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Severn Trent AD Upgrade ‘Presented Challenges’

Upgrading 12 anaerobic digesters at Severn Trent’s largest sewage treatment works (STW) presented a number of challenges but also allowed the company to realise several benefits.

Water & Wastewater Treatment revealed the extent of the work carried out by the company when it converted the conventional digesters it had been using into ones utilising a new thermal hydrolysis process.

The upgrade cost £60 million and the transition phase between the two systems was described as “difficult” by the news provider. One of the keys was specialist pump selection, it added.

The work at Severn Trent’s Minworth STW might have been challenging at times, but is already providing the company with a number of benefits. It has improved the efficiency of the AD process, and the sludge cake it produces is now classified as enhanced product fertiliser, which means it’s possible to use it for the production of crops for human consumption.

Once the new system was installed, the biggest challenge came from feeding the legacy sludge through the new thermal hydrolysis process that was installed upstream of the existing digesters.

To achieve this, the legacy sludge in the 12 digesters had to be fed back to the hydrolysis plant, so that it could be dewatered and diluted before being hydrolysed. This put a lot of pressure on the pumps being used, but now that Severn Trent has cleared all the legacy sludge from its digesters it has seen their efficiency improve further.

The company aims to generate 50 per cent of all the energy it uses across its business from renewable sources by 2020. Before this AD upgrade, it had achieved 34 per cent of energy from renewables.

AD is increasingly being used to help decarbonise a number of sectors. Farming is one area that can benefit from new AD technology, such as a hybrid solution launched recently that uses AD to produce biogas and solar panels to generate electricity.

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