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Abstract
The on-going development of microalgal wastewater treatment technology has seen the recent implementation of membrane photobioreactors (MPBR) and polyculture systems, that are designed to enhance biomass production, harvesting and nutrient uptake simultaneously. Previous research has attempted to assess these systems using conventional analytical techniques, for example, gravimetric methods. This project aims to extend previous research by providing advanced MPBR system characterisation that improves understanding of the impact of operating conditions, wastewater characteristics, and microalgal species on biomass production, wastewater treatment and membrane permeability (if applicable). In doing so, potential strategies to enhance overall system performance can be identified. Through application of flow cytometry, it was determined that the homogeneity of a microalgal culture tended to reduce during long-term operation. The heterogenous biomass increased the complexity of the MPBR system characteristics. It was identified that the system parameters and operating conditions of microalgal bioreactors need to be assessed holistically, rather than being considered independently. For example, the MPBRs were found to produce faster growing, more dewaterable and less heterogenous cultures with smaller flocs at lower SRTs or when fed with wastewater containing mainly nitrate (instead of ammonium). However, these conditions resulted in reduced nutrient removal, and higher concentrations of organic foulants (particularly biopolymers) that led to more severe membrane fouling during high-flux tests. Compared to homogeneous systems, the use of polycultures was able to achieve more balanced system performance, which appears to be a feasible strategy to address such trade-offs. Co-culturing microalgae with bacteria was also observed to improve bioreactor efficiency if controlled properly, where it was demonstrated that a microalgae/activated sludge inoculation ratio of 10:1 (w:w) provided better overall performance compared to other ratios (5:1 and 20:1). In addition, higher bacterial abundance in the cultures enhanced system recovery during simulated hazardous events. Overall, an appropriately designed microalgal consortia, rather than monocultures, is recommended for future implementation of MPBR-based wastewater treatment.