Ashford STC Expansion, United Kingdom

Southern Water provides water supply and sewerage services in the counties of Kent, East and West Sussex, Hampshire and the Isle of Wight, and small parts of Wiltshire, Berkshire and Surrey.

The purpose of the works was to increase the capacity of treating indignous sludge (300m³/d) and imported sludge (400m³/d) from 2.5 TDS/yr to 10.5 TDS/yr by the year 2015.

The key factor governing project execution was the need to maintain treatment capacity of the existing sludge treatment centre throughout the construction / commissioning of the new. This required considerable integrated planning & coordination with Southern Water at site level.

An integrated joint venture between VWS and Tilbury Douglas (now Interserve) acted as the main contractor to Southern Water. The project provided considerable upgrade and extension of the 2.5 TDS/yr existing sludge treatment centre to a new target 10.5 TDS/yr. Many of the existing tanks and structures were retained and refurbished to accomodate their amended duty.

The principal elements of the work included all MEICA and civil works for:

• Indigenous and import liquid sludge import, screening, storage
• Liquid sludge thickening to 7% via belt press
• Import cake reception / blending to 7% using liquid storage
• Anaerobic sludge digestion in three primary digesters (one new steel tank and two refurbished existing RC tanks) complete with biogas recovery / storage / digester heating
• Digested sludge dewatering via centrifuges to 25% ds
• Sludge liquor treatment (filtrate & centrate) in an SBR c/w upstream balancing tank
• Power / energy recovery via a 615 kWe biogas CHP set
• Odour control (2-stage peat & calcified media)