Millbrook Wastewater Treatment and Recycling Centre, United Kingdom

In March 2004, five years after Southern Water's Millbrook Wastewater Treatment and Recycling Centre was completed, it featured as one of a number of key European plants visited by a delegation from the China Municipal Engineering Association. The party was interested in many of the plant's features, particularly the approaches used for treatment and in the storage and management of excess stormwater.

Work to upgrade the Millbrook site to provide a regional sludge treatment centre began in May 1997, and was completed in early 1999 at a project cost of around £20 million. The work involved providing enhanced facilities for sludge screening, thickening and de-watering, together with upgraded anaerobic digestion incorporating biogas collection and heat recovery systems.

The plant currently treats wastewater from 138,000 people in the Southampton area and sludge from an additional 112,000, which is imported from the region's smaller WWTPs. A total of 14,000t of sludge is converted into 10,000t of fertiliser per year, which is sold to a variety of outlets for beneficial land use.

WASTEWATER TREATMENT AND RECYCLING CENTRE

The Millbrook treatment works was first built in 1936 and subsequently extended in 1965 to provide the area with treatment complying with the Royal Commission standard which applied at that time. Increasing population led to a further extension in 1991.

The next phase of the plant's development arose in response to the EU Urban Wastewater Treatment Directive, which heralded the end of the historic practice of disposing of the sludge at sea, in a licensed area six miles south east of the Isle of Wight. Establishing the Wastewater Treatment and Recycling Centre to enable sludge to be treated centrally was the first of a number of measures to safeguard sea quality throughout the region - a programme of improvements that remains ongoing.

WASTEWATER / SLUDGE TREATMENT

Wastewater treatment consists of largely traditional elements. Debris, grease and grit are removed from the influent at the inlet works with six primary tanks subsequently removing around 65% of solids. Aeration lanes provide biological treatment before a final settlement phase prepares the treated water for discharge into the River Test estuary.

A pipeline running beneath the River Test connects the Millbrook plant to the Slowhill Copse WWTP. This pipeline conveys separated solids from the Slowhill Copse works itself, which serves much of the New Forest area, as well as sludge brought by barge to Slowhill from the plants at Woolston and Portswood. The sludge input is blended and then thickened before entering the anaerobic digestion (AD) phase.

Three sequencing batch reactor (SBR) tanks, each equipped with gas collection and recycling systems stabilise and reduce the organic matter present. The AD phase has an average detention period of 14 days, after which time the material is separated by centrifuge, the resultant solid cake being dried at around 90°C in a thermal drier. The biogas produced during digestion is reused onsite to heat the digesters themselves to their 35°C operational temperature and to fuel the drier.

The resulting dry granulated product is marketed for fertiliser use on land, under the name "Bestway", having been used with considerable success, principally on golf courses and in land reclamation projects. In addition, it helped produce bumper crops of wheat and sunflowers on Southern Water's own trial farm near Horsham.

The plant also employs state-of-the-art SCADA and odour control systems.

CLEANER WATERS

One of the major factors behind the European legislation, which drove the establishment of the Recycling Centre, was the desire to improve the cleanliness of coastal waters. As a result of this project and others in Southern Water's ongoing multi-million pound investment programme, every beach in Southern Water's region met European standards on bathing water quality in 2003 - the first time the 79 beaches had achieved a 100% pass rate.

The allied and ongoing programme of work is extensive. Plans have been announced for a new £200 million environmental improvement - the Brighton and Hove Area Wastewater Treatment Scheme - to deliver even cleaner seas to the Sussex coast. The project will provide state-of-the-art treatment for the 95 million litres of wastewater generated every day by the 250,000 residents of Brighton and Hove, Woodingdean, Ovingdean, Rottingdean, Saltdean, Telscombe Cliffs and Peacehaven. A similar programme is planned for Margate and Broadstairs at a cost of £80 million. In addition, work began in 2004 on a £14 million tunnel scheme to upgrade the wastewater treatment system in Lewes, East Sussex, and improve river water quality, while work has also started on an £8 million scheme to safeguard the quality of drinking water in Southampton and Eastleigh.

KEY PLAYERS

Southern Water is the plant owner / operator and was responsible for financing the project. The main contractor was Purac, who supplied their proprietary 'Puractor' SBR technology. The civils work was undertaken by Harbour & General, and Montgomery Watson provided the design.