Energetic evaluation of wastewater treatment using microalgae, Chlorella vulgaris

Abstract

This research evaluates the feasibility of utilizing municipal wastewater source as growth medium for microalgae production and wastewater treatment. First, the effect of the addition of external organic carbon on the microalgae growth and wastewater treatment was studied. Second, actual municipal wastewater from two different (primary and secondary) levels of treatment of an oxidation ditch wastewater treatment plant was used as a growth medium to evaluate both the microalgae growth and wastewater treatment. Third, the effect of light under day/night cycles was evaluated using actual municipal wastewater. A comparative analysis of volumetric biomass productivities, specific growth rates and energetic evaluation of the three different experimental conditions was performed. COD removal rates were 37.5 mg/L, 87.5 mg/L, and 118.75 mg/L per day while microalgae volumetric biomass concentrations were approximately 50–80 mg/L per day with specific biomass growth rates of 0.81, 0.89, and 1.28 1/d for 125, 250, and 500 mg/L COD concentrations, respectively. COD removal, biomass production and specific growth rates are lower for the municipal wastewater due to poor availability of carbon source. However, microalgae present a promising alternative when the energetic evaluation is considered for both actual and synthetic wastewater treatment. A total of 13.5, 17.4, and 35.2% of net energy recovery was possible with microalgae harvested in synthetic wastewater at 125, 250, and 500 mg/L COD concentrations, respectively while a total of 4.7% net energy recovery was possible with microalgae harvested in raw wastewater after accounting for the energy required for wastewater treatment.