Optimisation of high-rate filamentous algal pond operating parameters for nutrient bioremediation of primary municipal wastewater

Effective management of operational parameters is crucial for optimising wastewater treatment in high-rate filamentous algal pond (HRFAP) systems. This study examined three key operational parameters - hydraulic retention time (HRT), stocking density, and harvest frequency - on the growth and nutrient bioremediation efficiency of Klebsormidium flaccidum cultivated in primary municipal wastewater in outdoor HRFAPs in summer and winter. Seasonal conditions significantly influenced biomass productivity, with productivity being 48.3 % higher in summer compared to winter across all experiments. The optimal HRT of 4 days for both seasons achieved the highest average reductions in total ammoniacal‑nitrogen (TAN) concentration (64.6 % day −1 ± 1.82 SE in summer, 32.3 % day −1 ± 1.82 SE in winter) and acceptable reductions in nitrate-N (66.6 % day −1 ± 3.54 SE in summer, 42.6 % day −1 ± 8.98 SE in winter) and dissolved reactive phosphorous (DRP, 19.8 % day −1 ± 1.01 SE in summer, 15 % day −1 ± 1.98 SE in winter). A stocking density of 0.25 g fresh weight (FW) L −1 was optimal in summer as it resulted in the highest reductions in TAN (75.9 % day −1 ± 2.49 SE), nitrate-N (43.8 % day −1 ± 3.31 SE), and DRP (21.6 % day −1 ± 1.37 SE). In winter, a stocking density of 0.5 g FW L −1 was optimal to mitigate the risk of primary wastewater toxicity during slower growth periods. Harvest frequency did not significantly affect nutrient removal rates across treatments and seasons; however, biomass productivity was significantly higher in summer with a 4-day harvest frequency (7.83 g dry weight (DW) m −2 day −1 ). Longer HRTs improved water quality variables, with the highest Escherichia coli (E. coli) reduction (99.9 %) observed with a stocking density of 0.25 g FW L −1 in winter. This study highlights the importance of seasonal optimisation of HRFAP operation to maximise biomass production and nutrient bioremediation for effective treatment of primary municipal wastewater. • We assessed key operational parameters in outdoor high-rate filamentous algal ponds. • Biomass productivity was 48.3 % higher in summer than in winter. • A 4-day hydraulic retention time was optimal in both summer and winter. • Nutrient removal was highest in summer at a stocking density of 0.25 g FW L −1 . • Harvest frequency did not significantly affect nutrient removal rates.