Assessing the performance of membrane photobioreactors (MPBR) for polishing effluents containing different types of nitrogen

Abstract

Membrane photobioreactors (MPBR) have the opportunity to remove various forms of nitrogen, such as ammonium and nitrate, found in high quantities in secondary effluent. This study provides a holistic comparison of the performance of two membrane photobioreactors treating secondary effluents containing ammonium (NH4+-N) or nitrate (NO3−-N) as the sole nitrogen source. Sophisticated analyses, such as flow cytometry, polymerase chain reaction denaturing gradient gel electrophoresis, liquid chromatography – organic carbon detection analysis and fluorescence excitation-emission matrix spectroscopy, were used to provide insight into system characteristics and performance. In terms of treatment efficiency, NH4+-N was better removed from the system (by 56%) compared to NO3−-N (33%). The MPBR fed with NH4+-N produced less microalgal cells (4.4 × 107 cells/ml compared to 5.3 × 107 cells/ml in that treating NO3−-N) but higher proportion of bacteria (6.3% compared to 2.1%), leading to a higher system heterogeneity. The biomass produced from the MPBR treating NH4+-N featured a higher level of extracellular materials and a lower dewaterability (as suggested by a significantly higher capillary suction time of 25.2 s than 13.1 s observed for the culture grown on NO3−-N), indicating a lower harvesting potential. As a result, faster membrane fouling was observed when NH4+-N was fed. This leads to a recommendation that operating conditions are optimised dependent on the form of nitrogen that needs to be treated.