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Water special: Bradford beauty

Extensive physical modelling has been used to refine and verify the hydraulics of Yorkshire Water's biggest treatment plant

Esholt wastewater treatment plant, just north of Bradford, is one of the most spacious in the UK and, oddly, for a sewage works, is one of the most picturesque. Long banks of trickling filters are sited along the scenic valley of the river Aire, the location used for TV soap Emmerdale is close by and the Liverpool–Leeds canal also hugs the contours of one of the hillsides.

Much of the treatment plant will soon be gone from this rural scene. Yorkshire Water is taking advantage of required upgrading to reduce the plant's footprint substantially. The sprawling 17ha of clinker-filled beds for secondary effluent treatment will be replaced by a modern activated sludge unit, which covers a more compact 16,200m2. It will have a flow of 3.2m3/s.

The driving factor for the upgrade is European Union-prompted legislation on freshwater fish. This demands a reduction of maximum ammonia levels in effluent from 5mg/litre to 3mg/litre. Other work is being tackled as part of the £71M project, including expanding and reconfiguring the inlet works to handle uncontrolled intermittent discharge requirements, and the construction of new modern primary settlement tanks.

"The old tanks are nearly 100 years old and they still require manual emptying," says Arup senior engineer Adrian Marsden. The consultant is the water company's framework technical advisor for Yorkshire Water's AMP4 spending round.

Further new sludge settlement tanks and digester plant for treating the solids emerging from the activated sludge process are also included in the work, and a pumphouse for recirculation of the sludge. Altogether, it is the largest part of a total £300M programme for Yorkshire Water.

Activated sludge has been chosen, says Marsden, because it is a fairly well understood process. It works by circulating still relatively dilute secondary effluent in a mixture with bacteria, fungi and other organisms, which break down the carbonaceous material and oxidise some of the ammonia. Partial denitrification also occurs.

Construction is design and build, says Marsden but "within very tight specifications for the process", which is essentially defined by the client's engineer. In the format used at Esholt, sewage will move around a U-shaped channel, beginning with a small anoxic section where the organisms grow, and then passing into a main aerobic section that is supplied with oxygen through aeration nozzles in the floor. The density of these drops off in three phases.

"The U-shape was selected because three tanks side by side could later be converted at low cost into a single long channel with a step feed," explains Marsden. That would allow greater levels of processing at little additional capital cost. Four sets of three tanks are to be used.

Making the biodegradation process work properly involves mixing the incoming flow with a proportion of the final sludge, which is rich in microrganisms. At the inlet, therefore, there is a special "selector" tank where the treated sludge is mixed into the raw effluent as it passes along a sinusoidal path around vertical baffles. As it tips over a weir at the end, more recycled sludge is mixed in and the flow is split for distribution to the tanks.

This creates some fairly complex paths for the effluent which can create eddies and vortices which disturb the hydraulic flows. "If you don't get those right it can greatly reduce the efficiency of the whole process," says Marsden.

It was decided early on that key structures would need modelling, mostly by physical models as the flows are too complex for computer simulation. For this Arup brought in Leeds based specialist Hydrotec Consultants, to create a variety of models.

First problem was to remodel the inlet works where flows had suffered as a result of alterations in the 1980s. Fine screens installed at the time caused water backup in the main supply sewer tunnel, and so they were replaced with coarse screens, with new fine screens added downstream in an extension to the inlet.

Contract team Mott MacDonald/Bentley wanted to confine the new works to the original site rather than let the expansion sprawl across a road which runs though it. As a result, compact hydraulics were needed. In particular, modelling helped develop a 3m-long flume for storm flow control rather than the standardised 10m design. The flume diverts additional stormflows, which can reach 12m3/s.

Most of the work is in a second phase which is being carried out by contractor Morgan-Est with designer Grontmij.

The main problem here was flows in the U-lanes, beginning with the inlet channel. The apparently straightforward issue of feeding water over a weir is complicated by a rolling effect induced in the water flow by a right-angled input pipe to the entry chamber.

"The angle is forced by site constraints, but it creates vortexes and effects, which mean flow is greater on one side of the channel than the other," explains Hydrotec analyst Tim Thornton.

The system was modified using a baffle in the middle of the chamber to steady the flow.

For the flows in the 7m-deep processing chambers themselves, a computational fluid dynamics (CFD) model was been used says Thornton.

The CFD work has produced cost savings. Baffles in the base of the chambers to even out the flows were not needed and a single short wall at the bend suffices. Equally, special impellers in the sludge flow, the mixers designed to blow back fast parts of the current to make sure there are no stagnant corners, were found to be needed only in higher flow levels, which will mean operational savings on power consumption in the future.

More physical models were needed of scales from 1:6 to 1:10.

One was for the complex "selection" chamber, where the fine-tuning of the microbial levels in the mix is done by adjusting flows and inputs. In particular, a high-level plate that splits the return activated sludge, was redesigned to prevent backflow into the main tank.

"It is impossible to calculate many of these weir effects," says Thornton.

The pumphouse at the end of the process was also modelled. This pumps material from the settlement tanks, which remove the solids from the treated effluent. Eight of these are being converted from the tanks for the old trickle filters, and four new ones are being added.

The pumping station has six 800l/s pumps, which are frequently used. "But they have to be kept to a small footprint," explains Thornton, "and that meant a small pumphouse box which was producing potential air entrainment, with possible cavitation consequences which would damage the pumps."

Changing the pump seating heights, adding baffles, and cones beneath the inlets eventually took out the vortexes, he says.

Most of the flow goes back into the process while a small 90l/s portion is skimmed off into the final sludge treatment area where thickeners and digesters will treat it.

An ultrasonic treatment to break down the sludge and comb thickeners will improve the efficiency of the digestion plant which will use the methane produced for power supply, saving an estimated £400,000 a year from Yorkshire’s operating costs.

Unusually, the plant will also add in £81,000 of power from a hydroelectric source driven by the incoming screened raw sewage.

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