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DISINFECTION BY-PRODUCT CONTROL THROUGH BIOLOGICAL FILTRATION
Citation:
Dugan*, N R. DISINFECTION BY-PRODUCT CONTROL THROUGH BIOLOGICAL FILTRATION. Chapter 7, Clark, R.M. and Boutin, B.K (ed.), Controlling Disinfection By-Products and Microbial Contaminants in Drinking Water (EPA/600/R-01/110). USEPA, Cincinnati, OH, (2002).
Description:
Disinfection by-product (DBP) control through biofiltration is defined as the removal of DBP precursor mateterial (PM) by bacteria attached to the filte nedia. The PM consists of dissolved organic matter (DOM) and is utilized by the filter bacteria as a substrate for cell maintenance, growth, and replication. The PM utilized by bacteria is no longer available to react with chlorine to form DBPs. All other things being equal, a water with lower PM concentration will yield lower DBP concentrations, at a given chlolrine dose, after a given time period. The biological filtration process is cost effective, since the facteria are naturally present in the water supply, can colonize existing filter media, do not produce a residual that needs disposal, and require almost no modification of ambient conditions. The only prerequisite for maximizing bacterial substrate utilization in filters is the absence of disinfectant in the filte influent or backwash water. The filter media colonized by bacteria can be sand, anthracite, or granular activated carbon (GAC). Anthracite and sand are considered inert because neither interacts chemically with PM. Thus, any removal of PM would be due solely to biological activity, GAC that has een colonized by bacgeria will initially remove DOM through adsorption and biological substrate utilization. After the GAC's adsorptive capacity has been exhausted, PM removal is achieved only through substrate utilization, and the GAC is defined as biological activated carbon (BAC). All drinking water filters will become biologically active in the absence of applied disinfectant residuals. The process of biological colonization and substrate utilization is enhanced by ozonating filter influent water. In the US, preozonation is practiced to remove color, taste, and odoe, to inactivate Giardia and Cryptosporidium, and to serve as an alternative to chlorine disinfection. Ozonation decreases the average molecular size and weight of the PM, allowing indigenous bacteria to utilize more of it as substrate in a given amount of contact time. Some fraction of microbes will always survive ozonation. As long as no liquid phase ozone residual is present in the filte influent, the surviving microbes will eventually colonie the filter media. A preozonated biological filter will achieve greater PM removals than one with influent that has not been preozonated. PM is measured as total organic carbon (TOC), dissolved organic carbon (DOC), trihalomethane (THM) formation potential (THMFP), or haloacetic acid (HAA) formation potential (HAAFP). All of the studies discussed in this chapter measure DBP control using one or some combination of the parameters TOC, DOC, THMFT, and HAAFP. Because of its potential to control DBP precursors and its economic advantages, the USEPA's WAter Supply and Water Resources Division has performed or funded a number of research studies to characterize the impact of biological filtration on the control of DBPs.