Performance of anaerobic digestion systems : A review

Anaerobic digesters contain extreme environments that change drastically during the production cycle. Organic material is broken down first into amino and fatty acids, then into volatile fatty acids, ammonia, CO2, H2S and other by-products. These acids and alcohols are converted to acetic acid as well as CO2 and H2, which is then used to create methane. All these biological processes mean that the pH, temperature and type of bacteria vary, creating conditions outside the scope of current standards, such as a concentration of ammonium ions 8 times greater than the upper limit of the XA3 class of highly aggressive chemical attack for concrete in BS EN 206-1:2000. Depending on the source, the concrete may be exposed to heavy metals, antibiotics or surfactants, which are not even considered by current standards. Anaerobic digestion (AD) is a growing industry, and this report gives estimates for investment in anaerobic digestion around the world: nearly £2.5 billion in India, over £3 billion in the UK and USA, and nearly £14.5 billion invested in Germany, with China becoming the largest AD market in the world. This means that anaerobic digestion has sizable economic value as well as positive environmental effects. However, as part of maximising these benefits, it is necessary to better understand the chemical and biological attack the concrete that is used to build these digesters undergoes, so that steps can be taken towards limiting premature deterioration. This article will show the current gaps in both knowledge and legislation, with the aim of promoting further research into the aforementioned areas.