Assessing the performance of membrane photobioreactors (MPBR) for polishing effluents containing different types of nitrogen
2020
Luo, Yunlong | Le-Clech, Pierre | Henderson, Rita K.
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.
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