GB2275921A

GB2275921A - Sewage treatment plant

Info

Publication number: GB2275921A
Application number: GB9305071A
Authority: GB
Inventors: John Laurence Nicholson
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-12
Filing date: 1993-03-12
Publication date: 1994-09-14
Anticipated expiration: 2013-03-12
Also published as: GB2275921B; GB9305071D0

Abstract

A sump housing of conventional septic tank shape incorporates: - A primary settlement tank 6 containing an apertured baffle 7 which divides the tank into a lower settlement part 10, which receives raw sewage, and an upper part 9 for receiving liquid effluent. - A first pump chamber 18 containing a pump 19 which operates intermittently, and has inlet ports 22 from the upper chamber 9, arranged to leave a substantial buffer volume in the upper chamber. A one-way drain valve 52 allows effluent to return to the tank 6 during de-sludging. - A filtration chamber 26 containing filter media, receives the pump output via an air-entraining venturi 34 and distributor 36. - A final settlement tank 38 of conical form receives filtered effluent from the filtration chamber, and has a one-way drain valve 54 to feed into the chamber 10 during de-sludging. Cleared supernatant liquid spills over a weir wall 40 into a channel 42 and exits through an outlet 43. - A further pump chamber 46 contains a second pump 49 which draws sediment from the final settlement tank 38 and returns it to the primary settlement tank. <IMAGE>

Description

SEWAGE TREATMENT APPARATUS TECHNICAL FIELD OF THE INVENTION This invention relates to apparatus for the treatment of raw sewage from a domestic dwelling house or similar premises having a relatively small number of users.

More particularly, the invention is concerned with sewage treatment apparatus including a sump housing which contains: - a primary settlement tank, the sump housing having an inlet for conducting raw sewage into the primary settlement tank, and the primary settlement tank including a base region for settlement of gross solids from the raw sewage; - a pump chamber containing pump means, and having a pump chamber inlet for receiving sewage effluent from the primary settlement tank from a higher level than said base region; - a filtration chamber containing filter media, the pump means being arranged to pump effluent from the pump chamber through conduit means to distribution means at the top of the filtration chamber, from whence the effluent trickles downwards through the filter media; - an additional settlement tank for settlement of particulate matter and arranged to receive effluent from the base of the filtration chamber, the sump housing having outlet means for conducting treated effluent from the additional settlement tank.

For convenience, such apparatus will be referred to hereinafter as "apparatus of the kind set forth".

BACKGROUND It is conventional to treat raw sewage by feeding the sewage to a primary settlement tank (commonly known as a septic tank) to allow settlement of gross solids. The liquid effluent is then passed to a separate sump containing a pump which operates periodically to remove the effluent.

It is known, e.g. from GB 2 218 082, to pump the effluent to a chamber containing random packed filter media which supports biological degradation of filtered particulate matter. After trickling through the filter media the effluent passes to an additional settlement tank, from which the final outfall is conducted away, but since such an arrangement uses several separate sump chambers it is relatively difficult and costly to install.

GB 2 239 237 discloses apparatus of the kind set forth, in which the various stages are all contained within a single sump housing. In this earlier proposal a vertical bulkhead wall divides the lower part of the sump housing into primary and additional settlement tanks. A portion of the primary settlement tank is partitioned off to form a pump chamber, and a continuously running pump recirculates effluent to a filter chamber formed in the upper portion of the housing, from which effluent enters the additional settlement tank. A conduit is provided to allow a free flow of effluent from the final settlement tank back to the pump chamber, and the conduit is provided with a break point in the additional settlement tank, which defines an equilibrium level for fluid in the pump chamber.

In an ideal setup, the treated effluent which is obtained from the final settlement tank should be clean enough to be discharged into a waterway.

The apparatus of GB 2 239 237 is perceived to have a number of operational drawbacks, which may be summarised as follows: 1. There is a tendency for particulate matter to remain in the additional settlement tank for prolonged periods, resulting in undesirable organic growth which can degrade the quality of the final effluent.

2. There is a risk that the treated effluent will contain substantial quantities of unsettled particulate organic matter.

3. Since the pump runs continuously the apparatus consumes a substantial amount of power.

4. During de-sludging of the apparatus, the primary and additional settlement tanks and the pump chamber must all be emptied via separate hatches.

5. The sump housing has a number of air vents which must be kept clear to permit air circulation through the filter media and enable the necessary aerobic biological degradation to take place in the filter chamber.

An aim of the present invention may be viewed as being to provide selfcontained sewage treatment apparatus which overcomes these drawbacks.

SUMMARY OF THE INVENTION The present invention proposes apparatus of the kind set forth, in which the additional settlement tank is of substantially conical shape.

This form of tank greatly assists in settlement of particulate material from the filtered effluent.

The additional settlement tank is preferably provided with an elongate wear wall, over which the supernatant fluid from the additional settlement tank flows to the outlet means. This minimises disturbance of the effluent in the additional settlement tank. The weir wall preferably extends continuously around the top region of the additional settlement tank and is preferably surrounded by a drainage channel leading to the outlet means.

The filter chamber preferably discharges into the additional settlement tank via an inlet which is located below the normal fluid level of the additional settlement tank.

The additional settlement tank is preferably provided with a sediment outlet from the bottom thereof. The sediment outlet preferably leads to a second pump chamber containing a recirculation pump arranged for periodic operation to remove sediment. The sediment is preferably returned to the primary settlement tank for further treatment.

The present invention also proposes that in apparatus of the kind set forth, the pump means is arranged to run intermittently for predetermined periods, and the pump chamber inlet is arranged to define a minimum fluid level in the primary settlement tank which provides for a substantial buffer volume for raw sewage above the minimum fluid level, the pump chamber having a low level detector which holds the pump inoperative until the fluid level rises.

Thus, the pump will only operate with a low duty cycle. A high level detector is preferably provided to cause the pump means to run continuously whilst the fluid level in the primary settlement tank or pump chamber exceeds a predetermined level.

The pump chamber is preferably provided with a one-way drain valve for draining fluid from the pump chamber into the primary settlement tank during de-sludging. The additional settlement tank is preferably also provided with a similar drain valve arrangement.

The invention further proposes that in apparatus of the kind set forth, the conduit which leads from the pump to the filter chamber includes venturi means for entraining air into the effluent flow.

It is also proposed that in apparatus of the kind set forth, the primary settlement tank comprises a lower settlement chamber and an upper settlement chamber separated by a partition wall which slopes towards aperture means therein, the arrangement being such that raw sewage from the inlet enters the lower chamber and the effluent therefrom enters the upper chamber through the aperture means.

In apparatus of the kind set forth, it is further proposed that the sump housing comprises a generally globular lower part surmounted by a generally upright cylindrical upper part.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a vertical section through a sewage treatment plant of the invention, Figure 2 is another vertical section through the treatment plant taken perpendicular to that of Fig. 1, and Figure 3 is a horizontal section Ill-HI through the treatment plant, indicating the position of the sections l-l of Fig. 1 and 11 11 of Fig. 2.

DETAILED DESCRIPTION OF THE DRAWINGS The sewage treatment plant includes a sump housing 1 which comprises a substantially spherical bottom part 2 surmounted by an upright generally cylindrical top part 3. In fact, the housing could be cylindrical or of any desired shape, but the present shape is preferred since it corresponds to that of a conventional septic tank. The bottom part 2 defines a primary settlement tank 6 which, at its widest part, contains a baffle 7 of frusto-conical shape, extending upwardly from the wall of the bottom part 2, and dividing the primary settlement tank into upper and lower chambers 9 and 10 which communicate via notches 11 in the periphery of the baffle. As will become clear below, the lower chamber 10 may be considered as a first settlement chamber for settlement of gross solids, with the upper chamber 9 providing a second settlement chamber. As shown in Fig. 1, a de-sludging pipe 12 extends from the lower chamber 10 through the baffle 7 and travels vertically up the outside of the top part 3 to be capped by a removable cover 14. Raw sewage enters the plant via an inlet pipe 15, tapped into the side of the desludging pipe 11, and then travels down the de-sludging pipe into the lower chamber 10. It will be noted that the inlet pipe 15 is above the normal maximum level F1 of sewage in the primary settlement tank 6. Gross solids settle out into the lower region of the bottom chamber 10, and the baffle 7 prevents large floating solids from entering the upper chamber 9. Since the baffle slopes downwardly towards the notches 11, further settlement of solids in the upper chamber 9 will result in the deposited solids moving downwards to return via the notches 11 into the lower chamber 10.

Referring to Fig. 2, a first cylindrical pump chamber 18 extends upwardly from the interior of the primary settlement tank 6 alongside the top part 3.

Two sets of shielded ports 20 and 22 permit liquid effluent to flow from the upper chamber 9 of the primary settlement tank into the pump chamber 18, which is otherwise sealed from the primary settlement tank 6. The lower set of ports 22 define the minimum, off-peak level F2 in the primary settlement tank (see below). A pump 19 stands on feet on the bottom of the pump chamber 18. The pump is controlled by a timer (not shown) to run for a short period several times a day (e.g. sixteen 30 second periods). A lower limit probe (also not shown) is provided to prevent the pump from coming on should the fluid level in the pump chamber fall below a predetermined minimum level, and a further, upper limit probe causes the pump to run continuously should the fluid level in the primary settlement tank and pump chamber exceed the normal maximum level F1. The output from the pump 19 passes via a conduit 24 to the upper region of a filtration chamber 26 disposed in the upper region of the cylindrical top part 3 of the housing 1.

The top part 3 of the sump housing is closed by a top wall 27 provided with an access cover 28. The top part 3 contains a conical wall 30 which is sealably joined to the side wall of the top part 3, and sealably passes through the mid region of the baffle 7. The conical wall 30 in turn contains and supports a shallow conical base wall 32 for the filtration chamber 26, which contains random-packed filter media 33 of known form for supporting a biomass. The conduit 24 passes above the filter media 33 via a venturi device 34 which entrains air into the effluent flow from an air induction pipe 35 extending through the top wall 27. The aerated effluent than passes to a rotary spray distributor 36 or similar distribution device located in the approximate centre of the top part 3, to trickle down through the filter media 33. The air entrainment provided by the venturi device 34 may be sufficient to support the biomass without further aeration measures, although an additional ventilation pipe 37 may be provided in the filter chamber 26 to permit an external duct pipe to be connected and lead to a suitable aboveground vent. If not used, the ventilation pipe can simply be capped off.

At the bottom of the filtration chamber 26 the effluent is collected by the bottom wall 32 and passes into a final settlement tank 38 formed by the lower portion of'the conical wall 30. The effluent is introduced into the tank 38 via a feed tube 39 which extends below the normal fluid level F3 to minimise disturbance of the effluent in the settlement tank 26. Any particulate matter in the effluent tends to settle out and move down the conical wall 30 to collect at the bottom of the tank 38. Thus, the supernatant fluid at the upper region of the tank 38 is relatively clear. The fluid level F3 is defined by a short vertical weir wall 40 which is joined to the conical wall 30 via an annular wall 41. As the effluent trickles into the final settlement tank 38 the cleared supernatant liquid weirs over the wall 40, again causing minimum disturbance to the contents of the tank. The walls 40 and 41 form an annular channel 42 which conducts the supernatant out of the plant via an outlet pipe 43 disposed substantially opposite to the inlet 15. The outlet 43 is arranged such that there cannot be any residual pooling of fluid in the channel 42, which would lead to undesirable organic growth and contamination of the outlet fluid.

The treated effluent from the plant is thus extremely clean with minimum contamination with undesirable organic materials.

A sediment outlet pipe 44 leads from the base of he final settlement tank 38 to the base of a second pump chamber 46 which again extends vertically alongside the top part 3, substantially opposite to the first pump chamber 18.

The second pump chamber 46 again contains a timer-operated pump 49 which is typically arranged to run for a short period several times a day. The pump outlet passes via a conduit 50 which runs beneath the top wall 27 and discharges into the primary settlement tank 6. The pump thus intermittently returns sediment to the primary settlement tank to be re-cycled through the filter medium 33 with further biological degradation.

Each time the first pump 19 operates, the fluid level in the first pump chamber 18 falls. Often, the periods of maximum sewage input are relatively short. In a domestic dwelling for example, the peak input occurs during early morning and early evening with a very low input level during the rest of the day. Under heavy periods of use when there is a large inflow of raw sewage, the fluid level in the primary settlement tank will tend to rise towards level F1, so that the first pump chamber will rapidly refill via the lower ports 22 and, eventually, through the upper ports 20. Further input of raw sewage will be sensed by the upper limit probe causing the pump to run continuously to reduce the volume. Under conditions of low input the pump chamber will still refill between periods of pump operation from the primary settlement tank via the lower openings 22, but eventually the level in the primary settlement tank 6 may fall to the level F2 defined by the lower openings 22, and the pump chamber 18 will cease to fill. The pump will then be held inactive by the lower limit probe as discussed above, and removal of effluent will cease until a further charge of sewage enters the tank 6. As soon as this occurs, the fluid level in the primary settlement tank 6 will rise and start to fill the pump chamber, whereupon pumping of effluent to the filter chamber 26 can recommence. Again, under periods of heavy use, the sewage level can rise towards the upper level F1, thereby providing a buffer volume which can be reduced by the pump during periods of low usage.

The plant is therefore extremely economical in terms of power consumption.

During periodic de-sludging of the plant, a flexible pipe is fed down the desludging pipe 11 into the bottom chamber 10 of the primary settlement tank.

The settled gross solids are then sucked out until the primary settlement tank is fully emptied of fluid. It will also be noted that one-way valves 52 and 54 are provided in the bottom of the pump chamber 18 and final settlement tank 38. These valves are normally held closed, but when the primary settlement tank is drained the hydrostatic head opens the valves so that the first pump chamber 18 drains via the valve 52 and, in addition, the second pump chamber 46, filter chamber 26 and final settlement tank 38 all drain through the valve 54. Thus, the entire plant can be substantially cleared of effluent in a single de-sludging operation.

Claims

1. Apparatus of the kind set forth, in which the additional settlement tank is of substantially conical shape.

2. Apparatus according to Claim 1, in which the additional settlement tank is provided with an elongate weir wall, over which the supernatant fluid from the additional settlement tank flows to the outlet means.

3. Apparatus according to Claim 2, in which the weir wall extends continuously around the top region of the additional settlement tank.

4. Apparatus according to Claim 3, in which the weir wall is surrounded by a drainage channel leading to the outlet means.

5. Apparatus according to any preceding claim, in which the filter chamber discharges into the additional settlement tank via an inlet which is located below the normal fluid level of the additional settlement tank.

6. Apparatus according to any preceding claim, in which the additional settlement tank is provided with a sediment outlet from the bottom thereof.

7. Apparatus according to Claim 6, in which the sediment outlet leads to a second pump chamber containing a recirculation pump arranged for periodic operation.

8. Apparatus according to Claim 7, in which the recirculation pump in the second pump chamber is arranged to return sediment to the primary settlement tank for further treatment.

9. Apparatus according to any preceding claim, in which the pump means of the first-mentioned pump chamber is arranged to run intermittently for predetermined periods, and the pump chamber inlet is arranged to define a minimum fluid level in the primary settlement tank which provides for a buffer volume for raw sewage in the primary settlement tank above the minimum fluid level, the first pump chamber having a low level detector which holds the pump inoperative until the fluid level rises therein.

10. Apparatus according to Claim 9, in which a high level detector is provided in the primary settlement tank or first pump chamber to cause the pump means to run continuously whilst the fluid level in said tank or pump chamber exceeds a predetermined level.

11. Apparatus according to any preceding claim, in which the first pump chamber is provided with a one-way drain valve for draining fluid therefrom into the primary settlement tank during de-sludging.

12. Apparatus according to any preceding claim, in which the additional settlement tank is provided with a one-way drain valve for draining fluid therefrom into the primary settlement tank during de-sludging.

13. Apparatus according to any preceding claim, in which the conduit means which leads from the pump to the filter chamber includes venturi means for entraining air into the effluent flow.

14. Apparatus according to any preceding claim, in which the primary settlement tank comprises a lower settlement chamber and an upper settlement chamber separated by a partition wall which slopes downwardly towards aperture means therein, the arrangement being such that raw sewage enters the lower chamber from the raw sewage inlet, and effluent from the lower chamber enters the upper chamber through the aperture means.

15. Apparatus accordingto any preceding claim, in which the sump housing comprises a generally globular lower part surmounted by a cylindrical upper part.

16. Sewage treatment apparatus substantially as described with reference to the drawings.