Nutrients Removal and Recovery in a Two-Stage High-Rate Algal Wastewater Treatment System

Abstract

Removal of wastewater borne nutrients by high rate algal ponds for the control of eutrophication in receiving bodies of water was studied under laboratory and outdoor conditions. Due to limitations of areal productivity and stoichiometry of nitrogen/algal bio-mass and phosphorus/algal biomass only less than 40 percent removal of nitrogen and phosphorus from strong municipal wastewater can be expected through biomass incorporation in one-stage high-rate algae ponds. Nitrogen removal of 74 percent can be attained by supplementary removal through ammonia atmospheric evolution from pond surface while phosphorus removal of 95 percent is attained following algae flocculation with alum. A second-stage pond following a “coarse” algae separation is needed to exert the full incorporation of nutrients into algal biomass. Effluent from the second stage, following “refined” algae separation, contain less than 4 mg/ℓ and less than 0.5 mg/ℓ of nitrogen and phosphorus, respectively. The final second stage effluent sustained reduced further algae productivity even when resupplemented by N and P and only addition of other macro and micro nutrients triggered additional significant algae growth, indicating depletion of algae growth potential factors other than N and P by the preceding two stages. The possibility of recovering the removed nutrient in the form of biomass proteinaceous animal feed is also discussed.