Incineration vs thermal hydrolysis

Bigot, B.1 and Taylor, T.2, 1Veolia Water Solutions & Technologies, 2Yorkshire Water Services, UK



Yorkshire Water Services (YWS) is seeking to move away from the use of incineration for sludge disposal in favour of Advanced Anaerobic Digestion (AAD) which comprises pre-treatment of the sludge using thermal hydrolysis, followed by mesophilic anaerobic digestion.  The first plant in this programme is to be built at Esholt STW, Bradford, using the Biothelys thermal hydrolysis process as supplied by Veolia Water Solutions & Technologies (VWS).

This paper discusses the rationale behind YWS’s decision to move to AAD together with the procedures which culminated in the choice of thermal hydrolysis process for Esholt, and also describes the Biothelys process in detail.

Key words

Thermal hydrolysis, incineration, anaerobic digestion, Biothelys, Enhanced Treated Sludge, Bio-Energy Strategy

What is Thermal Hydrolysis?

Thermal hydrolysis (subsequently abbreviated in this document to TH) is a process which is used to treat sewage sludge, either raw primary or secondary (e.g. surplus activated), or a mixture of both by the application of high temperature and pressure (i.e. approximately 140-170 oC and 6-8 bar) for a period of 20-30 minutes.

Its effect is to disintegrate the cellular structure of the sludge and break down high molecular weight organic materials into smaller molecules. It also dissolves naturally occurring cell polymers (a form of protein) into an easily digestible feed for anaerobic digestion, resulting in a considerable increase in biogas yield.

The sludge produced by the TH process is also much less viscous than raw sludge, allowing the mass loading of sludge fed to an anaerobic digestion plant to be considerably increased (in some cases even doubling digester throughput).  After digestion, the resulting biosolids material is also easier to dewater, with up to 40% dry solids being possible.

Biosolids material treated by the TH process is classified by the ADAS “Safe Sludge Matrix” as an “Enhanced Treated Sludge” which is free from Salmonella and where 99.9999% of the pathogens have been destroyed (i.e. a 6 log reduction). The material is, therefore, approved for use as a fertiliser/soil conditioner for all crops including cereals, vegetables and salads as well as for horticultural applications.

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