Biofouling of Reverse Osmosis Membranes

Abstract

Reverse osmosis (RO) membranes used for the treatment of industrial and municipal process waters often become biologically fouled. The development of a microbial biofilm on the feedwater surfaces of RO membranes results in several adverse effects, including: (i) a gradual decline in the membrane water flux, (ii) an increase in the transmembrane operating pressure [i.e. an increase in the membrane delta-p], and (iii) a reduction in membrane mineral rejection. The RO membrane polymer itself may also be directly or indirectly biodegraded by the adherent microorganisms. Bacterial colonization of the permeate [i.e. product-water] surfaces of RO membranes can also occur. Although the extent of biofilm formation on the permeate surface is typically quite low compared to that on the feedwater surface, it can result in microbial contamination of downstream processes, which may be of great concern in ultra-pure water applications.

Over the last decade, the Orange County water District in southern California has conducted basic and applied research on the mechanism of bacterial adhesion and biofilm formation on RO membranes employed in advanced wastewater treatment. Although this research has been performed principally at Water Factory 21, a 0.66 m³/s wastewater reclamation facility incorporating cellulose acetate [CA] type RO membranes, the general conclusions should extrapolate well to most other RO applications. The primary results of the research are summarized below:

• (1) RO Biofilm Bacteria: Early biofilm formation on cellulose acetate membranes used at Water Factory 21 is initiated by acid-fast mycobacteria, which can also be found in significant numbers in the RO feedwater. After some weeks or months of continuous operation, the mycobacteria are eventually replaced by a more diversified microbial community. Other researchers have demonstrated that different types of biofouling bacteria, such as species of Pseudomonas, Acinetobacter, Staphylococcus and others may predominate in early biofilm development at RO facilities located elsewhere. The type of biofouling bacteria that predominates at a particular RO facility depends on the physicochemical and microbiological composition of the feedwater and whether a biocide, such as chlorine, has been added.

• (2) Biofilm Growth Rate: Biofilm formation typically occurs in an exponential fashion when a new membrane element is placed into operation. The early increase in microbial biomass is correlated with a corresponding decline in RO membrane flux.

• (3) Bacterial Adhesion Kinetics: Laboratory tests indicate that mycobacterial adhesion to RO membranes occurs very rapidly with no discernable lag phase. An initial rapid rate of bacterial adhesion occurring over the first one or two hours is usually followed by a more gradual linear increase in adsorbed cells.

• (4) Adhesion Mechanism: Laboratory studies have also shown that the mycobacteria adhere to CA and possibly other RO membrane surfaces primarily by means of a hydrophobic interaction. Consistent with this hypothesis is the observation that adhesion can be largely inhibited by relatively low concentrations of certain non-ionic surfactants. Large changes in the medium pH or other ionic conditions generally result in much smaller inhibitory effects on mycobacterial adhesion. Furthermore, bacteria which exhibit a strongly hydrophobic cell surface, such as the mycobacteria, typically display more rapid adhesion kinetics than hydrophilic bacteria.

• (5) Adhesion to Different Membranes: Finally, there appears to be a direct correlation between the extent of mycobacterial attachment and the hydrophobicity of the RO membrane polymer itself. Other properties of the RO membrane which may also influence bacterial adhesion include (i) the magnitude and sign of the membrane charge, (ii) the charge orientation and distribution, (iii) the membrane porosity or density, and (iv) the surface ultrastructure of the membrane.

Several strategies are currently employed to prevent or control microbial biofilm formation in RO systems. These strategies include: (i) refinement of feedwater pretreatment, e.g. by improving prefiltration or disinfection, (ii) reducing the system operating pressure or recovery, (iii) increasing the frequency of membrane cleaning or improving the cleaning formulation, and (iv) changing the type of RO membrane. Additional research is needed to develop novel RO membrane polymers and module configurations having lower biofouling potentials.