GB2161154A - Process of treating wastewater containing biologically oxidisable material - Google Patents

Abstract

A process for treating wastewater containing biologically oxidisable material comprises the steps of contacting and mixing the wastewater with bacterial active sludge in a well-mixed, unoxygenated contact zone. Then allowing the resultant mixture to separate in a settling zone (13) downstream of the contact zone into a sludge containing biologically oxidisable material adsorbed and absorbed by bacterial material in the bacterial active sludge, and a clarified effluent. The clarified effluent is discharged from the settling zone and the sludge is withdrawn from the settling zone to a stabilising zone (16). The withdrawn sludge is contacted with oxygen-containing gas in the stabilisation zone which allows the adsorbed and absorbed material to be oxidised biologically to form the bacterial active sludge which is passed to the contact zone. <IMAGE>

Description

SPECIFICATION Process of treating wastewater containing biologically oxidisable material This invention relates to processes for treating wastewater containing biologically oxidisable material such as, for example, domestic sewage.

One process for the biological oxidation of wastewater which is very commonly used to treat domestic sewage and other biologically oxidisable industrial wastewater is the so-called "activated sludge process". This process works by contacting sewage with a bacterial active sludge in an aerated tank, the contents of which are well mixed. Oxygen-containing gas is dissolved into the contents of the aerated tank, the oxygen-containing gas either being pure oxygen or air. The oxygen dissolved in the contents of the aerated tank is used by the bacteria to respire as they adsorb and metabolise the organic contaminants of the sewage. The sewage flows continuously into the aerated tank and treated water mixed with the bacterial active sludge flows out to a quiescent settling tank.Here the bacterial active sludge settles and the clarified effluent overflows a weir of the settling tank to be discharged. The settled bacterial active sludge is collected from the settling tank and a portion transferred back to the aerated tank to treat further sewage.

A modification of the activated sludge process has found advantage in some situations. This is the so-called "contact-stabilisation process". This was originally called the "Biosoption" process developed in the early 1 950's by A.H. Ullrich and M.W. Smith in the U.S.A. The first full scale plant employing this process was installed in 1954. In this process instead of returning the settled bacterial active sludge immediately to the aeration tank with further sewage, the bacterial active sludge is aerated by itself in a separate stabilisation tank. This allows the size of a contact tank, where the bacterial active sludge is brought into contact with sewage, to be reduced. This is because a very large proporation of the organic contaminants of sewage are very rapidly adsorbed by the bacteria.Following settlement of the sludge this adsorbed material is metabolised aerobically in the stabilisation tank.

The contact-stabilisation process, as originally designed and tested, has a detention time of 20 to 40 minutes in the contact tank. Suspended and colloidal solids and some soluble material are adsorbed by the well-oxidised sludge coming from the stabilisation tank. If the detention time in the contact tank reaches 1.5 to 2 hours biological activity may start, however, and result in a high rate activated sludge process which may reduce the settleability of the bacterial active sludge and result in poor solid/liquid separation in the settling tank and a poorly clarified effluent flowing therefron. Due to the wide variations in flow common experienced at domestic sewage treatment plants and industrial wastewater treatment plants it is very difficult to maintain the desired detention time of 20 to 40 minutes in the contact tank.Commonly longer detention times are specified by regulatory authorities such as detention times of 2 to 3 hours in the contact tank which reduces the efficiency and effectiveness of the contact stabilisation process.

The detention times for oxygenation of the active sludge in the stabilisation tank has been recommended at between 4 to 8 hours. The source of oxygen has been provided from air using a mechanical mass transfer device such as a bubble diffuser system or surface aerators. The period of 4 to 8 hours in the stabilisation tank has been considered necessary to ensure that the bacterial active sludge is well oxygenated and that the bulk of the adsorbed organic material is either oxidised or converted to new bacteria. It has been considered unsatisfactory to attempt to stabilise the bacterial active sludge at detention times shorter than 4 hours because it is impracticable and uneconomic to dissolve oxygen from air at a rate sufficient to meet the oxygen demand of the bacterial active sludge where the detention time is relatively short.The result, however, of operation with long detention times is that a large proportion of the bacteria in the bacterial active sludge is held under aerobic conditions for a time sufficient not only to oxidise the organic material adsorbed during the contact phase of the process but also for the bacteria to undergo endogenous respiration for a long period of time. Under conditions of endogenous respiration the bacteria of the bacterial active sludge oxidise any stored organic material from within the bacterial cell, reduce their metabolic rate and ultimately die and lyse releasing their cell contents to be consumed by other bacteria.It is believed that a large proportion of the bacterial cells contained in a treatment plant operating in accordance with the contact-stabilisation process are subject to endogenous respiration conditions and are either nonviable or dead.

Due to the flocculant nature of the bacterial active sludge and the high cost of maintaining the dissolved oxygen concentration in the stabilisation tank at greater than 2 mg/l when using air as the oxygen source a proportion of the bacteria held at the central part of the bacterial active sludge does not experience aerobic conditions continuously for the detention time in the stabilisation tank. Under anaerobic conditions the metabolism of these bacteria will change and slow down compared to their metabolism under aerobic conditions. These bacteria will take longer to metabolise the organic material adsorbed during contact with sewage and will be less ready to adsorb fresh organic material during the next pass through the contact tank.

It is the result of these two effects that the performance of the contact-stabilisation process is limited and detention times of at least 20 to 40 minutes in the contact tank and 4 to 8 hours in the stabilisation tank are required. It has been believed that detention times shorter than 20 minutes in the contact tank would be insufficient to ensure that the bulk of the suspended and soluble organic materials are adsorbed by the bacteria, resulting in a high residual concentration of these materials overflowing with the effluent from the settling tank.

It has now been discovered that provided that the bacterial active sludge is well oxidised in the stabilisation tank and by ensuring that the detention time in the stabilisation tank is not so long that the activity of the bacteria in the bacterial active sludge is reduced by a prolonged period of endogenous respiration, the bacterial active sludge has the capacity to adsorb suspended and soluble organic material very quickly and without the need for oxygenation in a contact tank, such that the removal of suspended and dissolved organic material from the effluent overflowing from the settling tank is satisfactory.

The use of pure oxygen or oxygen enriched air for meeting the oxygen demand of respiring bacterial active sludge in the treatment of wastewater has been recognised since the early 1 970's as an economic process. In the most frequently employed process that uses oxygen for wastewater treatment, the wastewater is mixed with the bacterial active sludge and a feed gas containing at least 60% oxygen by volume in a gas-tight oxygenation zone. The feed gas above the oxygenation zone is continously recirculated to the bacterial active sludge in the zone to ensure that a high proportion of the oxygen is dissolved before venting a small proportion of gas which has become contaminated with products of the bacterial respiration or gases dissolved in the wastewater to atmosphere.This process has significant advantages for the treatment of wastewater in that the elevated concentration of oxygen in the feed gas allows high dissolved oxygen concentration in the bacterial active sludge to be maintained economically compared to aeration using air as the source of oxygen. Following oxygenation, the wastewater and bacterial active sludge in the oxygenation zone is then passed to a settling tank where the bacterial active sludge is separated from the treated water and a proportion of the bacterial active sludge is returned to the oxygenation zone as in the conventional activated sludge process. For economic and process reasons the oxygenation zone most usually comprises oxygenation tanks having between 2 to 6 stages, the feed gas and the sludge flowing co-currently through the stages.

Although the present invention is primarily directed to any novel integer or step, or combination of integers or steps, herein disclosed and/or as shown in the accompanying drawings, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a process of treating wastewater containing biologicallly oxidisable material comprising the steps of: contacting and mixing the wastewater with bacterial active sludge in an unoxygenated contact zone; allowing the resultant mixture to separate in a settling zone into a sludge containing biologically oxidisable material adsorbed and absorbed by bacterial material in the bacterial active sludge, and a clarified effluent; discharging the clarified effluent from the settling zone; withdrawing the sludge from the settling zone to a stabilisation zone; containing the withdrawn sludge with an oxygencontaining gas in the stabilisation zone which allows the adsorbed and absorbed material to be oxidised biologically to form the bacterial active sludge; and passing the bacterial active sludge to the contact zone.

The oxygen-containing gas may contain at least 25% and preferably at least 60% by volume of oxygen.

The stabilistion zone may comprise a plurality of stages in each of which the sludge therein is contacted with the oxygen-containing gas so that the amount of material to be oxidised biologically in any given stage is less than the amount of material to be oxidised biologically in the preceding stage in the direction of flow of sludge through the stabilisation zone.

The oxygen-containing gas at any given stage is preferably less than the amount of oxygen in the oxygen-containing gas in the preceding stage in the direction of flow of the oxygencontaining gas in the stabilisation zone.

Sludge may be passed from the last stage of the stabilisation zone to the contact zone and may contain between 10 and 30 mg/l dissolved oxygen.

The oxygen-containing gas, in the preferred embodiment, flows through the stabilisation zone in a direction counter to the direction of flow of sludge therethrough.

The sludge contained in the stabilisation zone may have a total suspended solids content between 5000 and 50000 mg/l.

The process may include the step of discharging sludge in an unstabilised condition upstream of the stabilisation zone for disposal or alternatively discharging sludge in a stabilised condition downstream of the stabilisation zone for disposal.

Preferably the oxygen dissolved in the sludge in the stabilisation zone is maintained at greater than 2 mg/l.

According to a further and non-restrictive aspect of the present invention there is provided a treatment plant when used for treating wastewater containing bacterial active material by a process according to the present invention.

The invention is illustrated, merely by way of example, in the accompanying drawings in which: Figure 1 is a schematic diagram illustrating a process according to the present invention of treating wastewater containing biologically oxidisable material; Figure 2 is a schematic diagram of a stabilisation tank for use with a process according to the present invention; and Figure 3 illustrates schematically a modification to a conventional sewage treatment plant to operate in accordance with a process according to the present invention.

One embodiment of a process according to the present invention of treating wastewater containing biologically oxidisable material is illustrated in Fig. 1 and comprises contacting wastewater with a well-oxidised bacterial active sludge upstream of the settling tank 13 in a well-mixed section or contact point 12 of an inlet channel 11 to the settling tank without the requirement of a purpose designed contact tank and without the requirement of oxygenating the contact point before the setling tank. A "contact loading rate" is defined as the ratio of biologically oxidisable material as measured by the biological oxygen demand (BOD) of the wastewater to a bacterial active sludge as measured by the sludge volatile suspended solids (SVSS) and prefetrably is in the range of 0.01 to 0.1 kg BOD/kg SVSS.

The bacterial active sludge settling in the settling tank 13 is then passed to a stabilisation tank 16 and the effluent is discharged to an outlet 14. In the stabilisation tank the material adsorbed at the contact point is oxidised or converted to new bacteria. The detention time, the mixing intensity and the dissolved oxygen concentration of the sludge in the stabilisation tank are controlled in order to limit the degree of endogenous respiration undergone by the bacteria and to ensure that a very high proportion of the bacteria held within the stabilisation tank are maintained continuously under aerobic conditions. It is believed that the most effective and economic method of ensuring that these conditions are maintained is by the use of a covered, gas-tight stabilisation tank provided with a feed gas containing at least 25% and preferably more than 60% oxygen by volume.The stabilisation tank may have 2 to 6 stages. The flow of feed gas may be counter to the flow of the bacterial active sludge through the stages of the stabilisation tank. This is to give the highest concentration of oxygen in the space above the sludge in the last stage of the stabilisation tank. This enables a very high dissolved oxygen concentration of 10 to 30 mg/l to be achieved in the bacterial active sludge before it passes to the contact point with the wastewater. This high dissolved oxygen concentration ensures that the bacteria of the bacterial active sludge is under extremely aerobic conditions before contact with the waste-water and provides a buffer of oxygen to maintain aerobic conditions for a period of time after the bacterial active sludge and wastewater mixture has entered the settling tank.

The main parameters that control the performance of the process are the contact load rate (CLR) defined above and the stabilisation loading rate (SLR). The importance of the contact loading rate it that is governs the efficiency of the bacterial active sludge for removal of suspended and dissolved organic material from the wastewater. The capacity for removal of organic material by the bacterial active sludge will also depend on other factors which are less within the control of the operator. These include the characteristics of the wastewater, the ratio of suspended to dissolved organic matter, temperature, pH, toxic contaminants of the wastewater and characteristics of the sludge. For a particular set of circumstances there will be an optimum contact loading rate required to achieve the desired removal of organic material from the wastewater.This will generally be in the range of 0.01 to 0.1 kg BOD/kg SVSS.

The stabilisation loading rate is defined as the ratio of mass of BOD contained in the wastewater in unit time to mass of SVSS contained within the stabilisation tank at any given time. The stabilisation loading rate has units of mass BOD/mass SVSS per unit time. This parameter is important because it governs the degree of stabilisation of the bacterial active sludge in the stabilisation tank, the degree of oxidation of the organic material adsorbed by the bacterial active sludge, the degree of endogenous respiration of the bacteria, the specific growth rate of the bacteria and the specific oxygen requirement of the bacterial active sludge held in the stabilisation tank.These parameters in turn have effects on the characteristics of the bacterial active sludge, such as the settling characteristics of the bacterial active sludge and the populations of bacteria, protozoa and other microorganisms contained within the bacterial active sludge. For most wastewaters the optimum average stabilisation loading factor with be in the range 0.1 to 2.0 kg BOD/kg SVSS. day although it may be allowed to come outside this range for a fe#w hours due to variations in the characteristics of the wastewater without serious deterioration of performance of the process.

Further parameters that may be controlled by the operator include the settling tank sludge blanket level (SBL) and the concentration of dissolved oxygen (DO) in the stabilisation tank. The sludge blanket level depends mainly upon the rate of return of bacterial active sludge from the base of the settling tank and the quantity of sludge contained in the system (controlled by the sludge wastage rate). The sludge blanket level controls the detention time of the bacterial active sludge in the settling tank.This is important because a very low sludge blanket level will lead to poor consilidation of the bacterial active sludge in the settling tank and hence a less concentrated SVSS and will also lead to a short detention time in the settling tank which reduces the period of contact between the bacterial active sludge and the wastewater which in turn reduces the efficiency for removal of suspended and dissolved organic material from the wastewater. If the sludge blanket level is high then there is an increased risk that some of the bacterial active sludge may overflow the settling tank and contaminate the effluent. Also a high sludge blanket level will lead to a long detention time of the bacterial active sludge in the settling tank.Under these conditions, the bacterial active sludge will eventually start to digest the adsorbed organic matter anaerobically with a consequent release of organic matter to the effluent and formation of gas bubbles which will cause the sludge to float and again contaminate the effluent. The optimum sludge blanket level would depend on the particular flow conditions and the characteristics of the wastewater and the bacterial active sludge. For a conventional designed 3m deep hopper bottomed circular settling tank the optimum sludge blanket level will be in the range 0.5 to 2.0 m above the base of the settling tank.

The concentration of dissolved oxygen (DO) in the stabilisation tank is important as it affects the penetration of oxygen into the bacterial active sludge flocs mixed in the stabilisation tank and the degree to which the bacteria experience aerobic conditions and are able to respire and oxidise the adsorbed organic material at the maximum rate. The concentration of dissolved oxygen in the last stage of the stabilisation tank will determine the concentration of dissolved oxygen in the bacterial sludge subsequently brought into contact with fresh wastewater at the contact point.

For the process employing a gas containing at least 60% oxygen in a covered stabilisation tank the concentration of dissolved oxygen in the first stages of the stabilisation tank should be maintained in the range of 4 mg/l to 10 mg/l for optimum economy and effectiveness. For the last stage of the stabilisation tank the concentration of dissolved oxygen should be maintained in the range 10 mg/l to 30 mg/l in order to maintain aerobic conditions for the significant period of time in the mixture of bacterial active sludge and wastewater after the contact point.

Fig. 2 illustrates a two-stage stabilisation tank of a treatment plant operating in accordance with a process according to the present invention. Unstabilised sludge from the settling tank enters a first stage 22A of the stabilisation tank through an inlet 21. The first stage 22A contains a ass transfer/mixing device 27A which mixes the bacterial active sludge therein and causes it to be oxidised by oxygen-containing feed gas in a gas space 24A. the bacterial active sludge in the first stage 22A of the stabilisation tank passes to a second stage 22B containing a mass/mixing device 27B which mixes the activated sludge therein and causes it to be oxygenated with feed gas in a gas space 24B. The feed gas enters the stabilisation tank through an inlet 25 and flows counter to the direction of flow of the bacterial active sludge through the stages 22A, 22B.Gas is vented from the stage 22A through an outlet 26. Bacterial active sludge leaves the stabilisation tank for the contact zone through an outlet 23.

The advantages of one embodiment of a process according to the present invention are as follows.

The volume of oxygenation zones is considerably reduced compared to conventional processes because only the settled sludge is oxygenated. This is because a much higher concentration of suspended solids can be maintained than would nornmally be experienced in conventional aeration tanks, for example, 5000 to 30000 mg/l suspended solids in the process according to the present invention compared to 1000 to 6000 mg/l in the conventional activated sludge process. Thus a typical fivefold increase in the sludge concentration may be achieved with a consequent saving in volume of sludge requiring oxygenation is possible.For example, at a typical sludge loading rate of 0.5 kg BOD/kg SVSS day, a wastewater of BOD 250 mg/l would require 4 hours to be treated by a sludge of SVSS 3000 mg/l, by the conventional activated sludge process, whereas at a typical sludge concentration of 15000 mg/l SVSS only 48 minutes detention in the stabilisation tank would be required in a process according to the present invention.

Control of the concentration of solids of bacterial active sludge in the aeration tank of the conventional activated sludge process by adjustment of the rate of return of settled sludge from the settling tank is limited because an increase in return flow tends to increase the mass loading on the settling tank and reduce the return sludge concentration, resulting in only a minor change to the concentration of bacterial active sludge in the aeration tank.

The sludge concentration in the stabilisation tank of a process according to the present invention may be controlled directly by adjustment of the rate of flow of sludge from the base of the settling tank. This adjustment however will have little effect on the sludge loading rate at the contact point because the mass flow of sludge around the system will only change slightly with changes to the sludge flow rate. This is demonstrated by the Example.

This degree of control over the sludge loading rate in a process according to the present invention is a very significant departure from the very limited degree of control possible with a conventional activated sludge process and enables control of the degree of oxidation and degree of endogenous respiration undergone in the stabilisation tank in order to optimise the capacity for the bacterial active sludge to adsorb the organic material from the wastewater at the contact point in the process.

The process according to the present invention is much less sensitive to variation in flow of wastewater than a conventional activated sludge process. This is because the flow of bacterial active sludge to the settling tank is controlled by the rate of flow of sludge to the stabilisation tank and is independent of the flow of wastewater. In the conventional activated sludge process when a sudden increase in flow of wastewater occurs there is a rapid increase of mass flow of bacterial active sludge to the settling tank. For sludges that settle poorly this very often results in overflow of bacterial active sludge from the settling tank and contamination of the effluent.

The main effect of increased flow on the process is to increase the volumetric contact loading, m3 wastewater/kg SVSS but since for domestic sewage the concentration of organic material in the wastewater tends to reduce with increased flow the organic contact loading, kg BOD/kg SVSS will tend not to increase significantly, the flow and concentration of the stabilised bacterial active sludge remaining constant. Hence the adsorption performance and the quality of the effluent will only be slightly affected by increases in flow.

In the conventional activated sludge process a proportion of the bacterial active sludge must be removed for disposal as the organic material removed from the wastewater accumulates or is converted to new bacterial cells, otherwise the mass of solids in the system would exceed the capacity of the settling tank and bacterial active sludge solids would overflow the settling tank and contaminate the effluent. The advantage of the process according to the present invention is that an operator may decide whether to dispose of the settled sludge either before or after stabilisation. This decision will depend on the chosen sludge disposal route. Thus if, for example, the surplus sludge is to be treated by an anaerobic digestion process then discharge of sludge immediately after settlement and before stabilisation may be desirable.This is because at this point the sludge will contain a high proportion of adsorbed organic matter which may be treated in an anaerobic digestor, increasing the methane generation potential and avoiding the need to supply oxygen to that portion of the sludge in the stabilisation tank. Should it be desirable to dispose of the sludge by some other route, then disposal after dstabilisation may be advantageous since sludge discharged after stabilisation will be slower to putrefy and will therefore tend to be less odourous and unpleasant after any period of storage.

The process according to the present invention is particularly suitable for the extension of an existing sewage treatment plant, either to increase the sewage throughput or to improve the quality of the final effluent. As shown in Fig. 3 this may be achieved by installing a stabilisation tank 30 in conjunction with an existing primary settling tank or tanks 35. Thus stabilised bacterial active sludge is brought into contact with the crude sewage at a contact point 33 upstream of the primary settling tank at a convenient point, for example, upstream of a detritor or grit remover 34. The sludge withdrawal system from the primary settling tank may be modified to handle a higher throughput of sludge and provided with a pump 38 to control the flow of sludge to the stabilisation tank 30, and a sludge outlet 39.The effluent flowing from the primary settling tank through an output 36 may then continue through the treatment plant for improvement of the final effluent quality by the existing treatment plant before disposal. The stabilisation tank has an inlet 31 for oxygen-containing gas and a vent 32.

It is estimated that a two- to three-fold increase in the load to a sewage treatment plant or the conversion of a sewage treatment plant from a non-nitrified effluent typically 20 mg/l BOD, 30 mg/l suspended solids, to a high quality fully nitrified effluent of less than 10 mg/l BOD, 15 mg/l suspended solids, 1 mg/l ammonia, may be accomplished at the very low capital cost of the small stabilisation tank, pumping system and oxygen supply and dissolution equipment. This saving in cost is estimated as of the order of two thirds of the cost of a conventional extension to a sewage treatment plant designed for the same duty.

The process according to the present invention is also particularly suited for the treatment of industrial wastewater containing a high concentration of biologically oxidisable material, such as a brewery or paper mill wastewater. The process is able to provide treatment of such wastewaters to the effluent quality typically required for sewer discharge and minimising water authority charges to an industrial discharger. The process enables this treatment to be achieved on a very limited area of land which is particularly advantageous on an industrial site where space is often at a premium and the use of a large area of any land that is available for a wastewater treatment plant is undesirable. The process is also most suited for widely varying wastewater since there is greater flexibility for control.The process is also better able to cope with the discharge of toxic or inhibitory material since the flow does not come into contact with all the sludge at once. The high concentration of sludge held in the stabilisation tank provides a buffer against any adverse effect by the inhibitory material.

EXAMPLE Bacterial active sludge settles typically according to the Kynch-Dick model: Vi = a cin where, Vi is the settling velocity of a sludge having a concentration Ci, a and n are constants which characterise the sludge and have typical values of: a = 0.00009 m/h n=1.9 The concentration of the return sludge is predicted by the equation: n/(n - 1)a(n - RSS = ~~~~~~~~~~~~~~~~~~~~~~ R/QXCOR 1/n where, RSS is the concentration of suspended solids in the return sludge stream (mass fraction), R/Q is the ratio of return sludge flow to feed wastewater flow, and COR is the setling tank up flow velocity (m/h) calculated by dividing the wastewater flow rate by the area of the settling tank.

For a typical wastewater BOD of 250 mg/l, a settling tank upflow velocity of 0.8 m/h (and a nominal retention time in the stabilisation tank of 30 minutes based on wastewater flow) the following relationship between return sludge to wastewater flow ratio and sludge loading rate can be determined: R/Q RSS Contact loading Rate Stabilisation mgXl kg BoDJkg SVSS loading Factor kg'#flO#Ag SV$S.d ~ kg BOkg St$S; ;d 0,25 35000 0.028 0.086 0.5 24000 0.02 0.25 0.75 19400 0.017 0.46 1.0 17000 0.015 0.7 1.5 12000 0.014 1.5 Hence over the range of sludge to wastewater flow ratios considered while a two-fold change in contact loading rate is predicted a seventeen-fold change in stabilisation loading factor may be anticipated.

Claims (16)

1. A process of treating wastewater containing biologically oxidisable material comprising the steps of: contacting and mixing the wastewater with bacterial active sludge in an unoxygenated contact zone; allowing the resultant mixture to separate in a settling zone into a sludge containing biologically oxidisable material adsorbed and absorbed by bacterial material in the bacterial active sludge, and a clarified effluent; discharging the clarified effluent from the settling zone; withdrawing the sludge from the settling zone to a stabilisation zone; contacting the withdrawn sludge with an oxygen-containing gas in the stabilisation zone which allows the adsorbed and absorbed material to be oxidised biologically to form the bacterial active sludge; and passing the bacterial active sludge to the contact zone.

2. A process as claimed in claim 1 in which the oxygen-containing gas contains at least 25% by volume of oxygen.

3. A process as claimed in claim 1 or 2 in which the oxygen-containing gas contains at least 60% by volume of oxygen.

4. A process as claimed in any preceding claim in which the stabilisation zone comprises a plurality of stages in each of which the sludge therein is contacted with the oxygen-containing gas so that the amount of material to be oxidised biologically in any given stage is less than the amount of material to be oxidised biologically in the preceding stage in the direction of flow of sludge through the stabilisation zone.

5. A process as claimed in claim 4 in which the amount of oxygen in the oxygen-containing gas in any given stage is less than the amount of oxygen in the oxygen-containing gas in the preceding stage in the direction of flow of the oxygen-containing gas in the stabilisation zone.

6. A process as claimed in claim 4 or 5 in which the sludge is passed from the last stage of stabilisation zone to the contact zone.

7. A process as claimed in claim 6 in which the sludge passed from the last stage of the stabilisation zone contains between 10 and 30 mg/l dissolved oxygen.

8. A process as claimed in any of claims 4 to 7 in which the oxygen-containing gas flows through the stabilisation zone in a direction counter to the direction of flow of sludge therethrough.

9. A process as claimed in any preceding claim in which the sludge contained in the stabilisation zone has a total suspended solids content between 5000 and 50000 mg/l.

10. A process as claimed in any preceding claim including the step of discharging sludge in an unstabilised condition upstream of the stabilisation zone for disposal.

11. A process as claimed in any of claims 1 to 9 including the step of discharging sludge in a stabilised condition downstream of the stabilisation zone for disposal.

12. A process as claimed in any preceding claim in which the oxygen dissolved in the sludge in the stabilisation zone is maintained at greater than 2 mg/l.

13. A process of treating wastewater containing biologically oxidisable material substantially as herein described with reference to the accompanying drawings.

14. A treatment plant when used for treating wastewater containing biologically oxidisable material by a process as claimed in any preceding claim.

15. A treatment plant as claimed in claim 14 and substantially as herein described with reference to and as shown in the accompanying drawings.

16. Any novel integer or step, or combination of integers or steps, hereinbefore described and/or as shown in the accompanying drawings, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.