Characterisation of organic matter in membrane photobioreactors (MPBRs) and its impact on membrane performance

Understanding the behaviour of organic materials in bioreactors is an important part of system assessment. This paper focuses on the combined use of liquid chromatography – organic carbon detection and fluorescence excitation–emission matrix spectroscopy for characterising the fate of dissolved organic compounds in membrane photobioreactors operated under different hydraulic and solids retention time. It was found that small amino acids/peptides were largely removed in the reactors, which coincided with the production of biopolymers. The biopolymers were effectively retained (>84%) by the ultrafiltration membrane, in contrast with other algal-derived substances, such as quinones, flavonoids and extracellular chlorophyll a. During long-term low-flux operation (4.5L/m²h), preferential deposition of organic matter (e.g. biopolymers) and particles on different fouling layers was observed. The upper fouling layer (obtained by rinsing) was found to contain the greatest amount of organic foulants (211–348mg/m²) and solids (7.5–15.5g/m²). By contrast, chemically desorbing layer showed the lowest degree of organic matter build-up, whereby low molecular weight acids were the most abundant organic material. The short-term flux-stepping trials conducted for the same microalgal biomass revealed distinct results, presumably due to the different fouling mechanism involved, where preferential deposition of foulants was not expected given the high driving force.