Bacterial community composition and structure of biofilms developing on nanofiltration membranes applied to wastewater treatment

The structure and microbial communities of biofilms developing on cross-flow nanofiltration (NF) membranes at different temperatures (20, 25 or 34 °C) and operation lengths (8 h–24 days) were studied. Feedwater comprised tertiary quality wastewater effluent or synthetic media mimicking effluents of intermediate quality. After each run, the membranes were autopsied for bacterial enumeration, bacterial community composition and microscopy visualization (SEM, CLSM and AFM/NSOM). Community composition was analyzed by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) coupled with sequence analysis of 16S rRNA gene fragments from dominant bands. Deposition of polysaccharides and initial bacterial colonization were observed within 8 h, whereas developed biofilms markedly affecting membrane permeability were evident from days 2–3 onwards. Regardless of applied conditions, the heterotrophic plate counts in the biofilm were 3–4×106 CFU/cm2 and the thickness of the biofouling layer was 20–30 μm. From a total of 22 sequences obtained from 14 independent experiments, most species identified were Gram negative (19 of 22 sequences). Proteobacteria were found to be a prevalent group in all cases (16 of 22 sequences) and among it, the β-subclass was the most predominant (8 sequences), followed by the γ-subclass (5 sequences). Pseudomonas/Burkholderia, Ralstonia, Bacteroidetes and Sphingomonas were the dominant groups found in most cases. Even though the microbial population might be important with respect to biofouling patterns, membrane permeability decline seems to be more substantially influenced by the formation and accumulation of exopolymeric substances (EPS).