Carbon-to-nitrogen and substrate-to-inoculum ratio adjustments can improve co-digestion performance of microalgal biomass obtained from domestic wastewater treatment

\textcopyright 2017 Informa UK Limited, trading as Taylor \& Francis Group This study comparatively evaluated the effect of co-substrates on anaerobic digestion (AD) and biochemical methane potential of wastewater-derived microalgal biomass, with an emphasis on carbon-to-nitrogen (C:N) and substrate-to-inoculum (S:I) ratios. A semi-continuous photobioreactor was inoculated with Chlorella vulgaris and the nutrient recovery potential was investigated. Derived microalgal slurry was subjected to AD in the absence and presence of co-substrates; model kitchen waste (MKW) and waste activated sludge (WAS). The results revealed that up to 99.6\% of nitrogen and 91.2\% of phosphorus could be removed from municipal wastewater using C. vulgaris. Biomethane yields were improved by co-digestion with both MKW and WAS. The maximum biomethane yield was observed as 523 ± 25.6 ml CH 4  g VS added −1 , by microalgal biomass and MKW co-digestion in 50:50 ratio, at an initial chemical oxygen demand (COD) concentration of 14.0 ± 0.1 g l −1 , C:N ratio of 22.0, and S:I ratio of 2.2. The observed biomethane yield was 80.7\% higher than that of the mono-digestion. The highest improvement achieved by 50:50 co-digestion of microalgal biomass and WAS was 15.5\%, with biomethane yield of 272 ± 11.3 ml CH 4  g VS added −1 at an initial COD concentration of 14.0 ± 0.1 g l −1 , C:N ratio of 13.0, and S:I of 2.3.