GB2142008A

GB2142008A - Treatment of water

Info

Publication number: GB2142008A
Application number: GB08408742A
Authority: GB
Inventors: M E Garrett; O A Kite
Original assignee: BOC Group Inc
Current assignee: Messer LLC
Priority date: 1983-04-29
Filing date: 1984-04-05
Publication date: 1985-01-09
Also published as: GB2142008B; GB8408742D0

Abstract

Water having a biochemical oxygen demand (eg sewage) is treated in a relatively shallow tank 102 which may for example take the form of a sewage lagoon. A wall 112 separates one treatment zone 114 from another treatment zone 116. Both the zones 114 and 116 communicate with a common inlet zone 118 into which water for treatment is fed from pipe 120. Both the zones 114 and 116 have outlet weirs 124 and 126 respectively. In operation, the water in one of the zones 114 and 116 is oxygenated in the presence of suspended bacterial solids that are effective to reduce the biochemical oxygen demand, while at the same time solids in the other of the zones 114 and 116 are allowed to settle, clear liquid flowing out of the tank over the outlet weir associated with that zone. At the end of a chosen period, the zone in which solids were settling is oxygenated, and the solids in the other zone are allowed to settle. In this way it is possible to treat the water continuously without the need to employ a separate settlement tank. <IMAGE>

Description

SPECIFICATION Treatment of water This invention relates to a method and apparatus for the treatment of water having a biochemical oxygen demand (BOD). The term water having a biochemical demand" is used throughout the specification and the accompanying claims to indicate water or some other naturally occuring source of water having a biochemical oxygen demand or sewage or other aqueous effluent having a biochemical oxygen demand.

It is currently known to treat water having a biochemical oxygen demand in a so-called "lagoon" or an oxidation pond. The rate at which such lagoons or ponds can treat the water is however relatively limited and there is a need to improve upon existing treatment methods as applied to lagoons and oxidation ponds. There is also a need to treat water having a biochemical oxygen demand in some vessels such as storm tanks conventionally used to hold the water until it is passed into a treatment vessel or plant. Lagoons, oxidation ponds and surplus tanks are all characterised in being relatively shallow. Such vessel(s) or container(s) shall be referred to herein as 'shallow tank(s)'.

Conventionally, treatment of waste water in order to reduce its biochemical oxygen demand may be performed by bacterial solids, sometimes known as activated sludge, effective to break down pollutants or other deleterious substances in the water. Such treatment typically therefore requires means for maintaining the bacterial solids in suspension in the water being treated and means for aerating or oxygenating the water so as to maintain the necessary aerobic conditions that the bacteria require for the purposes of respiration.

Moreover, in order to enable clear water to be withdrawn from the treatment vessel, means are necessary for separating water from the bacterial solids, typically by settlement of the solids. Such settlement is conventionally performed in a separate vessel from the treatment vessel. It is however possible with advantage to perform both treatment and settlement in the same tank by the process disclosed in our UK patent 1 596 311 or the process described in our co-pending U.K. patent application No. 2 11 8 449 (It is to be appreciated that knowledge of the aforementioned patent and patent application is not necessary for the understanding of the invention described in this specification).

Particularly, if it is required to increase the 'biochemical loading' of a lagoon or oxidation pond, the installation of separate settling tanks is likely to be a particularly expensive and capital intensive procedure. Moreover, it is difficult if not impossible to obtain adequate clarification if the process described in our aforementioned patent specification 1 596 311 (or that described in the aforementioned co-pending application) is performed in a shallow tank. Thus the known methods of treating water having a biochemical oxygen demand described herein above are not suited to the performance of such treatment in shallow tanks.

An aim of the present invention is to provide a method and apparatus for treating water having a biochemical oxygen demand that is suitable for use when it is required to treat the water in a shallow tank.

In its broadest aspect, the present invention provides a method of treating water having a biochemical oxygen demand, comprising repeatedly performing the steps of (a) oxygenating the water in the presence of suspended aerobic (or oxic) bacteria effective to reduce the biochemical oxygen demand of the water and (b) allowing bacterial solids to settle so as to leave a supernatant layer of treated water relatively free of such solids, in which method at least two volumes of such water are so treated generally simultaneously, the steps (a) and (b) being performed on one volume generally out of phase with the steps (a) and (b) on the other volume.

Both volumes may be defined in the same vessel thereby enabling the method to be performed on water being held in a shallow tank. Typically, but not necessarily the two volumes are served by a common inlet. Thus, as incoming water having a biochemical oxygen demand is received for treatment, so treated water may flow out of the volume in which settling of the solids is for the time being taking place. A substantially continuous treatment process is thus made possible by the method according to the invention.

It is generally important to introduce incoming water for treatment into a region of the tank remote from the discharge(s) from such tank. It will be appreciated that in a single vessel the incoming water will tend to flow towards the volume for the time being undergoing settlement. Accordingly, the steps (a) and (b) are arranged to have durations generally equal to one another and substantially less than the average residence time of the water in the tank. Typically, each step (a) or (b) of the method according to the invention is performed for a short period (say in the order of half an hour) before being succeeded by the other step to be performed on the same volume of water, the average residence time of the water in the tank amounting to several hours.The preferred duration of each step (a) and (b) depends on the biochemical oxygen demand of the water and on the ambient temperature. For water having a relatively large biochemical oxygen demand, said duration may in a hot climate need to be as short as fifteen minutes in order to keep the settled solids in good condition, whereas in more temperate climates and with lower biochemical oxygen demands each step (a) and (b) may have a duration in the order of one hour.

Typically, relatively clear water flows out of the shallow tank over a weir remote from the inlet. Preferably, an outlet channel or conduit is provided downstream of the weir.

In order to define the said volumes within a single tank, at least one dividing wall or baffle or other such means may be provided substantially to restrict or prevent the flow from one volume to the other. In particular, two different arrangements are envisaged. In one arrangement there are two longitudinally extending volumes communicating with outlet weirs (or like means) in the vicinity of one end of the tank and in the vicinity of the other end with a common inlet zone. In an alternatively preferred arrangement the two said volumes are separated from one another by a longitudinally extending intermediate volume into which incoming water for treatment is introduced.

The invention also provides an apparatus for treating water having a biochemical oxygen demand, comprising a shallow tank having at least two treatment zones; means for the withdrawal or outflow of water from the treatment zones; at least one water inlet remote from said withdrawal or overflow means; means for agitating water in the treatment zones; and means for oxygenating the treatment zones, said apparatus being adapted to perform the above defined method.

Typically, the oxygenation and agitation means are operable intermittently such that oxygenation and agitation takes place in only one of the treatment zones at any one time (that is if there are two treatment zones only).

The water in the other zone will thus remain relatively still enabling settlement to take place and relatively clear liquid to flow over an outlet weir or otherwise be discharged from the shallow tank. During agitation of the water containing suspended bacterial solids, some of the water (containing said solids) may pass over the outlet weir (or through any other outlet device provided) associated with that volume and typically collect in an outlet channel associated with the weir. Normally, such channel has a valve located in it, the valve only being open during periods in which clear liquid is being discharged over the associated weir and into the channel.During periods in which the volume of liquid associated with said weir and outlet channel is being oxygenated, the necessary agitation of the volume typically causes some water (containing suspended solids) to flow over the weir into the channel, where it is retained. It is generally desirable to withdraw such water containing bacterial solids from the channel and typically to return it to the shallow tank.

This is typically done immediately after step (a) so as to avoid significant contarnination of clear liquid run off from said volume during step (b). For this purpose, a sump may be associated with each such channel and water containing suspended bacterial solids passed by means of a pump or under gravity to the sump from where it can be returned to the shallow tank, as arid when desired.

Although separate means for oxygenating and agitating the water may be provided, the process as claimed and described in our UK patent 1 455 567 may be employed to perform both these functions. In this process a stream is withdrawn from the volume of liquid, the stream is then pressurised and oxygen introduced therein under turbulent conditions so as to form a dispersion of undissolved gas bubbles in the liquid (which will also contain dissolved oxygen) and such dispersion is then returned to the volume from which the stream is taken, being introduced into such volume such that the undissolved oxygen bubbles are sheared into fine bubbles that readily dissolve within or are consumed by the volume of liquid.By introducing the stream into the volume at several different points or locations in the form of jets comprising a dispersion of oxygen bubbles in the water, the desired degree of turbulence can be created in the volume of liquid being treated. The pressure to which the withdrawn stream is raised is typically chosen so as to provide the energy for agitating the water in the zone being oxygenated.

If desired, the water may be aerated as well as being oxygenated. Such aeration can be performed in the same zone as the oxygenation or in a separate zone. It offers the advantage of helping to counteract the carbon dioxide that is generated by the bacteria creating unfavourably acidic conditions within the water being treated. This is because it helps to displace such carbon dioxide from the solution.

It is an advantage of the method and apparatus according to the invention that existing lagoons or large tanks for holding or treating sewage can readily be converted to enable the invention to be performed without the need to incur large capital expenditure that would, for example, be necessary if settling tanks were introduced. It is therefore possible at relatively low capital cost to convert an existing holding tank to the treatment of water having a biochemical oxygen demand or substantially to increase the "biochemical load" with which an existing lagoon or oxidation pond has to deal.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic plan view of the sewage lagoon adapted to perform the invention; and Figure 2 is a schematic plan view of another sewage lagoon adapted to perform the invention.

Figure 3 is a schematic section through line Ill-Ill of Fig. 1.

Figure 4 is a schematic section through line IV-IV of Fig. 2.

Referring to Figs. 1 and 3 of the drawings, there is shown an elongate rectangular tank 102. The tank has side walls 104 and 106 and end walls 108 and 110. The dimensions of the tank may vary widely. However, in general the depth of the tank will be small in comparison with the length of the walls 104 and 106. Typically, the walls 104 and 106 may each be up to 50m long and the walls 108 and 110 may typically be a quarter or a fifth of this length, being say up to 1 Om long.

The tank 102 has an open top which is usually level with the surface of the ground; the base of the tank being below ground level. It is not necessary specially to install a tank 102 to perform the method according to the invention. Indeed, the method according to the invention is desirably performed in a tank that is already in place.

In order to adapt the tank 102 to perform the invention, a central wall (or baffle) 112 is formed in the tank. This wall 112 is equal distance between the walss 104 and 106 and extends generally parallel thereto from the wall 110 towards the wall 108. The length of the wall 112 may be about 80-90% of that of each of the walls 104 and 106. The wall 112 is thus effective to divide the tank 102 into two treatment zones 114 and 116 which have their ends remote from the wall 110 and communicate with an inlet zone 118 in which terminates an inlet pipe 120 which is employed to feed incoming water having a biochemical oxygen demand into the tank 102.

The inlet pipe 120 is provided with a valve 122 which is operable to shut off the feed whenever desired. Typically, the outlet end of the feed or inlet pipe 120 is disposed in the tank at a point colinear with the longitudinal axis of the wall 112. The height of the wall 112 is chosen to be such that in operation of the tank according to the present invention there is no flow of liquid directly over the wall. Likewise its construction is such that no passage thereunder is afforded for the flow of liquid from the zone 114 to the zone 116 or vice versa. It is not however necessary for the wall 112 to be impermeable to water flow therethrough as there is no differential liquid head between the zones 114 and 116. Indeed, the wall 112 may be of simple and lightweight construction, being formed of, for example, suitably anchored tarpaulin or plastic sheeting or of breeze blocks.Such walls are capable of performing the function of allowing a supernatant layer of clear liquid to be established in one of the zones 114 and 116, while the other is being oxygenated. The treatment zones 114 and 116 are provided with weirs 124 and 126 respectively over which treated water can flow out of the tank into respective channels 128 and 130 in operation of the process according to the invention. The weirs 124 and 126 and their respective channels 128 and 130 are provided around the two corners at the end of the tank 102 remote from the inlet zone.In the tank shown in Fig. 1 the weirs 124 and 126 are provided by appropriately dimensioning the wall 110 of the tank 102. (Alternatively, suitable weirs may be provided within the tank 102 itself, thus making it unnecessary to form parts of the walls 104, 106 and 110 such that they can act as weirs.) The channels 128 and 130 do not communicate with one another. The channel 128 does, however, communicate with outlet pipe 132 having a valve 136 disposed therein. Similarly the channel 130 communicates with an outlet pipe 134 having a valve 138 disposed therein. If desired, the channels 128 and 130 may be integral with the pipes 132 and 134.

The zones 114 and 116 are provided with oxygenation means as now described.

Each of the zones 114 and 116 has associated therewith a pump 140. Each pump 140 is provided with a feed conduit 142 leading from its respective treatment zone (114 or 116). Each pump 140 has a return conduit 143 having a venturi 144 formed therein at a region near to the pump 140. Each venturi 144 is provided with an oxygen pipeline 146 leading to its throat, whereby in operation of the oxygenator, oxygen is drawn into the return line 143 from the pipeline 146. Each oxygen pipeline 146 communicates with a source of commercially pure oxygen (not shown). The return line 143 ends in a sparge pipe 148 which is submerged beneath the level of the water in the tank 102.The sparge pipe 148 has a plurality of orifices 150 out of which a relatively high velocity dispersion of oxygen in water is propelled into the respective zones 114 and 116 in operation of the method according to the invention (although only one such zone is oxygenated at any one time). The construction of such a side stream oxygenation system is discussed in more detail in our UK patent specification No. 1 455 567.

The jets of oxygenated water provided by the sparge pipe 148 are effective to provide agitation in the respective zones 114 and 116. Thus, bacterial solids are kept in suspension in such zones during their oxygenation.

As shown in Fig. 1, surface aerators 152 of conventional design are provided in the zones 114 and 11 6. It is not essential to provide such aerators but it is to be appreciated that if they are provided they will also cause agitation of the water and help to keep bacterial solids in suspension. Such aerators 152 help to displace carbon dioxide generated by the bacteria out of solution and thus facilitate the keeping of the pH of the water within the desired limits.

During operation of the aerators and oxygenators in either of the zones 1 14 or 1 16 the resultant agitation will tend to cause some water with suspended solids to flow over the respective weir 124 or 126 into respective channel 128 or 130. It is to be appreciated that during oxygenation of each zone, the channel associated with that zone is kept closed (i.e. the valve 136 and 138, as the case may be, is kept closed). Accordingly, when after an oxygenation period, the suspended solids are allowed to settle, in order to prevent loss of solids the liquid containing suspended solids may be withdrawn from the channel and returned to the tank. For this purpose, the channels 128 and 130 communicate with a sump 154 via inlet pipes 156 and 158 respectively, having valves 160 and 162 respectively, disposed therein.The sump 154 is provided with a pump 164 for returning the liquor withdrawn from the channels 128 and 130 to the tank 102 via a return pipe 166 having a valve 170 disposed therein. The return pipe 166 typically terminates in the inlet pipe 120.

The operation of the apparatus shown in Figs. 1 and 3 will now be described.

The tank 102 contains waste water for treatment. A two stage treatment process is performed in each of the zones 1 14 and 1 16.

Each two stage treatment comprises successive steps of oxygenating the water in the presence of suspended bacterial solids which are effective to reduce the bio-chemical oxygen demand of the water and then allowing the solids to settle to enable relatively clear water to be run off from the tank. The treatment in each zone is phased with respect to the other zone, such that generally while settlement is being performed in one zone the oxygenation of the water takes place in the other zone and vice versa.

When settlement is taking place in the zone 116 and oxygenation in the zone 114, the valve 122 in the inlet pipe 120 is open as is the valve 138 associated with the channel 130, whereas the other valves 136, 1 60 and 162 are in a closed position. Further, the pump 140 and aerators 152 associated with the zone 114 are operated whereas those associated with the zone 116 are not. Similarly, the pump 164 associated with the sump 154 is not operating.

The pump 140 associated with the zone 114 is effective to withdraw water from that zone and raise it to a pressure typically in the range of two to four bars. Pressurised water is oxygenated to form an oxygen-in-water dispersion containing relatively small bubbles of oxygen. Typically, only about 40 to 60% of the oxygen introduced into the pressurised water is dissolved, the rest remaining in dispersion. This dispersion is then sparged into the volume of water in the zone 114 and the resultant drop in pressure causes the bubbles of oxygen to be sheared into even finer bubbles as the jets of the said dispersion enter the volume of liquid in the zone 114. This facilitates the solution of the bubbles of oxygen. A fuller description of this oxygenation technique is included in our UK patent specification No. 1 455 567 to which the reader is referred.The jets of the dispersion agitate the volume of water in the zone 114 and thus the bacterial solids that naturally occur in the water (or which can be produced by feeding the water with so called activated sludge) are kept in suspension. The bacteria metabolise the oxygen and are effective to breakdown polutants in the waste water, thereby reducing the biochemical oxygen demand of the water.

The solids tend to proliferate in the volume being oxygenated as a result of the respiration of oxygen and in addition carbon dioxide is formed by the bacteria. This carbon dioxide dissolves in the water but may at least in part be displaced therefrom by the action of surface aerators 152 which are operated during the oxygenation of the volume of water in the zone 114.

It will be appreciated that as water flows into the tank 102 for treatment through the inlet pipe 120 (typically continuously) so there will be a continuous outflow of water from the tank over the weir 126. Thus there will be a general drift of liquid from the inlet zone 120 towards the weir 126. The settlement period in the zone 116 and the rate of flow of water into the tank 102 are therefore arranged to be such that the volume of liquid entering the tank 102 during any one settlement period in the zone 116 is small compred with the volume of that zone. Thus, no more than an insignificant proportion of the incoming water can leave the process untreated. Typically the liquid in the zone 114 is oxygenated for a period in the range half hour to an hour while that in the zone 116 undergoes settlement.At the end of this period the inlet valve 11 2 and the outlet valve 138 are closed and the pump 140 and aerators 152 associated with the zone 114 are de-energised. Immediately afterwards, the pump 164 in the sump 154 is energised and the valve 160 opened to enable liquid in the channel 128 to be withdrawn to the sump. The liquid may then be returned to the tank at a later stage (or immediately) through pipe 168 (with the valve 170 being opened). Alternatively, any excess solids can be discharged to the environment. Once the channel 128 has been drained (which typically may take no more than a minute or two) the zone in which oxygenation has taken place immediately before the draining of the channel is switched to oxygenation (and aeration). Thus, the inlet valve 122 and the outlet valve 136 are opened and one of the pumps 140 and those of the aerators 152 associated with the zone 116 are energised. Water relatively free of solids thus flows out of the tank 102 over the weir 124 while the water in the zone 1 16 is oxygenated and aerated. At the end of the chosen time period (which is identical to that chosen for the oxygenation of the zone 1 14 and the settlement of solids in zone 11 6) the valve 122 and 136 are closed again, and that one of the pumps 140 and those of the aerators 152 associated with the zone 116 are de-energised. The pump 164 is then energised and the valve 162 is opened to allow liquid containing bacterial solids to be drained from the channel 130.Then the functions performed in the respective zones 140 and 116 can be switched again such that oxygenation and bacterial treatment of the water takes place in the zone 1 14 while solids are allowed to settle in the zone 116. It will thus be appreciated that a substantially continuous process for the treatment of water is thereby afforded.

If desired, control of the operation of the valves, pumps and aerators may be automatic. The valves 122,136 and 138 may be of the penstock type.

Referring now to Figs. 2 and 4 of the drawings, the tank 202 is substantially the same as the tank 102 shown in Fig. 1. Tank 202 has side walls 204 and 206 and end walls 208 and 210. However, instead of a wall exactly corresponding the wall 11 2 shown in Fig. 1. two walls 274 and 276 are provided. The walls 274 and 276 are spaced apart from one another and extend parallel to each other and to the walls 204 and 206.

The wall 274 extends from the wall 210 towards the wall 208 terminating short of the wall 208 terminating short of the wall 208.

Typically, its length is 80% of that of each of the walls 204 and 206.

The wall 276 is equal in length to the wall 274 but extends from the wall 208 towards the wall 210. There are thus then three zones defined in the tank 204. The wall 274 and the side wall 204 of the tank 202 bound a first treatment zone 216. There is thus an intermediate zone 218 into which incoming water for treatment is fed through an inlet pipe 220 having a valve 222 disposed therein. The intermediate or inlet zone 218 communicates at its end adjacent to the wall 204 of the tank 202 with the zone 216. The inlet pipe 220 extends into the centre of the zone 218. The walls 274 and 276 are constructed so as to prevent water passing thereover or thereunder in operation of the tank 202.

Whereas in Fig. 1 the outlet weirs are provided about corners at the same end of the tank, in the apparatus shown in Fig. 2, the outlet weirs are provided at diagonally opposite corners along the two side walls 204 and 206 of the tank 202. Water flowing over the weir 224 provided along the side wall 204 flows into the channel 228 and then to the outlet pipe 232 which has a valve 236 disposed therein. Likewise, water flowing into a channel 230 and then to an outlet pipe 234 having a valve 238 flows into a channel 230 and then to an outlet pipe 234 having a valve 238 disposed therein. The zones 214 and 216 are each provided with oxygenators substantially identical to those shown in and described with reference to Fig. 1. The construction and operation of such oxygenators can therefore be described only briefly with reference to Fig. 2.The oxygenators each comprise a pump 240 having an inlet 242 and a return pipeline 243 in which a venturi 244 is disposed. Each return pipeline 243 ends in a sparge pipe 248 which is disposed below the surface of the water in its respective treatment zone. Each sparge pipe 248 has spaced apart orifices 250.

Unlike the apparatus shown in Fig. 1 there are no aerators provided in the zones 214 and 216. Instead, a plurality of surface aerators 278 are provided in the intermediate or inlet zone 218.

An arrangement for draining the channels 228 and 230 is provided. This arrangement is similar to that shown in Fig. 1. A common sump 254 is provided with a pump 264 disposed therein. Sump 254 communicates with the channel 228 via an inlet pipe 256 having a valve 260 disposed therein and the channel 230 communicates with the sump 254 via a pipe 158 having a valve 162 disposed therein. There is a return pipe 266 having a valve 270 disposed therein. The return pipe 266 terminates in the inlet 220 to enable the solids to be returned to a central region of the intermediate or inlet zone 218.

The operation of the treatment zones 214 and 216 is substantially similar to the operation of the corresponding zones 11 4 and 116 described with reference to Fig. 1. That is, save for the fact that in the apparatus shown in Fig. 2 no aeration is provided in these zones 214 and 216. It is to be noted, however, that the incoming water for treatment is introduced into a region of the intermediate or inlet zone 218 which is relatively remote from the zones 214 and 216 in comparison with the position of introduction of the feed into the zone 118 in Fig. 1.The duration of the oxygenation periods and the rate of inflow of water into the tank 202 are chosen such that, when, say, solids are settling in the volume of water in the zone 214 and clear liquid is being run off from that zone, substantially none of the newly entering water reaches the zone 214 during the same settlement period. It is to be appreciated that the general direction of flow entering the tank 202 would be towards whichever of the zones 214 and 216 is for the time being the one in which settlement of solids and running off of clear liquid is taking place. Typically, the average residence time of the waste water in the intermediate or inlet zone 218 is equivalent to the length of several oxygenation periods.

The aerators 278 are operated continuously in the intermediate or inlet zone 218. In this respect, the apparatus shown in Fig. 2 is to be preferred to that shown in Fig. 1, as commercially available aerators are generally adapted to be operated continuously. The air dissolved in the water helps to counteract the tendency for unduly acidic conditions to be created in the water as a result of bacterial formation of carbon dioxide.

In other respects the operation of the apparatus shown in Fig. 2 is substantially equivalent to that shown in Fig. 1 and will not be described herein in further detail.

It is to be appreciated that it is not essential to stop all other flows into and out of the apparatus while liquid containing solids flows into the sump 254 from one of the channels 228 and 230. Thus the valve 222 may remain open throughout operation of the apparatus. At the end of an oxygenation period in say the zone 214, the valve 238 closes the and valve 236 opens. Simultaneously, the valve 260 opens. The sump 254 is in this instance not arranged as shown in Fig. 2.

Instead a pump withdraws liquid and solids from the channel 228 into the sump 254 and the suction provided is sufficient to prevent any significant flow of liquid out of the process via the pipe 232. Once the liquid withdrawn by the pump has become clear, the valve 260 closes, and the pump is de-energised, and clear liquid flows away through the pipe 232. The period required for the operation of the pump can be determined empirically or its operation and the closing of the valves 260 and 262 can be controlled automatically by a nephometer. Thus, the operation of the process can be totally continuous.

The method and apparatus according to the present invention may be put to a number of uses. They may be used to substantially operate an existing oxidation tank or to convert what is primarily a holding tank to a tank in which treatment of sewage can take place.

The clear liquid produced by the invention may be subjected to further treatment or may be discharged to a river or other source of naturally occuring water.

Claims

1. A method of treating water having a biochemical oxygen demand, comprising repeatedly performing the steps of (a) oxygenating the water in the presence of suspended aerobic (or oxic) bacteria effective to reduce the biochemical oxygen demand of the water and (b) allowing bacterial solids to settle so as to leave a supernatant layer of treated water relatively free of such solids, in which method at least two volumes of such water are so treated generally simultaneously, the steps (a) and (b) being performed on one volume generally out-of-phase with the steps (a) and (b) on the other volume.

2. A method as claimed in claim 1, in which both volumes are defined in the same vessel, said vessel being a shallow tank (as hereinbefore defined).

3. A method as claimed in claim 2, in which as incoming water having a biochemical oxygen demand is received for treatment, so treated water flows out of the volume in which settling of the solids is for the time being taking place, whereby a substantially continuous treatment is afforded.

4. A method as claimed in claim 2 or claim 3, in which water for treatment is introduced into the tank at a region remote from the discharge(s) from such tank.

5. A method as claimed in any one of claims 2 to 4, in which the steps (a) and (b) having durations generally equal to one another and substantially less than the average residence time of the water in the tank.

6. A method as claimed in any one of claims 2 to 5, in which at least one dividing wall or baffle is provided to prevent the flow of water from one of said volumes to the other.

7. A method as claimed in any one of claims 2 to 6, in which there are two longitudinally extending volumes from which water is withdrawn at the vicinity of one end of the tank, and to which water is fed at the other end of the tank.

8. A method as claimed in claim 7, in which the said step (a) additionally includes aerating the water.

9. A method as claimed in any one of claim 2 to 6, in which the said two volumes of water both extend longitudinally and communicate with an intermediate volume of water to which incoming water having a biochemical oxygen demand is fed.

10. A method as claimed in claim 9, in which said intermediate volume is aerated continuously.

11. A method as claimed in any one of claims 2 to 10, in which agitation of the water during step (a) caused some water containing suspended solids to flow over the outlet weir associated with the volume of water on which said step (a) is for the time being performed, such water containing suspended solids being passed into a sump.

12. A method as claimed in claim 11, in which the water and suspended solids are returned to the tank from the sump.

13. A method as claimd in any one of claims 2 to 12, in which the said step (a) includes withdrawing a stream of water from the volume for the time being undergoing said step (a); pressurising the stream; introducing oxygen into the stream under turbulent conditions so as to form a dispersion of undissolved gas bubbles in the water, and then returning the stream to said volume, the returned stream being introduced into such volume such that the undissolved oxygen bubbles are sheared into fine bubbles that readily dissolve in or are consumed by the volume of liquid.

14. A method as claimed in claim 13, in which the stream is reintroduced into the said volume at several different points and agitates said volume.

15. A method of treating water having a biochemical oxygen demand, substantially as described herein with reference to Figs. 1 nd 3 or Figs. 2 and 4 of the accompanying drawings.

16. Apparatus for treating water having a biochemical oxygen demand, comprising a shallow tank (as hereinbefore defined) having at least two treatment zones; means for effecting the withdrawal or outflow of water from the treatment zones; at least one water inlet remote from said withdrawal or overflow means; means for agitating water in the treatment zones; and means for oxygenating the treatment zones, said apparatus being adapted to perform a method as claimed in any one of the preceding claims.

17. Apparatus for treating water having a biochemical oxygen demand, substantially as described herein with reference to Figs. 1 and 3 or Figs. 2 and 4 of the accompanying drawings.