Case study: when PC correctness came to a sewage treatment works’ rescue

Thames Water has 15 million customers, 350 sewage treatment works (STW) and treats more than 4.2 billion litres of sewage annually.

Its strategy is to adopt treatment processes that maximise the generation of renewable energy and minimise the volume of residual sludge produced, and the company is using thermal hydrolysis of sludge before anaerobic digestion to achieve these aims, often on existing STW.

Use of high temperature heat treatment sterilises the sludge and changes its rheological properties. This makes the sludge more biodegradable and enables the loading rates to the anaerobic digesters to be doubled, delivering a higher gas yield.

Thames Water STW at Reading aimed to reduce onward sludge transport costs by increasing the ds% of dewatered sludge from 22 to 25%

A further benefit of the process is that the final sludge from the digester is pathogen free and can be easily dewatered, resulting in a more valuable final product for application to land.

Thames Water STW at Reading aimed to reduce onward sludge transport costs by increasing the ds% of dewatered sludge from 22 to 25%. However, the piston pumps that transferred the sludge were identified as a limitation due to frequent maintenance requirements and the high pressures generated.

While piston pumps are able to operate over a wide pressure range and can move viscous products over long distances, they do incur high operating and maintenance costs and have a pulsating flow.

This is due to the action of filling and emptying a chamber of product that is then pushed through the pipe by reciprocating action, requiring check valves to regulate and direct flow through the system.

Pulsation creates a high operating pressure – the Reading, piston pumps generated pressures up to 64 bar and likely more if the solids ds% was higher.

Furthermore, they had hydraulic drives and drew absorbed power of 31 kW. These factors, along with the high maintenance requirements, added up to a high operational cost for Thames Water.

The new pump system has delivered a hat-trick of cost reductions for sludge disposal, lower energy use and maintenance

Progressive cavity pumps are also able to operate over a wide pressure range and can move viscous and abrasive products over long distances. However, they have a different principle of operation and employ a rotary action via a series of cavities that are filled and emptied continuously as they are ‘progressed’ through the pump.

This results in low pulsation that generates lower pressure in the pipework that can be further reduced by boundary layer injection due to smooth product flow.

PC pumps have lower operating and maintenance costs and in this case drew absorbed power of 18 kW.

Thames Water replaced the piston pumps, which transferred dewatered digested sludge from the energy generation plant to storage silos, with Seepex PC pumps. Seepex worked with approved contractors to deliver a turnkey system.

The new pumps transfer higher ds% sludge with a lower energy demand, bringing the additional benefit of a reduction in the system’s pressure rating, enabling de-rating of pipework and valves to suit, says Seepex.

The pumps have been in use for more than two years and required no maintenance interventions in this time. The new pump system has delivered a hat-trick of cost reductions for sludge disposal, lower energy use and maintenance.