Wessex Water expects washing costs to plummet

11 April 2008

Wessex Water can now use final effluent, as opposed to clean water, for screen washing operations at its Bridport Sewage Treatment Works. The development, which is hoped to greatly cut the cost of washing the site’s drum thickeners, is due to the installation of a Cross automatic backwashing filter.

A service engineer records the backwash count on the Cross Phoenix filter installed at Wessex Water Bridport Sewage Treatment Works

Drum thickeners demand a constant supply of clean water to maintain efficient operation by keeping the drain holes in the rotating drums free from the build-up of debris. The water is sprayed under pressure through nozzles, which themselves will become blocked if the water is not of a good, clean quality. By filtering out solids with a size in excess of 200 microns, the Cross filter is designed to ensure that final effluent can be safely used for this application without the risk of interruption to normal operations.

With a flow capacity of up to 18 litres/second, the compact Phoenix filter installed at Bridport is hoped to provide dramatic savings in terms of reduced clean water consumption. There are no additional running costs since the pump, which supplies the water to the thickeners, creates enough pressure in the system to backwash the filter. The Phoenix is designed to backwash without interrupting the forward flow of filtered water and using only minimal quantities of backwash water. An adjustable pressure differential switch, with a set time lapse back up, triggers backwashing.

Central to Cross filtration technology is a ‘zero-gravity’ stainless steel coil with raised nodes on the surface which define a precise filter rating. Selectable in seven ratings between 12 and 400 microns, the coil is designed to open slightly and evenly along its total length when the flow across it is reversed during backwashing, ensuring that the build-up of particles is completely washed away.

Cross claims it can tailor its filter systems to meet any size of duty by adding enough coils to meet the required flow capacity. In the case of Phoenix, six coils are fitted in a specially designed housing that creates a cyclonic flow path for the water passing through. This helps to keep suspended solids away from the surface of the coils and therefore extending the length of time before a backwash is required.