Comprehensive respirometric approach to assess photosynthetic, heterotrophic and nitrifying activity in microalgal-bacterial consortia treating real municipal wastewater

The design of photobioreactors (PBRs) to treat municipal wastewater is in the early stages of development. Kinetics of activated sludge cannot be used for PBRs because biomass differs in composition, growth and decay rates. In this paper, respirometry was extended to a microalgal-bacterial consortium in a long-term PBR treating municipal wastewater. Kinetic and stoichiometric parameters (20 °C) were estimated for endogenous respiration, biodegradable COD oxidation and nitrification. The comprehensive respirometric approach was based on the Oxygen Production Rate (OPR) of photosynthetic microorganisms, and the Oxygen Uptake Rate (OUR) of the overall biomass, including data processing and corrections. The gross OPR (gOPR) was 9.8 ± 0.2 mg O2 g TSS−1 h−1; using microalgae quantified by flow cytometry, the per-cell gOPR was 11 fmol O2 cell−1 h−1. This oxygen was utilized for COD oxidation with rate of 18.0 and 19.3 mg g TSS−1 h−1 under light and dark, respectively. Biomass yield was 0.66 on average, similar to the typical values of activated sludge. The nitrification rate in the dark was 0.92 mg NH4+-N g TSS−1 h−1. The stoichiometric oxygen-over-ammonium coefficient was 4.4 g O2/g N under dark conditions (similar to activated sludge), but lower under light ones (2.8 g O2/g N). The respirometric approach proposed here added experimental kinetic results needed in the PBR design.