WET News

10 WET NEWS SEPTEMBER 2014 Davyhulme makes advances with thermal hydrolysis The thermal hydrolysis plant reaction vessels • Anaerobic digestion has been around for 100 years and it seems fitting that Davyhulme, having just celebrated its centenary, has taken the process to the next stage. Maureen Gaines has been finding out how. U nited Utilities (UU) pro- duces "lots of sludge" – around 200,000 tonnes dry solids (tds) a year, in fact. "If that's taken in wet tonnes that is enough to fill Old Trafford ten times over," says Richard Lan- caster, UU's regional sludge manager. To help deal with this amount of sludge, UU has invested £100M at its Davy- hulme, Manchester site to create a thermal hydrolysis and diges- tion plant. The Sludge Recycling Centre (SRC) project, under- taken by Black & Veatch, has created the largest thermal hydrolysis plant on the planet "at this moment in time", says Lancaster. It enables Davyhulme to pro- vide farmers with a high quality soil conditioner as well as gen- erate renewable electricity from the biogas produced during the process. Agricultural land The company has a balance strategy of recycling and dis- posal. To deal with this amount of sludge, 21,000 hectares a year would be needed to recycle the sludge that it takes to agricul- ture. "That's a huge amount of agricultural land," says Lancaster. In addition to Davyhulme, UU has a further 18 digestion plants and in the past year, the company produced 122 gigawatt hours from sewage sludge across the region. "That's a lot of energy, and we're determined to outperform that figure signifi- cantly in this financial year," says Lancaster. "We're on track to do that." Generating renewable energy has also had a beneficial impact on customer bills to the tune of £11M a year. In comparison to the rest of the UK, UU has got less land for recycling to agriculture and the predominance of that smaller amount of land is grass. With grassland you have to surface spread, says Lancaster, and so a suitable product is needed to facilitate that. Also, there are more stringent regulations; nitrate vulnerable zones; timing restrictions where sludge can be stored on agricultural land but it cannot be applied, for instance. Elsewhere in its region, instead of using digestion UU had increased the PH of the sludge using lime. However, that process added more vol- ume, which meant there was more to dispose of. This process also did not make use of the energy that is inherently within sludge. "That was a waste," says Lancaster. United Utilities also recog- nised that energy prices were rising, and that it uses a lot of power. Plus with more stringent regulations coming on the wastewater side, the company TEchkNoW • The plant capacity has increased from 39,000tds/a to 121,000tds/a • Four thermal hydrolysis streams feed the eight digesters • The maximum energy production of the new plant is 11.5MW • The process enables a 6log reduction in pathogens for land application • There are less dry solids and less volume of wet sludge for disposal ThE vErdicT • "This was a strategy, a vision. It's now been implemented, it's been commissioned, it's optimised, it's part of our day-to-day business now. It's given us some flexibility. It's the absolute balance in our strategy, and it's also stopped us from having to build a huge raw incineration facility to hit that strategy in the centre of our region" Richard Lancaster, United Utilities ONSITE ADVANCED DIGESTION projEcT SpEcS • Create a Sludge Recycling Centre at Davyhulme • Implement a thermal hydrolysis plant at the site • Make use of the energy within sludge • Produce a high quality fertiliser for agricultural use NEEd To kNoW • The Davyhulme thermal hydrolysis plant is currently the largest of its type in the world • The project cost £100M, less than the £170M- plus if UU had opted for a new incineration plant • United Utilities has produced 122 GigaWatt hours from sewage sludge in the past year

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