Predicting attenuation of solar radiation (UV-B, UV-A and PAR) in waste stabilization ponds under Sahelian climatic conditions.

Because of its importance in pathogen removal and algal productivity in waste stabilization ponds, sunlight penetration was measured in microcosms and in situ under Sahelian climatic conditions. The different wavelengths were detected using a submersible radiometer equipped with three sensors: UV-B (311 nm), UV-A (369 nm) and photosynthetically available radiation (PAR, 400-700 nm). UV-B was more attenuated than UV-A and PAR. Facultative pond was more light-attenuating than maturation pond. The mean euphotic depths for UV-B were 0.20 and 0.31 m, respectively, in the facultative and maturation ponds; PAR penetrated deeper with mean euphotic depths of 0.27 and 0.42 m, respectively. The mean Secchi depths were 0.16 and 0.10 m in the maturation and facultative ponds waters, respectively. In view of the reported results, the contribution of the deeper sections of ponds to pathogen removal mediated by sunlight seems negligible. Therefore, when designing WSPs, these findings should be considered to increase the penetration of damaging wavelengths in order to ensure efficient microbial removal. For more pathogen elimination, downstream shallow ponds could be considered. The paper also shows how suspended solids, turbidity, and Secchi depth are related to the attenuation coefficients and euphotic depths. The developed models could be used to predict light penetration and then algal growth and pathogen removal mediated by sunlight in waste stabilization ponds located in Sahelian climate.