Impact of salt acclimation on anaerobic co-digestion of sewage sludge and saline microalgae biomass for biogas production

The biogas sector is expanding rapidly at international level and the search for new biomasses is becoming a key issue for the viability of existing and future projects. Among them, saline biomasses and effluents are promising because they are not widely used even if they present some bottlenecks like high salinity and sulfur content. The objective of this study was to acclimate anaerobic bacteria to saline conditions. Semi-continuous feeding experiments were carried out with two 1.5 L mesophilic reactors for 10 weeks, with a hydraulic retention time of 21 days. The first reactor was fed only on sewage sludge (control), the second one was fed on a mixture of sewage sludge and microalgal biomass (80/20 % w/w) grown at 70 g.L-1 salinity. The in-reactor salinity reached after acclimation phase was 14 g.L-1. Biomethane production was similar in control and acclimated reactors (205 +/- 29 NmLCH4.gVS-1). The activity of the methanogenic archaea was investigated by batch tests at salinities up to 30 g.L-1 using inoculum before and after acclimation and methane production rates were fitted with Hill Model. The effective concentration reducing 50% of the methanogenic activity was 10 g.L-1 of sea salts before acclimation, and increased to 27 g.L-1 after acclimation, confirming the ability of the procaryotic community to adapt to high salinities. Microbial diversity analyses (16S MiSeq) revealed changes in methanogenic archaea populations in the acclimated reactor, particularly methylotrophic (+27%) and acetotrophic (-26%) methanogens, compared to the control reactor.