Semi-continuous removal of nutrients and biomass production from domestic wastewater in raceway reactors using Chlorella variabilis TH03-bacteria consortia

Abstract

Abstract Production of algae biomass from wastewaters have been considered as an economical strategy for feasible commercialization of microalgae industry. Technically, large scale cultivation of microalgae in wastewaters should be conducted in semi-continuous and/or continuous modes, in which stable health of algal cultures of previous generations for the use of later generations needs to be ensured. In this regards, this study has investigated stability of growth and pollutant removal capability of Chlorella variabilis TH03-bacteria consortia using domestic wastewater as the sole of nutrient source in raceway ponds. Experiments were performed in semi-continuous mode with the aim of recovering valuable nutrients including carbon, nitrogen and phosphorous from the waste stream to algal biomass. Data revealed that prior domestication of C. variabilis TH03 in unsterilized domestic wastewaters established seed cultures of C. variabilis TH03–bacteria consortia, which enabled to grow effectively in new wastewaters under highly variable conditions. Moreover, once nutrient levels dropped below those required by the national technical regulation on domestic wastewater QCVN 14 : 2008/BTNMT, the replacement of 80% volume of the algal–bacteria culture with new wastewater media was recommended to ensure both culture stability as well as high pollutant treatment capacity of the raceway systems. Under the optimal replacement, biomass concentration and areal biomass productivity of 1.67–1.85 g/L and 11.1–15.3 g/m2 day, respectively, were steadily achieved over 148 days of the outdoor cultivations. Removal efficiencies of nutrients in the form of chemical oxygen demand (COD), total nitrogen (TN) and total phosphorous (TP) removed by the C. variabilis TH03-bacteria consortia in the reactor systems were determined as 64.7–90.7, 85.1–96.8 and 99.7%–100%, respectively. The obtained results are useful for further optimization including design and manage C. variabilis TH03-bacteria consortia based continuous processes in larger scales for the production of high volume and low price biomass, helping to obtain returns from treatment of the waste stream, which is currently considered as a major source of water pollution in Vietnam.