Thermal Hydrolysis Plant New process provides
Yorkshire Water is moving away from reliance on incineration for the disposal of sewage sludge and towards its use for the production of "green" electrical energy.The first stage of this process is the building of a Veolia Biothelys thermal hydrolysis plant at its major sewage treatment works at Esholt, near Bradford.
The Biothelys thermal hydrolysis plant at Esholt is designed to treat a nominal throughput of 30,000 tonnes of dry solids per year of a mixture of primary and secondary sludges which are produced both at the Esholt works itself as well as sludge imported from other sites.
The plant comprises 3 pairs of Biothelys reactors, each with a volume of approximately 20m3, together with a hydrolysed sludge buffer tank of approximately 40m3 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 1650C 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.
The effect of thermal hydrolysis is to disintegrate the cellular structure of the sludge into an easily digestible feed for anaerobic digestion.This results in a considerable increase in biogas yield, as well as producing a product which is classed as an "Enhanced Treated Sludge" and is free from Salmonella and where 99.9999% of the pathogens have been destroyed. This biosolids material is approved under the ADAS "Safe Sludge Matrix" for use as a fertiliser or soil conditioner for all crops including cereals, vegetables and salads as well as for horticultural applications.
The biogas produced by the anaerobic digestion process can be used to fuel a combined heat and power (CHP) plant or cleaned and injected directly into the national gas grid, thus providing a source of "green energy".
A number of UK water utilities have recognised the benefits of the use of thermal hydrolysis prior to anaerobic digestion, and are in the process of adopting it either as a replacement for other treatment routes such as drying or incineration, or simply to take advantage of the energy benefits and security of sludge disposal which it provides.