Removal of nutrients and COD through co-culturing activated sludge and immobilized Chlorella vulgaris

Simultaneous removal of inorganic nutrients (nitrogen and phosphorus) and organic carbon (glucose) was realized from synthetically-made municipal wastewater using co-culture of suspended activated sludge and immobilized Chlorella vulgaris in a single reactor. Use of immobilized microalgae and suspended activated sludge can help in separating microalgae biomass from the culture broth at the end of treatment. Efficient removal of nutrients and COD was achieved by the symbiotic co-culture than stand-alone cultures of C. vulgaris and activated sludge after 2-d of retention time. By decreasing inoculum ratio of suspended activated sludge to immobilized Chlorella vulgaris (from 5.0 to 0.2), the performance of nutrients removal was enhanced significantly, and the co-culture at 0.5 inoculum ratio accomplished highest removal of nitrogen (99.8%) and phosphorus (100%) within 2 days of incubation. In the case for the removal of COD, co-culture with different inoculum ratios demonstrated virtually similar performance (showing 90-95% removal after 2-d treatment) which is indicating that there was no robust association between carbon degradation and inoculum ratio. The maximum growth of microalgae (0.76 from initial 0.4 g L-1) was obtained from the co-culture having 0.5 inoculum ratio indicating significance of proper inoculation. Co-culture at 0.5 inoculum ratio of suspended activated sludge and immobilized C. vulgaris was advanced in the semi-continuous mode treatment. By repeating batch cycles for three times, the co-culture considerably eliminated 98-100% nitrogen, 92-100% phosphorus, and 94-96% COD and accumulated 2.2 g L-1 of biomass production confirming the stability of the current co-culture system in municipal wastewater medium.