Extra-cellular polysaccharides, soluble microbial products, and natural organic matter impact on nanofiltration membranes flux decline.

Extra-cellular polysaccharides (EPS), soluble microbiological products (SMP), dispersed bacterial cells, and a well-characterized natural organic matter (NOM) isolate were observed to determine their influence on the flux decline of model nanofiltration membrane systems. Biofouling tests were conducted using bench-scale, flat-sheet membrane modules, fed with particle-free (laboratory) waters and natural waters, some of which were augmented with readily biodegradable organic carbon. The modules were operated 6.7 x 10(5) Pa, and 21+/- 2 degrees C. Membrane flux-decline was associated with increases in surface EPS mass: between 30 and 80% of normalized flux decline occurred when membrane-associated EPS content increased from 5to 50 microg/ cm2. As judged by standard culturing, heterotrophic cell densities recovered from membrane biofilm samples showed no significant correlations with the different carbon sources present in the feedwaters, or flux decline rates. Results suggested that, in the absence of microbiological activity, SMP and NOM have intrinsic membrane fouling properties at levels that are operationally significant to commercial-scale membrane treatment practices. Results also suggested that SMP may have a biofouling potential significantly greater than some types of NOM. Trends obtained relating these compounds with flux decline were successfully described by expanding existing resistance-in-series models.

Ultrasonic monitoring of early-stage biofilm growth on polymeric surfaces.

Biofilm growth on polymeric surfaces was monitored using ultrasonic frequency-domain reflectometry (UFDR). The materials utilized for this study included nonporous polycarbonate (PC) sheets, polyamide (PA) nanofiltration composite membranes and porous polyvinylidene fluoride (PVDF) microfiltration membranes (nominal pore size: 0.65 microm). Coupons of each material were placed in a biologically active annular reactor for up to 300 days, and subjected to a constant shear field (0.12 N m(-2)), which induced sessile microbial growth from acetate amended municipal tap water. Acoustic monitoring was non-destructively executed by traversing coupons in a constant temperature water bath using a spherically focused 20-MHz immersion transducer. This semi-automated system was configured to obtain reflections from 50 regions (c.a. 120x10(3) microm2) distributed evenly near the centerline of each coupon. The resulting reflected power distributions were compared with standard biochemical and microscopic assays that described surface associated biofilms. When compared to clean (virgin) conditions, biofilms growing on coupons induced consistent attenuations in reflection amplitude, which caused statistically significant shifts in reflected power (p<0.01). Using exocellular polysaccharides as a surrogate measure of total biofilm mass, UFDR was able to detect biofilms developing on any of the materials tested at surface-averaged masses < or = 150 microg cm(-2). Above these threshold levels, increasing amounts of exocellular polysaccharides correlated with significant decreases in total reflected power (TRP). The distribution of biomass on the coupon surfaces determined by acoustic spectra was consistent with that observed using environmental scanning electron microscopy (ESEM). These results suggest that UFDR may be used as a non-destructive tool to monitor biofouling in a wide variety of applications.