Combined experiences of thermal hydrolysis and anaerobic digestion – Latest thinking on thermal hydrolysis of secondary sludge only for optimum dewatering and digestion

The basic principles of batch thermal hydrolysis process (THP) are well known as a pre-treatment to anaerobic digestion (AD) . Early work done in the 70s in the USA and the 80s in Japan demonstrated the optimum time and temperature for thermal hydrolysis being 340 F for 30minutes. These principles have been put into full practice in 20 plants around the world at plants ranging from 5 mgd to 150 mgd. In this case the mixed raw sludge is both thermally hydrolysed and sterilised to give a digester feed that is optimised for digestion, dewatering and pathogen control. 

This paper reviews the historical development of the process and the latest thinking on the application of THP from a number of sources in Europe. An independent laboratory investigation of the claims support the data for the full scale benefits claimed to date. In particular emphasis is given to the effect of THP on dewaterability. Belt Filter Press performance is cited up to 32% DS and up to 35% with centrifuges with THP digested sludge. In general terms adding THP as a pretreatment to existing AD increases dewaterability by 10-12% points over conventional digestion and dewatering. The main effect on digestibility and dewaterability is on the secondary fraction of mixed sludge. This has been demonstrated in tests and at full scale on plants operating only on secondary sludge. 

The need for energy efficiency suggests that THP of secondary sludge only is a good way of getting maximum benefits for minimum cost and energy demand for a mixed raw sludge. THP applied to WAS only yielded 8% points improvement in dewaterability plus 25% increase in biogas production in a laboratory scale simulation of a full scale plant with 50:50 mixed sludge. The authors propose that the main mechanism for THP is in changing the structure of water binding extra cellular polymer (ECP) that binds water mainly to secondary sludge and limits dewaterability and compressibility of the mixed sludge. In this case pathogen control is not ensured. The main aim is to improve the mass and energy balance of AD. A full scale operation is being implemented based on this approach. Furthermore the benefits can be obtained using continuous THP that would more cost effective to operate. This approach has been demonstrated in another laboratory scale simulation. It is expected that there will be a number of plants in the future using continuous THP of secondary sludge mixed with unhydrolysed primary sludge as a pre-treatment to AD. This is analogous to other disintegration technologies used as a pre-treatment to AD but with the added advantage being dewaterability. The advantage of this approach is that heat required for the process of THP is reused in admixture with ambient temperature primary sludge to give 'free' heat to the digester. This heat can be generated easily as part of cogeneration project and used for digester heating and/or prepasteurisation. A typical mass and energy balance is shown for this approach. Therefore the energy demand of the process is not parasitic as the digesters must be heated anyway. All other costs are low, suggesting that this is a very cost effective approach where increased dewaterability is required as an alternative to replacing dewatering equipment (particularly belt filter presses) and as a way of improving digestion.