Calcium effect on microbial activity and biomass aggregation during anaerobic digestion at high salinity

The potential effect of different Ca²⁺ additions (150, 300, 450, 600 and 1000 mg/L) on microbial activity and aggregation, during anaerobic digestion at moderate (8 g/L Na⁺) and high salinity (20 g/L Na⁺) has been investigated. Batch tests were carried out in duplicate serum bottles and operated for 30 days at 37 °C. At 8 g/L Na⁺, methanogenic activity and protein degradation were comparable from 150 to 450 mg/L Ca²⁺, and a significant inhibition was only observed at a Ca²⁺concentration of 1000 mg/L. In contrast, at 20 g/L Na⁺, 150 to 300 mg/L were the only Ca²⁺ concentrations to maintain chemical oxygen demand (COD) removal, protein hydrolysis and methane production. Overall, increasing Ca²⁺ concentrations had a larger impact on acetotrophic methanogenesis at 20 g/L than at 8 g/L Na⁺. Increasing Ca²⁺ had a negative effect on the aggregation behaviour of the dominant methanogen Methanosaeta when working at 8 g/L Na⁺. At 20 g/L Na⁺ the aggregation of Methanosaeta was less affected by addition of Ca²⁺ than at 8 g/L Na⁺. The negative effect appeared to be connected with Ca²⁺ precipitation and its impact on cell-to cell communication. The results highlight the importance of ionic balance for microbial aggregation at high salinity, bringing to the forefront the effect on Methanosaeta cells, known to be important to obtain anaerobic granules.