Articles & Case Studies

MOTNEY HILL TURNS FULL CIRCLE

Posted: Thursday 16th April 2009

Motney Hill STW

Southern Water’s Motney Hill Wastewater Treatment works treats 32.5 million litres of wastewater every day and serves a population of 240,000.

Original arrangement

The area serving the work in Kent is very flat, with the sewerage system prone to silting up with large amounts of debris. Heavy rain, especially after a dry spell, flushes the build-up of material to Motney Hill where the screens filtering the wastewater would struggle to cope and become blocked.

The debris would also find its way into downstream processes, blocking tanks, pipes and pumps.

System installed

A £4.2 million scheme to improve the screens and install new ones that are remotely activated at the first sign of a storm has transformed the site.

The screens now feed into 7 SPIRAC shaftless screw conveyors. SPIRAC’s shaftless conveyors were selected because they do not require intermediate or end bearings, offer low rpm’s and low power usage and were thus a space-saving and extremely reliable conveying option for Motney Hill.

Maintenance has been significantly reduced at the site because the elimination of a central shaft allows a much higher fill rate resulting in lower rpm’s; more efficient conveying and consequently less wear and power usage. In addition, eliminating intermediate and end bearings also reduces maintenance work and SPIRAC’s direct-drive design allows a clean and efficient transmission without the maintenance required with belt and chain drives.

By using shaftless conveyors Southern Water is saving both power and money as they do not require any wash water. The SPIRAC conveyors are also totally enclosed so there is no spillage of the material being conveyed and odours are completely contained.

Megawashers

The screenings are then conveyed to one of 4 SPIRAC Megawashers, each with a 10m3/hr capacity. The Megawasher washes out organic matter from sewage screenings by means of two slowly rotating spirals, whilst mechanically compacting and dewatering the remaining inorganic portion, reducing the volume and weight of the screenings by up to 70%. It then compacts them to a high dryness screenings plug whose weight allows it to fall directly into the skip and not be prone to any wind blown effects. Moreover, the Megawasher is not sensitive to varying qualities of screenings and grit or fat contents.

The Megawasher operates automatically according to screenings level, thus minimising running time and wear but ensuring that large screenings volumes can be accommodated in storm conditions. It was thus an ideal choice of equipment for a site with a long history of large storm screenings events handling shock loadings from two treatment streams.

Southern Water Section leader Lee Denham said: “We have already seen a dramatic improvement at the works. Previously, our average monthly reactive call-outs to deal with the screens took up to 25 hours each month, but this has dropped to about 5 hours with the new plant. We are also improving the quality of the water we treat because debris is not making its way through to other treatment stages and causing problems further along the process.

The scheme is a good example of how Southern Water is investing for the benefit of the environment and our customers.”

The project formed part of Southern Water’s £2 billion capital investment programme between 2005 and 2010.




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