The influence of heat balance on the economics of advanced anaerobic digestion processes

Mills, N.1,2, Pearce, P.1, Farrow, J.1, Thorpe, R.1 and Kirkby, N.2, 1Thames Water Utilities Limited, 2University of Surrey, UK



The UK Water Industry currently generates approximately 800 GWh pa of electrical energy from sewage sludge, a renewable by-product from wastewater treatment. Advanced anaerobic digestion processes are beginning to improve energy yields from this resource which the industry has in abundance. These processes are being implemented across the UK and within Thames Water. However, these processes are still being driven by the operational and regulatory requirements for sludge disposal and significant opportunities for increased energy extraction are not being exploited.
A model has been created which compares current advanced sludge to energy processes, based around anaerobic digestion. Particular attention is made to the use of heat within these advanced processes such as the Thermal Hydrolysis Process. Factors that affect the heat balance and the subsequent economics are explored. CHP engine selection and the combined heat and power configuration are critical to optimising the operational economics.

Key words

Sewage sludge, Thermal Hydrolysis Process, Heat Balance, Combined Heat & Power, OpEx


This paper presents some preliminary results from the first of a four year collaborative research project between Thames Water and the Centre for Environmental Strategy at the University of Surrey; employing an engineering doctorate student, the primary author of this paper.

Anaerobic Digestion

Anaerobic Digestion (AD) is a very effective stabilisation technique for sewage sludge that has been used by the water industry for over 100years (McCabe 1957). Traditionally this complex biological process has been implemented primarily to provide a pathogen kill and odour reduction to allow sludge to be disposed to agricultural land. AD has the added benefit of producing sustainable energy in the form of a methane rich bio-gas which can be used in gas engines to generate electricity and heat to maintain the process, commonly referred to as combined heat and power (CHP). Currently 90% of the UK’s biogas is produced from the AD of sewage sludge (Andrews 2008).

Advanced AD processes began to appear in the UK 15 years ago (Riches 2010) these include Enzymatic Hydrolysis and Thermal Hydrolysis. These processes and some variants are now well established across Europe and offer similar benefits over basic AD, mainly:

• Improved volatile solids destruction
• Improved dewaterability of digested sludge
• Reduced disposal costs
• Higher loading rates in existing assets
• Increased energy yields
Thermal Hydrolysis Process

The research for this paper has concentrated on Thermal Hydrolysis Process (THP), one of the most effective advanced processes currently available to the industry that enables high volatile solids destruction and a grade-A sludge favoured by farmers. It is also very effective at treating surplus activated sludge (SAS): the root cause of many sludge treatment issues.

The additional biogas yield using THP is 35% (Merete) and the process offers the potential for the water industry to become a large renewable energy generator.

THP forms a large part of Thames Water’s sludge strategy and it is very important that its implementation and operation is optimised to maximise the performance to reduce costs and increase revenue. This research focuses on the energy and heat balance of THP and the parameters that influence performance. It is a relatively unexplored and unpublished area and the potential to make a contribution to knowledge is high. Although progress has been made there is much work still to be completed, it is planned that this paper will be followed by several more over the course of the four year research project.

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