Submerged hollow-fiber-ultrafiltration for harvesting microalgae used for bioremediation of a secondary wastewater

Microalgae were employed to remove nutrients from a pre-treated wastewater (so-called anaerobic membrane bioreactor effluent, AnMBR effluent) over a 24-day semi-continuous cultivation with varied N:P ratios. NH₄⁺-N reduction percentage ranged from 68 to 94% with average removal rates varying from 8 to 26 mgN·L^-1d^-1 at different stages. Total phosphorus was almost completely removed shortly after inoculation. Biomass productivity ranged from 0.13 to 0.19 g·L^-1d^-1 during the cultivation. A submerged membrane-based filtration device was used to investigate the formation of biofouling, which occurred on the membrane surface during filtration and is considered as a bottleneck in downstream processing. Results indicated most severe fouling was caused by original microalgal suspension, compared to its individual fractions (cellular resuspensions, supernatant and AnMBR effluent). Irreversible fouling mainly occurred at the beginning of the filtration process; extracellular organic matter and cellular debris in the supernatant changed the fouling behavior more significantly than morphological properties of the algae cells; AnMBR effluent also led to a nonnegligible fouling effect. To minimize biofouling, TOC content in wastewater should be carefully controlled before and during cultivation. The current membrane can be proposed for long-term filtration, as irreversible fouling only occurred at the beginning and reversible fouling can be controlled effectively by backflushing and aeration. The biomass, characterized by relatively high protein and carbohydrate but low heavy metal content, indicated its potential as feeds and feedstock for bioenergy production. The present work provides novel insights into the coupling of wastewater treatment and fouling control, which has been rarely studied.