Energy saving water works

Severn Trent Water’s Market Drayton sewage treatment works (STW) serves a population of 18,400 and discharges into the river Tern, a breeding ground for Brown Trout.

The facility has been recently upgraded, with three existing oxidation ditches undergoing a maintenance scheme needed to sustain capacity at the works.

The improvement to the site was due to the fact Market Drayton STW was failing on its consents and eight action limit breaches for ammonia and biochemical oxygen demand (BOD) in 2010.

It was the tenth most vulnerable treatment works in Severn Trent’s West region and was very close to the limit with the power available to the site.

Contractor JN Bentley asked aeration specialist Suprafilt to develop a solution to save energy, improve treatments and the reliability of treatments, as well as improving the phosphorous removal.

The performance of the oxidation ditches and final settlement tanks was being affected by mixer and blower failures.

Diffuser blinding and blockage of Final Settlement Tank bell mouths exacerbated these problems.

Due to these issues, there was poor dissolved oxygen control in the ditches resulting in filamentous bacterial growth and associated solids in the final effluent leading to compliance risks.

Furthermore, local neighbours had complained due to the noise from the constant running of the generator.

Upon evaluation, it was clear that the existing mixers and aeration system were not designed to work together and were “fighting against each other,” Suprafilt says.

This resulted in poor treatment and power wastage.

Suprafilt took a holistic approach and selected appropriate mixers and aeration diffusers and produced a computational fluid dynamics analysis to model the process and ensure that both mixers and aeration were designed to work together.

The company designed, manufactured and fitted the new system working in close partnership with pumps manufacturer Wilo.

The Suprafilt 9” diameter membrane disc diffusers not only increased oxygen transfer efficiency but also had Teflon PTFE coating to prevent the blinding of diffusers.

Wilo mixers were selected to give high efficiency and reliability, but were also fitted with back curved blades to eliminate rag build-up.

The speed of the blades was also optimised to give the required tank velocity when working with the aeration system and not against it.

The new Flow Inducers from Wilo improved effluent quality and impact energy efficiency so much so that Market Drayton is set to be the new standard for oxidation ditches.

The 45% reduction in energy at Market Drayton represented an annual saving of £68, 975 per annum for Severn Trent, at a cost of 10 pence for every kilowatt of energy saved.

This equates to a return on the water company’s investment within 2.4 years.

“These are fantastic figures and a real vindication of the research and development we’ve been doing,” says Graeme Fielden, sales director.

Not only has the Suprafilt and Wilo pump system improved Market Drayton’s accounts, it has also helped create a more sustainable environment.

This initiative has also reduced energy levels and therefore the carbon footprint at the site.

The installation is also 100% recyclable and environmentally friendly materials were used. The feedback from Market Drayton was very positive and Severn Trent indicated it would like to use the system on all its oxidation ditches.

Severn Trent has also expressed an interest in rolling this technology out across similar sites.