High rate algal pond operating strategies for urban wastewater nitrogen removal

Two experimental high rate algal ponds (HRAPs) (1.5 m(2), 570 L per unit), each with a secondary clarifier for algal biomass separation (0.025 m(2), without recirculation), were fed with urban wastewater for a one-year period (June 1993 to July 1994). The HRAPs were installed on the roof of the Department of Hydraulic, Coastal and Environmental Engineering of the Technical University of Catalonia, Barcelona, Spain (lat. 41 degrees 24' 42 " N; long. 2 degrees 7' 42 " E). Nitrogen removal efficiency and changes in total nitrogen, total organic nitrogen, NH4+-N, and oxidized nitrogen under different hydraulic retention times (HRTs) were compared. HRAP A was always operated at a higher HRT than HRAP B. Both HRAPs were subjected to the same environmental conditions of solar radiation, air temperature and influent water quality. Grab samples of influent, effluent of the HRAP (mixed liquor) and final effluent from the clarifiers were taken once a week. The annual average nitrogen removal was 73% for HRAP A, and 57% for HRAP B. Higher removal in HRAP A was due to a lower inorganic nitrogen concentration in its effluent. Significant differences (p < 0.05) in the relative proportions of nitrogen forms between the two HRAPs were observed only in autumn and winter. This was mainly because HRAP B did not achieve a high level of NH4+-N removal by stripping and algal uptake, as observed in HRAP A. NH4+-N stripping was the most important mechanism for nitrogen removal (mean efficiency of 47% and 32% in HRAP A and B, respectively) followed by algal uptake, and subsequent algal separation in the clarifiers (mean efficiency of 26% and 25% in HRAP A and B respectively). The conclusion of this study is that HRT determines both the nitrogen removal efficiency and the distribution of nitrogen forms in the effluent of a HRAP. The nitrogen removal level can be controlled through suitable HRT operating strategies. By operating at a HRT of 4 days in spring and summer, and 10 days in autumn and winter, nitrogen concentration in the effluent of a HRAP system can be reduced to less than 15 mg L-1 N.