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

•Wastewater grown microalgae was digested in unmixed anaerobic reactors.•Anaerobic digestion at 8-21 °C was compared with conventional digestion at 37 °C.•Low-temperature thermal and freeze-thaw pretreatments enhanced algal degradability.•AVRs separate liquid and solid phases, enabling longer solid retention time.

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.

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