Microalgal biomass grown in wastewater treatment raceway ponds may be valorised producing bioenergy through anaerobic digestion. However, pretreatment techniques seem to be necessary for enhancing microalgae methane yield. In this study, hydrothermal pretreatment was studied prior to batch and continuous reactors. The pretreatment increased organic matter solubilisation (8-13%), anaerobic digestion rate (30-90%) and final methane yield (17-39%) in batch tests. The highest increase was attained with the pretreatment at 130 °C for 15 min, which was attested in a laboratory-scale continuous reactor operated at a hydraulic retention time of 20 days with an average organic loading rate of 0.7 g VS/L·day. The methane yield increased from 0.12 to 0.17 L CH₄/g VS (41%) in the pretreated digester as compared to the control. Microscopic images of microalgal biomass showed that pretreated cells had unstructured organelles and disrupted cell wall external layer, which may enhance the hydrolysis. Indeed, images of the pretreated reactor digestate showed how cells were more degraded than in the control reactor.

The anaerobic digestion of microalgae is hindered by its complex cell wall structure and composition. Thus, several pretreatment methods have been used for increasing microalgae anaerobic biodegradability. Since the methane yield depends on biomass characteristics, pretreatments should be compared using the same microalgal biomass. In this study, physical pretreatments including thermal (95 degrees C; 10 h), hydrothermal (130 degrees C; 15 min), microwave irradiation (900 W; 3 min; 34.3 MJ/kg TS) and ultrasonication (70 W; 30 min; 26.7 MJ/kg TS) were evaluated in terms of microalgae solubilisation and methane yield increase in batch tests. Organic matter solubilisation was improved in all cases, with the highest increase on soluble proteins, followed by soluble carbohydrates and soluble lipids. This was attributed to the macromolecular and cell wall composition of the main microalgae species composing the biomass, i.e. Monoraphidium sp. and Stigeoclonium sp. Furthermore, the methane yield was increased by 72% for thermal, 28% for hydrothermal and 21% for microwave pretreatments, whereas no significant increase was found for ultrasonication as compared to control. Outstanding results of the thermal pretreatment should be validated in prospective pilot-scale studies in order to quantify the potential increase in biogas production upon continuous operation. (C) 2015 Elsevier B.V. All rights reserved.Postprint (published version