Incorporating saline microalgae biomass in anaerobic digester treating sewage sludge: Impact on performance and microbial populations

Abstract

The aim of this study was to acclimate anaerobic prokaryotes to saline microalgae biomass. Semi-continuous experiments were conducted using two 1.5 L mesophilic reactors for 10 weeks, (hydraulic retention time of 21 days). The first reactor was solely fed with sewage sludge (control), while the second received a mixture of sewage sludge and microalgal biomass (80/20 %w/w) cultivated at 70 g·L^-1 salinity. The in-reactor salinity reached after the acclimation phase was 14 g·L^-1. Biomethane production was comparable between the control and acclimated reactors (205 ± 29 NmL_Methane·g_{VolatileSolids}^-1). Salinity tolerance assessment of methanogenic archaea revealed that salinity causing 50% inhibition of methane production increased from 10 to 27 g·L^-1 after acclimation. Microbial diversity analyses revealed notable changes in methanogenic archaea populations during co-digestion of saline microalgae biomass, particularly methylotrophic (+27%) and acetotrophic (-26%) methanogens. This study has highlighted the possibility of treating efficiently saline microalgae in co-digestion with sewage sludge in future industrial biogas plants.

Keywords: Anaerobic co-digestion; Methanogenic activity; Microalgae; Microbial diversity; Salt acclimation.