Effect of the organic carbon to nutrient (N and P) ratio on the biological performance of a microalgal-bacterial membrane photobioreactor

This paper provides new insights into the effect of chemical oxygen demand (COD) to nitrogen (N) ratios on the biological performance of a novel microalgal-bacterial membrane photobioreactor (MB-MPBR) system for removing COD and nutrients TN (total nitrogen) and TP (total phosphorus) simultaneously in one single reactor over a long-term period of around 300 days. The COD/N ratios in synthetic municipal wastewater were set and controlled at 10.3 : 1, 13.9 : 1, and 16.8 : 1, respectively, in each of the three phases. Results show that an organic matter COD removal efficiency of above 96.0% was achieved, denoting that the COD removal was independent of the COD/N ratio. The TN removal was enhanced significantly in the MB-MPBR when the COD/N ratio was increased (i.e., more COD), starting with 65.8 +/- 8.8% removal efficiency in the first phase, and reaching 72.0 +/- 12.8% and 81.1 +/- 9.1% in the second and third phases, respectively. The TP removal started at 66.1 +/- 8.1% in the first phase, and then a high uptake reached above 95%, which indicates the positive effect of the rising COD/N ratio. The superior efficiency of the MB-MPBR relied on the employment of the membrane module, which acted as a liquid-solid barrier, as well as the consortium of microalgae and bacteria. Overall, this study proved the potential of MB-MPBR technology to achieve simultaneous COD and nutrient (N and P) reduction in one reactor with high effluent quality.