Wastewater treatment High Rate Algal Ponds with CO2 addition could provide cost-effective and efficient tertiary-level wastewater treatment with the co-benefit of algal biomass production for biofuel use. Wastewater grown algal biomass can have a lipid content of 10–30% of dry weight, which could be used to make biodiesel. This research investigated algal biomass and total lipid production by two pilot-scale wastewater treatment HRAPs (4-day HRT) with and without CO2 addition under New Zealand mid summer (Nov–Jan) conditions. The influence of CO2 addition on wastewater treatment performance was also determined. CO2 was added to one of the HRAPs (the HRAPE) by maintaining the maximum pH of the pond below 8. Measurements of HRAP influent and effluent water qualities, total lipid content and algal biomass production were made twice a week over the experimental period.

Both HRAPs achieved high levels of organic compound and nutrient removal, with >85% SBOD5, >92 NH4+-N and >70% DRP removal. Algal/bacterial biomass production in the HRAPE (15.2 g/m2/d) was improved by CO2 addition by ∼30% compared with that of the control HRAPW (10.6 g/m2/d). Total lipid content of the biomass grown on both HRAPs was slightly reduced (from 25% to 20%) with CO2 addition and the maximum total lipid content of ∼40% was observed in the HRAPW when low NH4+-N concentration (<0.5 mg/L) and high maximum pH (>10.0) occurred. Total lipid content of the biomass increased by ∼15% under nitrogen limiting conditions, however, overall algal/bacterial biomass production was reduced by half during the period of nitrogen limitation. More research is required to maintain algal production under near nitrogenlimiting conditions.

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