How a High-Purity Chlorine Dioxide Solution Can Remove Biofilms and Prevent Regrowth without Membrane Damage
By Thomas E. McWhorter
Published test results demonstrate that pure chlorine dioxide supplied in readyto-use aqueous solution can remove biofouling and prevent its regrowth on thin-film membranes and filters in reverse osmosis (RO) or ultrafiltration (UF) systems without damage to membranes. Previous results showing membrane damage from chlorine dioxide are believed to be caused by impurities such as chlorine or chlorous acid in chlorine dioxide made at the point of use (POU).
One of the biggest problems plaguing the fast-growing field of membranebased water purification is biofouling. Bacteria and other microorganisms form a slimy layer that adheres to the surfaces of membranes, filters, pipes, and other components of an RO or UF system. Biofilm consists of bacteria and other microorganisms bound together and to a surface by polysaccharide “glue”. Biofilm formation is especially problematic in water containing high levels of nutrients such as tertiary municipal wastewater and other wastewater streams. Biofilm blocks the flow of water through membranes and filters and necessitates frequent shutdown for off-line cleaning or replacement of elements. Off-line cleaning often results in damage to expensive membranes.
Biofilm usually forms most quickly on the feed side of the membrane, but it can also form in the permeate stream or even within the membrane (1). Biofilm in the permeate stream can reach a thickness where it sloughs off, resulting in bacterialaden particles in the product stream.
Chlorine, hypochlorite, and related chemicals are not effective at controlling biofilm and can quickly damage membranes (1). Some disinfectants are only somewhat effective against biofilm in part because the disinfectants cannot penetrate the polysaccharide matrix. Therefore, when membrane performance is compromised by biofilm, the membrane must be taken off-line for mechanical cleaning and disinfection. During the shutdown for off-line cleaning, the total system must be shut down, causing loss of production, or a secondary set of membranes must be provided, often at substantial cost. Off-line disinfectants never remove 100% of the biofilm so biofilm re-seeds itself and grows back faster after every cleaning (1).
In many membrane systems the water is prefiltered using cartridge filters to remove particles as small as a fraction of a micron before the water passes through the membranes. These filters are often subject to rapid biofouling by microorganisms that grow in the nutrient-rich media. This fouling usually necessitates shutdown and replacement of filter cartridges.
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