ACWA Services’ Amtreat activated sludge process has been successfully installed at Southern Water’s Ashford wastewater treatment works.

Although Ashford WwTW predominantly treats domestic wastewater, the site also contains a sludge treatment centre, which treats sludge not only from the Ashford site but imported sludge tankered in from the surrounding area as well. 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.

Southern Water decided to expand its sludge treatment centre and install additional drying capacity, commissioning 4Delivery to undertake this work. 4Delivery, the consortium comprising United Utilities, Costain and MWH, is carrying out a programme of environmental improvement and water quality schemes for Southern Water until 2015.

The improvements are taking place across Kent, Sussex, Hampshire and the Isle of Wight, and include the £300 million scheme to bring Cleaner Seas to Sussex.

Optimum level
Black & Veatch (B&V) constructed and commissioned the project for 4Delivery.

B&V opted to install ACWA’s Amtreat system to ensure that the wastewater treatment works would not experience excess ammonia and nitrogen loads.

ACWA’s proposals manager, Gary Harris, says: “Amtreat provides a dedicated, cost-effective means of treating sludge liquors from sludge thickening, dewatering and drying processes. 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.”

Harris continues: “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 3,000mg/l can be achieved, with typical ammoniacal nitrogen removal rates in excess of 97%.”

The system at Ashford can treat a maximum flow of 2,100m3/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 that 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.

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

Detrimental effect
Harris says: “Amtreat is a purpose-designed, high-rate activated sludge process targeted at treating high strength ammonia wastewaters. 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.

“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.”

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