Abstract
The present study examined fouling in a microalgal membrane bioreactor under mixotrophic, heterotrophic and photoautotrophic conditions. N-enriched wastewater, containing nitrate source, was used as a nutrient source for cultivation of microalgae. The results confirmed that the membrane fouling rates increased under mixotrophic cultivation through enhanced production of carbohydrates in soluble microbial products (SMPc) and protein in extracellular polymeric substances (EPSp). The transmembrane pressure (TMP) jumping was observed under mixotrophic and photoautotrophic cultivation after 31 and 47 days of operation, respectively, while the TMP of heterotrophic cultivation did not exceed 10 kPa throughout 51 days. The highest EPSp was produced under mixotrophic condition due to high nitrogen removal rate. Also, the results of resistance analysis indicated that cake resistance was the main fouling resistance in all cultivation types and the latter result was confirmed by SEM analysis. In addition, higher protein fraction of cake layer on membrane foulants in comparison to carbohydrates fraction increased the hydrophobicity of membrane's surface in all cultivations (except heterotrophic culture). Compared to mixotrophic and photoautotrophic cultures, hydrophobic properties and cell size of heterotrophic microalgae increased and resulted in low membrane fouling rates.
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