Microbial acclimation to salinity
In recent years anaerobic digestion (AD) has been investigated as suitable biotechnology to treat wastewater at elevated salinities. However, when starting up AD reactors with inocula that are not adapted to salinity, low concentrations of sodium (Na+) in the influent can already cause disintegration of microbial aggregates and wash-out. In this study, biomass acclimation to low Na+ concentrations (5 g/L) of two different non-adapted inocula was investigated in two lab-scale hybrid expanded granular sludge bed (EGSB)-anaerobic filter (AF) reactors fed with synthetic wastewater. After an initial biomass disintegration, new aggregates were formed relatively fast (i.e., after 95 days of operation), indicating adaptation of the microbial community. The newly formed microbial aggregates accumulated Na+ at the expenses of calcium (Ca2+), but this did not hamper biomass retention or process performance. The hybrid reactor configuration, that included a pumice stone filter in the upper section, and the low up-flow velocities applied, were key features for the retention of the biomass within the system. This reactor configuration can be an easily applied and represents a low-cost alternative for the acclimation of biomass to saline effluents, even in existing digesters. When the acclimated biomass was transferred from EGSB to an up flow anaerobic sludge blanket (UASB) reactor configuration, more dense aggregates in the form of granules were obtained. The performances of the UASB inoculated with the acclimated biomass was comparable to another reactor seeded with saline-adapted granular sludge from a full-scale plant. Regardless of the inoculum origin, a defined core microbiome of Bacteria (Thermovirga, Bacteroidetes vadinHA17, Blvii28 wastewater-sludge group, Mesotoga, and Synergistaceae) and Archaea (Methanosaeta and Methanobacterium) was detected, highlighting the importance of these microbial groups in developing halotolerance and maintaining AD process stability.