Application of forward osmosis (FO) membrane to microalgae cultivation processes enables concentration of microalgae and nutrients with low energy consumption. To understand fouling characteristics of FO membrane in concentration of microalgae culture, we studied flux decline, flux recovery by cleaning, and foulants characteristics, in different membrane orientation of active-layer-facing-feed-solution (AL-FS) and active-layer-facing-draw-solution (AL-DS) modes. Batch concentration of Chlorella vulgaris was conducted with a cellulose-triacetate FO membrane. Rapid flux decline and lower flux recovery was observed in AL-DS mode because of inner-membrane fouling including internal pore clogging, adsorption and internal concentration polarization in the support layer. A proportion of polysaccharides in extracellular polymeric substances to soluble microbial products were larger in chemical cleaning effluent than physical one in AL-DS mode, although those were not significantly different in AL-FS mode. Excitation-emission matrix analysis revealed that proteins and humic-like substances were also possible irreversible foulants both in AL-DS and AL-FS modes.

Treated sewage is a promising source of nitrogen and phosphorus in microalgae biomass production for carbon-neutral biofuel and chemical products. In this study, Chlorella vulgaris was continuously cultivated in membrane photobioreactors (MPBRs) under short hydraulic retention times (HRTs) and with different numbers of submerged membrane modules to investigate potential microalgae productivity when treated sewage was used as a nutrient source. Microalgae biomass concentrations were independent of HRT in MPBRs with one membrane module owing to microalgae biomass deposition on the membrane. Installation of an additional submerged membrane module effectively reduced deposition on the submerged membrane, resulting in increased microalgae biomass concentration and volumetric productivity. Growth kinetics suggested that HRT is the essential parameter influencing the volumetric productivity of microalgae under nutrient-limited conditions, and that optimization of the biomass concentration, which depends on the surface/volume ratio of the photobioreactor and initial light intensity, is critical to maximization of the volumetric productivity under light-limited conditions.

The green microalgae Chlorella vulgaris, which is known to accumulate valuable substrates, are one of the promising bio-resources. To achieve the mass cultivation of microalgae, anaerobic digestion supernatant was applied as cultivation media in membrane photobioreactor system. In the experiment, the synthesized anaerobic digestion supernatant mimicked from activated sludge collected from an actual wastewater treatment plant in Ishikawa, Japan, was used as cultivation media. In the point of view of nutrient loading, the diluted digestion supernatant satisfied the level for growth. Especially, N/P ratio was 3.8 with high concentration of P, which was previously reported as limiting factor when treated sewage was used as media. Although the maximum biomass productivity (5.58 mg/L/day) was below the previously reported data with treated sewage, the high P concentration in anaerobic digestion supernatant could be utilized as a "supplement" in media.

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