GB2329634A

GB2329634A - Activated sludge treatment plant

Info

Publication number: GB2329634A
Application number: GB9821183A
Authority: GB
Inventors: Jonathan Edward Tul Strickland
Original assignee: AW Creative Technologies Ltd
Current assignee: AW Creative Technologies Ltd
Priority date: 1997-09-30
Filing date: 1998-09-30
Publication date: 1999-03-31
Anticipated expiration: 2018-09-30
Also published as: GB9720787D0; GB2329634B; GB9821183D0

Abstract

An activated sludge treatment plant in the form of an annulus comprises an anoxic zone 2, an aeration zone 3, a sludge storage zone 7, and a central settlement zone 4. Sludge may be wasted directly from the aeration zone, so that sludge age is controlled. In use, sewage may be passed firstly into the anoxic zone, then to the aeration zone and finally to the central settlement zone where sludge is collected in a collection area 4a at the base of the settlement zone and liquor passes over an annular weir 5 through a wash water zone 6.

Description

ACTIVATED SLUDGE PLANT Field of the Invention This invention relates to the treatment of sewage by an activated sludge process, and to apparatus for use in such treatment.

Backqround of the Invention The treatment of sewage by the activated sludge process is well known. In this process, sewage is retained in an aeration tank, with activated sludge, where it undergoes microbiological oxidation to give clarified effluent and further activated sludge. The duration of time the sludge spends in the aeration tank, termed the solids retention time, is an important factor in the efficiency of the treatment process. Most activated sludge treatment works use a solids retention time of between 4-15 days. This ensures the development of a population of bacteria appropriate to the required treatment level. For example, nitrification requires a sludge age in excess of 10 days, but long sludge age can result in poor sludge settling and mousse problems.

In conventional treatment works the activated sludge in the aeration tank is mixed fully and then pumped from a return line either to a settlement tank or to a sludge waste tank and then flows to a settlement tank where a settled sludge is removed. This is either returned to the inlet of the process (Returned Activated Sludge) or intermittently wasted (Waste Activated Sludge). A disadvantage of this method is that the settled sludge varies in solids content and it is therefore difficult to control the amount of sludge wasted. This can mean that sludge age is poorly controlled and settling characteristics less than optimal. In large treatment works these problems can be overcome by making adjustments to the operating conditions. There is however a growing need for small treatment works designed for populations in the range 500 to 5,000. To achieve the necessary efficiency in these plants it is desirable that they incorporate the same process steps as conventional treatment works. However, constant monitoring of sludge characteristics is impractical in small treatment works.

Summary of the Invention It is now realised that the components of the activated sludge treatment works, incorporated in an annular design, around a central settler, can provide an efficient activated sludge system which helps regulate sludge age. The annular design is compact and requires minimal construction of pipelines and pumping stations.

Further, the design allows for a continuous, reliable treatment of sewage, without the need for constant monitoring of the process steps.

Description of the Invention The invention will now be described by way of example only with reference to the accompanying drawing, in which: Fig. 1 is a schematic plan view of an embodiment of the present invention.

Fig. 1 shows an annular treatment plant with incorporated process stages. The sewage treatment plant is designed with an activated sludge aeration tank positioned around a central settler. Aeration can be provided by any conventional means.

In use of the plant shown in Fig. 1, sewage may be pumped via an inlet 1 into an anoxic zone 2 occupying a sector of the annulus. From the anoxic zone 2, sewage passes in the direction of the curved arrow into an aeration zone 3, and then into a central settlement zone 4 via a channel (not shown). The settlement zone 4 includes, at its base, a collection area 4a from which sludge can be pumped via a channel (not shown) underneath the annular plant, for recycling etc. An annular weir 5 allows liquor to pass out of the settlement zone 4, through a wash water zone 6.

In the anoxic zone, sewage may be mixed with recycled activated sludge, encouraging denitrification and discouraging the growth of filamentous bacteria which cause poor settling sludge. As shown in Fig. 1, a sludge storage tank 7 is also present in a section of the annulus; an overflow 8 allows sludge to pass directly into the anoxic zone. A sludge thickener (not shown) can be installed directly above the tank 7; after the addition of, say, polyelectrolyte, the discharge of sludge is to the storage tank 7 and liquor is passed to the anoxic zone 2.

Preferably, sludge thickening is carried out using a drumthickener, which can result in significantly reduced costs compared to tankering away gravity-thickened surplus activated sludge. To protect equipment, and to produce a more refined effluent, a fine screen with a mesh size down to 6 mm may be provided together with a washer-dewaterer and, if desired, a handling plant. The screen may be sited directly over the anoxic stage, so that sewage is discharged into the latter, and cleaned/bagged screenings may be discharged to the adjacent ground level. A settlement tank scraper may be supported on a bridge over the central settler.

The transfer of effluent from one stage into the next is typically carried out by submersible centrifugal pumps under timer control. Waste activated sludge can be drawn from the aeration zone directly to the thickener which is designed to operate for a chosen number of hours per day.

A dissolved oxygen (DO) control system for monitoring the aeration conditions can be provided to control duty/assist blowers, with a central standby unit. Direct wasting of the sludge from the aerobic zone allows accurate, consistent maintenance of sludge age.

Operation of the submersible pump in the aerated zone, pumping activated sludge to the thickener, ensures that the solids retention time in the tank is controlled. Because the sludge wasted is at the same concentration as that in the aeration tank, the sludge age depends purely on the volume of sludge wasted, determined in this case by either the run time of the submersible pump or a flowmeter on that line. This can be illustrated by examination of the formula for sludge age, i.e.

Sludge age = mass of sludhe under aeration mass of sludge wasted each day In a conventional plant: Sludge age = volume of aeration basin x mixed liquor suspended solids volume of sludge wasted each day x RAS suspended solids In an embodiment of the present invention: Sludge age = volume of aeration basin x mixed liquor suspended solids volume of sludge wasted each day x mixed liquor suspended solids = volume of aeration basin volume of sludge wasted Thus, the system is controlled absolutely by the volume wasted.

Claims

1. An activated sludge treatment plant in the form of an annulus comprising an anoxic zone, an aeration zone and a sludge storage zone, and a central settlement zone.

2. A plant according to claim 1, which comprises means for wasting sludge directly from the aeration zone.

3. A plant according to claim 1 or claim 2, which comprises a sludge thickener positioned above the sludge storage zone, and a pump for passing sludge to the thickener.

4. A plant according to claim 1, substantially as herein described with reference to the accompanying drawing.