Analysis of key variables controlling phosphorus removal in high rate oxidation ponds provided with clarifiers

Abstract

This study evaluates the influence of hydraulic retention time (HRT), solar radiation, and water temperature on phosphorus removal from two experimental high rate oxidation ponds (HROP) with clarifiers. Both HROPs were operated for a period of one year with different HRTs (3 to 10 d), but under the same environmental conditions. Phosphorus species, phytoplankton biomass, solar cumulative radiation, water temperature and pH were measured once a week. Average total phosphorus removal (TP) was higher in the HROP operated with a higher HRT (43%) than in that using a lower HRT (32%). TP removal was due to dissolved reactive phosphorus (DRP) removal in the mixed liquor of the HROPs and transformation of DRP into particulate phosphorus (PP), with subsequent sedimentation of PP in the clarifiers. The influence of HRT on TP removal was due mainly to its control over DRP removal, which was observed to be more important in autumn and winter. The lowering of solar radiation and temperature in autumn and winter and their influence on DRP removal can barely be compensated by HRT to obtain a significant TP removal. This work indicates that DRP transformation into PP depends mainly on environmental factors through their mediated influence on pH and chemical precipitation.


WaterSA Vol.28(1) 2002: 55-62