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Chemical and physical disinfection of feedwater to a reverse osmosis (RO) membrane helps to prevent fouling and maintain efficient operation.
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The bane of existence for users of reverse osmosis (RO) membrane systems (Figure 1) is controlling membrane fouling from microorganisms. Autopsies of 150 membranes found that all of the membranes had some degree of membrane biofouling (Figure 2) (1). Forty-nine had microbial colony densities greater than I05 CFU/cm2, which was the direct cause of the membranes' performance decline. Biofouling was a contributing factor to the performance decline of the other 101 systems.
Biofouling is the irreversible adhesion on a membrane of microorganisms and the extracellular polymers (ECPs, Le., biofilm) that they produce. The process of adhesion involves three steps (2):
1. bacterial adhesion, which can become irreversible in just hours, even without nutrients present
2. micro-colony formation
3. biofilm maturation and the formation of ECP, which protects the bacteria from biocides, flow shear, and predators.
Properties of membranes that favor adhesion and biofilm formation include:
* surface roughness (Figure 3) - the rougher the surface, the more adhesion
* surface charge - the more neutral the charge, the more adhesion of bacteria (which are negatively charged)
* hydrophobicity - the more hydrophobic the membrane, the more adhesion.
Biofilm adhesion is also promoted by dissolved nutrients in the laminar boundary layer next to the membrane, which is called the concentration polarization layer (3). Because no convection occurs at the membrane surface, dissolved and suspended solids, including nutrients, build up (Figure 4).
Once a biofilm forms, it protects the underlying bacteria from disinfecting chemicals and flow shear forces that could disrupt the film.
Aerobic bacteria in seawater, brackish water, and wastewater systems, and anaerobic bacteria (e.giron- and sulfate- reducing bacteria) in wells, can all cause membrane biofouling. Reverse osmosis membranes used in recycle/reuse processes are exposed to an even broader range of microbes, including aerobic, anoxic, and anaerobic species.
The presence of biofouling can be indicated by (4):
* higher differential pressure, which can be evident within a few days of inoculation
* lower membrane flux, which may or may not occur prior to the change in differential pressure
* higher operating pressure required to maintain the product flowrate
* localized...