Algae scrubbers for phosphorus removal in impaired waters

Algae scrubbers are a developing technology used for nutrient removal from many different impaired waters, including: agricultural runoff, wastewater, and animal operation waste. Algae thrive in environments with high flow velocities, allowing treatment of large volumes of nutrient-rich water. This adaptation helps overcome space issues associated with other biological treatment systems. Although algae scrubbers have shown promise for recovering phosphorus (P), no detailed studies have been completed to research conditions for maximum total P (TP) removal. Previous studies have suggested that faster flow rates and pulsed inflow conditions are necessary for high biomass production. Studies have also theorized that the calcium-phosphorus (Ca-P) co-precipitation is the driving TP removal mechanism. As a result, the goal of this study was to determine the effect of the following operating parameters on TP removal: (1) flow rate, (2) pulsed versus constant inflow, (3) calcium addition to inflow water, (4) calcium seed addition, and (5) 24-h versus 12-h operation. Results show that 12-h operation significantly increases TP removal in algae scrubber systems from an average of 24g/m²/y to 52g/m²/y. The high TP removal rate can be attributed to the stabilization of Ca-P minerals during 12-h operation. Algal photosynthesis increases daytime pH values (9-9.5) driving Ca-P co-precipitation, and 12-h operation prevents these minerals from re-dissolving at night as the pH decreases back to neutral (7-7.5). The maintenance of an algal seed on the scrubbers significantly decreased the amount of time necessary between harvests, increasing algal productivity.