Influence of temperature and pretreatments on the anaerobic digestion of wastewater grown microalgae in a laboratory-scale accumulating-volume reactor

Abstract

This laboratory-scale study investigated the performance of a low-cost anaerobic digester for microalgae. Low (∼2%) solids content wastewater-grown microalgal biomass (MB) was digested in an unmixed, accumulating-volume reactor (AVR) with solid and liquid separation that enabled a long solids retention time. AVRs (2 or 20 L) were operated at 20 °C, 37 °C or ambient temperature (8–21 °C), and the influence of two pretreatments – low-temperature thermal (50–57 °C) and freeze-thaw – on algal digestion were studied. The highest methane yield from untreated MB was in the 37 °C AVR with 225 L CH4 kg volatile solids (VS)−1, compared with 180 L CH4 kg VS−1added in a conventional, 37 °C completely stirred tank reactor (CSTR), and 101 L CH4 kg VS−1added in the 20 °C AVR. Freeze-thaw and low-temperature thermal pretreatments promoted protein hydrolysis and increased methane yields by 32–50% at 20 °C, compared with untreated MB. Pretreatments also increased the mineralisation of nitrogen (41–57%) and phosphorus (76–84%) during digestion. MB digestion at ambient temperature was comparable with digestion at 20 °C, until temperature dropped below 16 °C.