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.