Optimizing high-rate algae pond for resource recovery from municipal wastewater

The study focuses on optimising high-rate algae ponds (HRAP) for resource recovery from municipal wastewater. The research aims to optimise operational parameters such as hydraulic retention time (HRT) and to assess the option of sludge return, to maximise algal biomass production while meeting effluent standards. For this, an intensive measurement campaign on a pilot HRAP in Ajdovščina, Slovenia, was performed, which will be used to calibrate a mathematical model to simulate algae-bacteria dynamics. Measurements were performed from 2019 to 2025. The measurements from 2019 to 2024 are taken at single operational mode, i.e., at HRT of 35 days, while in 2025 the campaign was conducted under two different HRT, 35 and 10 days. Monitoring results from 2019 to 2024 indicated variability in treatment performance, however, there was efficient removal of BOD₅ and NH₄-N despite fluctuations in inflow concentrations. The results from 2025 indicate that HRT can be as low as 10 days to achieve acceptable efficiency for BOD removal. However, the concentration of bacteria-microalgae biomass was expectedly lower at lower HTR. There was a clear need for optimisation, particularly in increasing bioactive biomass in the HRAP through, for example, sludge return and varying HRT.