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Active improvement Treatment beds at Severn Trents largest sewage works are being gradually replaced in an attempt to improve the quality of the effluent,

Sewage from the whole of Birmingham and much of the Black Country passes through the massive Minworth sewage treatment works where eventually, after treatment, it tips into the River Tame.

New standards set by the Environment Agency for the quality of the discharge must be met by the end of the century, and it is this need to improve the effluent which is driving upgrading works now being built. Biological oxygen demand, suspended solids and ammonia levels must all be reduced to meet the new discharge consent.

Secondary treatment at the Minworth plant is currently split between the huge bacteria beds which dominate the 300ha site and the three small activated sludge beds which are at the centre of the current construction.

The aim of the rebuilding is to replace all of the bacteria beds with activated sludge beds, which treat the effluent more efficiently, producing a higher quality discharge, and using less land.

But the process has to be done gradually because the facility which treats about 540M litre every day from a population of 1.3M must be kept in operation for the whole time. There are very short windows during which it is possible to divert the flow and carry out essential connections; effluent can be stored in the storm tanks but capacity is very limited.

A joint venture between Nuttall and Norwest Holst won the 30M contract for the main works which started on site last Easter. The scheme involves construction of three more activated sludge plants and removal of all of the bacteria beds a four phase process which has to be done step by step. The initial phase is almost complete with the first of three new ASPs almost ready to begin commissioning and expected to come into use later this year.

And the first of the six old bacteria beds, each of which is made up of 16 sections, is now being removed to make room for the second ASP.

When the first phase ASP comes into operation, the bacteria beds on the remaining ASP land will be broken out so that the last phase can start. Completion is scheduled for October 1999.

In the existing system, the bacteria live on the stones in the bed, over which the effluent is trickled and broken down by the bugs. There is a limit to the quality of final effluent that can be achieved by this method. But in the activated sludge plant, there are two treatment zones; first the anoxic zone in which the sludge is mixed with the return activated sludge, which contains the bacteria. It then passes into the aeration zone, where oxygen is bubbled through the liquid and the bugs break down the effluent before it goes on to final settlement tanks.

Methane from the sludge digestion is used to fuel a power station at the treatment works. The plant generates about 95% of the electricity that is needed at Minworth.

According to Norwest Holst/ Nuttall deputy project manager Stephen Fox, removal of the bacteria beds is producing a large amount of waste which the contractor is doing its best to recycle. A crusher is being used to reduce the stone from the beds, and the concrete of which they are made, to type 1 material for re-use either in the works or as landscaping.

But this is proving more difficult than expected, Fox says. The filter material coming out of the beds is largely river gravels, and is proving very difficult to crush with the impact crusher. Its very hard, says Fox, but Im not sure if the crusher will survive. Whatever the outcome, none of the material will go off site.

Each of the three ASPs consists of an aeration tank, a return activated sludge pumping station, and eight final settlement tanks. The reinforced concrete aeration tank is 185m long, 79m wide and 4m deep, and contains eight anoxic zones and four aeration lanes. Settlement tanks are over 31m in diameter, at the centre extending 12m below ground level. Construction of these elements is fairly straightforward, says Fox, but the pipework associated with them is extremely complex.

But once the land is released for each ASP, construction work can at least go ahead with minimal interruption. Other parts of the contract are more difficult because they have to be built around the existing treatment plant.

One of the more complicated elements to build is the distribution chamber, which splits the incoming flow into 12 lanes which feed the three existing ASPs. Under this contract, it will be extended to handle the increased flow, and 12 more lanes added with connections to the new ASPs.

This was one of the first parts of the contract that we started, says Fox, and we have been working on it all the time. But it is very fiddly, he says, and can only be done a bit at a time because all the input to the existing activated sludge plants passes through the chamber. In another six months we will actually finish this part, he says. The final bite will take out the small road that runs alongside, and cannot be built until a new road is in place.

Design of the new scheme is by Severn Trent Engineering and project manager Mark Edwards explains that the introduction of the new ASPs will allow more efficient use of land at the plant. The Norwest Holst/Nuttall contract is only one of a number of contracts which will result in a total upgrade of the plant, and in the future there will be much more automation.

At the moment, the aeration tank blowers which bubble the air through the sludge are switched on and off manually. Future plans mean that the operation will be governed by the oxygen level in the tanks with automatic sampling to control the switching on and off of the blowers as required.

Replacement of the existing blowers is also part of the main contract. The existing eight machines have to be removed and new models installed, and four more are to be added in a new blower house.

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