Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | december 2014 | 19 The esholt THP plant cooks sludge to 165 degrees celsius. © richard bird Photography T he completion last month of York- shire Water's Esholt Wastewater Treatment Works, following a £34M project, represents a major step forward for the utility and the way it deals with the sludge produced in the wastewater treatment process. The upgraded works, near Bradford, will use a Thermal Hydrolysis Process (THP) to generate renewable energy from the 30,000 tonnes of sludge that comes through the site each year. It will produce as much energy from biogas as the 750-acre site consumes in heat and power, effectively meaning that the site is self-sufficient. Built by a joint venture between Morgan Sindall and Grontmij, Esholt uses Veolia's BioThelys technology. The genesis of the project dates back four years, when Yorkshire Water started looking for a greener and more ef- ficient alternative to the incineration of sludge, which had been used at Esholt since 1977. "Previously we were heavily reliant on incineration for treatment of sludge at Esholt," explains Andrew Calvert, Yorkshire Water's Investment and Im- provement Manager. "Incineration has many disadvantages. It is expensive to operate and the plant was becoming less reliable as it aged, increasing costs. Incineration also didn't provide the op- portunity to generate renewable energy and to recycle nutrients to agriculture; it was less sustainable as well as more expensive." Wastewater treatment Thermal hydrolysis provides Esholt with sludge solution Consultants Arup were engaged to investigate the options for upgrading the plant, with a focus on technologies that could break down the sludge to prepare it for an anaerobic digestion (AD) process. Technologies which use radiation fired at the sludge, or high- pressure ruption to crush the cells in the biosolids, were both considered. However, ultimately Yorkshire Water opted for THP, under which the sludge is 'cooked' at 165ºC for 30 minutes with pressure release aœerwards. This combination bursts the cell membranes and leaves the sludge with a soup-like consistency from which it is easier to extract biogas. One of the key reasons for the choice was that 'enhanced treated sludge' which has been through THP is high enough quality to be later sold as fertiliser to farmers, maximising the sustainability benefits. "Thermal Hydrolysis produces an excellent quality product for recycling and optimises the amount of energy we can generate from the sludge," contin- ues Calvert. "It also replaced an ageing asset, allowing us to process the sludge production with more reliability." Morgan Sindall Grontmij, who were awarded the main delivery contract for the project in August 2011, were already familiar with Esholt having worked there extensively during AMP4 in improvements linked to the Freshwater Fish Directive (FFD). The firm oversaw the appointment of Veolia as preferred technology partner. Project focus ● £34m investment in a Thermal Hydrolysis Process (THP) to treat sludge ● biogas produced is enough for the plant to be self-powered ● Treated sludge is high enough quality to be sold as fertiliser James Brockett edITOr, WATer & WASTeWATer TreATmeNT • Innovations the system consists of three pairs of Biothelys reactors, each with a volume of approximately 20 cubic metres, together with a hydrolysed sludge buffer tank of approximately 40 cubic metres in capacity. raw sludge, at 16% dry solids, is pumped directly into one of the pairs of reactors which operate in parallel. It is heated, with live steam, to the required temperature of 165 c and held for a "lock-in" period of 30 minutes. Following completion of the reaction, thermal energy from the flash steam is recovered from one reactor to its paired unit, and the treated sludge discharged to the hydrolysed sludge buffer tank. The process is then repeated for the paired reactor. Hydrolysed sludge is then continuously pumped from the Hydrolysed Sludge Buffer Tank to the anaerobic digestion plant via a heat exchanger, and the recovered heat re- used in the process.

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