Digester Gas Measurement

Auty, D.1, and Fountain, P.2, 1Entec UK Ltd, 2Thames Water, UK



This paper provides a relevant and applicable example of the adage that ‘you can’t manage what you can’t measure’.  The highly incentivised value of digester gas means that operators want to manage anaerobic digesters better so that they produce more electricity to sell and gain ROCs or FiTs from.  However, very few people realise just how poorly gas flows are measured and what tricks are being missed as a result.

This paper is based on the interesting incongruities found in some excellent and detailed Thames Water data that forms part of their highly praised Cockpit system of treatment performance enhancement.  This relies on data being gathered that is analysed in clever ways to show to everybody from the site operator to the senior management just how well a works is performing.  The study carried out on behalf of Thames Water revealed some interesting facts regarding measurement techniques, installations and the understanding of what the data is telling us.  In addition, it has revealed much about the direct link between how a digester is operated and how much energy can be gained from the gas.

Keywords: Sewage sludge, waste treatment, anaerobic digestion, optimisation, digester gas, costs

Anaerobic Digestion – What’s the Fuss all About?

Anaerobic digestion (AD) is a well established process that has been widely applied to waste treatment for many years, but it has recently seen a significant resurgence of interest.  This has been provoked by the improved economics arising from the renewable energy incentives, particularly Renewable Obligation Certificates (ROCs).  Broadly speaking, ROCs have made AD attractive to anybody with access to sufficient digestible organic material and so water companies, local authorities, waste companies and farmers are all interested in applying it to their particular fields of interest.  Since the banking crisis provoked the current economic downturn, there are also many groups and individuals that are looking to AD as a means of getting a greater return on their wealth than is available from the banks.

Of course, AD is not all about gaining an income stream from the renewable energy.  It also offers the benefits of producing a stable product that has none of the wastes’ propensity to produce malodours and has excellent soil improvement qualities as well as useful fertiliser value.  This product – digestate – is easy to handle and generally has a lower volume than the original waste making it an acceptable product for agriculture that is just beginning to have a value because of its benefits.

However, it is the gas that is of greatest interest to those investing in and operating digestion plants.  The gas is very easily converted into renewable electricity using a Combined Heat and Power (CHP) unit and this then enables it to claim one of the fiscal incentives for renewable electricity.  The principal incentive is the ROC scheme, but AD plants are also eligible for the Feed in Tariff (FiT) which, in financial terms is similar to ROCs (there are many actual differences between the ROC and FiT incentive schemes and choosing between them could be the subject of another paper). The ROC value attached to the gas varies such that existing (prior to April 2009) water company CHP installations can claim ‘one ROC’ worth about £45 for each MWh of electricity produced.  Installations registered with OFGEM after April 2009 will be able to claim ‘half a ROC’ for each MWh of electricity produced.  Investors interested in digesting food waste, farm wastes and/or energy crops are eligible to claim ‘two ROCs’ for each MWh of electricity produced.  The difference in the degree of incentivisation is intended to differentiate between the established water company AD industry and the emerging use of AD to treat other organic waste.  There are other incentives and a few grants (those that avoid ‘state subsidy’ rules) that can also be applied.

The existing incentives only apply to renewable electricity.  Renewable heat will be incentivised by the Renewable Heat Incentive (RHI) which is due to come into force in April 2011.  This is primarily aimed at burning biomass in district heating systems, but digester gas will be eligible either as heat in such district heating systems (the ‘waste’ heat from a CHP, for example) or as a substitute for natural gas in the national gas grid.  To replace natural gas, the digester gas has to be upgraded to biomethane and meet similar standards to those applied to natural gas.  This is relatively easy to achieve using technologies that are well established in other fields and have some track record in creating biomethane.  The level of the RHI is expected to be set by the October 2010 budget so it should be known by the time you are reading this.  It is widely expected to be set to mimic the level of incentive offered by ROCs and FiTs (with some exceptions) so for the purposes of this paper, we will assume there is no difference in the fiscal benefits to be gained from the renewable energy incentive schemes as they apply to digester gas.

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