United Utilities

Quantifying the impacts of co-digesiton of waste streams with sewage sludge

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In order to slow down the impending impacts of global climate change due to consumption of fossil fuels, governments are looking at ways to develop and incentivise alternative renewable energy sources. Subsequently, the UK has imposed renewable energy targets of 15% of all energy consumption by the year 2020, but admit that – in order to meet carbon dioxide reductions of 20% by 2020 – renewable use will have to account for 30-35% of all energy use. Current levels of renewable energy use are below 5%. One form of renewable energy generation which has gained considerable publicity recently is the production of methane enriched biogas from the digestion of wastes which would otherwise be put down the sewer or landfilled. Subsequently, in a recent energy white paper, the UK government has decided that biogas generated from waste is entitled to receive economic incentives in the form of 2 Renewables Obligation Certificates (ROCs), which can then be redeemed for profit. Ironically, the energy generated from the anaerobic digestion of sewage sludge, which currently accounts for over 90% of the UK’s biogas production has been devalued to 0.5 ROCs as it is considered a mature market place.

A great deal is mentioned about the additional biogas potential when waste streams are co-digested with sewage sludge at existing facitlities. However, apart from many business and regulatory concerns, the addition of waste streams to sludge digesters has a number of other – albeit less obvious – influences. Addition of material to an existing sewage digester (assuming regulatory issues are in place and that the digestion plant inclusive of ancillaries has sufficient capacity) may increase: the need for additional gas cleanup (e.g. siloxane removal); sludge production (which has to be recycled or disposed); dewatering liquors (which require treatment); upstream processing requirements (e.g. pasteurisation), and the need for chemicals amongst others.

This paper presents the issues concerning co-digestion of waste with sewage sludge and shows the results of a detailed model which aims to quantify the other, less obvious impacts of co-digestion.

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