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

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 degrees C, 37 degrees C or ambient temperature (8-21 degrees C), and the influence of two pretreatments lowtemperature thermal (50-57 degrees C) and freeze-thaw on algal digestion were studied. The highest methane yield from untreated MB was in the 37 degrees C AVR with 225 L CH4 kg volatile solids (VS)(-1), compared with 180 L CH4 kg VSadded-1 in a conventional, 37 degrees C completely stirred tank reactor (CSTR), and 101 L CH4 kg VSadded-1 in the 20 degrees C AVR. Freeze-thaw and low-temperature thermal pretreatments promoted protein hydrolysis and increased methane yields by 32-50% at 20 degrees 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 degrees C, until temperature dropped below 16 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.