Acid phase digestion at Derby STW
Wild, R.1 and Bjorn, A.2, 1Severn Trent Water Ltd, 2Atkins Ltd, UK
(free)
Renewable energy credits provide a significant income stream for the water companies in the UK. Maximising this income stream is increasingly important with the drive for increased efficiencies through innovation in AMP5, and the challenging wider economic landscape. Severn Trent Water processes around 150,000 tonnes dry solids sewage sludge per annum, the majority of which is anaerobically digested, with currently over 30MW of installed CHP capacity to harness the energy in the resultant biogas.
Acid phase digestion (APD) separates the hydrolysis and acidogenesis stages from the methanogenesis stage of digestion. This allows for the provision of optimal conditions for the two different cultures of biomass, optimising the production of biogas.
With this economic driver in mind, and following research into existing systems on the market, an APD plant of Severn Trent Water’s own design was constructed upstream of the existing Mesophilic Anaerobic Digesters at Derby STW. This is a 490,000 population equivalent works, processing between 500-700 m3/d of thickened sludge, including imports.
The plant was designed with the flexibility to operate under varying configuration and hydraulic retention times. Since commissioning in February 2009, a programme of optimisation has been progressing. This paper builds upon the one delivered at the 12th European Biosolids Conference by relaying the operational experience from the first stage of the optimisation programme, including a comparison of recent data with benchmark pre-APD performance.
