Evaluation of the simultaneous biogas upgrading and treatment of centrates in a high-rate algal pond through C, N and P mass balances

The simultaneous capture of CO₂ from biogas and removal of carbon and nutrients from diluted centrates in a 180 L high-rate algal pond (HRAP) interconnected to a 2.5 L absorption column were evaluated using a C, N and P mass balance approach. The experimental set-up was operated indoors at 75 μE/m²·s for 24 h/d at 20 days of hydraulic retention time for 2 months of steady state, and supported a C-CO₂ removal in the absorption column of 55 ± 6%. Carbon fixation into biomass only accounted for 9 ± 2% of the total C input, which explains the low biomass productivity recorded in the HRAP. In this context, the low impinging light intensity along with the high turbulence in the culture broth entailed a C stripping as CO₂ of 49 ± 5% of the total carbon input. Nitrification was the main NH₄⁺ removal mechanism and accounted for 47 ± 2% of the inlet N-NH₄⁺, while N removal as biomass represented 14 ± 2% of the total nitrogen input. A luxury P uptake was recorded, which resulted in a P-PO₄⁻³ biomass content over structural requirements (2.5 ± 0.1%). Phosphorus assimilation corresponded to a 77 ± 2% of the inlet dissolved P-PO₄⁻³ removed.