The treatment of municipal wastewater by the macroalga Oedogonium sp. and its potential for the production of biocrude

Municipal wastewater represents a significant yet under-utilised resource for the commercial production of algae. However, in order to realise this potential, two constraints must first be addressed - the treatment of wastewater to levels comparable with existing technologies and the reliable production of monocultures as the basis for commercial products. In this study, three municipal wastewater sources were investigated at various water exchange rates for the cultivation of the filamentous freshwater macroalga Oedogonium sp., demonstrating that the delivery of nutrients through primary effluent at a low exchange rate of 5% volume per day supported high biomass productivity. A continuous high-density culture of Oedogonium (0.25-1.5 g/L fresh weight) in a pilot-scale pond system yielded biomass productivities of 7-10 g/m2/d dry weight and nutrient removal rates of 0.50 g nitrogen/m2/d and 0.11 g phosphorus/m2/d without the addition of CO2. Nitrogen, phosphorus and chemical oxygen demand were reduced by 62%, 75% and 57% respectively in the treated water. The concentration of microbes was reduced by 99% and some metal contaminants that can impede the reuse of treated water were also reduced by 99%. The biomass produced had a relatively consistent biochemical composition that would yield 26-27% of the dry weight as biocrude oil through hydrothermal liquefaction. The results demonstrate that monocultures of Oedogonium sp. can be used to treat multiple components of municipal wastewater and simultaneously deliver biomass that can be conveniently harvested and converted to biocrude for the production of drop-in fuels.