This schematic for an advanced wastewater treatment process uses a multi-stage pond system for complete organic waste degradation and nutrient removal. The initial wastewater treatment ponds are shown, followed by a smaller intermediate “green algae” pond for N depletion and a final pond for cultivating N-fixing blue-green algae and removing residual phosphates. CO2 supplementation would be required in the last two ponds, and could increase productivity in the initial pond. (Source: Benemann et al. 1978.)

Figure III. A.4. Growth of microalgae on pond effluents.

Graph showing typical changes in density, chloropyll a content and ammonia concentrations during batch growth of green microalgae on effluents from wastewater treatment ponds after settling of the primary culture. As the culture grew from the residual concentrations of about 20 mg to about 170 mg/L, ammonia concentrations and chlorophyll a levels decreased. A 4- or 5- day culture period appears to be optimal. (Source: Benemann et al. 1978.)