Frequency of CO2 supply affects wastewater microalgal photosynthesis, productivity and nutrient removal efficiency in mesocosms: implications for full-scale high rate algal ponds

Abstract

Carbon limitation in domestic wastewater high rate algal ponds (HRAPs) is thought to constrain both microalgal photosynthesis and biomass production. CO2 augmentation is one way of overcoming this carbon limitation; however, high carbon demands due to large microalgal biomass means that the frequency of CO2 supply is likely to be critical to its success. This paper investigates the hypothesis that decreasing the frequency of CO2 addition reduces the performance of wastewater microalgae. Microalgae were cultured in outdoor high rate algal mesocosms to assess the frequency of carbon dioxide (CO2) addition on photosynthetic performance, biomass production and nutrient removal. Microalgal productivity significantly increased with high frequency of CO2 addition. Organic biomass was 120 % higher, and total microalgal biovolume was 157 % higher at high-frequency compared to low-frequency CO2 addition. Photosynthetic efficiency increased, and algae were less photo-inhibited as the frequency of CO2 addition increased, while microalgae with low CO2 addition frequency did not differ from those without CO2 addition. In conclusion, low-frequency CO2 addition, such as that which could be expected in large HRAPs with long circuit times and a single point of CO2 addition, did not improve microalgal productivity. While CO2 addition enhances microalgal photosynthesis and productivity when good pH control is maintained through day-time, this study has demonstrated that, due to rapid assimilation of CO2 by the microalgae, having an effective CO2 addition system in a full-scale HRAP is important if the benefits of CO2 addition are to be realised.