Proceedings

Factors Influencing Energy Generation from Municipal Sludge Digestion

Barber, W.P.F., United Utilities, UK

(free)

Abstract

Renewable energy and carbon reduction targets are generating a great deal of interest in the production of a methane-enriched biogas from anaerobic digestion. Currently, the Water Industry generates the majority of this biogas using infrastructure which was not designed for either, energy generation or carbon footprint reduction. This paper highlights the results of a model which predicts potential renewable energy generation from sewage sludge based on a number of variables. Sludge type was f the most influential parameter of those tested with primary sludge generating over twice the energy of secondary sludge. As type and quantity of sludge produced is fundamentally influenced by wastewater treatment it is suggested that they be looked at in combination in order to maximise synergism.  Pre-treatment technology and highly efficient CHP engines were next most beneficial.  Additionally, biogas diversion from CHP (to grid for example) should be limited to approximately 20% to avoid requirement of auxiliary fuel.

Keywords 

Advanced anaerobic digestion; municipal sludge digestion; CHP; gas-to-grid; primary sludge; renewable energy; secondary sludge.

Introduction 

Anaerobic digestion, an intricate compilation of series and parallel biological reactions degrading organic material into a methane-rich biogas in the absence of oxygen, is nearly as old as the earth itself. Organisms responsible for anaerobic digestion have been carbon dated back to 3.8 billion years (Hahn and Haug, 1986) which is approximately 2 billion years before oxygen was in the earth’s atmosphere. In spite of their age, the complexity of the biological reactions involved is such that, many of these organisms have not sufficiently evolved to convert simple materials directly to methane without the presence of other anaerobic consortia (McCarty, 1982). In the modern sense, anaerobic digestion “started” when Alessandro Volta discovered “flammable air” in 1776, however the connection with microbial activity was not linked until a century later (Zehnder et al., 1982). During the 20th century, anaerobic digestion was embraced by the Water Industry for the treatment of sewage sludges generated from wastewater purification. However, original drivers revolved around using anaerobic digestion to stabilise the sewage sludge by reducing the levels of harmful bacteria, odour, and as a consequence, the sludge quantity itself. Biogas evolved from the process was of secondary importance and the plants were not designed to enhance its production.

The Water Industry, with close to a hundred years experience, currently accounts for over 90% of all biogas produced in the UK (Andrews, 2008), and according to Water UK (2009), this is generated (using CHP) from 60% of the 1.6 million tonnes dry solids sewage sludge produced annually generating 515 GWhr/yr (2008 data). This is sufficient power to supply 110,000 homes (calculated from data in Digest of UK Energy Statistics, 2005). According to OFWAT – the Water Industry Regulator – (Fergusson, 2009), the Water Industry is increasing its renewable energy generation infrastructure by a further 260 GWhr/y an increase of 38% over the next Asset Management Period (AMP – 5 year economic regulatory period in the UK).

The UK government has identified the importance of anaerobic digestion to assist with: meeting renewable energy targets; reducing carbon footprint, and diverting waste from landfill and has aspirations that “by 2020 anaerobic digestion will be an established technology in this country” [presumably outside the Water Industry] (Defra, 2009). The same document acknowledges the importance of the Water Industry and describes it as a key stakeholder in the implementation of its vision statement on anaerobic digestion.

This paper presents the outputs of a mathematical model based on both, actual plant performance, and bacteriological kinetics, set up to predict the influence of a number of parameters on the generation of biogas and ultimately renewable energy.

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