elements through which an inflating gas is supplied or withdrawn to inflate or deflate said tubular elements; the openings of said 180° bend and said closure plug forming complementary halves which are accommodated, respectively, within the ends of said pondage upper and lower chambers with said membrane captured between their abutting faces, the named components being sealingly retained in assembly by a clamping band passing completely around all; the inner face of said closure plug being provided with a curved inner deflection face which directs flow upwardly to prevent water stagnation in the end zone of said pondage lower chamber, that part of said membrane above said end zone being provided with a plurality of closely-spaced apertures covered by hinged flaps or transversely- arranged, hinged strips, said strips being unseated by increased pressure in said lower chamber but closing said apertures otherwise to prevent backflow to said lower chamber, said apertures also acting to prevent water stagnation in said lower chamber end zone.

33. Apparatus according to Claim 32 in which said apertures are made more or less circular with diameters in the range 2.5 to 7.5 millimetres.

34. Apparatus according to Claim 32 in which moulded filler pieces of a suitable resilient material are positioned along the sides of said openings of said 180° bend and said closure plug in their assembled positions to provide a smoothly curving surface to receive the ends of said upper and lower chambers, a suitable sealant being provided between all sealingly joined surfaces.

35. Apparatus according to Claim 34 in which said sealant is a room temperature- vulcanising silicone rubber or a thin, soft, polymer foam material impregnated with a suitable non-setting sealant.

36. Apparatus according to Claim 32 in which said openings of said 180° bend and said closure plug are reinforced internally by a plurality of bracing webs which support said openings against the crushing force of said clamping band.

37. Apparatus according to Claim 1 and Claim 32 in which said 180° bend, said gas collection shroud, said water take-off or return well and said closure plug are made by spraying a thermo-setting resin reinforced with chopped strand glass fibre into suitable moulds.

38. Apparatus according to Claim 1 and Claim 32 in which said 180° bend, said gas collection shroud, said water take-off or return well and said closure plug are fabricated from suitable metal alloy sheet material, fabricated from a suitable thermoplastic sheet material or rotationally moulded from a suitable thermoplastic material.

39. Apparatus according to Claim 1 in which said membrane takes the form of a sheet of polymer film material provided over the greater part of its area with a large plurality of short slits arranged in staggered parallel arrays, with a slit in one row located adjacent substantially un-slit areas in adjacent rows, said rows of slits being arranged longitudinally, obliquely, transversely.

40. Apparatus according to Claim 39 in which said slits are arranged in herringbone arrays with the ends of parallel, angled slits in one line of slits being positioned adjacent un-slit areas between the ends of parallel, oppositely-angled slits in adjacent lines of slits.

41. Apparatus according to Claim 39 in which the length of said slits is in the range 10 to 30 millimetres, the separation (end-to-end) distance of said slits is in the range 10 to 50 millimetres and the separation (lateral) distance between rows of said slits is in the range 5 to 50 millimetres.

42. Apparatus according to Claim 39 in which irregular arrangements of said slits in relation to slit length, end-to-end separation and lateral separation are employed in different parts of said pondage, providing for increased water flow across said membrane in zones along the sides of said pondage chambers to prevent stagnation in those zones, decreased water flow in zones adjacent points of inflow of water to said lower chamber and increased flow of water across said membrane in zones remote from points of inflow of water to said lower chamber.

43. Apparatus according to Claim 39 in which said membrane is left un-slit along a plurality of narrow longitudinal zones in which said tubular elements are fixed to it.

44. Apparatus according to Claims 1, 32 and 57 in which water is drawn off from said pondage upper chamber via said well and pumped to means in which its temperature is adjusted and carbon dioxide and other compounds are dissolved in it, said water being returned to said pondage lower chamber via a second well; during the hours of darkness, said tubular elements are deflated thereby permitting said membrane to rise freely and accommodate said returning water flow in the increased capacity of said pondage lower chamber, as no pressure differential exists across said membrane, no flow across it occurs; with the return of daylight, said tubular elements are progressively inflated to their full, round cross-sectional shape, increasing their volume displacement and drawing said membrane down, both acting to reduce the capacity of said pondage lower chamber such that the pressure of inflowing water to said pondage lower chamber acts to create a pressure differential across said membrane, slightly stretching it and thereby opening said slits and permitting a flow to occur across said membrane; additional inflows of water to said pondage lower chamber passing along said lower chamber between said inflated tubular elements are up through said membrane into said pondage upper chamber.

45. Apparatus according to Claim 44 in which the rate of inflation of said tubular elements is regulated in relation to the sensed pH of said water in said pondage upper chamber.

46. Apparatus according to Claim 44 in which all said tubular elements are inflated simultaneously or some said tubular elements are inflated selectively.

47. Apparatus according to Claim 44 in which the maximum volume flow across said membrane per unit area and unit time is small in relation to the longitudinal flow of water per unit area and unit time in said pondage upper chamber, the consequent high rate of dilution achieved acting to minimise shock to microalgae.

48. Apparatus according to Claim 1 in which the use of said two-chambered arrangement provides pondage of improved thermal stability.

49. Apparatus according to Claim 44 in which lateral stress induced in said membrane is accommodated by drawing in of the sides of said pondage tubes and longitudinal stress is accommodated by elastic compliance throughout the length of said pondage runs.

50. Apparatus according to Claim 1 in which the superincumbent load of said biological activity water stream in said pondage upper chamber upon said water in said pondage lower chamber slightly increases the pressure in said water in said pondage lower chamber, thereby increasing the solubility of carbon dioxide in it.

51. Apparatus according to Claim 1 in which small amounts of carbon dioxide outgasing from said water in said pondage lower chamber tend to diffuse through said membrane slits in fine bubbles which are readily taken up in said biological activity water stream in said pondage upper chamber and rapidly re- dissolved.

52. Apparatus according to Claims 1 and 31 in which said membrane is made impervious and water flows are transferred to said pondage upper chamber from said pondage lower chamber or from external means via a plurality of longitudinally-arranged diffusers provided in said pondage upper chamber.

53. Apparatus according to Claim 1 in which said upper panel of polymer film material, said lower of polymer film material, said membrane of polymer film material and said tubular elements of polymer film material are supplied from continuous lengths carried on rolls and machine-welded together, said tubular elements in flattened form firstly being simultaneously welded to said membrane and said lower panel followed by the welding together along their edges of said upper panel, said membrane and said lower panel.

54. Apparatus according to Claim 53 in which a suitable separation material is provided internally in said inflatable elements to prevent adhesion of their opposing surfaces during welding, said separation material taking the form of paper or other film material coated with graphite, talcum, silicone or the like.

55. Apparatus according to Claim 53 in which means to control the entry of solar irradiance to said pondage upper chamber are welded to the upper surface of said pondage upper panel.

56. Apparatus according to Claim 53 in which said components are welded together using any of direct thermal welding, ultrasonic welding, induction welding, laser welding, radio-frequency welding or bonding with adhesives which are optionally radiation-cured.

57. Apparatus according to Claim 53 in which strips of thermoplastic material are positioned in welding zones during said welding process, their inclusion serving to strengthening the welded joints made between said components.

58. Apparatus according to Claim 1 in which a sub-surface take-off or return well unit is provided intermediate of the ends of a said pondage run, the ends of said pondage upper chamber being sealingly connected to openings on each side of an upper part of said well unit connected to said well by a plurality of narrow vertical ducts, the upper openings of which are covered by a foraminous plate, a sub-surface water take-off or return conduit being connected to said well; the upper panel of said well unit being made foraminous and covered by a gas collection shroud; the ends of said pondage lower chamber being sealingly connected to openings on each side of a lower part of said well unit connected to a plurality of horizontally-arranged ducts carrying the flow of water in said pondage lower chamber across the width of said well unit and passing between said narrow vertical ducts; said openings of said upper and lower parts of said well unit forming complementary halves which are accommodated, respectively, within the ends of said pondage upper and lower chambers with said membrane captured between their abutting faces, the named components being sealingly retained in assembly by a clamping band passing completely around all.

59. Apparatus according to Claim 1 in which a plurality of leaves of a thin, light, flexible material coated with a suitable reflective material is fixed to the upper surface of said pondage upper chamber and joined by electrical conductors, a static charge being applied to said conductors to cause said leaves to be deployed by repulsion to an erect position in which they impose minimal impedance to light entry, or said static charge is removed, thereby permitting said leaves to relax onto said upper surface in which position they impose maximum impedance to light entry.

60. Apparatus according to Claim 1 in which a plurality of longitudinally-arranged, folded, tubular, inflatable elements is fixed to the upper surface of said pondage upper chamber, said elements being partially coated with a suitable reflective material; said elements being evacuated to collapse them into a folded state in which condition said reflective material imposes maximum impedance to light entry, or said elements being inflated in which condition said reflective material imposes minimum impedance to light entry.

61. Apparatus according to Claim 60 in which said inflatable tubular elements incorporate two folds, four folds or five folds and have one or two parts coated with said reflective material.

62. Apparatus according to Claim 60 in which said inflatable tubular elements are provided on said upper surface of said pondage upper chamber in closely- spaced, parallel arrays such that they abut when inflated.

63. Apparatus according to Claim 60 in which the ends of said inflatable tubular elements are folded across and sealingly bonded to shoes of a suitable flexible polymer material which are bonded to said upper surface of said pondage upper chamber, said shoes incorporating suitable inflation and deflation means.

64. Apparatus according to Claim 1 in which said upper polymer film panel is made replaceable, side panels of said pondage being brought up and their edges folded with the edges of said upper polymer film panel across a bead, stapling strips being positioned on either side of said folded components and the combination being fixed together by closely-spaced stapling.

65. Apparatus according to Claim 64 in which all polymer film components of said pondage excepting said upper panel are coated with a suitable light-obscuring material to minimise deterioration resulting from ultra-violet light exposure.

66. Apparatus according to Claim 64 in which said stapling strips are separated to dislodge said stapling, thereby permitting the removal and replacement of said pondage upper panel in the manner described.

67. Apparatus according to Claim 64 in which the joining of said pondage upper panel to said side panel or the breaking of said stapled joints and replacement of said pondage upper panel is performed by mechanised means.

68. Apparatus according to Claim 1 in which access is gained to parts of a large area of said pondage by means of a service vehicle provided with soft, smooth, low pressure tyres or smooth caterpillar tracks of a soft polymer material, said vehicle driving along said pondage runs using them as access ways.

69. Apparatus according to Claim 68 in which a separate bridging vehicle is employed to deploy folding access ramps permitting said service vehicle to gain access to said pondage runs.

70. Apparatus according to Claim 1 in which mixing is induced in said biological activity water flow in said pondage upper chamber by means of a plurality of vertically-arranged fins fixed to a transversely-arranged baseplate, said fins extending through the depth of said water flow and being partially split to permit their upper parts to be deflected in one sense and the lower parts deflected in the opposite sense, the flow of water past said deflected parts inducing vortices in said water flow.

71. Apparatus according to Claim 1 in which mixing is induced in said biological activity water flow by dense clouds of bubbles emitted from a transversely arranged, foraminous duct situated on the floor of said upper pondage chamber, said bubbles being air or carbon dioxide.

72. Apparatus according to Claim 71 in which said dense clouds of bubbles promote mixing throughout the depth of said biological activity stream, thereby preventing stratification and promoting the outgasing of oxygen generated by biological activity.

73. Apparatus according to Claim 1 in which said upper panel of said pondage upper chamber is made with two transparent, polymer film layers having narrow strips of light reflective material applied to them, the arrangement of said strips being such that, when said film layers are in abutment, said strips substantially impede the entry of light into said pondage upper chamber; the separation of said film layers by light gas pressure permitting the entry of light into said pondage upper chamber.

74. Apparatus according to Claim 1 in which, to promote stratification of algae, a zone of suitable length upstream of a water take-off point is made deeper and more tranquil with no turbulence or mixing, flow guides and straighteners being employed for the same purpose.

75. Apparatus according to Claim 1 in which, to prevent wind damage to said pondage, strong metal stanchions are driven into the supporting surface at suitable intervals and a protective netting is fixed to them over the upper panel of said pondage, said netting is made from a UV-stabilised polymer material with a mesh in the size range 100 to 600 millimetres.

76. Apparatus according to Claim 1 in which access ports are provided to said pondage tubes, said access ports taking the form of suitable apertures closed by flaps sealingly secured by polymer slide fasteners or of flexible entry chutes closed by folding and clamping.

77. Apparatus according to Claim 1 in which access ports are provided to said pondage tubes by incorporating a straight, rigid sections containing removable entry hatches into said pondage.

78. A method for the large-scale, intensive cultivation of microalgae in which pondage containers in the form of wide, substantially flat, transparent or translucent polymer film tubes are laid in a plurality of long, parallel runs upon a graded, flat, level surface, the extreme ends of a plurality of said tube runs being connected to permit a continuous circulation of shallow water through said tube runs; said pondage tubes being divided by a selectively-permeable membrane into an upper biological activity chamber and a lower chamber employed as a conduit for the delivery of carbonated water or vessel for the storage of carbonated water during periods of darkness; the volume of said lower chamber being controllable by the deflation or inflation of longitudinally-arranged tubular elements fixed to said membrane and the lower film surface of said lower chamber such that water is either stored in said lower chamber or transferred from it to said upper chamber at a controlled rate; the upper film surfaces of said upper chamber being maintained clear of the water surface by light gas pressure and the entry of solar irradiance to said upper chamber being optionally regulated by the deployment of light-reflecting elements; adjacent ends of said pondage upper chambers being joined by suitable flow-transferring 180° bends, flow impelling means being provided to maintain said water flow through said pondage upper chambers and said bends and mixing means being provided to generate a mixing effect within said body of flowing water, said pondage lower chambers being closed at the ends of each said pondage run; means being provided to draw off water from said upper chamber, to supply water to said lower chamber or to draw off oxygen generated by biological activity within said upper chamber; the possible adverse effects of mass extinctions of microalgae being minimised by sub-dividing large areas of pondage into discrete, smaller, closed-circuit flow units, each sub-division being biologically separate from others and having its own water circulation and management means and continuous monitoring and adjustment of its aquatic environment.

79.