Biochemical methane potential of wastewater-grown microalgal bacterial flocs: influence of pretreatments

A sequencing batch reactor with microalgal bacterial flocs (MaB-floc SBR) is a novel solar-based system to treat wastewater. During this aerobic treatment process, solar energy is harnessed and wastewater nutrients are recovered into microalgal bacterial biomass. To further improve the economics of this MaB-floc SBR system, the produced biomass should be valorized, for example as feedstock for anaerobic digestion to biogas. Therefore, batch experiments were carried out to evaluate the biochemical methane potential (BMP) of MaB-flocs originating from treatment of aquaculture wastewater in an outdoor pilot reactor in Belgium. Significant differences were found between the BMP of MaB-flocs harvested on different seasons, ranging from 0.143 to 0.205 NL CH4 g-1 volatile solids. The low chemical methane conversion efficiency of these untreated MaB-flocs of 27-34 % highlights the importance of pretreating MaB-flocs before being digested to biogas. Of all physical and chemical pretreatment methods screened in this study, only microwave pretreatment showed a significant - but still low - improvement of 9.4 ± 5.4 % of the BMP. Because of the low BMP and low chemical methane conversion efficiency of wastewater-grown MaB-flocs, the anaerobic digestion of this novel biomass feedstock to methane should be granted a supporting role within a biorefinery concept