SAS only thermal hydrolysis with series digestion – project results confirm lab predictions
Shana, A.1, Fountain, P.1, Christie, I.1, Panter, K.2, 1Thames Water, UK, 2Cambi, UK
(free)
Abstract:
The amount of energy generated in conventional anaerobic digestion (AD) depends on the primary and WAS ratio of the digester feed. Higher proportion of WAS produces low energy and it also produces wetter cake that increases the cost of sludge transportation to land. Thermal Hydrolysis Process (THP) pre-treatment of WAS only and blending with Primary sludge and digestion in series was predicted to increase the energy generated regardless of primary to WAS ratio in the blend feed and could reduce the sludge to land transport cost. Digestion in series avoids sludge short circuiting and maintains the HRT that in turn maintains enhanced gas production and coliform die out.
Lab work carried out by Thames Water’s R&D department in 2013 using a bench scale THP and laboratory chemostats confirmed the predicted process improvements. This in turn led to the development of the Long Reach project. Thames water selected AECOM as main contractor with Cambi as its specialist sub-contractor to build an upgrade to its sludge digestion plant in at Long Reach STW , in NW Kent near London. The plant was completed in April 2015 and handed over to Thames Water. The plant has sewage treatment design capacity for 840,000 people equivalent and digests its sludge for use in agriculture as a fertiliser.
The aim of the project was to make the site more self-sufficient in energy, and to produce a lower volume of final biosolids that can meet a higher treated standard and that has superior storage qualities without building further digesters. The volume of digester feed was reduced to about 70% of previous volume therefore extending the digestion time; the effect of Cambi’s THP has made the sludge much more digestible producing more biogas; the dewaterability of the agricultural product has increased to 35% DS reducing final quantities by about 1/3; the THP system operates on waste heat from two 1MW Caterpillar engine generators supply the heat for the THP process that in turn will be mainly responsible for the digester heating (as the heat from THP sludge in admixture wit the ambient temperature primary sludge achieves near digestion temperature)..
The THP system is a second generation Cambi B6 x 3 continuous batch system that is extremely compact and that was assembled on site in a matter of days. The system is designed to operate on steady steam consumption from the engine waste heat boilers
The plant has been operated by Thames Water now for 2 years and is achieving 62% VSR and dewatering up to 35% DS using Bucher twist presses.
