GB2178333A

GB2178333A - Settling tank

Info

Publication number: GB2178333A
Application number: GB8617613A
Authority: GB
Inventors: Michael Ian Robert Smith
Original assignee: Klargester Environmental Engineering Ltd
Current assignee: Klargester Environmental Engineering Ltd
Priority date: 1985-07-19
Filing date: 1986-07-18
Publication date: 1987-02-11
Also published as: GB8518316D0; NO170536B; GB2178333B; GB8617613D0; NO862916D0; NO170536C; NO862916L

Abstract

A sewage settling or septic tank is divided by at least one partition 3, 4 into chambers 7, 8, 9 which act in series on liquid flowing from inlet 12 to outlet 13. The chambers communicate by restricted openings 10, 11 to allow liquid flow while retaining solids mainly in chamber 7. Sludge is periodically pumped from chamber 7 by a pipe lowered through passage 4a, which an upward extension of partition 4 and extends above the liquid level 16. During such desludging, flap valves 20 open to allow sludge from chamber 9 to fall into chamber 7. Openings 11 between chambers 8 and 9 are confined to the region opposite outlet 13. Valve 20 (Figs. 3, 4) comprises a moulded rubber cup having a slit (25) which is constrained to a closed position when the mouth of the cup is engaged with a hole through a flat wall. <IMAGE>

Description

SPECIFICATION Sewage treatment plant This invention relates to piant for the treatment of sewage or waste water of the kind utilising a settlement or septic tank in which precipitation of sludge takes place continually. In this specification, for convenience, the term "sewage" will be employed as a generic term which includes waste water requiring treatment by primary settlement. In such a plant the sludge precipitant is removed from time to time for disposal and the liquid effluent may be disposed of in any suitable known way, for example passed into a suitable sub-surface absorption field or delivered to a bacteriological filter bed for further processing and/or reuse for any purpose.

Sewage treatment plant of the kind mentioned above is described and claimed in British Patent Specifications Nos. 1,297,959 and 1,597,167. The preferred embodiment of the plant described in these Specifications comprises a settlement tank provided by two frusto-conical partitions into a final settlement chamber, an annular intermediate chamber and a lower chamber, the frusto-conical partitions being provided with slots therethrough which allow the passage of effluent liquid from the lower chamber to the intermediate chamber and from the intermediate chamber to the final settlement chamber.The frusto-conical partitions are arranged so that the lower partition has its larger diameter end attached to the lower edge of an upper tank part and its smaller diameter end uppermost, and the larger diameter end of the upper frustoconical partition is attached adjacent to the top of the tank whence the upper partition projects downwardly and is sealed to the lower partition. The upper end of the lower chamber is provided with a plug to prevent the passage of floating conglomerate and suspended matter directly from the lower or sludge digestion chamber to the final settlement chamber. Under normal domestic flow conditions this arrangement of frusto-conical partitions with slots for the passage of effluent liquid produces a final effluent of satisfactory and acceptable quality.However, under abnormal flow conditions, such as when a plurality of sanitary fittings are discharged into the tank at the same time or in rapid succession, the hydraulic conditions change and this can result in an increase in the quantity of suspended matterand floating conglomerate passing through the slots into the intermediate and final settlement chambers, thus causing a reduction in the quality of the final effluent. The problem arises when the influent velocity is above average, due to the conditions described above, resulting in increased turbulence in the lower chamber which adversely affects the hydraulic conditions in the other two chambers.

Occasionally conditions in use of the tank can arise in which the aforementioned plug becomes ineffective and suspended matter, paper and other solids pass directly from the lower chamber to the final settlement chamber.

The present invention aims to provide a sewage treatment plant which considerably reduces or eliminates the occasional disadvantages described above.

According to the invention a sewage treatment plant cemprises a settlement tank provided by at least one partition, into a final settlement chamber, and an initial chamber, means for delivering raw sewage into the initial chamber, means allowing the passage of effluent liquid through or past said at least one partition from the initial chamber to the final settlement chamber but substantially preventing the passage of solid matter from the initial chamber into the other chamber(s) and a liquid effluent outlet from said final settlement chamber, said at least one partition being arranged so that a sludge-removal passage leading to the initial chamber extends upwardly through the final settlement chamber and terminates at a level above the liquid effluent outlet.

In a sewage treatment plant in accordance with the invention the arrangement of the at least one partition dispenses with the need for a flap valve (as described in 1297959) or a floating ball valve (as described in 1597167) in the sludge-removal passage to prevent flow of effluent liquid between the final settlement chamber and the initial chamber. This can be a significant advantage in practice since the plugs used for this purpose in the known tanks can become jammed in a partially open condition and, in the case of a ball valve of a foamed plastics material, can become distorted by organic solvents in the sewage discharged for treatment in the tank.

A desirable arrangement of tank in accordance with this invention has two partitions which divides the tank into an initial, an intermediate and a final settlement chamber and which leaves each of the three chambers accessible through the open top of the tank to facilitate removal of sludge precipitant therefrom.

Conveniently where there are three chambers, the intermediate chamber directly surrounds a sludgeremoval passage leading to the initial chamber and is itself surrounded by the final settlement chamber.

Suitably flap valves of rubber or a similar organicsolvent resistant elastomeric material are used to prevent back-flow of sewage from the initial chamber to the final settlement chamber.

Where first and second partitions are employed these preferably each include a frusto-conical region. They may, however, each include a frustopyramidal region. The tank and the at least one partition may be made of fibre-reinforced synthetic resin material.

Sewage treatment plant in accordance with the invention is particularly suitable for the treatment of sewage from dwelling houses, and may be made in sizes suitable for a single house or up to 60 or more houses.

Sewage treatment plant in accordance with the invention may also be made of sizes suitable for a petrol service station, a factory, a hotel or a restaurant or indeed any other installation which has sewage requiring treatment by primary settlement.

The effluent outlet may be arranged to discharge directly to any suitable disposal point, for example a biological filter or sub-surface irrigation system.

Alternatively, the effluent outlet may be connected to the inlet side of a pump which is arranged to recycle part of the effluent back to one of the chambers of the tank to achieve a further reduction in suspended solids. Alternatively, the effluent outlet may be arranged to discharge into a filtering device, for example a biological rotating contactor, if necessary a pump being incorporated between the effluent outlet and the filtering device. In this case the liquid effluent from the filtering device may all be discharged to a suitable disposal point, or a proportion of it may be re-cycled to one of the chambers of the tank to achieve a still further reduction in suspended solids.

The invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a first form of sewage treatment plant in accordance with the invention, Figure 2 is a section on the line Il-Il of Figure 1, Figure 3 is an enlarged cross-section of one of the flap valves used in the tank of Figure 1 prior to fitting in place in a tank partition, Figure 4 is a view corresponding to Figure 3 showing the flap valve in place in the partition, Figure 5 is a schematic sectional view of a second form of sewage treatment plant in accordance with the invention, and Figure 6 is a section on the line VI-VI of Figure 5.

Referring to Figures 1 and 2 of the drawing, the sewage treatment plant shown comprises a closed tank 1 of circular cross-section formed by a substantially hemi-spherical lower tank part 1a and an inverted cup-shaped upper part 1 b which are connected together by a spigot and socket joint 2.

The tank is made of reinforced synthetic resin, for example, polyester reinforced with glass fibre or any other suitable material, and the two parts are bonded together at the joint 2 with an adhesive.

Within the tank 1 are two partitions 3, 4 which include frusto-conical regions arranged with their axes coincident with the vertical axis of the tank. The partition 3 has its circumferential edge of larger diameter lowermost, this edge being joined to the inner surface of the upper tank part 1 b. A central part 3a of the partition 3 is cylindrical (but includes four vertical grooves 3b) and is bonded to the upper part 1 b of the tank close to its upper narrow end. The partition 3 may also be made of reinforced polyester resin and preferably it is bonded to the tank part 1 b, for example with polyester resin.

The partition 4 has its circumferential edge of larger diameter bonded to the partition 3 with an adhesive. A central part 4a of the partition 4 is also frusto-conical this part 4a narrowing in the upward direction to terminate in an open upper end 4b. The partition 4 may be constructed of glass fibrereinforced polyester resin or any other suitable plastics material or other material and can be moulded with the part 4a, or the latter part can be moulded separately and adhesively secured in place.

The partitions 3 and 4 define three chambers within the tank 1, namely a lower or initial chamber 7, an intermediate annular chamber 8 and an annular final settlement chamber 9.

The chambers 7 and 8 are in communication with each other via slots 10 in the partition 4.

The chambers 8 and 9 are in communication with each other via slots 11 in the part 3a of the partition 3. The slots 11 are disposed equidistantly from one another around just one half of the part 3a intermediate the lower and upper circumferential edges of the latter. Chambers 7 and 9 are in communication by means of a ring of non-return valves 20 whose flow direction is downwards. These valves can be of any convenient design but flap valves of rubber have proved to be effective. A suitable design for the valves 20 is shown in Figures 3and4.

The numeral 12 designates a vertically disposed inlet pipe which enters the part 1 b of the tank 1, passes through the partitions 3 and 4 and discharges into the chamber 7. The numeral 13 designates a vertically disposed outlet pipe which communicates with the chamber 9 and passes through the part 1 b of the tank. These pipes 12 and 13 may be made of synthetic resin, for example polyvinyl chloride, or paper impregnated with pitch, and may be bonded to the part 1 b of the tank.

The numeral 14 designates a vertically disposed access shaft formed integrally with, or secured to, the upper tank part 1 b, the axis of this shaft being aligned with the tank axis. The upper end of the shaft 14 is closed by a removable lid or manhole cover 15.

The outlet pipe 13 has a branch pipe 13a which determines the top water level (shown at 16) within the tank and the upper end 4b of the central part of the partition should terminate above this level 16.

To ensure that the end 4b is above the water level in all circumstances of use, it is desirable to locate the end 4b above the highest part of the branch pipe 13a (i.e. as shown in Figure 1).

Aclamping ring 17 (e.g. of galvanised steel) and radial spider arms 18 are used to support the part 4a within the tank, the arms 18 also serving to prevent a child failing into the chambers 7 or 8.

Two of the grooves 3b in the central cylindrical part 3a of the partition accommodate the pipes 12, 13, but the other two provide access from above to the chamber 9. Thus, for example, a hose can be inserted into the chamber 9 through the shaft 14 and a groove 3b to wash sediment off the frusto-conical surfaces of the partition 3 down through the valves 20 into the chamber 7 when the tank is being desludged.

In use of the above described tank, raw sewage enters through the inlet pipe 12 and passes straight to the lower chamber 7 which is the sludge storage and digestion chamber. The solids are deposited in this chamber and retained in it.

The settled effluent, largely free from solids, flows vertically into the intermediate chamber 8 which is the secondary settlement chamber, admission of the effluent to this chamber being through the slots 10. These slots assist in reducing velocity within the tank thus encouraging settlement and clarification of the effluent.

From the chamber 8 the effluent then passes into the chamber 9 through the slots 11 which also assist in further reducing velocity and promoting greater clarification of the effluent. Since the slots 11 are located in the cylindrical part 3a only over the half which is furthest from the outlet pipe 13, effluent liquid entering the chamber 9 has to flow some distance through the chamber before it can leave the tank via the pipe 13.

Finally the clarified liquid effluent passes out of the tank through the pipe 13.

Any fine suspended matter passing into the chamber 8 will settle down and return to the chamber 7 through the slots 10.

When it is desired to empty accumulated solid matter from the chamber 7, a pipe (not shown) from a suitable suction device is passed into the chamber 7 through the access shaft 14.

In one particular sewage treatment plant constructed as shown in Figure 1 and having a capacity of 4500 litres, the tank has a maximum internal diameter of 2.12 m.

The branch pipe 13a may discharge the liquid effluent direct to any suitable dispersal system, for example a subsurface irrigation system.

Alternatively the branch pipe 13a may discharge to a pumpwell (not shown) located beside the settlement tank, and a pump installed in the pumpwell may discharge the effluent to a subsurface irrigation system located at a site remote from the settlement tank or to a biological filter disposed either adjacent to the settlement tank and pumpwell or remotely from it. The biological filtration of the effluent will further reduce suspended solids and biological oxygen demand to produce a higher quality effluent than that which is discharged from the settlement tank. The liquid discharged from the filter may flow to a ditch, a humus tank, a subsurface irrigation system or any other approved disposal means.

Alternatively a proportion of the liquid discharged from the filter may be returned to the chamber 7 or to the chamber 9 of the tank to achieve a still further reduction in B.O.D. and suspended solids. This recycled effluent may be fed into chambers 7 to 9 through a return pipe (not shown) similar to pipe 12 or 13. The remainder of the effluent discharged from the filter would be led to a suitable disposal point.

The valves 20 can be formed from a moulded rubber cup having a slit provided therein. A preferred form of valve is shown in Figures 3 and 4.

Figure 4 shows the valve held in place in an aperture in the partition 3 (the chamber 9 being above the partition and the chamber 7 below) by engagement of the partition in a groove 21 of the cup. A diametrically disposed slit 22 formed adjacent to flat faced webs 23, 24, provides the normally-closed valve opening which opens only when there is a sufficient pressure difference across the cup (e.g.

when the tank is being emptied via the chamber 7).

To ensure the slit remains closed when there is no hydraulic pressure difference inside and outside the cup, the cup can be moulded as shown in Figure 3 with a V-shaped clearance 25 between the webs 23, 24 but with a bias in the rim so that the groove 21 does not lie in one plane. In deforming the groove 21 to accommodate the partition 3, the clearance 25 disappears. In a typical small tank of capacity 2700 litres the hole diameter in the partition would be of the order of 750 mm and this could increase to 1000 mm for a 6000 litre tank.

Generally speaking, the slot area in each partition is chosen so that, when the tank is in use, there will be an adequate velocity of the liquid flowing through the slots, so that the latter are kept free from clogging.

Figures 5 and 6 show a second embodiment of tank which utilises just one partition and thus operates with just an initial chamber and a final settlement chamber. For convenience in describing the second embodiment the same reference numerals will be used to designate integers in Figures 5 and 6 as have been used in Figures 1 and 2 to designate similar or equivalent integers but with the addition of a prime and only the features of difference will be referred to.

The single partition 3' smoothly curves outwardly from its upper end 4b' to the point where it connects to the upper part 1b' of the tank 1', to facilitate the wash down of sediment when desludging of the tank takes place. The partition is symmetrical about the vertical axis of the tank (shown dotted at 30' in Figure 5) and is thus substantially completely defined bythe rotation of a curved line aboutthe axis 30'. A plurality of valves 20' (e.g. as shown in Figures 3 and 4) are located in pockets in the partition 3' to allow washed down sediment to fall into the initial chamber 7'. A hose 31 has been shown in Figure 5 to indicate how it can be led from the access shaft 14' into the chamber 9'.

The slots 11' are formed in the sludge-removal passage 4a defined by the partition 3' on the side of a diametric plane through the axis 30' which is opposite to the side in which the outlet pipe 13' is located. This ensures that the settled effluent which enters the final settlement chamber 9' does so in positions which are radially displaced with respect to the outlet pipe 13'.

The upper edge 4b' of the partition 3' is well above the normal liquid level 16' and even above the highest part of the branch pipe 13a'.

It will of course be appreciated that the invention is not limited to the plants described in detail above with reference to the drawings. Thus, for example, the tank may be of rectangular or other polygonal cross-section, in which case the partitions 3 and 4 may be partly shaped as frusto-pyramids. The valves 20 and 20', although desirable need not be provided.

Claims

1. A sewage treatment plant comprising a settlement tank divided by at least one partition, into a final settlement chamber, and an initial chamber, means for delivering raw sewage into the initial chamber, means allowing the passage of effluent liquid through or past said at least one partition from the initial chamber to the final settlement chamber but substantially preventing the passage of solid matter from the initial chamber into the otherchamber(s) and a liquid effluent outlet from said final settlement chamber, said at least one partition being arranged so that a sludge-removal passage leading to the initial chamber extends upwardly through the final settlement chamber and terminates at a level above the liquid effluent outlet.

2. A tank as claimed in claim 1, in which two partitions divide the tank into an initial, an intermediate and a final settlement chamber, in such a way that all three chambers are accessible through the open top of the tank.

3. A tank as claimed in claim 2, in which the intermediate chamber directly surrounds a sludgeremoval passage leading to the initial chamber and is itself surrounded by the final settlement chamber.

4. A tank as claimed in claim 2 or claim 3, in which each partition includes a frusto-conical or frustopyramidal region.

5. A tank as claimed in claim 1, in which just one partition is provided, the final settlement chamber surrounding the sludge-removal passage.

6. A tank as claimed in claim 5, in which the one partition narrows in the upward direction and is substantially completely defined by the rotation of a curved line about an axis of the tank, which axis in the installed position of the tank is vertical, the means allowing passage of effluent liquid from the initial chamber into the final settlement chamber being openings formed in the partition on the side of a plane through the axis which is opposite to that side in which the liquid effluent outlet is located.

7. A tank as claimed in any preceding claim, in which flap valves of rubber or a similar organicsolvent resistant elastomeric material are used to prevent back-flow of sewage from the initial chamber to the final settlement chamber.

8. A tank as claimed in any preceding claim, in which the tank and the or each partition are made of fibre-reinforced synthetic resin material.

9. A sewage treatment plant substantially as hereinbefore described with reference to, and as illustrated in, Figures 1 and 2 of the accompanying drawings.

10. A sewage treatment plant substantially as hereinbefore described with reference to, and as illustrated in, Figures 5 and 6 of the accompanying drawings.

11. A plant as claimed in claim 9 or claim 10, in which the valves are substantially as shown in Figures 3 and 4.