Twin–stream System Tackles Sludge in Southern England

ACWA Services' activated sludge process has been successfully installed at Southern Water's Ashford wastewater treatment centre to help treat sludge liquors from sludge thickening, dewatering and drying processes.

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ACWA Services' activated sludge process has been successfully installed at Southern Water's Ashford wastewater treatment centre to help treat sludge liquors from sludge thickening, dewatering and drying processes.

The Ashford wastewater treatment works is an important area facility treating predominantly domestic wastewater. The site also contains a sludge treatment centre, which treats sludge from the Ashford site as well as imported sludge tankered in from the surrounding area. The liquors from the sludge treatment process were originally released through an existing SBR (sequencing batch reactor) to the head of the wastewater treatment works for treatment.

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Southern Water decided to expand its sludge treatment centre and install additional drying capacity. Black & Veatch constructed and commissioned the project for 4Delivery, a consortium comprising United Utilities, Costain and MWH, who carried out the programme on behalf of Southern Water. It was decided that ACWA Services' AMTREAT® system was installed to ensure the wastewater treatment works would not experience excess ammonia and nitrogen loads.

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Gary Harris, ACWA Services proposals manager, said: "The twin–stream system is designed to operate at its optimum level when the influent liquor temperature is 25ºC and is effective between 20ºC and 35ºC, so it is ideal for the warm liquors that the sludge drying process generates."

He went onto say: "The compact, high–rate ammonia treatment system has a relatively small footprint compared to conventional activated sludge treatment processes, though being based on the same control systems it is easy to operate.

"Full nitrification of effluent streams with ammonia concentrations of up to 3000mg/l can be achieved, with typical ammoniacal nitrogen removal rates in excess of 97%." The system at Ashford is able to treat a maximum flow of 2100m3/day at typical removal rates in excess of 75% COD and BOD. Flows are divided equally between the twin streams, although each stream is hydraulically able to accept 100% of the flow if required.

The Ashford process comprises a balance tank with mixing system, a hot and cold liquor blending system, followed by an anoxic tank which provides denitrification upstream of the AMTREAT reactor. The AMTREAT reactor is designed as a fully–mixed vessel, where carefully–controlled conditions ensure the optimum amount of nitrifying bacteria are present in suspended aerobic cultures.

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A proportion of the mixed–liquor suspended solids (MLSS) and return activated sludge (RAS) are recycled to the anoxic tanks. Solids separation is achieved using conventional secondary clarifiers.

Harris added: "As legislation on allowable levels of ammonia and total nitrogen in wastewaters become ever more stringent, increasing amounts of sludge liquors are being diverted to main wastewater treatment works. In many cases this can have a detrimental effect on treated effluent quality."

The proposals manager added: "In such situations, installing AMTREAT can provide a cost–effective solution for reducing ammonia and total nitrogen to acceptable levels in sludge liquors which, when returned to the wastewater treatment works, will not impact on either the processes or the final effluent discharge consent."


WWi Author¹s note: ACWA Services is a process, mechanical, electrical, instrumentation, control and automation (MEICA) engineering company providing a design and build service for air, water and wastewater treatment systems.

A subsidiary of Consolidated Contractors International (UK), ACWA operates throughout the whole of the UK, the Middle East and other overseas countries.

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