Developing algae/bacteria membrane photobioreactors for wastewater treatment : treatment efficiency, microbiological insights, and membrane fouling characterization

Abstract

Membrane bioreactors (MBRs) have gained widespread application in wastewater treatment. However, they come with disadvantages, such as membrane fouling, requiring the use of intensive mechanical aeration to manage this fouling issue. The aim of this thesis was to investigate how microalgae can be introduced into MBRs to improve the performance of MBR.

This thesis evaluated the effect of various aeration rates and the ratio of algae to total biomass on membrane fouling and purification efficiencies in algae-activated sludge membrane bioreactors (AB-MBRs). Results showed that introducing microalgae to conventional-MBRs (C-MBRs) with an appropriate inoculation ratio led to a 60% reduction in mechanical aeration without compromising purification efficiency or increasing membrane fouling. Controlling mechanical aeration allowed for the adjustment of algae cell growth compared to bacteria cells, maintaining the algaebacterial cells ratio in the MBR. In addition, this thesis explored hybrid biocarrier algae membrane bioreactors as alternatives to C-MBRs and AB-MBRs. The hybrid AB-MBR, inoculated with polyethylene biocarriers and algal cells, demonstrated superior performance for nutrient removal efficiency, balanced microbial growth, and lower membrane fouling. It also showed enhanced nitrification and ammonium biomass uptake. Bacterial sequencing analysis indicated that the bacterial communities altered in hybrid AB-MBRs, including Candidatus, Cloacibacterium, and Falavobacterium, leading to improved performance of the MBRs. Furthermore, the thesis examined the performance of seven microalgal cells collected from Finland's lakes and inoculated with activated sludge in batch and continuous bioreactors. Mixed-algae cultures (Chlamydomonas and Selenastrum) outperformed solo algae MBRs and C-MBR. Mixed algae-MBRs had a greater diversity of bacteria than solo algae MBRs. More fatty acids exist in batch algaesludge reactors than in the sludge reactor. Moreover, mixing microalgae with activated sludge revealed a reduction in both extracellular polymeric substances (EPSs) and Nhexanoyl-L-Homoserine lactone (bacterial communicating molecule) that may also reduce biofilm formation.