Impact of temperature and photoperiod on anaerobic biodegradability of microalgae grown in urban wastewater

• Tested temperatures and photoperiods did not affect final biomass yield.
• Main mechanism identified for nutrients removals was biomass assimilation.
• Different cultivation scenarios evidenced different biomass macromolecular profile.
• Methane productivity was different for the three collected biomass.

This study was designed to elucidate how temperature and photoperiod, two of the principal parameters affecting microalgae culture conditions influenced the anaerobic digestion of harvested biomass when grown in wastewater under different scenarios (I: 23°C/14 h illumination, II: 15 °C/14 h and III: 15 °C, 11 h). With respect to biomass cultivation, temperature affected biomass productivity but not final biomass concentration. Scenario I mediated faster ammonium and phosphate removal (100% for all the evaluated scenarios) and greater organic matter removal (80.5% compared to 56.5% and 70.8% obtained for Scenario II and III, respectively). Biomass grown under unfavorable conditions of light and temperature (Scenario III) evidenced the highest nitrogen assimilation due to the lowest ammonia stripping (6%). Different cultivation scenarios resulted in a different macromolecular profile of the harvested biomass. Carbohydrates accumulation prevailed under Scenario I while low temperature (Scenario II) and short photoperiod (Scenario III) increased lipid and protein content. Harvested biomass was subjected to anaerobic digestion. Anaerobic biodegradability of the three types of biomass remained in the narrow range of 36–42%, however different hydrolysis constant rates were calculated. Comparison between the theoretically calculated and experimentally obtained methane yield values showed that biomass collected at Scenario III only reached 36.1% of the theoretical methane yield achievable compared to 46.5% attained with the biomass collected at Scenario I. Further research on microalgae communities and cell wall composition is required to understand the methane yield mismatch.