One of the biggest problems plaguing the fast-growing field of membrane-based water purification is bio-fouling. Bacteria and other microorganisms form a slimy layer that adheres to the surfaces of membranes, filters, pipes and other components of a reverse osmosis or ultra-filtration 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 shut-down 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. Biofilm in the permeate stream can reach a thickness where it sloughs off, resulting in bacteria-laden particles in the product stream.
Chlorine, hypochlorite, and related chemicals are not effective at controlling biofilm and can quickly damage membranes. Some disinfectants are only partly 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.
Published test results demonstrate that pure chlorine dioxide supplied in ready-to-use aqueous solution can remove bio-fouling and prevent its regrowth on Thin Film Composite (TFC) membranes and filters in reverse osmosis or ultrafiltration system 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. These impurities are not present in CDG Environmental products.