From piggery wastewater nutrients to biogas: Microalgae biomass revalorization through anaerobic digestion

Microalgae grown in swine wastewater were used as a promising strategy to produce renewable energy by coupling wastewater bioremediation and biomass revalorization. The efficiency of a microalgae consortium treating swine slurry at different temperature (15 and 23 degrees C) and illumination periods (11 and 14 h) was assessed for biomass growth and nutrient removal at two NH4+ initial concentrations (80 and 250 mg L-1 NH4+). Favourable culture conditions (23 degrees C and 14 h of illumination) and high ammonium loads resulted in higher biomass production and greater nutrients removal rates. The initial N-NH4+ load determined the removal mechanism, thus ammonia stripping and nitrogen uptake accounted similarly in the case of high NH4+ load, while nitrogen uptake prevailed at low NH4+ load. Under favourable conditions, nitrogen availability in the media determined the composition of the biomass. In this context, carbohydrate-rich biomass was obtained in batch mode while semi-continuous operation resulted in protein-rich biomass. The revalorization of the resultant biomass was evaluated for biogas production. Methane yields in the range of 106-146 and 171 ml CH4 g COD-1 were obtained for the biomasses grown in batch and semi-continuous mode, respectively. Biomass grown under favourable conditions resulted in higher methane yields and closer to the theoretically achievable. (C) 2016 Elsevier Ltd. All rights reserved.