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Whiff of a good idea

Water Sewage treatment

A major reference point in Reading's olfactory landscape, Manor Farm sewage works will soon disappear.

Andrew Mylius took a deep breath and went to find out what's going on.

Locals call the lavatorial pong that hangs over south Reading the 'Whitley whiff'. Sewage treatment works were first built in suburban Whitley a century ago and the present Manor Farm plant was put into operation in the mid 1950s. As Reading's population increased Manor Farm was expanded. And the odour problem grew in proportion.

This is something of an embarrassment to local water and sewerage utility Thames Water, which is headquartered a stone's throw away in Reading city centre. But Manor Farm has other short-comings: the quality of water discharged from the plant falls short of standards demanded by the European Union's Urban Waste Water Treatment Directive. And Reading's ongoing sprawl is daily increasing the volume of raw sewage to be dealt with.

To meet discharge standards and remove the shameful stench, Thames is building a new £80M, 3.426 cumec treatment plant across the road from the Manor Farm works, which is slated for demolition in 2004. It will be, says Thames' engineering programme manager Terry Bane, a model facility.

For the occupants of executive housing estates and gleaming new business parks springing up all over Whitley, the most important detail of the new works is that the primary settlement tanks and dewatering process will be hermetically sealed from the outside world. Architect Broadway Malyan has designed sleek, modern buildings that will be fitted with air handling plant to scrub out nasty niffs. Sewage held in the plant's four colossal underground storm tanks - combined they have a holding capacity of 21,400m 3- will be chemically dosed and aerated to keep it 'sweet'.

Pleasing the eye and preventing offence to the nose, however, are least of the challenges addressed by joint venture engineers for the scheme Black & Veatch and Taylor Woodrow.

Work started in March 2001 with a major site remediation campaign.

Since around 1900, sludge had been dumped in lagoons there, and the site was later used to landfill domestic waste. Taylor Woodrow project director Graham Stanley reports that, with a chalk drinking water aquifer directly below, there were fears that ammonia from the sludge was leaching into surrounding ground. Meanwhile, nobody really knew how much domestic waste had been deposited, what it contained, or how hazardous it was.

A 1.6km long bentonite concrete cut off wall was installed to encircle the 10ha site and toed into an underlying clay stratum 12m-16m down. 'We created a bathtub to prevent pollution migrating, ' Stanley explains. To prevent the bathtub filling with rainwater, three small pumps have been installed. Ground water levels on site will be kept permanently lower than those in the surrounding soils to ensure that, if there is any water migration at all, it will be inward, not outward. Water will be put through the full sewage treatment process once the plant is complete.

All the domestic waste and around 180,000t of sludge was dug out and taken off to a new landfill tip. With little certainty about exactly how much material would have to be dealt with, Stanley describes this phase of operations as the riskiest the JV has undertaken. The contract is based on a target price with pain and gain sharing arrangements.

Taking large volumes of material off site meant hefty landfill tax penalties, eating into the project team's contingency fund and cutting margins. To minimise the hit, a small mountain of sludge was kept back for landscaping.

The works will be partially screened from view by large bunds, and planting trials are being carried out to discover what kind of vegetation will grow best in the nutrient-rich medium.

Sewage flows will be diverted from the existing Manor Farm works to the new plant via four 1m diameter steel lined concrete micro-tunnels that cross the A33 Reading relief road. And to get the treated flow to the existing outfall point on a tributary of the River Kennet, a further three tunnels, 1.2m in diameter, must be built back across the busy traffic artery.

Cut and cover tunnelling was unthinkable because of the road chaos it would have caused, says Bane. Conventional tunnelling was too costly, and the diameter too great to make directional tunnelling an option. The jury is still out on whether welded steel or ductile iron pipes will be used to line the micro-tunnels, says Black & Veatch project director John Aldridge. Detailed design and construction are overlapping and, while suppliers and subcontractors are being brought in early to minimise the need for design revisions as work advances, the project is subject to continual value engineering.

Once it has been through grit and rag screens the raw sewage will pass into a primary settlement tank. Lamella plates are being used to increase the available settlement area. The lamellas are about four times more efficient than a conventional settlement pond and take up only a quarter as much space. This is important, says Bane, as local real estate prices are soaring.

Thames is financing the new works by selling off its Manor Farm site to developers, and keeping land take to a minimum at the new site is critical to the project's cost-effectiveness.

Thames carried out extensive outline design before Black & Veatch and Taylor Woodrow were appointed. It had decided, for instance, to use a 'Bardenpho' process to remove nitrates and phosphates from the sewage. The effluent stream is put through aerobic and anoxic cycles in which bacteria respond to the contrasting oxygen-rich and oxygen-starved conditions by attacking nitrates and phosphates. The waste stream is so rich with useful bacteria by the end of the cycle it is possible to harvest liquors and recycle them back into the primary sewage stream to accelerate the treatment process.

The effluent stream is put through secondary settlement in eight circular tanks, and is screened for salts and fine suspended particles before being discharged. Sludge, meanwhile, is dewatered using belt presses, increasing the solid content from around 6% to 9%. It is then put into egg-shaped digesters where bacteria break down pathogens, and the copious amounts of methane produced are siphoned off to drive a combined heat and power plant.

Power from the CHP plant makes up about half of the energy needed to pasteurise the sludge, done by holding it at 70infinityC for an hour. As the sludge cools following treatment, the warmth it gives off is used to pre-heat sludge going into the pasteurisation phase. Sludge comes out of the pasteuriser about 25% solid, and is given a final drying out in a centrifuge, increasing the solid content to 50%, before being sold to farmers for fertiliser.

The sludge treatment process is state of the art, claims Bane, but in the main the civil engineering is straight-forward. All structures are founded on reinforced concrete raft foundations. A 2m deep stratum of river gravels provides good bearing capacity.

There are a couple of features of which the JV is especially proud, however. The first is invisible. By reviewing Thames Water's outline design of the plant, Black & Veatch managed to cut 800m from the total length of pipeline installed to shift sludge from the primary and secondary settlement tanks to the dewatering presses. To keep those 800m of pipeline blockagefree, 16 pigs would have been needed. Eliminating the lot saved around £800,000.

Second of the JV's major sources of pride are the four eggshaped sludge digesters. In the UK, digesters are normally cylindrical. Grit tends to settle out of the sludge and build up in the corners, forming deposits which can be up to 2m deep. This reduces the digesters' efficiency, digging is a costly, time-consuming and hugely unpleasant job.

With smooth, rounded bases, egg-shaped digesters circumvent this problem - grit exits with the rest of the sludge.

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