Influence of C/N ratios on treatment performance and biomass production during co-culture of microalgae and activated sludge

Novel phycosphere associated bacteria processes are being regarded as a potential and cost-effective strategy for controlling anthropogenic contaminants in wastewater treatment. However, the underlying concern with the process is its vulnerability to improper organic or nutrient intake. This study established a synergistic interaction between microalgae and activated sludge in a three-photobioreactor system (without external aeration) to understand how pollutants could be mitigated whilst simultaneously yielding biomass under different C/N ratios of 1:1, 5:1 and 10:1. The result showed that the superior biomass productivity was facilitated at a C/N ratio of 5:1 (106 mg L-1 d-1), and the high degradation rate constants (kCOD = 0.25 d-1, kTN = 0.29 d-1, kTP = 0.35 d-1) was approximated using a first-order kinetic model. The removal of pollutants was remarkably high, exceeding 90% (COD), 93% (TN), and 96% (TP). Nevertheless, the C/N ratio of 1:1 resulted in a threefold drop in biomass-specific growth rate (μ = 0.07 d-1). Microalgal assimilation, followed by bacterial denitrification, is the major pathway of removing total nitrogen when the C/N ratio exceeds 5:1. Activated sludge plays an important role in improving microalgae tolerance to high concentration of ammonia nitrogen and boosting nitrification (light phase) and denitrification (dark phase). The use of phycosphere associated bacteria could be a promising strategy for controlling nutrients pollution and other environmental considerations in wastewater.