Smyth, M.1, Kabir, M.1, Inman, D.2, 1Aqua Enviro, UK, 2Anglian Water, UK
(free)One tonne of dry solids contains anywhere between five and seven megawatt hours of energy; of this
perhaps as little as 20% or as much as 80% may be recovered through anaerobic digestion. How much
energy is recovered is a function of the AD process (advanced or conventional), digester retention time
and organic loading rate and the feedstock’s composition and biodegradability.
The best performing sites in the UK consistently produce 3-3.2 MWh/TDS as biogas (equivalent to
~1.25 MWh/TDS after CHP), but many operate at half of this figure, which means that as little as 25%
of the total energy content of the feedstock may be converted to biogas.
In this study, BioWin models for seven Anglian Water Sludge Treatment Centres (STCs) were built
and calibrated using historical site data and from a 3-week sampling period. COD fractionation is core
to the BioWin model and has been used to rank different sludges in terms of their amenability to
digestion. Investigation of the data showed that ‘cycling’ of sludges upstream of the sludge treatment
centre or at the site itself can significantly worsen the characteristics of the sludge.
The calibrated model can be used to show numerous scenarios, including:
1. The difference between the energy a site is and should be producing, which can lead to short term improvement projects.
2. The increase in energy that could be achieved by improving the composition and biodegradability of the feedstock, resulting in ‘spend to save’ investment.
3. The energy benefit of employing pre-treatment.
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