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Special treatment

The largest sequencing batch reactor plant in Europe has been built as part of the £200M Humbercare sewage treatment scheme. Richard Bennett reports from Hull.

Peas are big business for farmers in East Yorkshire. In summer the Birds Eye factory in Hull works night and day freezing peas as soon they are picked, to get that 'fresh from the pod' taste. But before freezing they have to be washed - and this uses a lot of water.

So when the engineers came to plan Hull's new £54M treatment works at Hedon Road on the outskirts of Hull, they were not simply designing for a domestic population of 340,000.

They also had to account for a doubling water demand over a few weeks in summer, the peawashing season.

Kvaerner is building the new plant as part of Yorkshire Water's £200M Humbercare scheme, to bring Hull into line with the European Union wastewater directive that comes into force at the end of the year. The directive is also driving the replacement of Hull's two existing sewage outfalls and pumping stations, with a new 8.7km, 3.2m diameter tunnelled sewer - to feed the new treatment works with domestic and trade wastewater from the east and west Hull catchments - and new outfall into the Humber estuary.

Average full treatment flows of 2m 3/s will enter the works through a vast underground inlet pumping station, where it is lifted out through screens for grit and grease removal. The flow then passes back into an intermediate pumping station and into solids separation using lamella tanks - which offer solid separation in a small footprint - before entering the sequencing batch reactors for secondary treatment. Sludge from the lamella tanks and SBRs are combined, anaerobically digested and dried to create 30t/day of soil conditioning pellets. The plant will have a five year storm capacity of 22m 3/s of screened flow.

In designing the plant engineers had to contend with very difficult ground conditions. A water table at 2m, along with soft glacial clays and alluvial sands and gravels, required all the tanks to be above ground with most of the structures on site founded on 3,500 18m piles. 'In some areas the ground was like soft porridge, ' says Kvaerner senior project manager Colin Nicol.

Ground conditions were also a particular problem during construction of the inlet pumping station, which takes the form of a vast underground chamber, 26m deep and 24m in diameter.

The chamber was sunk down to 26m within a 31m deep, 1m thick diaphragm wall. During the final stages of excavation heave of the clay floor began to occur, and caused some headaches for the team.

'We had a few tense moments and were levelling four times a day, ' says Nicol.

Overall the floor came up about 70mm in four days, and there were fears that this heave would crack the concrete slab floor before it had time to gain strength.

However a solution was found by casting the floor slab over a layer of compressible polystyrene sheeting laid onto the ground that absorbed any upward ground movement while the concrete went off. Once the slab had achieved full strength it was tied down with 70 Macalloy tension piles grouted into the chalk below.

Another civils problem facing the team was to provide secondary treatment capacity within a relatively small footprint. Eight SBRs have been used to aerate, settle and decant the water all in one tank - which in this case saved having to find room for a further eight settlement tanks. Each 40m diameter, 11m high SBR tank is founded on 430 driven cast in-situ piles.

The tanks were constructed with a post tensioning system designed to prevent thermal cracking but also allowed a relatively thin wall thickness of 300mm. 'Tanks this large haven't been built this way before, ' says Nicol. 'Yorkshire Water had to satisfy itself that it would work.'

The main civils work has now been completed and the plant is being fitted out and commissioned. Last year's collapse of the feeder tunnel and the ongoing recovery will delay flows from the western catchment of Hull, but will have no effect on flows from the eastern catchment due to start flowing into the works at the end of the year.

The first flows through the plant will allow Yorkshire Water to test the market for its soil conditioning by-product, which it hopes to sell at low cost to local growers. It should make the pea farmers happy.

On the spot

Name: Colin C Nicol

Age: 47

Qualifications: BSc Hons Civil Engineering

Company: Kvaerner Construction

Current position: Senior project manager

Best thing about the job: The diversity of the challenges and the quality of the people I work with.

. . . and the worst: Time is always at a premium and family life can sometimes suffer.

Most useful lesson learnt as an engineer: It's people who make the difference.

Anything else: I am a keen hillwalker and golfer - although I probably walk further on the golf course.

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