Quantification of methane losses from the acclimatisation of anaerobic digestion to marine salt concentrations

The research assessed losses in methane production as a result of raising digester salt concentrations to marine values, and of increasing the feedstock sulphate concentration. Acclimatisation of inoculum from a municipal wastewater biosolids digester was begun by raising the concentration of chloride salts (Na, Mg, Ca and K) to 6-9 g L-1, as initial experiments showed higher concentrations caused severe inhibition. After stable operation for four retention times salt content in the reactors and the feed was increased by 1 g L-1 every 14 days, up to 31.1 g L-1. The digesters were fed daily in semi-continuous mode and monitored for performance and stability criteria including specific methane production (SMP). SMP was 6-7% less than in controls using the same feedstock without saline addition. After steady-state conditions were achieved at high chloride salinity, magnesium chloride was partially replaced by magnesium sulphate to give a range of sulphate concentrations. Higher sulphate concentrations caused initial instability, indicated by volatile fatty acid accumulation. This subsequently reduced and stable operation was achieved at marine sulphate concentrations, but with a 5% loss in SMP due to interspecies substrate competition. High sulphate also affected pH, leading to gaseous H2S production proportional to the applied sulphate load. (C) 2015 Elsevier Ltd. All rights reserved.