Water & Wastewater Treatment Magazine
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20 | NOVEMBER 2017 | WWT | www.wwtonline.co.uk Transformational change for sludge JON BRIGG MANAGER OF INNOVATION YORKSHIRE WATER "When I started in this role in 2007, we were incinerating about 50% of our sludge and putting about 50% of it into anaerobic digestion (AD). AD is good, solid technology, and we see it as a mainstay of our sludge process: all of our incinerators are now shut down, and in two years' time we will be 100% AD. "However, our R&D programme is all about transformational change: doing different things, as opposed to incremental improvements. It's the Henry Ford thing – if you ask people what they want, they want a faster horse. You can give them a faster horse, that's incremental, or you can be transformational and produce the model T, which is what we are trying to do. "Yorkshire Water uses about 600GwH of energy a year, costing almost £60M, and if we put all our sludge through AD, even with thermal hydrolysis up front, we would only ever be 26% self-sufficient in energy. The cost of electricity is only going to go up, so there's a compelling, burning platform to maximise the energy recovery from what is a resource in our business. "In a hierarchy of technologies that are emerging for energy recovery from sludge, pyrolysis is the most efficient, and gasification is a close second. At the end of 2008 we partnered with a company called Intervate, and we tried pyrolysis, at significant pilot scale at our Esholt site at Bradford. We operated that plant for nearly a year, but we couldn't make it work – it's an incredibly difficult process, and getting the chemical output right required us to move away from our core skills as a business. So we hit the stop button. "We then moved to the next in the hierarchy, gasification, and put in a pilot plant. We got some very positive results in terms of gas quality, but also some negatives, in that if the temperature around the grate wasn't controlled to a very fine degree, then the ash would melt and it would clog up very quickly. Given the probable downtime from having to clean the gasifier out frequently, we were on the verge of giving up on this as well, when in 2013 we were made aware of Enertecgreen. "It's a family business in Latvia started by Eriks Samulis, who has the IP on a gasification system which he designed, built and operated himself. We visited it in Riga and it had operated for 30,000 hours - the efficiency levels were beyond anything that we'd seen before globally in any system, and it could be operated remotely from a mobile phone. "At that point we started building the business case for a significant investment of around £8M. We think it was the biggest investment that anybody's made in the R&D arena from the UK water sector, so it attracted quite a lot of challenge and governance process from the business. We brought in significant external expertise to design a commercial demonstration plant which had the Enertecgreen gasifier at its heart. "Over the last three years, there have been some niggles which we have ironed out, but the exciting thing is the consistent high quality of gas coming out the gasifier. From the moment it was switched on it has consistently produced a flat line graph of gas quality, which is incredible." • BIG PICTURE The Works: sludge and energy • TECHNOLOGIES Yorkshire Water approached its R&D work using a hierarchy of sludge treatment technologies produced by Cranfield and Sheffield universities: 1 Pyrolysis: Thermal degra- dation of the sludge in the absence of oxygen. This process is the most efficient in terms of producing energy, but its operation is the most challenging to achieve. 2 Gasification: A similar process but with a controlled, reduced oxygen flow and a slightly higher temperature. This trade-off results in a cleaner gas being produced. 3 Advanced Anaerobic Diges- tion: Anaerobic digestion of sludge that has been pre-treated with thermal hydrolysis. This produces a better yield of biomethane which is used to create energy. 4 Standard anaerobic diges- tion: The most common sludge treatment used in the industry, AD works similarly to a cow's stomach, producing meth- ane and solid digestate that can be recycled to agricultural land. 5 Incineration: Burning sludge produces some energy, but is the least efficient as well as the least sustainable method of sludge disposal, and is therefore being phased out. 6 Landfill: With sludge now widely seen as a resource, dis- posal to landfill is an option to be avoided. There are two CHP engines at the ATC plant