Biofouling in forward osmosis and reverse osmosis:
Measurements and mechanisms
Sarah E. Kwan, Edo Bar-Zeev, Menachem Elimelech
We have investigated biofouling behavior in forward osmosis (FO) and reverse osmosis (RO) membrane systems. Analysis of these two systems was done via comparison of biofilm structure and membrane permeate water flux decline. Experiments were performed using a model wastewater solution inoculated with Pseudomonas aeruginosain laboratory-scale FO and RO test cells. For a meaningful comparison, biofouling runs were carried out using the same thin-film composite polyamide FO membrane, identical hydrodynamic conditions, and an initial permeate water flux of 19 L m−2 h−1. Water flux decline after 600 mL of cumulative permeate volume (or ~17–18 h of fouling) was significantly lower in FO (~10%) compared to RO (~30%). FO biofilms grew in a loosely organized thick layer, with eminent mushroom-shaped structures, while RO biofilms grew in tightly organized mats encased in larger amounts of extracellular polymeric substances (EPS) per cell. The more compact biofilms in RO induced greater biofilm-enhanced osmotic pressure and hydraulic resistance to water flow compared to FO, which resulted in higher flux decline. We attribute the differences in biofouling behaviors in FO and RO to the different driving forces: osmotic pressure in FO and hydraulic pressure in RO.
The importance of microscopic characterization of membrane biofilms in an unconfined environment
Edo Bar-Zeev, Katherine R. Zodrow, Sarah E. Kwan, Menachem Elimelech
Confocal laser scanning microscopy (CLSM) is often used to evaluate biofilm development or biofouling mitigation in membrane systems. However, several methods of CLSM sample preparation exist. In this paper, we evaluate the effects of three preparation techniques — dry, confined (wet), and unconfined (immersed) mounting — on CLSM-derived biofilm architecture and dimensions. Although placing a wet or dry biofilm between a slide and a coverslip before viewing is relatively common, our results show that this confinement significantly alters the biofilm observed. Therefore, biofilms should be viewed in an unconfined and hydrated state that allows for full extension of the biofilm structure in a media-filled viewing well of fixed depth (~ 250 μm). Pseudomonas aeruginosa biofilms were grown on thin-film composite reverse osmosis membranes and glass coupons. Dry and confined mounting of 24 and 48 h biofilms resulted in biofilms with low 3-D complexity and thickness (14 and 18 μm, respectively). Measured biofilm thickness was significantly higher on samples prepared using unconfined mounting (55 μm). Additionally, the reduction in biofilm thickness and biovolume observed after treatment with biocidal compounds was significantly less on the dry and confined biofilms than the unconfined samples. Our results strongly suggest that biofilms on membranes be prepared for microscopy using unconfined mounting to accurately assess biofilm structure and dimensions. Unconfined mounting will allow for accurate CLSM assessment of membrane biofilm structure, dimensions, and biofouling mitigation measures in membrane systems.