GB2237565A

GB2237565A - Aeration of sewage

Info

Publication number: GB2237565A
Application number: GB9022846A
Authority: GB
Inventors: Edward Powell
Original assignee: Severn Trent Water Ltd
Current assignee: Severn Trent Water Ltd
Priority date: 1989-10-21
Filing date: 1990-10-19
Publication date: 1991-05-08
Anticipated expiration: 2010-10-19
Also published as: GB9022846D0; GB8923748D0; GB2237565B

Abstract

in an installation for aerating activated sewage sludge separate arrays (5, 6) of two different types of diffuser are provided, the air supply to each array being under independent control of a control system. The first type of diffuser (7) is fixed and the corresponding array (5) is fed with a continuous (but possibly variable-quantity) supply of air. The second type of diffuser (8) has a flexible membrane which constitutes a non-return valve and the air supply to the corresponding array (6) is controlled according to the aeration requirements of the sludge. The system for controlling the air supply includes a dissolved oxygen sensor. The control system may be a computer-based export system to tailor the system performance to the specific installation. <IMAGE>

Description

AERATION OF SEWAGE This invention relates to the activated sludge treatment of sewage by aerobic breakdown, which requires continuous oxidation. Conventionally the air is introduced by the use of arrays of aeration devices placed below the surface of the liquid and fed from a grid of pipes connected to supplies of air under pressure.

The aeration devices used are in the form of diffusers comprising tubes, discs or domes made of ceramic, plastics or rubber. They are generally of two basic types, either of fixed construction, rigid and permanently open, or made with elastic material over a perforated support to constitute a non-return valve. The former kind are used in installations where one can be sure that under all conditions air under pressure will be supplied continuously and at above a specified flow rate as long - as- subme-rged in sewage, so that -one can be- sure that-sewage will not find its way into, and block, the air supply pipe and diffuser system. The latter type have the advantåge that the air supply can be shut off safely without risk of contamination of the supply pipes caused by ingress of sewage.

The profile of demand in an activated sludge aeration basin changes in response to diurnal and seasonal variations in load and is related to the shape of the aeration vessel. The variability in the profile of demand is usually such that the normal aeration systems, which provide a fixed profile of air input and are thus operationally inflexible, operate below peak efficiency. The actual rate of air supply is varied in response to demand as indicated by DO (dissolved oxygen) measurement at appropriate locations.

It is also known that the aeration system can be resolved into a series of independently controllable zones, each with its own DO control, acting on the blower output. Although it can be shown that this method of operation can achieve a saving in energy usage of as much as 25% over the method described in the preceding paragraph, it is operationally complex.

The aim of the present invention is to develop a system of aeration in the activated sludge treatment of sewage in which there is sufficient flexibility of control to allow operation at peak or near-peak efficiency despite wide variations in demand and without the complexity of the -system described above.

According to the invention in its broadest sense we achieve this by employing, in one and the same installation, a-carefully selected mixture of arrays of rigid diffusers and of the type with a flexible membrane -(i.e. non-return-valve diffusers), in combination also with a DO system controlling the total air supply. In a first aspect the present invention provides an installation for aerating activated sludge in the treatment of sewage comprising at least one treatment zone equipped with submerged diffusers fed with air under pressure having at least one array of diffusers of a first type which is of fixed construction, permanently open, and at least one array of diffusers of a second type constituting non-return valves, and a system for controlling the total air supply to the arrays in response to the amount of dissolved oxygen in the sludge.

Furthermore in a second aspect the invention also provides a method of aerating activated sludge in the treatment of sewage comprising passing the sewage over at least one array of diffusers of a first type which are permanently open and continuously supplied with air and at least one array of diffusers of a second type which constitute non-return valves, measuring the dissolved oxygen in the sewage and controlling the total supply of air to the diffusers in response to the dissolved oxygen measurement, the control including, under some conditions, reducing the supply of air to the diffusers of the second type, but not those of the first type, to zero.

The two sets of diffusers may be fed from the same air supply or from different supplies. Generally speaking, the rigid type will handle the base load, whilst the -other type are switched on and off or modulated, by' appropriately placed valves, to cater for the diurnal and seasonal variations in the load.

Activated sludge installations usually comprise a series of zones, not necessarily of the same size, for example four, through which the sewage passes in turn, and in which the oxygen demand diminishes as sewage proceeds from the inlet zone to the outlet zone. In a preferred arrangement the early zones, indeed possibly all of them, may have both types of diffuser, although the final zone, i.e. the one with the lowest oxygen demand, may have solely the rigid type, the variation over a twenty-four period being catered for solely by varying the air supply.

In the remaining zones, apart from the control of the air supply, the matching of the air supply to the profile of the demand may be met by closing off selected groups of the collapsible membrane diffusers for the necessary periods when the demand is low, or by cycling them on and off for predetermined short periods, e.g. a minute at a time, under the control of a timer, or by modulating the output from them, using a variable valve.

The overall control is preferably automatic.

Sensors measuring the oxygen demand in the different zones feed the information to a central processor which uses it, in accordance with a stored program, to control modulating valves -in the air supply to the different groups of flexible-membrane diffusers in such a manner as to obtain as far as possible maximum efficiency of operation over a wide range of flow conditions.

The invention -will now be further-described by way of example with reference to the accompanying drawings in which: Figure 1 is a graph illustrating a typical variation in oxygen requirements along the length of the tank and in the different zones of the treatment tank; Figure 2 is a simplified diagrammatic plan view of an activated sludge aeration tank in accordance with the invention; Figure 3 shows to larger scale a detail of the layout shown in Figure 2; Figure 4 shows possible variations in dissolved oxygen with time in the first zone of the installation of Figure 2; and Figure 5 shows a possible alternative mode of operation of the installation, using a timer.

Referring first to Figure 1, this shows the variation in oxygen demand as one proceeds from the inlet end to the outlet end of a typical treatment unit. The upper line shows the demand in a typical summer condition and the lower line the corresponding variation in winter. It will be seen that the turn-down ratio, i.e. the amount by which the summer demand exceeds the winter demand is about 1.7 for the inlet end of the bed, whereas at the outlet end of the bed this ratio is as much as 2.5.

Thus a control system which simply varies the overall supply of air by controlling its pressure cannot, without manual intervention, produce the optimum conditions in all zones of the tank and under all flow conditions that are to be expected.

In the layout according' to the invention, shown only by way of example in Figure 2, we overcome this by separately and automatically controlling two groups of arrays of diffusers. The installation comprises an elongated tank made up of several zones. The waste water under treatment enters an initial anoxic zone A, provided with stirrers (not shown), followed by four successive aerobic treatment zones 1 to 4. An air supply from a blower (likewise not shown) is fed, in each of the first three zones, not to a complete ring main, but through separate control valves V1 and V2 to respective manifolds 5 and 6 on opposite sides of the zone.Each manifold leads into a group of pipes and those from the manifold 5 carry, distributed along the length of each, an array of rigid diffuser domes 7, whilst the pipes connected to the other manifold 6 carry diffusers 8 of the flexible membrane type.

In the fourth zone there is a complete ring main 9 controlled through a single butterfly valve 10, although there may be a certain number of manually operated valves 11 to allow some of the pipes to be isolated. In this zone all the diffusers may be of the standard rigid type.

Figure 3 illustrates how in each of the first three zones, after passing through a meter M which gives a signal representing the total quantity of air flow, the flow is -split, with some of it going through the modulating valve V1 to the rigid diffusers 7 whilst the rest, through a modulating valve V2, feeds the flexible membrane diffusers 8. Instead of having separate valves in each of the supplies, #i.e. one in the supply to the rigid diffusers and one in the supply to the flexible membrane diffusers, we could omit one of these, preferably the one to the rigid diffusers, and achieve the same overall degree of control by having a main valve in series with'the meter M, i.e. in the common supply to both.

Figure 4 shows the effect of applying a cycling control to the modulating valve V2 controlling the flexible membrane diffusers 8 of zone 1. The graph shows dissolved oxygen content measured against time.

At a given oxygenation rate, from the rigid diffusers alone if the respiration rate (the rate at which the oxygen is consumed by the bacteria), is greater than this the amount of dissolved oxygen will fall. If we assume that the maximum amount of dissolved oxygen that need be present is 2 units, then from a given starting instant at this value, the amount will fall as shown by the curve C. / If a sensor is arranged to detect when the DO falls below one unit, this sensor can then open the valve to allow air in also from the flexible-membrane diffusers. In the example shown this takes place at the point K, five minutes from the starting point. The oxygen supply then exceeds the respiration rate substantially and so the DO rises.When it reaches the value 2 again, the sensor closes the valve to the flexible-membrane diffusers (at point L) and the DO starts to fall again.

Thus the valve controlling the supply of air through the flexible-membrane -is -cycled on and off every few minutes to keep the DO within a given control band B.

If, for the same oxygen rate from the fixed diffusers, the respiration rate rises, then the fall with the other diffusers off will be steeper, for example as shown by the curve D. Accordingly the flexible-membrane diffusers will be introduced sooner, and the rise in DO when they are on will be less steep, but the DO will still be kept within the same control band as before, the only difference being that, overall, the flexible-membrane diffusers will be on for a greater proportion of the time.

If the respiration rate rises to a value such as to exceed the maximum oxygenation rate with both sets of diffusers in operation, then the main supply valve can be opened further, or the power to the blower or blowers can be increased, to increase the total volume of air delivered.

It will be understood that control of the valves and blower or blowers will be from a Central processor receiving information from DO sensors in the different zones, and possibly information on other parameters such as temperature, rate of flow incoming effluent, and even from predictions based on previous experience of conditions to be expected at certain times of the day or year, and weather conditions. It can form a so-called 'expert system' learning all the time, and thereby progressively increasing the efficiency at which the treatment installation operates, closer and closer to an ideal optimum.

Although four zones have been shown in the tank or bed illustrated- by way - of example, it will be understood that there could be greater or smaller number; also the final zone could have a mixture of the two types of diffuser, just like the other zones. The example described is of a possible system based on a common air supply to both sets of diffusers; it would equally well be possible to supply air independently to each type of diffuser system from separate blower systems.

Finally, Figure 5 shows a method of operation alternative to that of Figure 4, and again a tank with four aerobic zones is taken as an example. The four graphs illustrate the oxygen supply to each of the four zones over a twenty four hour period.

This method is suitable where the pattern of oxygen demand over a day is predictable. A base load of oxygen is provided by the fixed diffusers and is indicated by the lower cross-hatched area; it is greatest for the first zone and least for the fourth zone and is maintained throughout the twenty four hours, but its quantity does not remain constant, the air supply being modulated in all/ four zones in accordance with need. In addition, in each of the first three zones, extra air is fed in only over certain periods of the day, controlled by a timer, from the flexible-membrane diffusers. Again, the actual quantity may be controlled by the demand, but it will be seen that for substantial parts of the day the flexible-membrane diffusers are closed altogether, and so again optimum efficiency is obtained.

Claims

1. An installation for aerating activated sludge in the treatment of sewage comprising at least one treatment zone equipped with submerged diffusers fed with air under pressure having at least one array of diffusers of a first type which is of fixed construction, permanently open, and at least one array of diffusers of a second type constituting non-return valves, and a system for controlling the total air supply to the arrays in response to the amount of dissolved oxygen in the sludge.

2. An installation according to claim 1 in which the controlling system includes- means to vary the supply of air to the or eadh-array of diffusers of the first type independently of the -supply of air to the or each array of diffusers of the second type.

3. An installation -according to claim 2 in which the air supply to the diffusers of the first type is continuous, though variable in quantity, whereas that to the diffusers of the- second type is, under some conditions, reduced to zero.

4. An installation according to claim 1 comprising a series of zones through which the sewage passes in turn, at least one of the zones having both an array of diffusers of the first type and on array of diffusers of the second type, the controlling system being adapted to control the supply of air to the array of diffusers of the second type independently of the supply of air to the other array.

5. An installation according to claim 4 in which the final zone in the series has one or more arrays of diffusers of the first type only.

6. An installation according to any /preceding claim in which the control system modulates the supply of air to the or each array of diffusers of the second type between a maximum and zero.

7. An installation ~-according to any preceding claim in which the control system comprises a processor arranged to receive an input signal from at least one dissolved oxygen sensor and a stored program arranged to generate output signals to control the supply of air to the arrays# of diffusers.

8. An -installation according to any preceding claim in which the or each' array- of diffusers of the first type is supplied by an air supply separate from that supplying the or each array of the second type.

9. An installation according to any preceding claim further comprising an anoxic zone through which the sewage passes prior to aeration by the diffusers, the anoxic zone being provided-with stirrers.

10. A method of aerating activated sludge in the treatment of sewage comprising passing the sewage over at least one array of diffusers of a first type which are permanently open and continuously supplied with air and at least one array of diffusers of a second type which constitute non-return valves, measuring the dissolved oxygen in the sewage and controlling the total supply of air to the diffusers in response to the dissolved oxygen measurement, the control including, under some conditions, reducing the supply of air to the diffusers of the second type, but not those of the first type, to zero.

11. A method according to claim 10 in which the sewage is passed through a series of zones Seach having an array of diffusers of the first type and at least one of which has an array of diffusers of the second type.

12. A method according to claim 10 or claim 11 in which the supply of air to -the or each array of diffusers of the first type is maintained at a constant rate corresponding to a minimum required aeration rate, whereas the air supply to the or each array of diffusers of the second type is controlled in response to the measured dissolved oxygen.

13. A method according to claim 12 in which the supply of air to the or each array of diffusers of the second type is switched solely between a fully flowing and a non-flowing state.

14. A method according -to claim~12 in which the supply of air to the or each array of diffusers of the second type is varied from a non-flowing state, through a range of intermediate flow rates to a maximum flow rate.

15. A method according to any one of claims 11 to 14 as dependent on claim 10 in which two or more zones have arrays of diffusers of the second type and the supply of air to each of those arrays is controlled independently of the others.

16. A method according to any one of claims 10 to 15 in which a microprocessor receives a signal from a dissolved oxygen sensor, processers the signal using a stored program and generates an output designed to control the supply of air to least one of the arrays.

17. A method according to claim 16 in which the program includes processing steps t6 evaluate the effect on the dissolved oxygen of past output signals so as to modify and improve to processing of the input signal to achieve more nearly the designed dissolved oxygen value.

18. An installation for aerating activated sludge in sewage treatment substantially as described with reference to the drawings.

19. A method for aerating activated sludge in sewage treatment substantially as described with reference to the drawings.