WO2002004089A1

WO2002004089A1 - Filtration apparatus and process with collapsible filer tubes

Info

Publication number: WO2002004089A1
Application number: PCT/IB2001/001241
Authority: WO
Inventors: Visvanathan Lingamu Pillay; Chettiyappan Visvanathan
Original assignee: Water Research Commission
Current assignee: Water Research Commission
Priority date: 2000-07-12
Filing date: 2001-07-12
Publication date: 2002-01-17
Anticipated expiration: 2003-01-12
Also published as: AU2001272684A1

Abstract

This invention relates to a jigsaw puzzle, a method of assembling a jigsaw puzzle and to a jigsaw puzzle set. The pieces of the jigsaw puzzle include a backing element which defines a first surface and an opposed second surface. Part of a picture for a complete jigsaw puzzle is provided on the first surface and there is a coating of adhesive on the second surface. The adhesive surface on the second surface is covered with a protective strip. In use, the protective strip is removed from the jigsaw puzzle piece and the piece is placed onto a mounting sheet to which the adhesive bonds. Once the puzzle is assembled, it is adhered to the mounting sheet, so that it can be kept in an assembled state.

Description

FILTRATION APPARATUS AND PROCESS WITH COLLAPSIBLE FILER TUBES

BACKGROUND OF THE INVENTION

THIS invention relates to an apparatus for separating solids from a feed liquid having the solids entrained therein.

The demand for higher pollution control and greater amounts of usable water has created a need for the more effective treatment of domestic and industrial waste-water. For the past twenty or thirty year's researchers have investigated the use of membranes in waste-water treatment. One approach has been to combine membranes with biological treatment methods. The conventional method of treatment was modified by submerging a membrane filtration unit in the aeration tank of a waste-water treatment installation, thereby eliminating the need for a settling unit. Such units are referred to as membrane bioreactors.

In comparison with a conventional activated sludge system, a membrane bioreactor system of this type is free from the limitations that are imposed by a settling unit. Thus, when a settling unit is used, the average biomass concentration in the activated sludge, measured as mixed liquor suspended solids (MLSS) should not exceed 5 g MLSS/L and should preferably be between 3 and 4 g MLSS/L, in order to achieve safe separation of treated water from the activated sludge. For a membrane bioreactor system a mixed liquor suspended solids content of up to 16 g MLSS/L is manageable. A membrane filtration unit allows an effectively complete retention of biomass in the bioreactor, regardless of the properties (such as size of floes, age, and so on) of the biomass, and ensures a high quality treated water. The process enables a combination of high volumetric throughputs and high mass loadings that ensure good operational reliability and stability. Almost complete nitrification is possible as a result of the high sludge age. Sludge entrainment from the settling tank into the effluent resulting in insufficient denitrification is thus no longer possible.

However, membrane fouling and polarization still present a major problem in membrane bioreactor systems, and this often necessitates a high cross flow velocity and frequent cleaning of the membrane. Various operational techniques have been devised to reduce fouling, for example, the use of air and liquid back-washing, and air agitation on the outside of the membrane.

It is an object of the present invention to provide an immersed membrane filter for the recovery of a solids-free liquid from a solid-liquid suspension. A further object of the invention is to provide a membrane bioreactor system for waste-water treatment which has a long operational life and in which the problem of membrane fouling and polarization is minimised.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided apparatus for separating solids from a feed liquid having the solids entrained therein, the apparatus comprising a collapsible tube of filter material which can be immersed in the feed liquid, an outlet line in fluid communication with the lumen of the filter tube, a porous support located inside the filter tube, to prevent the filter tube from collapsing due to a pressure differential between the inside and outside of the filter tube, and control means for cyclically reversing the pressure differential across the filter tube for back-washing purposes, wherein the circumferential extent, in cross-section, of the inside of the filter tube is greater than the circumferential extent, in cross-section, of the outside of the porous support so that, during back-washing, the material of the filter tube can inflate and separate from the porous support, whereby solid material that may have collected on the outside surface of the filter tube is dislodged as a result of this movement. Preferably the porous support comprises a length of tubing having a porous side wall such as a length of pipe which may be circular in cross-section and has holes located along its length. Conveniently the apparatus may comprise a plurality of said filter tubes, the filter tubes being spaced from and parallel to one another.

Preferably, a second end of each of the filter tubes is connected to an outlet manifold in fluid communication with the outlet line. Typically, a first end of each of the filter tubes is connected to an inlet manifold in fluid communication with an inlet line.

According to another aspect of the invention there is provided a method of separating solids from a feed liquid having the solids entrained therein, the method comprising the following steps: immersing a collapsible tube of filter material in the feed liquid; locating a porous support inside the filter tube to prevent the filter tube from collapsing; applying a pressure differential between the inside and outside of the filter tube; withdrawing a filtered liquid from the inside of the filter tube; and inflating the filter tube by reversing the pressure differential between the inside and outside of the filter tube to dislodge intermittently accumulated solids from the outer surface of the filter tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which:

Figure 1 diagrammatically illustrates a waste-water treatment installation which includes a membrane filter in accordance with an embodiment of the invention;

Figure 2 is a detail section on ll-ll in Figure 1 ; Figure 3 is a cross-sectional side view of a manifold for the membrane filter illustrated in Figure 1 ;

Figure 4 is a detail section on IV-IV in Figure 3;

Figure 5 diagrammatically illustrates a bioreactor which includes a membrane filter in accordance with an embodiment of the invention;

Figure 6 is a graph in which the variation of influent and effluent ammonia and nitrate is plotted against time;

Figure 7 is a graph in which COD removal is plotted against time, showing the COD removal at various recirculation rates; and

Figure 8 is a graph illustrating the effect of the recirculation rate on nitrogen removal.

Embodiments of the invention are described in detail in the following passages of the specification which refer to the accompanying drawings. The drawings however, are merely illustrative of how the invention might be put into effect, so that the specific form and arrangement of the features shown is not to be understood as limiting on the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figure 1, reference numeral 10 indicates a waste-water treatment installation, which comprises a pair of reactor vessels 12 and 14. In the example illustrated the total working volume of the two reactor vessels 12 and 14 is 40 litres, with the volume of the second reactor vessel 14 making up 55% of the total volume. A feed pump 16 is provided for pumping raw effluent into the first reactor vessel 12 via a feed line 18. For screening out coarse particles, there is a 750μm circular disc screen 20 in the feed line 18. Overflow from the first reactor vessel 12 flows into the second reactor vessel 14, as indicated by the line 22.

The first reactor vessel 12 forms an anoxic zone in which denitrification of the effluent takes place. The first reactor vessel 12 is provided with an electric stirrer 24 for keeping solids in the effluent in suspension.

The second reactor vessel 14 forms an aerobic zone where carbon oxidation and nitrification of the effluent takes place. To this end, the second reactor vessel 14 is provided with an air diffuser 26 through which air is introduced into the liquid in the vessel 14.

Filtered water is withdrawn from the reactor vessel 14 via a membrane filter 28 according to the invention, which is immersed in the liquid in the reactor vessel 14.

The membrane filter 28 has a construction in cross-section which is illustrated in Figure 2. Thus, it comprises a series of side by side filter tubes 30 of woven filter cloth. In the lumen of each of the filter tubes 30 there is an internal support 32 which is in the form of a PVC tube having a wall which has holes drilled therein along the length of the tube. It should be understood that the internal support need not be a PVC tube, but could be any tubular porous support. Examples of porous supports are porous tubes, pipes with holes, springs, meshes, extruded or injection moulded formers etc. In the example illustrated the filter tubes 30 have a pore size of 5μm, and a total surface area of 98 cm². It should be understood that the filter tubes could be made of any suitable flexible material including paper, a textile or another fabric of natural or man-made origin. The material must be liquid permeable and have pores which can retain the solids entrained in the liquid feed. The inside circumferential extent of each of the filter tubes 30 is greater than the outside circumferential extent of the porous supports 32. In this embodiment the filter tubes 30, when fully inflated, have an inside diameter of 25 mm, whereas the porous supports 32 have an outside diameter of 20 mm. It should be understood that the exact dimensions of the filter tubes and porous supports depend on the specific application.

Referring back to Figure 1 , the lumens of the filter tubes 30 lead to a top manifold 34 along the upper edge of the membrane filter 28. The manifold 34 is connected via a line 36 to the suction inlet of a suction pump 38, which pumps the filtered water that is withdrawn from the membrane filter 28 in reactor vessel 14 into a storage tank 40. Clarified water can be withdrawn from the tank 40 via an outlet valve 50.

The lumens of the filter tubes 30 further lead to a bottom manifold 42 along the lower edge of the membrane filter 28, the bottom manifold being, for purposes of back-washing, connected to the delivery outlet of a back- washing pump 44 via a line 46. It should be understood that the bottom manifold 42 could act as an end-plate for sealing the ends of the lumens of the filter tubes 30 and the back-wash process could then be conducted by reversing the flow through the top manifold 34.

Referring to Figure 3, the top manifold 34 and bottom manifold 42 mentioned above may have the same construction and, for convenience, will be described in relation to the manifold 52 shown in this Figure. The manifold 52 comprises a casing 54 which is connected to an end block 56. An inlet/outlet pipe 58 extends from the manifold 52, and the filter tubes 30 are connected to the end plate 56, with the lumens of the filter tubes 30 in fluid communication with the manifold 52. Porous supports 32 are located within the filter tubes 32. Referring to Figure 4, an aperture 60 is provided in the end block 56 for each filter tube 30. The filter tube 30 is connected to the end block 56 by way of a tapering PVC tube 62 so that the filter tube 30 is in fluid communication with the manifold 52. A recess 64 surrounds the aperture 60 and the filter tube 30 is attached to the recess 64 in the end block 56 by way of an adhesive. The porous support 32 fits loosely about the tapering PVC tube 62. Referring back to Figure 1 , a timer 66 is provided to control operation of the pumps 38 and 44. The installation 10 is able to operate in a filtering mode and in a back-washing mode. When operating in the filtering mode, the pump 38 is switched on and the pump 44 is switched off, the pump 38 withdrawing filtered water from the reactor vessel 14 via the membrane filter 28 and pumping it to the storage tank 40. When operating in the backwash mode, the pump 38 is switched off and the pump 44 is switched on, the pump 42 pumping filtered water from the storage tank 40 via the line 46 into the lumens of the filter tubes 30 of the membrane filter 28, via the manifold 42. The timer 66 serves to switch the operation cyclically from one mode to the other, at predetermined intervals of time. It should be understood that, instead of having the timer 66 controlling the pumps, the pumps could be controlled by a controller which measures the pressure differential across the membrane filter.

When operating in the filtering mode the walls of the filter tubes 30 collapse onto the porous supports 32, the porous supports 32 serving to keep the lumens of the filter tubes open, i.e. to prevent the filter tubes 30 from collapsing completely. Solids which are unable to pass through the walls of the filter tubes 30 remain behind and collect on the outside surfaces of the filter tubes 30.

When switching from operation in the filtering mode to operation in the back-washing mode, the filter tubes 30 inflate, causing the filter tubes 30 to expand and separate from the internal supports 32. This has the effect of dislodging solids that have collected on the outside surfaces of the filter tubes 30. Furthermore, clarified water now starts to flow in a reverse direction through the walls of the filter tubes 30, cleaning the filter tubes 30 through back-rinsing.

Owing to the fact that the solids are collected on the outside of the filter tubes 30, it is also possible to minimise the degree of clogging by providing some form of agitation around the outside of the filter tubes 30, for instance air agitation. In addition, if over time fouling becomes significant, and is not removed by the back-washing procedure, the filter tubes 30 are simply allowed to dry out. It has been found that dried solids are very easily removed from the walls of the filter tubes 30 by vibration or by the back- washing procedure. Hence, in most applications, the system will not require a chemical clean.

Referring now to Figure 5, there is shown a bioreactor vessel 68 which has a membrane filter 70 according to the invention immersed therein.

The membrane filter 70 has a construction similar to that described above with reference to Figures 1 and 4, having a top manifold 72 and a bottom manifold 74. A pump 76 is provided for pumping raw liquor into the vessel 68, and a pump 78 for withdrawing clarified water from the vessel 68 via the membrane filter 70. A further pump 80 is provided for pumping clarified water into the membrane filter via the manifold 74, for back-washing purposes.

Means (not illustrated) may be provided for entraining air in the flow stream of clarified water pumped into the membrane filter during back-washing. It has been found that this increases the effectiveness of the back-washing procedure.

EXAMPLE:

The following results were obtained in tests carried out with the installation described above with reference to Figures 1 and 5, on waste-water in the form of dairy effluent.

The volumetric loading into the system was between 0,4 and 0,9 kg COD/m³/d. The concentration of the sludge in the system was between 4 and 6,5 g MLSS/L. The pH was maintained between 7,2 and 8,2, and dissolved oxygen between 2,5 and 4,5 mg/L in the reactor vessel 14 and below 0,5 in the reactor vessel 12. Measurements were taken at various values of the mixed liquor recirculation rate. Tests were done at various values of the mixed liquor recirculation rate, i.e. at values of 1 , 3, and 5. Back-washing as described above was carried out at a cycle time of 120:15 (i.e. 120 minutes of filtration and 15 minutes of back-washing). This gave a stable flux of 214 L/m²/h at a pressure of not more than 40 kPa across the membrane. Proper aeration around the membrane was found to enhance good flux recovery. More than 95% of the filtered water was recovered after each cycle of filtration and back-washing.

Table 1 , below, sets out the quality of the raw feed water, and Table 2 sets out the quality of the treated water extracted via the membrane filter, for the three operating conditions at which measurements were made. All the analyses were based on the average of samples taken every 24 hours. From the test results it can be seen that the COD reduction does not follow the operating condition used as shown in the graph of Figure 7. The residual COD represents the soluble non-biodegradable COD of the treated effluent.

Table 1. Quality of raw feed water

Nitrification was completed under the three conditions at which measurements were made, and are set out in the graph of Figure 6. In all three cases the concentration of NH -N in the treated water was less than 1 ,0 mg/L. The NO₃-N concentration in the treated effluent changes with the mixed liquor recirculation rate. The recirculation rate of 3 gives a better effluent quality, giving a concentration of less than 10mg NO₃-N/L as shown in the graph of Figure 8. This shows that at a certain value of the recirculation rate higher percentage of nitrogen removal can be achieved from the system.

From Table 2, below, it can be seen that the suspended solids were removed almost completely from the treated water. The turbidity of the treated water was equal to or less than 1 ,0 NTU.

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Claims

1. An apparatus for separating solids from a feed liquid having solids entrained therein, the apparatus including:

a collapsible filter tube of filter material which can be immersed in the feed liquid;

an outlet line in fluid communication with the lumen of the filter tube;

a porous support located inside the filter tube, to prevent the filter tube from collapsing due to a pressure differential between the inside and outside of the filter tube; and

control means for cyclically reversing the pressure differential across the filter tube for back-washing purposes,

wherein the circumferential extent, in cross-section, of the inside of the filter tube is greater than the circumferential extent, in cross- section, of the outside of the porous support so that, during back- washing, the material of the filter tube can inflate and separate from the porous support, whereby solid material that may have collected on the outside surface of the filter tube is dislodged as a result of this movement.

2. An apparatus according to claim 1 wherein the porous support is a length of tubing having a porous side wall.

3. An apparatus according to claim 1 or 2 comprising a plurality of said filter tubes.

4. An apparatus according to claim 3 wherein the plurality of filter tubes are spaced apart from, and parallel to, one another.

5. An apparatus according to claim 3 or 4 wherein a second end of each of the filter tubes is connected to an outlet manifold which is in fluid communication with the outlet line.

6. An apparatus according to claim 5 wherein a first end of each of the filter tubes is connected to an inlet manifold which is in fluid communication with an inlet line.

7. A method of separating solids from a feed liquid having solids entrained therein, the method including the following steps:

immersing a collapsible tube of filter material in the feed liquid;

locating a porous support inside the filter tube to prevent the filter tube from collapsing;

applying a pressure differential between the inside and outside of the filter tube;

withdrawing a filtered liquid from the inside of the filter tube; and

inflating the filter tube by reversing the pressure differential between the inside and outside of the filter tube to dislodge accumulated solids on the filter tube.

8. A method according to claim 7 wherein the filtered liquid from the inside of the filter tube is withdrawn through an outlet line and wherein the pressure differential between the inside and the outside of the filter tube is reversed by reversing the flow of liquid through the outlet line.

9. A method according to claim 7 wherein the filtered liquid is withdrawn from the inside of the filter tube via an outlet line and the pressured differential in the filter tube is reversed by pumping a liquid into the filter tube via an inlet line.

10. A method of separating solids from a feed liquid having solids entrained therein using an apparatus including:

at least one collapsible filter tube of filter material having a first end and a second end and a porous support having a first end and a second end located inside the filter; and

an outlet manifold which is connected to the second end of the at least one filter tube;

the method including the steps of immersing the apparatus in the feed liquid, withdrawing filtered liquid from the feed liquid, through the apparatus via the outlet manifold, and thereby entraining solids from the feed liquid on the filter tube of the apparatus and, when necessary, pumping a liquid into the outlet manifold of the apparatus, causing the filter tube to expand and separate from the porous support to dislodge accumulated solids from the filter tube.

11. A method of separating solids from a feed liquid having solids entrained therein using an apparatus including:

at least one collapsible filter tube of filter material having a first end and a second end and a porous support also having a first end and a second end located inside the filter tube;

an inlet manifold which is connected to the first end of the at least one filter tube; and an outlet manifold which is connected to the second end of the at least one filter tube;

the method including the steps of immersing the apparatus in the feed liquid, withdrawing filtered liquid from the feed liquid, through the apparatus via the outlet manifold and thereby entraining solids from the feed liquid on the filter tube of the apparatus and, when necessary, pumping a liquid through the inlet manifold of the apparatus, causing the filter tube to expand and separate from the porous support to dislodge accumulated solids from the filter tube.