Experimental study for the reduction of CO2 emissions in wastewater treatment plant using microalgal cultivation

Wastewater treatment plants (WWTPs) contribute to CO2 emissions in atmosphere through direct (biological metabolism) and indirect (fuel combustion) oxidation of organic carbon. This detrimental effect of WWTPs operation can be mitigated by integrating the traditional treatment with a microalgae cultivation pond where CO2 is fixed into autotrophic biomass and the positive side effect of removing nutrients also takes place. To test the feasibility of this modified WWTPs configuration, a pilot-scale 200 L raceway pond, operating outdoor, was designed and used for biomass cultivation in untreated urban wastewater. Nitrogen gas enriched with 20% CO2, simulating the exhausted gas of biogas combustion, was supplied continuously during daytime at different flowrates. The dynamics of microalgae growth as well as inorganic carbon and nutrients uptake were studied during the pond start-up and semi-continuous feeding conditions. The absorbed bio-available CO2 was monitored during daylight for different gas flowrates (0.2, 0.4 and 1.0 L/min) and for wastewater semi-continuous feeding conditions (0.8 L/h). The highest efficiency, equal to 83%, of bio-available CO2 fixation was obtained for the lowest gas flowrate of 0.2 L/min., whereas the highest CO2 removal rate of 24.6 mg/L/min was reached for the highest gas flowrate of 1.0 L/min. Furthermore, this operating condition resulted in the highest microalgae biomass productivity of 28.3 g/d/m2. Nutrients removal was complete for each operating condition tested.