A fully automatic Cross filter has been installed at the Thames Water Deephams STW in North London to enable treated final effluent to be used to clean recently upgraded works inlet screens.

A fully automatic Cross filter has been installed at the Thames Water Deephams sewage treatment Works in North London to enable treated final effluent to be used to clean recently upgraded inlet screens at the head of the works. Using final effluent for this task saves the considerable expense of using fresh town water, but it is essential to ensure that it doesn't contain any suspended solids that are large enough to block up the spray nozzles. At Deephams this is achieved by using a Cross Phoenix filter, rated at 400 microns.

Sewage treatment at Deephams has been upgraded to meet strict new Environment Agency consent targets.

Treatment entails screening and grit removal, primary sedimentation, aeration and final sedimentation at a designed daily capacity of up to 460,000 tonnes (cubic metres).

A proportion of the final effluent has always been used as the source of cooling water in the site's powerhouse, where electricity is generated from methane gas produced during the treatment processes.

The same water supply is now also used for inlet screen cleaning, passing through the Cross filter on its way from the powerhouse to the head of the works.

Despite its compact size, the Cross Phoenix filter is capable of filtering at a flow rate of up to 85,000 litres per hour, without interruption, even during automatic backwashing.

Filtering is performed by patented Cross 'zero gravity' coils, six of which are fitted in the Phoenix.

Backwashing is triggered by an adjustable pressure differential signal, supported by a time lapse back up.

Each coil is individually backwashed in a sequential programme, which reverses the flow, enabling the coil to open slightly along its entire length, ensuring that the debris build-up is completely flushed away.

Since only one coil is backwashed at a time, five coils always remain on-line, maintaining an uninterrupted flow through the system at all times.

The complete backwashing cycle takes less than a minute and uses a minimal amount of water.