Characteristics of an immobilized microalgae membrane bioreactor (iMBR): Nutrient removal, microalgae growth, and membrane fouling under continuous operation

Abstract

Microalgae immobilization has been developed as a prospective method to improve the culturing and harvesting of microalgae cells in wastewater; it is also an effective method to control membrane fouling in an algal membrane system. In this study, a continuous immobilized microalgae membrane bioreactor with alginate-immobilized Chlorella vulgaris beads was developed for investigating the progress on nutrient removal and the characteristics of membrane fouling through synthetic wastewater treatment. High initial total biomass (more than 0.52 g/L) led to significant fluctuations in nutrient removal. The growth pattern of microalgae cells within the beads was explored for a better understanding of the fate of immobilized microalgae. The results showed that a moderate initial cell density of microalgae cells (7.4 × 105 cell/bead) in the immobilized beads achieved relatively higher specific growth rate than that of a high density, and higher nutrient removal efficiency than that of a low density. The fluorescence microscopy results of a hemisphere cut from the immobilized beads indicated that a shading layer was formed with high-density microalgae cells after a 2-day cultivation. Thus, setting a proper initial cell density of microalgae in immobilized beads was vital to achieve better performance on nutrient removal and microalgae cultivation and harvesting in the continuous operation of a reactor. Immobilization is highly effective for retarding membrane fouling through the reduction in suspended microalgal cells, mitigating the release of macromolecule algal organic matter from the immobilized beads, and regular replacement of the immobilized beads. The results and optimized directions discussed in this paper provide integrated clues and suggestions for future microalgal immobilization-related development.