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 sp ecies composition on biomass removal from high-rate oxidation ponds or lago ons (HROPs) by means of sedimentation with spontaneous flocculation. Two ex perimental 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 p hytoplankton biomass removal were operated with different hydraulic retenti on time criteria over a period of a year. Phytoplankton biomass removal rat es 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 a s 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 a nd 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 clarif iers.