GB2392909A

GB2392909A - A portable sewage treatment unit

Info

Publication number: GB2392909A
Application number: GB0320685A
Authority: GB
Inventors: Tanya Yvonne Paston
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-09-04
Filing date: 2003-09-03
Publication date: 2004-03-17
Anticipated expiration: 2023-09-03
Also published as: GB2392909B; GB0220478D0; GB0320685D0

Abstract

A portable sewage treatment unit, comprising a housing containing a sewage digestion tank (1) having a plurality of sequentially arranged digestion compartments (2,3,4), by internal dividing walls (5) such that the sewage flow between compartments (2,3,4) is driven by the inflow of sewage. Each of the digestion compartments is provided with means to controllably aerate sewage resident therein. The aeration means, for each compartment (2,3,4), may consist of a manifold (13), connected to a series of nozzles (14) that are independently controllable, and can be operated in a predetermined timed sequence, to interrupt aeration and digestion in a respective compartment. A mobile settler/clarifier is also detailed (figure 2).

Description

1 2392909

PORTABLE SEWAGE TREATMENT UNIT

The present invention relates to a transportable self-contained unit for the treatment of sewage and the like. More particularly, but not exclusively, it relates to a transportable self-

contained sewage digestion unit for temporary use, possibly in place of a corresponding element of a conventional sewage treatment plant. It further relates to a method of operation of said transportable unit.

A conventional sewage treatment plant performs a series of operations on sewage fed into it, to produce a substantially clean aqueous effluent which may safely be discharged to a watercourse or the like. An important operation is sewage digestion, in which the organic matter in the sewage is digested by aerobic microorganisms, which are provided with a supply of oxygen, typically by blowing air through the sewage. A standard measurement applied to sewage is its BOD (biochemical oxygen demand), which must be reduced as far as possible before it is discharged to the environment.

If the digestion section of a sewage plant were to be out of action for any reason, the sewage would either have to be to be stored until the digestion section is repaired, which may not be practicable, or discharged untreated, which is usually impermissible. There is hence a need for a unit which can be temporarily installed in place of a conventional digestion section while repairs or maintenance are carried out.

Such a unit would also be of use in situations in which sewage cannot be piped to a conventional sewage treatment plant, for example where a sewage pipe has been fractured.

Temporary encampments, for example set up during military operations, after natural disasters or for refugees, would also benefit from a mobile sewage treatment unit which could be set up on site.

Temporary toilet facilities are a feature of many large public events, and might also benefit from on-site treatment of their contents, rather than having to empty the waste material and ship it away for treatment.

There are other operations of a sewage treatment plant that may require temporary replacement, such as settling/clarification to remove suspended matter and chlorination to kill harmful microorganisms before effluent discharge. There is hence also a need for a transportable unit capable of carrying out such functions.

It would be convenient if the transportable unit were transportable and could be put in place and recovered after use without requiring specialised equipment. A transportable unit also needs to be robust.

A standard piece of equipment in the waste disposal industry is a "hookloading skip".

Unlike a builder's skip, this is a fully enclosed container. A hookloading (or "roll on, roll off") skip has supporting wheels or rollers at one end, and can be connected at its opposite end to a winch mounted on a flatbed lorry. The hook-loading skip can thus be drawn on to the flatbed of the lorry by the winch, and can be deposited again by rolling it off the back of the lorry. A so-called "40 yard" skip, for example, has a nominal internal volume of 40 cubic yards, or almost 31m3, and is around 6m in length and 2.5m wide. This is easily transportable on a conventional four-axle flatbed lorry.

It is an object of the present invention to provide a transportable sewage treatment unit which may be installed in place of an inoperative element of an existing sewage treatment plant or may be installed to treat sewage which cannot be brought to such a plant. It is also an object of the present invention to provide a method of operation for such a transportable sewage treatment unit.

According to a first aspect of the present invention, there is provided a transportable sewage treatment unit comprising a transportable housing, a sewage digestion tank having a plurality of sequentially arranged digestion compartments, each said digestion compartment being provided with means controllably to aerate sewage resident therein.

Preferably, each said aeration means is independently controllable.

Each said aeration means may be provided with a plurality of nozzle means through which air may be directed into the sewage, each said nozzle means being directed substantially downwardly towards a floor of a respective compartment.

The aeration means of each compartment may be provided with valve means, optionally controllable in a predetermined timed sequence, to interrupt aeration and hence digestion in a respective compartment.

Preferably, the plurality of sequential digestion compartments are separated by dividing walls, such that sewage may flow from one compartment to a next compartment in the sequence over an upper rim of a respective wall therebetween.

Advantageously, a first compartment in said sequence is provided with at least one inlet means and a last compartment in said sequence is provided with at least one outlet means, and sewage flow through the sequence of compartments is driven by inflow of sewage through the inlet means.

Barrier means may be provided adjacent each upper rim of a dividing wall and upstream thereof, extending above and below a level of said upper rim, to block material floating on a surface of the sewage in a compartment from flowing directly into a next compartment.

An inoculum of aerobic microorganisms capable of digesting sewage is preferably provided in at least the first compartment.

The common transportable housing may be provided with wheel means.

The common housing may be provided with means by which it may be hauled on to a vehicle for transportation.

Preferably, the common housing comprises a hook loading skip as hereinabove defined.

The transportable sewage treatment unit may further comprise at least one settlement tank, mounted within a separate transportable housing.

Clarifier means may be provided connected to the settlement tank.

The transportable sewage treatment unit may be provided with chlorination means.

According to a second aspect of the present invention, there is provided a method of treating sewage comprising providing a transportable sewage treatment unit having a plurality of digestion compartments as described in the first aspect above, transporting said unit to a desired location, delivering sewage into the unit so that it flows through each said compartment in sequence, and aerating said sewage such that aerobic microorganisms therein digest organic matter in the sewage.

Preferably, the method comprises providing a controlled air flow independently to each said compartment. Advantageously, the method comprises periodically selectively interrupting said air flow to a compartment. The method may optionally comprise inoculating at least a first compartment with aerobic microorganisms capable of digesting sewage.

The method may comprise carrying out a settlement step on material exiting said sequence of digestion compartments.

The method may then comprise an additional step of further providing at least one transportable settlement tank.

The method may comprise a chlorination step prior to discharge of the treated sewage.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional elevation of a mobile sewage treatment unit embodying the invention; Figure 2 is a schematic cross-sectional elevation of a mobile settler/clarifier unit usable in conjunction with the unit of Figure 1, Figure 3 is a scrap view of a conventional sewage aeration head; and Figure 4 is a scrap view of a sewage aeration head of the present invention.

Turning now to the Figures, and to Figure 1 in particular, a transportable sewage treatment unit comprises a substantially enclosed digestion tank 1, divided into three compartments 2, 3 and 4 by internal dividing walls 5. Sewage flows into the tank 1 via an inlet pipe 6 connected to first compartment 2. It then flows from the first compartment 2 to second compartment 3, and from the second compartment 3 to third compartment 4, in each case by flowing over an

upper edge of a respective dividing wall 5, which thus acts as a weir. Material is discharged from the third compartment 4 via an outlet pipe 7.

A baffle 8 is provided in the first compartment 2 to ensure that sewage flowing into a full first compartment 2 mixes with sewage already present, rather than flowing directly across the compartment 2. A scum board 9 extends across the tank 1 adjacent the upper edge of each dividing wall 5. The scum boards 9 create a barrier against scum or objects floating on a surface of the contents of a respective compartment 2, 3 passing over a dividing wall 5 to a subsequent compartment 3, 4. While a small proportion of such material may evade the scum boards 9 by becoming entrained in the flow under the scum boards 9, or by becoming at least partially waterlogged, the scum boards 9 ensure that such material is at least wetted.

The outlet pipe 7 may be protected by a weir arrangement 29.

An air pump 10 is mounted to the tank 1 or, in a preferred embodiment, is mounted within an accompanying settler unit 19. In either case, it is powered either by a mains electrical supply or an electrical generator unit, depending on where the unit is deployed. The pump 10 delivers ambient air to each compartment 2, 3, 4 via a respective motorised valve 11. A downpipe 12 leads downwardly from a respective valve 11 into each compartment 2, 3, 4, and is provided at a lower end with a manifold 13 having a plurality of downwardly-directed nozzles 14 (here shown for clarity as only five nozzles 14 per tank) through which air is expelled into the contents of the compartment 2, 3, 4.

A set of wheels 15 is mounted to an underside of the tank 1 adjacent a first end thereof, and a hauling loop 16 is mounted to an opposite end of the tank 1. As access may be required to a l

roof of the tank 1, a ladder 17 is provided on an outer wall of the tank 1 and a safety rail 18 is mounted around a perimeter of the roof.

In operation, sewage is piped into the first compartment 2, which has preferably been primed with an inoculum of suitable microorganisms. (The sewage already contains such microorganisms, but effective operation benefits from such enhancement). As the compartment 2 fills, air is blown into the sewage through the nozzles 14.

Potentially treated sewage then overflows from the first compartment 2 to the second compartment 3, which may be provided with its own inoculum of micro-organisms or may rely on those transferred over with the sewage from the first compartment 2. The sewage in the second compartment 3 is aerated as described for the first compartment 2, and digestion of the sewage continues. Similarly, the third compartment 4 fills with sewage from the second compartment 3, and aeration and digestion continues therein. The material flowing out of the third compartment 4 through the outlet pipe 7 will be substantially free of digestible organic matter, and can then be passed to a settler (see below).

The aerobic microorganisms used in sewage digestion may require periods of dormancy between periods of consumption and growth. Hence, the air supply to each compartment 2, 3, 4 is controlled by timing mechanisms incorporated into or operatively connected to each valve 11. The microorganisms in each compartment 2, 3, 4 may thus be "rested" by turning off the air supply to a respective compartment.

The three-compartment tank l shown allows one compartment 2, 3, 4 at a time to be "rested" anoxically in this way. As the three compartments 2, 3, 4 are independently aerated, the

digestion process in each can be "tuned" as required, which may, for example, require more aeration in the first compartment 2 than in the third 4 which holds sewage which has already undergone considerable digestion. Other advantages of such a multi-compartment arrangement are that there is very little chance of material travelling directly from the inlet pipe 6 to the outlet pipe 7, and the scum boards 9 in conjunction with the dividing walls 5 greatly reduce the chance of floating untreated detritus reaching the outlet pipe 7.

The entire tank 1 is conveniently constructed within a conventional hookloading/"roll-on, roll-off" skip. These are robust, readily available, and can easily be loaded on to and dropped off from a flatbed lorry equipped with a winch connectable to the haulage loop 16. The hook- loading skip is supported by the wheels 15 while being rolled on or off the lorry.

A tank 1 constructed in a conventional "40 yard" (30.5m3 nominal capacity) skip can process the sewage produced by a population of at least three hundred people. An average sewage retention time of around nine hours is normally required to reduce BOD to acceptable levels.

A standard set of assumptions for domestic sewage is based on 65g BOD per head per day, and a total sewage volume of between 180 and 225 litres per head per day. For a population of 300, this means a daily BOD of l9.5kg on a total sewage flow of between 54 and 67.5m3.

This is well within the capabilities of the unit described, which can be run with far higher levels of"mixed liquor suspended solids", approaching 3000mg/1, or around ten times the standard levels listed above. (Note: these calculations are all based on a"non-nitrifying" works). A further refinement of the unit shown is that the downpipe 12 of each compartment 2, 3, 4 is detachably mounted to a respective valve 11, for example via a threaded coupling. Each

downpipe 12 and associated manifold 13 can, if necessary, be removed for maintenance or repair by a user working entirely above the level of the sewage in the respective compartment 2, 3, 4. This minimises the time needed to deal with such problems and the consequent downtime for the compartment in question. There is no need to work in the sewage or to drain all or part of the unit, which is particularly important as the unit will usually be operating without backup facilities.

As mentioned above, the material leaving the digester unit is normally passed to a settler/clarifier and to a chlorinator before discharge to a watercourse. Where the digester unit is being used at a conventional sewage works, such plant sections will usually be available and in operation. However, where the unit is being used away from an existing sewage works, a corresponding transportable settler and chlorinator unit may be required, an example of which is shown in Figure 2.

A settler unit 19 comprises a settling tank arrangement 20, which may comprise a pair of pyramidal settlement tanks. It is fed by an inlet pipe 21, connected, for example, to an outlet pipe 7 of the digester unit. Suspended solids will settle out in this tank arrangement, optionally after treatment with flocculating agents, leaving a substantially clear supernatant which overflows out of the settling tank 20 over an upper edge of a dividing wall 5. The dividing wall 5 is protected by a scum board 9 against material floating on the surface, as described above.

The supernatant is then pumped through a media bed 22 by a pump 23. Alternatively the supernatant may flow through a conventional brush clarifier. The clear water then leaves the settler unit 19 via an outlet pipe 24. The water leaving through the outlet pipe 24 may be

passed through a separate chlorinator before being released to the environment, or a chlorinator may be incorporated into the clarifier bed 22.

As described above, a preferred location for the aeration pump 10 is below the clarifier, beside or instead of the water pump 23.

A settler unit 19 built into a hook-loading skip is well matched in treatment capacity with a digester unit built in a hook-loading skip of the same size. It is equally easy to load on to an appropriate lorry, to transport and to drop off in place where required.

Figures 3 and 4 show a conventional aeration head for a digester and an aeration head for the digester unit of the present invention, respectively. A conventional aeration head comprises a domed diffuser 25 fed with air by a manifold 13. The diffuser 25 emits air bubbles 26 upwardly, which continue to rise through the sewage. There is hence a "dead zone" 27 below the diffuser 25, which is not aerated, and in which no digestion takes place. There is little liquid flow in this zone, and sediment may build up. In a conventional digester, the "dead zone" 27 typically extends 250 to 300mm above a floor of the digester. There is also a risk that the bubbles 26 may entrain in a column 28, possibly leaving poorly aerated zones between adjacent columns.

The aeration head shown in Figure 4, on the other hand, has a downwardly directed nozzle 14 fed with air by the manifold l 3. A variety of conventional nozzles 14 have been found to be suitable for the purpose and a custom built diffuser 25 is not necessary. The air bubbles 26 are emitted in a stream directed downwardly towards the floor of the digester. There is hence no "dead zone" below the aeration head which is not aerated. Turbulent flow and improved

mixing may result, and any settlement will probably be scoured from the floor of the digester.

The bubbles 26 from such a head will also probably tend to spread out further as they rise than those from the conventional head of Figure 3. The elimination of an inoperative "dead zone" in the digester means that an increased treatment capacity is available from a given size of tank, or a smaller and cheaper tank can be used. This form of aeration head would be of use in any digester system, but is of particular benefit in the transportable unit of the present invention, in which efficiency is at a premium.

The transportable sewage treatment unit disclosed herein may rapidly and conveniently be deployed at a desired location, either as a temporary replacement for an inoperative section of an existing sewage treatment facility or in a location remote from such a facility. It may be constructed using commonly available parts and transported on existing flatbed lorries. The unit can digest sewage as effectively as a fixed unit, and may also separate out suspended solids and chlorinate treated material before its discharge to the environment.

Claims

1. A transportable sewage treatment unit comprising a transportable housing containing a sewage digestion tank having a plurality of sequentially arranged digestion compartments, each said digestion compartment being provided with means controllably to aerate sewage resident therein.

2. A unit as claimed in claim 1, wherein each said aeration means is provided with a plurality of nozzle means through which air may be directed into the sewage, each said nozzle means being directed substantially downwardly towards a floor of a respective compartment.

3. A unit as claimed in either claim 1 or claim 2, wherein each said aeration means is independently controllable.

4. A unit as claimed in claim 3, wherein the aeration means of each compartment is provided with valve means, optionally controllable in a predetermined timed sequence, to interrupt aeration and hence digestion in a respective compartment.

5. A unit as claimed in any one of the preceding claims, wherein the plurality of sequential digestion compartments are separated by dividing walls, such that sewage may flow from one compartment to a next compartment in the sequence over an upper rim of a respective wall therebetween.

6. A unit as claimed in any one of the preceding claims, wherein a first compartment in said sequence is provided with at least one inlet means and a last compartment in said sequence is provided with at least one outlet means, and sewage flow through the sequence of compartments is driven by inflow of sewage through the inlet means.

7. A unit as claimed in either claim S or claim 6, further comprising barrier means adjacent each upper rim of a dividing wall and upstream thereof, extending above and below a level of said upper rim, to block material floating on a surface of the sewage in a compartment from flowing directly into a next compartment.

8. A unit as claimed in any one of the preceding claims, wherein an inoculum of aerobic microorganisms capable of digesting sewage is provided in at least the first compartment.

9. A unit as claimed in any one of the preceding claims, further comprising a settlement tank mounted within a separate transportable housing.

10. A unit as claimed in claim 9, further comprising clarifier means connected to the settlement tank.

11. A transportable sewage treatment unit substantially as described with reference to the Figures of the accompanying drawings.

12. A method of treating sewage comprising providing a transportable sewage treatment unit having a plurality of digestion compartments as claimed in any one of claims 1 to 8, transporting said unit to a desired location, delivering sewage into the unit so that it flows through each said compartment in sequence, and aerating said sewage such that aerobic microorganisms therein digest organic matter in the sewage.

13. A method as claimed in claim 12, wherein a controlled air flow is provided independently to each said compartment.

14. A method as claimed in either claim 12 or claim 13, which comprises periodically selectively interrupting said air flow to a compartment or the compartments.

15. A method as claimed in any one of claims 12 to 14 comprising inoculating at least a first compartment with aerobic microorganisms capable of digesting sewage.

16. A method as claimed in any one of claims 12 to 15, comprising the further step of carrying out a settlement step on material exiting said sequence of digestion compartments.