Integrated nutrient removal and carbon dioxide sequestration by using mixed microalgae culture

Çakırlar, Şükrü Burak
Microalgae can remove nitrogen (N) and phosphorus (P) in domestic and industrial wastewaters, which cause eutrophication in rivers, lakes and seas. Microalgae have also been recognized as a promising alternative for carbon dioxide (CO2) sequestration from flue gas. However, it is necessary to design flexible and low-cost cultivation systems and, use suitable operating conditions to achieve enhanced biomass productivities and high CO2 fixation efficiencies. The aims of this study were: (i) to determine optimum hydraulic retention times (HRTs) for cultivation of microalgae in different types of wastewaters; (ii) to compare the microalgal productivities and nutrient removal rates at different N:P ratios (iii) to propose an integrated system for the utilization of wastewater and CO2 in flue gas for the production of microalgae. Within this context, a mixed microalgae culture collected from Araç Creek in Karabük Province in Turkey was grown under batch and semi-continuous operation modes. Two types of culture mediums were used in the experiments: (i) primary treated domestic wastewater from Ankara Tatlar WWTP and (ii) KARDEMIR Coke Plant wastewater diluted with supernatant of sludge thickener tanks of Ankara Tatlar WWTP. While ambient air (0.03% CO2) was supplied to the cultures grown with primary treated domestic wastewater, CO2 enriched air (4% CO2) was sparged into the cultures grown with diluted industrial wastewater. Light and mixing (aeration) conditions were the same in all set-ups. The optimum inoculum volume was determined as 10% (v/v) conducting a batch study and was used in all experiments. The optimum HRT was found to be 2 days for cultivation of microalgae in primary treated domestic wastewater. Mixed microalgae culture was able to remove 94.7% of Total Ammonia Nitrogen (TAN) and 93.8% of orthophosphate (PO4-P) from domestic wastewater at a HRT of 2 days. Although almost complete nutrient removal efficiencies were observed during steady conditions of the cultures with 4- and 8-day HRT, the steady-state conditions could not be maintained and cell washout was observed in the reactors due to nutrient limitation. The TAN/PO4-P (g/g) ratio of 6 resulted in the maximum nutrient removal efficiency when the diluted coke plant wastewater was used in the batch-mode operation. Results of the semi-continuous study conducted with diluted coke plant wastewater revealed that HRT should be kept 8 days at minimum in order to achieve efficient TAN and PO4-P removal (>98%) and high steady-state biomass concentrations (>2.4 mg TS/L). The CO2 removal rates were highest in the culture with 12 day-HRT and, it was obtained as 0.436 g CO2/h. The results demonstrated both effectiveness and potential application of the coupled system to remove nutrients from domestic and industrial wastewaters and simultaneous CO2 removal from a point source.