New twist on wastewater treatment with picket fence filter

United Utilities' Warrington North Sewage Treatment Works has an innovative approach to handling primary sludge - a high level of solids is created by the use of picket fence thickeners in the primary desludging tanks

United Utilities' Warrington North Sewage Treatment Works, which serves a population equivalent of more than 310,000, has an innovative approach to handling primary sludge. The primary sludge from the sedimentation tanks is drawn off at between 4-6% total dry solids. This high level of solids is achieved by the incorporation of picket fence thickeners within the primary desludging tanks.

It is held in two storage tanks where imported sludge of a similar thickness is introduced from smaller treatment works in the area. From the holding tanks, the sludge needs to be pumped to two sludge screen strain presses to extract the high level of rag; this is prior to digestion.

As the site was continually let down by the failure of its existing screw impeller primary sludge transfer pumps, ITT Water & Wastewater suggested its Flygt N-Pump as a solution.

Daily unblocking
As the pumps are located in a 7m-deep pump chamber adjacent to the sludge-holding tanks, which is prone to flooding, IP 68 pumps were required to ensure continued trouble-free operation. With the original pumps requiring daily unblocking, it was imperative that the site owners were provided with a solution they could rely upon.

As the pumps have to deliver a flow rate of not greater than 25l/s against a variable head, dependent upon the sludge level within the sludge holding tanks and the sludge thickness, two Flygt NT3153 dry-well submersible pumps were selected.

ITT Water & Wastewater says its Flyps selection program sludge module was used for the selection of these pumps as it de-rates pump curves according to sludge thickness and applies the coefficients within TR185 for calculating losses through pipes and fittings. The installation was carried out by a contractor appointed by United Utilities and the pumps set to work in June 2007.

On the Warrington North site the United Utilities' operations team is saving about 32 man-hours a week in labour. As there are no digester disruptions, cost savings also come from the now undisrupted process.

Municipal sludge
Tony Price, market manager - sludge & wastewater for ITT Water & Wastewater, said: "The Flygt N-Pump is capable of handling municipal sludge of up to 8% total dry solids, and is therefore suitable for around 75% of treatment works pumping applications."

Chris Connor, process controller for United Utilities, said: "We are really impressed with the reduction in blockages and resulting increase in efficiency and decrease in costs which the Flygt N-Pump has brought to our Warrington North site. The ITT Water & Wastewater team have been invaluable in providing a solution to deal with our individual site needs including the heavy sludge and high rag content at the works."

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