Sandown - an investment in the environment

High capacity, compact design

The new wastewater treatment works were constructed on a disused solids waste disposal site. Constructing the plant on a brown field site meant it was necessary to design the works in a very compact form. For its capacity Sandown Wastewater Treatment Works is one of the smallest plants within the South West of England.

The WWTW provides preliminary and primary treatment prior to discharge to sea via a new 3km outfall. The process begins with preliminary treatment through drum screens that provide screening to 6mm in two directions. The screenings are washed and compacted prior to discharge to skip. Screened wastewater is treated by detritors for grit removal, the grit is then washed and transferred to skip.

The optimum flow for preliminary treatment is 3145 litres per second. After storm flow splitting, the optimum flow for primary and secondary treatment is 1240 l/s. Six storm storage tanks are provided to retain high flows for subsequent treatment. The first of these operates as a blind tank, designed to take the first flush of each storm event. Sludge and cess liquors from smaller communities around the island are imported into the inlet works via air locks. The imported sludge and imported cess receives screening to 6mm or less.

Chemically assisted primary settlement is achieved by lamella plant settlement tanks, which provide a small footprint solution. The lamella plant provides sludge thickening to between 3% and 5% dry solids (w/w). The plant was designed for saline influent, and this impacted on material selection and primary settlement process design. The solids removed from the flow are combined with the imported sludge and passed to the SRC.

Secondary treatment plant required

Two years into the contract, Government re-designation of high natural dispersion areas meant full secondary treatment was required to provide secondary fine screening and biological activated flooded filtration (BAFF), prior to pumping to the new, long sea outfall. The secondary treatment plant was also designed to comply with the UWWTD.

The BAFF plant offers high biological and solids removal capabilities combined with negligible odour emissions (closely following the philosophy adopted on the primary treatment works) – all within a small footprint plant.

Return liquors from the WWTW, SRC and dryer are collected and returned upstream of the primary settlement tanks. The control system ensures flows are balanced prior to their return for treatment.

Washwater from the primary works is used for general process washwater requirements and washdown for the site. In addition, the site is provided with a clean water ringmain.

Efficient recycling

The SRC receives indigenous sludge, grease from primary treatment, imported sludge and surplus activated sludge. The blended sludge is thickened to 7-8% dry solids (w/w) by belt thickeners. Thickened sludge is digested in three mesophillic, anaerobic primary digesters, and then gravitates to two secondary digesters.

The digested sludge is dewatered (using centrifuges from the secondary digesters) and then dried to produce granular organic soil conditioner predominantly used by the Isle of Wight farming community. The drying plant provides a suitable sludge disposal route that reflects the introduction of the latest legislation. The dryer has a capacity of two tonnes of water evaporation per hour, and this achieves 92% dry solids by a direct heating system.

Biogas from the digesters is collected and stored in a common gas holder for use as a fuel to provide heat to the digestion plant and the dryer. Any excess biogas is treated via a flare stack. In addition, waste heat from the dryer is recycled to add to the heating process.

Odour control

All process units, including the inlet works and primary treatment plant, are fully enclosed and foul air is extracted into an odour control system. This system reduces the potential odours from the works to near undetectable levels at the site boundary, via a two stage wet chemical scrubber package - an acidic scrubber and then an alkaline scrubber. In addition, the odorous air is passed through a catalyst reactor vessel and finally through a polishing carbon filter. The inlet works and sludge buildings are ventilated and the air is treated via dry physi-sorption, carbon filter units.

Reducing pollution

The works has substantially reduced pollution - solids and bacteria - entering the Solent and the English Channel. Solids entering the sea were reduced by up to 50% by the primary treatment works, and with the addition of secondary treatment, solids reduction has now been increased to 85%. Delivering the flows to the sea 3km out from the shoreline has also provided better protection for the Island’s beaches.