The Influence on Digestion and Advanced Digestion on The Environmental Impacts of Incinerating Sewage Sludge – A Case Study from the UK

Traditionally, when incinerating sewage sludge, it was preferable to burn raw undigested sludge. Digestion processes extract some of the energy from sludge and subsequently reduce the sludge's heating value. However, when considering incineration as part of a wider strategy, the decision whether or not to include digestion becomes less clear. Since, virtually all of the requirements of incineration are directly related to throughput, i.e. quantity of flue gas produced; power required; consumption of chemicals; production of effluent from flue-gas abatement, it becomes evident that upstream processing capable of reducing sludge quantity may be beneficial. Furthermore, whilst more energy is generated from raw sludge than digested, when the energy extracted from digestion is also considered, it then becomes evident that more energy is recovered by combining digestion with incineration.Furthermore, advanced digestion processes exist which improve dewatering of the sludge to levels where the digested cake has significantly less water than the raw equivalent and this influences the energy content, by volume, of the sludge. United Utilities owns and operates a sludge incinerator known as the Mersey Valley Processing Centre (MVPC) in Widnes, which is being upgraded to burn 75,000 tones of digested dry solids of sludge per year. This paper presents details of studies conducted by United Utilities to determine if additional capacity could be gained within the incineration plant through installation of advanced digestion on one of the feeder sites. The studies also compared raw to digested and advanced digested sludge incineration and quantified the impacts of these three modes of operation. Additionally, the work demonstrated that up to 15,000 additional tones of sludge could be accounted for within the MVPC, due to a combination of reduced load to incineration (by converting more sludge to biogas upstream) and altered dewatering characteristics of the cake entering the facility.