Methane production by anaerobic digestion of wastewater and solid wastes

Methane production by anaerobic digestion of wastewater and solid wastes

Methane production by anaerobic digestion of wastewater and solid wastes 

T.Z.D. de Mes, A.J.M. Stams, J.H. Reith and G. Zeeman 

Anaerobic digestion is an established technology for the treatment of wastes and wastewater. The final product is biogas: a mixture of methane (55-75 vol%) and carbon dioxide (25-45 vol%) that can be used for heating, upgrading to natural gas quality or co-generation of electricity and heat. Digestion installations are technologically simple with low energy and space requirements. Anaerobic treatment systems are divided into 'high-rate' systems involving biomass retention and 'low-rate' systems without biomass retention. High-rate systems are characterised by a relatively short hydraulic retention time but long sludge retention time and can be used to treat many types of wastewater. Low-rate systems are generally used to digest slurries and solid wastes and are characterised by a long hydraulic retention time, equal to the sludge retention time. The biogas yield varies with the type and concentration of the feedstock and process conditions. For the organic fraction of municipal solid waste and animal manure biogas yields of 80-200 m3 per tonne and 2-45 m3 per m3 are reported, respectively. Co-digestion is an important factor for improving reactor efficiency and economic feasibility. In The Netherlands co-digestion is only allowed for a limited range of substrates, due to legislation on the use of digested substrate in agriculture. Maximising the sale of all usable co-products will improve the economic merits of anaerobic treatment. Furthermore, financial incentives for renewable energy production will enhance the competitiveness of anaerobic digestion versus aerobic composting. Anaerobic digestion systems currently operational in Europe have a total capacity of 1,500 MW, while the potential deployment in 2010 is estimated at 5,300-6,300 MW. Worldwide a capacity up to 20,000 MW could be realised by 2010. Environmental pressures to improve waste management and production of sustainable energy as well as improving the technology’s economics will contribute to broader application.