Cultivation and anaerobic digestion of Scenedesmus spp. grown in a pilot-scale open raceway

Digestibility of a micro-algal mixture was evaluated by mesophilic anaerobic digestion in continuously-stirred tank reactors. The culture consisted primarily of Scenedesmus spp. continuously cultivated over a 6-month period in a 100 m(2) raceway reactor instrumented to record pH, dissolved oxygen and temperature. The raceway received supplementary carbon in the form of flue gas from a diesel boiler (10% CO2) injected into a 1-m deep sump to control pH in the range 7.88.0. Dilution was optimised to biomass productivity and gave values of 1015 and 2025 g total suspended solids (TSS) m(-2) day(-1) in winter (DecemberFebruary) and spring (AprilMay), respectively. The culture for the anaerobic digestion trial was harvested in February by centrifugation to give an algal paste containing 4.3% volatile solids (VS). Semi-continuous digestion at organic loading rates of 2.00, 2.75 and 3.50 g VS l(-1) day(-1) gave volumetric biogas productions of similar to 0.66, similar to 0.83 and similar to 0.99 l l(-1) day(-1), respectively. Specific methane yield ranged from 0.13 to 0.14 l CH4 g(-1) VSadded with biogas methane content similar to 62%. Overall the digestion process was stable, but only similar to 30% VS destruction was achieved indicating low biodegradability, due to the short retention times and the recalcitrant nature of this type of biomass. (C) 2014 Elsevier B.V. All rights reserved.