Application of an innovative process for improving mesophilic anaerobic digestion of sewage sludge

Shana, A.1, Ouki, S.2, Asaadi, M.3 and Pearce, P.1, 1Thames Water, 2University of Surrey, 3AD Technologies Ltd, UK



Over the last 16 years, different sludge pre-treatment processes have been retrofitted to existing sewage sludge treatment plants. The drivers for these initiatives were to improve pathogen kill during sludge digestion process, enhanced biogas production and improved sludge dewaterability. Some of these pre-treatment technologies have doubled the sludge throughput without adversely impacting the biology of sludge digestion process.  Although they have allowed more efficient utilisation of treatment assets, the digestion process itself may not be as efficient as it should be. Whilst the digester sludge throughput is doubled, the biogas yield (expressed as m3 per dry tonne of sludge (TDS) fed) was not increased significantly or expected high volatile solid reduction was not achieved.

To investigate sludge pre-treatment process efficiencies, a novel sludge treatment configuration has been developed which consists of an intermediate step of thermal hydrolysis. The purpose of this paper is to report the potential benefits of this new configuration referred to as the intermediate thermal hydrolysis process (ITHP).

The results obtained so far have shown that the ITHP process achieved an overall volatile matter reduction (VSR) of 68% in comparison with 60% VSR reported from the THP process. The ITHP configuration enhanced the biogas production by 28 – 59% compared to the conventional mesophilic anaerobic digestion (MAD). The ITHP overall biogas yield achieved was in excess of 450 m3/tds fed, compared with 350  and 387 m3/tds biogas achieved from the conventional MAD and THP  processes respectively.

Key words:

Double digestion, Intermediate thermal hydrolysis process, ITHP, re-digestion


The cost of treatment and disposal of sludge represents almost half the total operating cost of sewage treatment works (STWs) (Appeles et al, 2008;Spinosa et al, 2011). This cost could be offset or even savings could be made by one or combination of several measures including optimising the sludge treatment process, investing in novel treatment technologies or changing existing process configuration. These steps could lead to more sustainable overall sludge treatment process.

Water companies in the UK utilise anaerobic sludge digestion assets in most of their sludge treatment centres (STC); most of which were built during 1940s through to 1990s. Since then, the quantity of sludge produced has increased due to population growth, improved treatment standards due to tightening of wastewater treatment consents as a result of ever increasing legislation. To deal with the increasing amount of sludge produced and to more effectively use assets available, an innovative way of sludge treatment is required.

In the conventional mesophilic anaerobic sludge digestion process, between 35 – 45 % of the initial biodegradable organic matter in the sludge feed is converted to biogas. The remaining stabilised organic matter in the treated sludge is mainly recycled to land or disposed of through a variety of other routes.  According to Water UK (2006), in the UK over 1 million tonnes dry solid of sludge is produced every year, of which about 62% is recycled to agricultural land. Almost 1.4 million tonnes (dry solids) of sludge was produced in 2008 in the UK (Water UK, 2010), of which 77% was recycled to agricultural land.  The sludge recycled to land often contains about 60% organic matter (Deryck H., 2005). This remaining quantity of energy rich sludge could be further subjected to anaerobic digestion process using innovative technologies with the view of obtaining additional biogas and reducing the sludge volume for tankering.

Throughout the 1970s and 1980s, numerous studies were conducted to determine the effects of thermal treatment on sludge digestion process. During this period reducing digester volume and sludge digestion retention time had become additional drivers for further improvement in sludge treatment and development of processes such as the thermal hydrolysis process (Panter K., 2008).

During the last 10 years, a number of sludge pre-treatment processes have been retrofitted to existing plant in order to improve pathogen kill, to secure continued use of land for sustainable sludge recycling and to increase biogas production for power generation.  Acid Phase Anaerobic Digestion, Thermal hydrolysis and sludge pasteurisation are examples of these processes.  The invention of thermal hydrolysis process (THP) has brought an advanced technology in the sludge digestion process as it has a superior performance amongst available sludge pre-treatment technologies (Shana A., 2001). It has increased sludge solid throughput and maximised asset efficiency. However, the use of new sludge pre-treatment processes, despite achieving reduced digester volume and sludge digestion retention time, has not resulted in increased digestion process efficiency in terms of doubling the gas production per tonne dry solid fed as well as enhanced volatile solid destruction. To address this area of shortcoming, a novel sludge treatment configuration has been proposed; the use of THP as an intermediate thermal hydrolysis process step (ITHP). ITHP is a post primary sludge digestion treatment technology that is installed after the conventional sludge digestion process. The overall process is a two stage sludge digestion treatment. Its function is to further break down the already biologically weakened sludge components which have resisted microbial breakdown in the first stage of digestion.

The purpose of this paper is, therefore to report the potential benefits of an intermediate thermal hydrolysis process (ITHP). In this paper experimental results carried out during the last two years, particularly in the past six months are presented.

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