Algal bioflocculation and energy conservation in microalgal sewage ponds

It is estimated that, under suitable climate conditions, a high-rate pond system employing bioflocculation-sedimentation for algal removal would require less than one-half the direct energy input of an equivalently sized activated sludge or trickling filter plant.

Abstract

Controlled bioflocculation for harvesting of microalgae produced during municipal wastewater treatment in high-rate ponds was investigated. Nonflocculant algal cultures were produced in high-rate ponds operated at very high dilution rates or with poor mixing. Bioflocculation of such cultures was achieved by isolating them in secondary ponds, but isolation periods of up to 29 days were required. In-pond sedimentation of flocculant algal cultures produced by the isolation technique resulted in algal removals consistently exceeding 80%. When high-rate ponds were operated with improved mixing and at moderate-to-high dilution rates, flocculant algal cultures were developed. The settleability of flocculant algal cultures produced in this manner averaged 76 to 80% when measured in 24-h-detention Imhoff cones and 71% when measured in 48-h-detention settling ponds. It is estimated that, under suitable climate conditions, a high-rate pond system employing bioflocculation-sedimentation for algal removal would require less than one-half the direct energy input of an equivalently sized activated sludge or trickling filter plant. This requirement could be provided entirely through complete utilization of biogas produced from anaerobic digestion of primary (sewage) sludge.