GB2224728A

GB2224728A - Sewage treatment plant

Info

Publication number: GB2224728A
Application number: GB8824212A
Authority: GB
Inventors: Andrew John Skilling
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-10-15
Filing date: 1988-10-15
Publication date: 1990-05-16
Anticipated expiration: 2008-10-15
Also published as: GB8824212D0; GB2224728B

Abstract

Sewage treatment plant suitable for installation below ground has a primary settlement chamber 12, a percolating filter chamber 13 and a secondary settlement chamber 14. Sewage introduced through inlet 19 is transferred by an air lift to the top of the filter chamber 13, and is allowed to percolate over filter media 28 whilst air is pumped to the lower region of that chamber 13, to rise through the percolating liquid. Pump 32 serves to transfer filtered liquid to the secondary settlement chamber 14, and a further air lift then discharges liquid from that chamber to a water-course. Solid matter settling in secondary settlement chamber is transferred back to the primary settlement chamber 12, to prevent the accumulation of solids in the secondary settlement chamber and to ensure that there will be a relatively constant liquid flow over the filter media 28. <IMAGE>

Description

SEWAGE TREATMENT PLANTS This invention relates to sewage treatment plants, and in particular concerns such plants which are configured to be suitable for installation below ground.

In particular, but not exclusively, the invention relates to relatively small-scale sewage treatment plants which are able to service the output of just a few or even only one dwelling.

In recent years, there has been an increasing emphasis- placed on pollution control of all kinds. One area which has received particular attention is the treatment of the foul water output from dwellings, where such dwellings are not connected to a main sewer.

Conventionally, the foul water is either collected in a cess-pit which has to be emptied periodically by means of a road tanker, or the sewage has been passed through a multi-stage septic tank installation. However, cesspits often are not emptied frequently enough, with the result that foul - and perhaps toxic - waste water overflows into a natural water course. On the other hand, the biological oxygen demand (BOD) of the liquor from a septic tank very often is too high for discharge directly into a natural water course, especially during the winter months when the bacteriological activity in the septic tank may be at a too low level.

In an attempt to improve this situation, there have been proposals to use percolating filter beds in small scale treatment plants, but such plants have proved to be unsatisfactory. A percolating filter bed is commonly used for the treatment of sewage in relatively large scale sewage treatment plants. With the kind of filter media which has to be used in such plants, it is most important that the media at no time is allowed to dry out, to maintain the bacteriological action. In the case of a large scale sewage treatment installation, this presents little problem for the outflow from the relatively large number of dwellings serviced will provide sufficient effluent to ensure that liquid may continuously be passed over the percolating filter media.However, if such a bed is used on a small-scale plant, in view of the irregular outflow which can be expected from only a few dwellings, acute problems can be experienced in maintaining a flow over the filter media. One proposal employed a reservoir tank collecting all the outflow, and liquid being pumped from the reservoir tank at a constant rate over the percolating filter media - but experience has shown that it is impractical to provide a sufficiently large reservoir tank able adequately to cope with the peaks and troughs of the outflow.

It is an object of the present invention to provide a sewage treatment plant which is suitable for installation below ground, for example to service a relatively small number of dwellings, which plant overcomes the disadvantages described above of relatively low-volume sewage treatment installations.

According to one aspect of this invention, there is provided an integrated sewage treatment plant suitable for installation below ground, which plant comprises: a primary settlement chamber; a percolating filter chamber containing percolating filter media; means to transfer liquid from the primary settlement chamber to the percolating filter chamber, for percolation over the filter media; a secondary settlement chamber; means to transfer liquid from the percolating filter chamber, following the percolation thereof over the filter media, to the secondary settlement chamber; and means to transfer back to the primary settlement chamber both solids settling in the secondary settlement chamber and a portion of the liquid therein.

The sewage treatment plant of this invention as described above may be constructed as a single selfcontained unit, either by appropriate partitioning of a single vessel or by conjoining together separate vessels defining the respective chambers. Either way, the integrated plant, when completed, may be buried below ground, with just access hatches to the various chambers left exposed, so that the plant lends itself to installation in sensitive areas - such as in the garden of a dwelling.

In the present invention, the difficulty of maintaining a wflow over the filter media of the percolating filter is overcome by recirculating a portion of the liquid which has already trickled over the filter media, so as thereby to ensure that there is always a sufficient flow of liquid over the media to maintain the efficiency of the percolating filter.

A further advantage of the sewage treatment plant of this invention is that by returning to the primary settlement chamber solids collecting in the secondary settlement chamber, no decomposition of those solids occurs in the secondary settlement chamber: this prevents degradation of the quality of the final effluent discharged from the treatment plant.

Moreover, all solids may be periodically removed from the treatment plant, by one simple operation of extracting those solids from a single chamber.

A further aspect of the present invention provides an integrated sewage treatment plant suitable for installation below ground, which plant comprises: a primary settlement chamber; a percolating filter chamber containing percolating filter media; means to transfer liquid from the primary settlement chamber to the percolating filter chamber, for percolation over the filter media; air delivery means arranged to deliver air under pressure to a lower part of the percolating filter chamber, whereby the delivered air may rise upwardly through the filter media; a secondary settlement chamber; and means to transfer liquid to the secondary settlement chamber from the percolating filter chamber; following the percolation of that liquid over the filter media.

The biological process which takes place in a percolating filter bed is essentially an aerobic process and in known forms of such filter beds, whilst the l quor is allowed to percolate downwardly through the filter media, air is allowed to circulate freely through the filter bed. This may be achieved by providing appropriate vents in the bed, such that a natural updraught of air may be established through the filter media. In a below-ground installation, it is not possible to allow for natural ventilation in this way, and so this problem is overcome in the present invention by providing a forced draught, with air delivered under pressure to a lower part of the percolating filter, the air then rising upwardly through the filter media, so ensuring that the required aerobic treatment process is efficiently carried out.Thus, a sewage treatment plant as described above and according to this second aspect of the present invention particularly lends itself for installation below ground.

Most preferably, an integrated sewage treatment plant for below-ground installation and of this invention includes both novel aspects as described above, and thus has the means to transfer to the primary settlement chamber both solids settling in the secondary settlement chamber and a portion of the liquid therein, and the air delivery means for the percolating filter media.

Most advantageously, the transfer means for liquid from the primary settlement chamber to the percolating filter chamber, and for solids and liquid from the secondary settlement chamber to the primary settlement chamber each comprise air lifts, where air is introduced into a generally vertically extending pipe immersed in the liquid, such that as the air rises, liquid is carried with the rising bubbles of air. An advantage of such transfer means is that only a simple air pump is required, which may be located externally of the respective chambers of the plant, air being conducted to the air lifts by appropriate pipelines. Moreover, the same air pump may be used to provide a source of air under pressure for delivery to the lower part of the percolating filter chamber, for a plant where a forced air supply is employed in that chamber.

In order to minimise the likelihood of odours arising from the use of a percolating filter chamber, it is preferred for there to be an exit port from the percolating filter chamber which port is fitted with an activated carbon air filter. Preferably, a pre-filter is provided for the activated carbon filter, in order to prevent excessive moisture reaching the carbon filter.

By way of example only, one specific embodiment of sewage treatment plant constructed in accordance with this invention will now be described in detail, reference being made to the accompanying drawings in which:- Figure 1 is a longitudinal vertical section through the embodiment of sewage treatment plant of this invention, adapted for installation below ground; Figure 2 is a plan view on the plant shown in Figure 1; and Figures 3 to 5 are sectional views through the plant, taken on lines B-B, C-C, and D-D respectively, marked on Figure 2.

The embodiment of sewage treatment plant shown in the drawings comprises a main cylindrical vessel 10 arranged with its axis horizontal, and a further cylindrical vessel 11 mounted on the main vessel 10 partway between the ends thereof, with the axes of the two vessels intersecting substantially at right angles.

The part of the main vessel 10 to the left (in Figures 1 and 2) of the further vessel 11 defines a primary settlement chamber 12, the further vessel 11 defines a percolating filter chamber 13 and the part of the main vessel 10 to the right (in Figures 1 and 2) of the further vessel 11 defines a secondary settlement chamber 14. Each of these chambers is fitted with an access shaft 15, 16 and 17 respectively, and the primary settlement chamber 12 is further fitted with a access pipe 18, having a fluid inlet port 19.

A wall 20 is mounted in the primary settlement chamber 12 adjacent the further vessel 11 so as to define in conjunction therewith a primary effluent air lift chamber 21, into which liquid may flow from the primary settlement chamber 12 through an opening 22 arranged behind a baffle 23, which baffle serves to prevent floating matter entering the primary effluent air lift chamber 21. A U-shaped air lift pipe 24, having a liquid opening 25 and an air injection pipe 26 is arranged to feed liquid drawn from the primary settlement chamber 12 to a distributor arrangement 27, within the percolating filter chamber 13. Plastic filter media 28 is disposed in the filter chamber 13, the distributor arrangement 27 serving to distribute transferred liquid over the filter media 28, such that the liquid trickles own through that media.An air line 29 is disposed below the filter media 28, and is connected back to a manifold 30 disposed in the access shaft 15, a suitable valve being provided to control the flow of air to the air line 29. The air injection pipe 26 is also connected to the manifold 30 again through a suitable control valve.

A central passageway 31 extends from below the access shaft 16 down through the filter media 28, and an electric pump 32, fitted with a float switch is disposed below that passageway to pump liquid collecting in the bottom of the filter chamber 13 to an effluent duct 33, leading to the secondary settlement chamber 14.

A conical lower wall 34 is fitted in the secondary settlement chamber 14 and rising from the lowermost part thereof is a pipe 35 having apertures 36 at the bottom thereof and further apertures 36A below the normal liquid level in the chamber. A U-shaped air lift pipe 37 is disposed within the pipe 35 (shown in section in Figure 1), the open end 38 of the pipe 37 defining the liquid level in the secondary settlement chamber 14. The output end 39 of the air lift pipe 37 extends through the wall of the access shaft 17 to the secondary settlement chamber, to allow final effluent to be discharged from the sewage treatment plant, for example to a suitable water course.

Also mounted within the pipe 35 is a second air lift 40, discharging into a pipe 40A leading back to the primary settlement chamber 12. This secondary air lift operates directly in conjunction with the pipe 35, drawing through the apertures 36 into the pipe 35 liquid and any solids (sludge) which may settle within the conical wall 34. Both air lifts are connected through suitable control valves back to the manifold 30 in the access shaft 15.

The access shaft 16, mounted above the percolating filter chamber 13, is fitted with an activated carbon deodorising filter 41, that filter having a moisture pre-filter 42 to excessive moisture reaching and blinding the deodorising filter. Removal of the filter assembly allows access to the interior of the percolating filter chamber and also to the pump 32.

In use, the plant described above is buried below ground and the foul water from one or more dwellings is directed into the primary settlement chamber through the inlet port 19. The manifold 30 is connected to an external air pump or blower, and the pump 32 is connected to a suitable electrical supply.

Solids will separate from liquid in the primary settlement chamber 12, and the liquid is removed by the primary effluent air lift 24 from the primary settlement chamber to be distributed over the plastic filter media 28, to trickle downwardly therethrough. At the same time, air from the air line 29 rises upwardly through the plastic filter media, so ensuring an active aerobic biological process takes place. From the bottom of the percolating filter chamber 13, the treated liquid is pumped into the secondary settlement chamber. There, any solids will collect at the bottom of the lower conical wall 34, in that chamber 14.Those solids and some liquid will be returned to the primary settlement chamber by the secondary air lift 39 and pipe 40, but when the level in the secondary settlement chamber rises above the open end 38 of the air lift pipe 37, final effluent will be discharged from the plant, through the output end 39 of pipe 37.

It will be appreciated that once the plant described above has properly been installed and set up, only minimal maintenance is required, other than occasional emptying of solids from the primary settlement chamber. By maintaining a liquid flow over the plastic filter media, that media may be maintained at optimum operating conditions and moreover degradation of the final effluent discharge by decomposing sludge in the secondary settlement chamber is avoided, because any solids separating in that secondary settlement chamber are returned to the primary settlement chamber 12.

Claims

1. An integrated sewage treatment plant suitable for installation below ground, which plant comprises: a primary settlement chamber; a percolating filter chamber containing percolating filter media; means to transfer liquid from the primary settlement chamber to the percolating filter chamber, for percolation over the filter media; a secondary settlement chamber; means to transfer liquid from the percolating filter chamber, following the percolation thereof over the filter media, to the secondary settlement chamber; and means to transfer back to the primary settlement chamber both solids settling in the secondary settlement chamber and a portion of the liquid therein.

2. A sewage treatment plant according to claim 1, wherein the chambers thereof are defined by two or more separate vessels conjoined together to form a single self-contained unit.

3. A sewage treatment plant according to claim 1 or claim 2, wherein air lift means are provided to transfer solids and liquid from the secondary settlement chamber to the primary settlement chamber.

4. A sewage treatment plant according to any of the preceding claims, wherein air lift means are provided to transfer liquid from the primary settlement chamber to the percolating filter chamber.

5. A sewage treatment plant according to any of the preceding claims, wherein air delivery means is arranged to deliver air to the base of the percolating filter chamber, so that aerobic digestion may take place in that chamber.

6. An integrated sewage treatment plant suitable for installation below ground, which plant comprises: a primary settlement chamber; a percolating filter chamber containing percolating filter media; means to transfer liquid from the primary settlement chamber to the percolating filter chamber, for percolation over the filter media; air delivery means arranged to deliver air under pressure to a lower part of the percolating filter chamber, whereby the delivered air may rise upwardly through the filter media; a secondary settlement chamber; and means to transfer liquid to the secondary settlement chamber from the percolating filter chamber; following the percolation of that liquid over the filter media.

7. A sewage treatment plant according to claim 6, wherein there is provided means to transfer to the primary settlement chamber both solids settling in the secondary settlement chamber and a portion of the liquid therein.

8. A sewage treatment plant according to claim 6 or claim 7, wherein the transfer means for liquid from the primary settlement chamber to the percolating filter chamber, and for solids and liquid from the secondary settlement chamber to the primary settlement chamber each comprise air lift means.

9. A sewage treatment plant according to claim 8, wherein each air lift means includes a generally vertically extending pipe immersed when in use in liquid in the respective chamber, and means to introduce air into a lower portion of that pipe.

10. A sewage treatment plant according to any of claims 6 to 9, wherein a single air pump is provided externally of the respective chambers of the plant, pipe lines being provided to conduct air from the pump to the air lift means and to the lower part of the percolating filter chamber.

11. A sewage treatment plant according to any of the preceding claims wherein there is an exit port from the percolating filter chamber which port is fitted with an activated carbon air filter.

12. A sewage treatment plant according to claim 11, wherein a pre-filter is provided for the activated carbon filter, in order to prevent excessive moisture reaching the carbon filter.

13. A sewage treatment plant substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.