Wastewater treatment and algal production in high rate algal ponds with carbon dioxide addition

High rate algal ponds (HRAPs) provide improved wastewater treatment over conventional wastewater stabilisation ponds; however, algal production and recovery of wastewater nutrients as algal biomass is limited by the low carbon:nitrogen ratio of wastewater. This paper investigates the influence of CO₂ addition (to augment daytime carbon availability) on wastewater treatment performance and algal production of two pilot-scale HRAPs operated with different hydraulic retention times (4 and 8 days) over a New Zealand Summer (November–March, 07/08). Weekly measurements were made of influent and effluent flow rate and water qualities, algal and bacterial biomass production, and the percentage of algae biomass harvested in gravity settling units. This research shows that the wastewater treatment HRAPs with CO₂ addition achieved a mean algal productivity of 16.7 g/m²/d for the HRAP_4d (4 d HRT, maximum algae productivity of 24.7 g/m²/d measured in January 08) and 9.0 g/m²/d for the HRAP_8d (8 d HRT)). Algae biomass produced in the HRAPs was efficiently harvested by simple gravity settling units (mean harvested algal productivity: 11.5 g/m²/d for the HRAP_4d and 7.5 g/m²/d for the HRAP_8d respectively). Higher bacterial composition and the larger size of algal/bacterial flocs of the HRAP_8d biomass increased harvestability (83%) compared to that of HRAP_4d biomass (69%).