|M.Sc Student||Mizrahi Roi|
|Subject||Evaluation of Feed Water Parameters to Determine Potential|
for Biofilm Development and Membrane Clogging
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Carlos Dosoretz|
|Professor Tom Berman|
|Full Thesis text - in Hebrew|
The phenomenon called biofilm (BF) causes huge damage to the water industry. Biofilm is caused by bacteria that are able to adhere to surfaces and secrete an extracellular matrix called extracellular polymeric substances (EPS).
The goal of this study was to examine possible relationships between water biological quality parameters and the rate of biofilm formation and membrane clogging. Six parameters were examined: Transparent exopolymer particles (TEP), Bacterial activity (BA), Chlorophyll (Chl), Turbidity (Tu), Particle count (PC) and Bacterial count (BC). TEP are negatively charged, transparent organic particles composed mostly of polysaccharides, ranging from 0.4-200 μm in size.
The influence of these parameters on BF formation was examined by utilizing a Cross Flow Membrane Array system Lake Kinneret water equipped with tubular ultrafiltration membranes (100 kDa cut off). The rate of membrane clogging (NΔL) was quantified as the coefficient of the exponential equation describing the decrease in membrane permeability with time.
Three series of runs were performed: Series 1 (19 experiments) with raw and diluted Lake water (~8 hours in full batch mode at Reynolds=600); Series 2 (18 experiments) with micro-filtered (~1 μm) Lake water (72 hours in full batch mode at Reynolds=2500); Series 3 (16 experiments) with coarse-filtered (20?m) Lake water (~50 hours in a continuous flow regime at Reynolds=4500). In all three series NΔL was significantly correlated with feed water concentrations of TEP (between 40%-50% of the variability). Bacterial counts in Series 1 and 2 were also significantly correlated with NΔL.
A fourth series of experiments was aimed to study the effect of bacterial inactivation (e.g., chlorination) on BF development. The biofilm that had formed on the membranes was evaluated by confocal laser scanning microscopy which provided quantitative data on the contributions of EPS and bacteria to the biofilm. The results showed that regardless of whether bacteria were metabolically active or dead, the contribution of EPS was similar. These experiments indicate that most of the EPS in the initial BF originated from TEP in the feed water and not from secretion by attached bacteria.
Taken together the results of this study indicate that TEP levels in feedwater are directly related to the rate of membrane clogging and also the important role played by TEP in the formation of aquatic biofim. Furthermore, the results presented here contribute to general understanding about the process of aquatic BF formation and highlight the role of TEP as a factor in causing biofouling.