Effects of series operation of high-rate algal ponds treating sanitary sewage: hydrodynamics, remediation potential, and biomass production

Operational modifications are a promising, cost-effective strategy to increase the efficiency of algal systems applied to sewage treatment. In this context, this study investigates the effectiveness of operating High-rate Algal Ponds (HRAPs) in series for sanitary sewage treatment, focusing on improving technical performance by assessing pollutant removal efficiencies and biomass production. A hydrodynamic analysis determined Bodenstein's value of 9.7 for HRAPs, indicating a complete mixing regime and suggesting potential benefits to series operation. Subsequently, two systems were operated, each comprising two HRAPs: one operated in series (with a hydraulic retention time (HRT) of 5 days in each HRAP) and one operated in parallel (with an HRT of 10 days in each HRAP). The series operation achieved higher coliforms and E. coli removal (97.79 % and 98.33 % compared to 84.09 % and 89.56 % in parallel), while no statistically significant differences were observed in nutrient removal or biomass production. However, the parallel system exhibited 14 % higher TOC removal and biomass with greater lipid (19 %), protein (76 %), and carbohydrate (44 %) content, while the series had higher ash content (65 % higher). Discussions approach the roles of hydrodynamics, nutrient balance, and senescence to explain the performance of each system. Future research should focus on reaching a plug flow regime by optimizing mixing, besides testing more than two ponds in series.