Assessment of the influence of the hydraulic retention time on the tertiary and quaternary treatment of a high rate algal pond

The influence of the hydraulic retention time (HRT) on the performance of a high-rate algal pond (HRAP) with biomass settling and recycling for tertiary and quaternary treatment of domestic wastewater was herein evaluated. A decrease in the HRT from 4 days (P1) to 2 days (P2) did not entail substantial differences in the removal efficiencies of nitrogen (54.3 ± 11.6 and 54.0 ± 7.3 %), phosphorous (42.6 ± 21.6 and 39.3 ± 35.8 %), organic carbon (33.5 ± 12.2 and 38.4 ± 26.4 %) and inorganic carbon (55.6 ± 10.8 and 50.5 ± 26.8 %). Escherichia coli (0.7 ± 1.4 and 3.8 ± 5.2 CFU-mL⁻¹, respectively) and suspended solid (10 mg TSS-L⁻1 average) concentrations in the effluent remained below European discharge limits. However, the effluent total nitrogen (TN) concentration exceeded those limits, despite the high NH₄⁺ removal capacity (> 95 %) of the HRAP. Regarding CECs, 9 compounds were removed below their quantification limit in P1, and 5 compounds in P2. The HRT exhibited a low impact on the total removal of CECs, with removal efficiencies of 57.6 ± 4.0 % and 63.7 ± 3.1 % during P1 and P2, respectively. The CEC concentrations retained in biomass increased at decreasing the HRT, with 723 and 1026 μg L⁻¹ at P1 and P2, respectively. Most of these compounds belonged to the group of fluoroquinolones or anti-inflammatory drugs. Overall, HRAPs represent a cost-effective technology platform for tertiary and quaternary treatment of domestic wastewaters.