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Abstract:With the increasing demand for renewable energy and sustainable waste treatment, biogas production is expanding. Approximately four billion litres of bio-ethanol are produced annually for vehicle fuel in Europe, resulting in the production of large amounts of stillage residues. This stillage is energy-rich and can be used for biogas production, but is a challenging substrate due to its high levels of nitrogen and sulphate. At the full-scale biogas production plant in Norrköping, Sweden (Svensk Biogas i Linköping AB), thin grain stillage is used as a biogas substrate. This paper describes the plant operation and strategies that have been implemented to digest thin stillage successfully. High ammonia concentrations in the digester have resulted in syntrophic acetate oxidation (SAO) becoming the major pathway for acetate degradation. Therefore, a long hydraulic retention time (HRT) (40-60 days) is used to allow the syntrophic acetate-oxidising bacteria time to grow. The high sulphate levels in thin stillage result in high levels of hydrogen sulphide following degradation of protein and the activity of sulphate-reducing bacteria (SRB), the presence of which has been confirmed by quantitative polymerase chain reaction (qPCR) analysis. To optimise biogas production and maintain a stable process, the substrate is diluted with tap water and co-digested with grain residues and glycerine to keep the ammonium nitrogen (NH 4 -N) concentration below 6 g L −1 . Combined addition of iron, hydrochloric acid and cobalt successfully precipitates sulphides, reduces ammonia toxicity and supplies microorganisms with trace element. Mesophilic temperature (38 °C) is employed to further OPEN ACCESSEnergies 2013, 6 5643 avoid ammonia toxicity. Together, these measures and doubling the digester volume have made it possible to increase annual biogas production from 27.7 TJ to 69.1 TJ.

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BACKGROUND: This study evaluated a two-stage process as an alternative to single-stage anaerobic degradation, with the aim of separating sulphidogenesis and methanogenesis while treating a mixture of organic fraction municipal solid waste (OFMSW) and thin stillage. For full-scale applications, no pH regulation was applied. The hydrolytic/acidogenic stage was evaluated with a hydraulic retention time (HRT) between 3 and 15 days, with and without recirculation of methanogenic stage reactor material. RESULTS: Homoacetogenesis dominated in the hydrolytic/acidogenic stage. However, sulphate reduction was unsuccessful. Using a two-stage approach with OFMSW at a HRT of 10 days in the first stage and 28 days in the second stage resulted in a 12% increase in specific methane production and a 6% increase in methane content. The two-stage process with thin stillage was less effective, probably because of decreasing pH (<4) in the first stage. CONCLUSION: Two-stage operation resulted in a pH too low (∼4) for successful sulphate reduction in the first stage. However the resulting pH and applied organic loading rate caused homoacetogenesis to dominate, producing acetate as a favourable energy carrier between the stages.