Influence of phytoplankton composition on biomass removal from high-rate oxidation lagoons by means of sedimentation and spontaneous flocculation

The objective of this study is to analyze the influence of phytoplankton species composition on biomass removal from high-rate oxidation ponds or lagoons (HROPs) by means of sedimentation with spontaneous flocculation. Two experimental systems each consisting of an HROP (with a surface area of 1.54 m(2) and depth of 0.3 m) with a clarifier (surface area = 0.025 m(2)) for phytoplankton biomass removal were operated with different hydraulic retention time criteria over a period of a year. Phytoplankton biomass removal rates reached a yearly average of 70 to 80% in both HROPs. During periods with large amounts of the algal species Scenedesmus acutus, which was present as discrete, unflocculated particles, both systems operated unsatisfactorily, with effluent total suspended solids (TSS) concentrations greater than 30 mg/L. Phytoplankton biomass removal from HROPs by means of sedimentation and spontaneous flocculation is unreliable for year-round use (at least for 6 months of the year) if TSS effluent concentration is required to be less than 30 mg/L. Additional treatment, such as pH or chemical augmentation, is needed to reduce TSS and improve the effluent quality of HROPs with clarifiers.