GB2270909A

GB2270909A - Sewage treatment

Info

Publication number: GB2270909A
Application number: GB9220187A
Authority: GB
Inventors: David Peter Froud
Original assignee: COPA HOLDINGS Ltd
Current assignee: COPA HOLDINGS Ltd
Priority date: 1992-09-24
Filing date: 1992-09-24
Publication date: 1994-03-30
Anticipated expiration: 2012-09-24
Also published as: GB2270909B; GB9220187D0

Abstract

A sewage treatment plant comprises a primary sedimentation tank, intermediate treatment means, and a humus tank, and is characterised in that a biological aerated filter is installed in the primary sedimentation tank or in the humus tank.

Description

SEWAGE TREATMENT This invention relates to sewage treatment, and more particularly to a process for improving the quality of effluent from a sewage treatment works.

In a conventional sewage treatment plant, crude sewage is first screened to remove grit and other hard solids. It is then passed to a primary sedimentation tank, where suspended solids are allowed to gradually settle and are removed in the form of a liquid raw sludge. The raw sludge is passed to a sludge digestion plant and the digested sludge is subsequently dried.

Effluent from the primary tank is subjected to secondary treatment. A commonly used form of secondary treatment is a biological filter. This is not a filter as such, but is a tank containing solid media providing a large surface area on which colonies of microorganisms form a biomass and are exposed to the liquid effluent and to the air. These microorganisms break down noxious substances in the effluent. Filter effluent from the biological filter is passed to a humus tank where settlement again is allowed to take place. Nitrification (conversion of ammonia to nitrates) may also take place in the humus tank. Settled sludge from the humus tank is returned to the primary sedimentation tank. Effluent from the humus tank is discharged to a water course. Such basic processes are described in "An Introduction to Sewage Treatment", published by the Institute of Water Pollution Control, 1987.

One problem faced by sewage treatment plants is the variability of the incoming crude sewage. The volume and concentration of incoming crude sewage can vary depending on weather conditions and seasonal variations, for example in holiday resorts. It is also desirable to be able to up-grade sewage treatment plants using existing void structures, and so eliminate the need for expensive civil engineering work to increase the biological capacity of a works.

The present invention overcomes these problems by the use of a biological aerated filter in a primary sedimentation tank or in a humus tank of a sewage treatment plant.

Thus, the invention provides a sewage treatment plant comprising a primary sedimentation tank, intermediate treatment means, and a humus tank, characterised in that a biological aerated filter is installed in the primary sedimentation tank or in the humus tank.

A biological aerated filter (hereinafter BAF) is a biological filter including means for introducing additional air into the effluent under treatment. It will generally comprise a self-supporting solid matrix, for example corrugated layers of plastics material, such as rigid PVC, which acts as support for microbial biomass. Air diffusers are introduced into the treatment tank beneath the solid media, enabling a constant stream of air bubbles to be blown through the effluent in and around the solid media. The BAF may be a permanent fixture in the primary tank or humus tank.

Alternatively, it can be provided as a movable modular unit which can be suspended in the tanks from a support beam spanning the tank. The advantage of this second embodiment is that existing tanks can be upgraded within a few hours. There are no moving parts within the tank, and additional modules can be fitted as required or removed when not required, e.g. to take into account seasonal variations in load.

BAF modules can be installed in trailers or skidmounted tanks to provide rapid upgrading of works, or to provide emergency capacity. The BAF module can be installed on screened crude sewage, settled sewage, or on humus effluent to effect nitrification.

It is surprising that the use of a BAF in a primary tank or humus tank has proved so effective in reducing the biochemical oxygen demand (BOD) and suspended solids of effluent. It has always been accepted practice in this field that effluent in a primary tank or in a humus tank should not be disturbed in any way, in order to allow sedimentation to take place. It would therefore have been expected that introducing a BAF into a primary tank or humus tank would upset the sedimentation of the sludge to an unacceptable extent. However, the use of a BAF in this way according to the present invention has been shown to provide a dramatic improvement in the treatment of sewage.

Reference is now made to the accompanying drawings, in which: Figures 1A and 1B are respectively a part sectional view and a plan view of a primary sedimentation tank according to one embodiment of the invention; Figure 2 is a side view showing in more detail the use of a BAF in a primary sedimentation tank; Figures 3A, 3B and 3C are respectively a front view, a side view and a plan view of a single BAF module for use in an embodiment of the invention; Figure 4 corresponds to Figure 2 and shows an alternative flow pattern; and Figure 5 shows an alternative embodiment of a BAF module with a single diffuser.

Referring to Figure 1, there is shown a rectangular primary sedimentation tank 1, having around its periphery a plurality of BAF units 2, one of which is shown in section on the lined X-X in Figure 1A. Each BAF unit has an upright support frame 3 housing several layers of self-supporting solid media 4. Suitable solid media is that marketed under the trade mark BIODEK, which consists of adjoining sheets of corrugated rigid PVC. In the base of the BAF unit there are several air diffusers 5.

Suitable air diffusers are disc diffusers marketed under the trade mark DIDIER. The air diffusers are supplied with air under pressure and this bubbles up through the liquid effluent surrounding the solid media.

Colonies of microorganisms form as biomass on the solid media 4 and there break down noxious elements in the effluent with the assistance of the air supplied from the air diffusers.

Figure 2 shows one possible arrangement for adjoining BAF units. Effluent enters from an inlet 6 and passes through successive media compartment 7. Baffles 8 are arranged between alternate compartments so that the liquid effluent passes from the bottom of one compartment to the next, and then as an overflow into a subsequent compartment, as shown by the arrows 9. Effluent exits by an outlet 10. Aerators or air diffusers 5 are arranged at the bottom of each compartment, and receive air under pressure from an adjoining blower compartment 11, the air escaping as a stream of upward bubbles through each compartment.

Figures 3A, 3B and 3C show a single BAF module.

This includes a support frame 12 housing self-supporting solid media 4, similar to that described in relation to Figure 1. Aerators 13 are arranged at the bottom of the module and communicate with compressed air pipework 14.

Attached on the outside towards the top of the media support frame is a module support framework 15, adapted to rest on the edge of beams spanning the primary sedimentation tank or humus tank, so that the BAF module is able to operate temporarily in such tank.

In Figure 4, the numbered parts correspond to those shown in Figure 2. However, the media compartments and baffles are arranged so that the flow of waste water is always upward through the media. This is a preferred embodiment of the invention. Also, the diffusers are spread at maximum possible density so as to ensure aeration over all the base area of the media.

In Figure 5, a BAF module comprises blocks of media 4, as described above, surrounded by plastic shrink wrap, and supported by a hanger 16 and a grill support 17 at the base. A single diffuser 13 is positioned beneath the base. The diffuser is preferably of the same shape and size as the media transverse cross section, to ensure maximum surface area aeration.

To maximise the efficiency of the biological process, it is important to obtain the optimum air/liquid velocity through the system. This ensures that surplus biomass is stripped steadily from the media, and that oxygen transfer from the air to the influent waste water is maximised. The preferred arrangements shown in Figures 4 and 5 help to achieve these optimum conditions.

EXAMPLES A conventional sewage treatment works was modified by introducing a BAF into a primary settlement tank.

Measurement were made on different dates for biochemical oxygen demand (BOD) and suspended solids of the crude influent, the effluent from a primary settlement tank without any modification, and the effluent from a primary settlement tank modified by addition of a BAF. The BAF modification resulted in a substantial reduction both of BOD and suspended solids in all cases. The results are shown in the following Table.

DATE SAMPLE LOCATION BOD SUSPENDED SOLIDS (1050C) 13/08/92 Crude Influent 455 916 Effluent from Primary Settlement Tank 74 56 Effluent from BAF Primary Settlement Tank 48 48 14/08/92 Crude Influent 352 340 Effluent from Primary Settlement Tank 232 196 Effluent from BAF Primary Settlement Tank 143 86 17/08/92 Crude Influent 371 280 Effluent from Primary Settlement Tank 323 164 Effluent from BAF Primary Settlement Tank 275 120 18/08/92 Crude Influent 300 258 Effluent from Primary Settlement Tank 265 119 Effluent from BAF Primary Settlement Tank 146 76 20/08/92 Crude Influent 324 366 Effluent from Primary Settlement Tank 170 - Effluent from BAF Primary Settlement Tank 75 94

Claims

CLAIMS: 1. A sewage treatment plant comprising a primary sedimentation tank, intermediate treatment means, and a humus tank, characterised in that a biological aerated filter is installed in the primary sedimentation tank or in the humus tank.
2. A sewage treatment plant according to Claim 1, in which the biological aerated filter comprises a selfsupporting solid matrix, which acts as support for microbial biomass, and at least one air diffuser beneath the solid matrix, enabling a stream of air bubbles to be blown through the effluent in and around the solid matrix.
3. A sewage treatment plant according to Claim 1 or 2, in which the biological aerated filter is provided as a movable modular unit which can be temporarily suspended in the tanks.
4. The use of a biological aerated filter in a primary sedimentation tank or in a humus tank of a sewage treatment plant.
5. A sewage treatment plant substantially as hereinbefore described with reference to and as shown in the accompanying drawings.