Proceedings

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, (which is the UK’s share of the EU binding targets) but admit that – in order to meet carbon dioxide reductions of 26% 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. (There are also targets in place to reduce landfill use and these are assisted by heavy taxes). 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.

DEFRA published an AD vision statement for England, the key message is that anaerobic digestion has a huge part to play in meeting Landfill Directive targets and achieving key national targets set in the climate act (by managing the waste removed from landfill). DEFRA have identified the UK Water industry as one of the key stakeholders in this vision statement due to their experience in this field. The mixing of biodegradable waste with sewage sludge is known as co-digestion.

A great deal is mentioned about the additional biogas potential when waste streams are co-digested with sewage sludge at existing facilities. 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 clean up (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.