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The practice of primary disinfection and the maintenance of a disinfectant residual withinthe distribution system are important in the control of microbial contaminants andbacterial re-growth. Chlorine dioxide (ClOsub2/sub) is a strong disinfectant and oxidant that hasdemonstrated promise as a secondary disinfectant in full-scale distribution systems (Volket al., 2002). The formation of organohalogens (e.g., trihalomethanes) with ClOsub2/sub istypically much lower when compared to the use of free chlorine (Cl(sub2/sub) (Hofmann et al.,1999; Werdehoff and Singer, 1987).Chlorite (ClOsub2/subsup-/sup) is a known byproduct of ClOsub2/sub (Gordon, 2001). When applied todrinking water, a portion of the ClOsub2/sub will form ClOsub2/subsup-/sup upon reaction with natural organicmatter (NOM). ClOsub2/subsup-/sup has been suggested to have potential benefits as a biocide formitigating ammonia oxidizing bacteria (AOB) which are known to cause nitrification indistribution systems. In particular, McGuire et al. (1999) reported that the occurrence ofnitrification in full-scale systems could be acutely mitigated by switching fromchloramines to chlorite. In that study no information was provided concerning long-termaffect of ClOsub2/subsup-/sup on suppressing heterotrophic microorganisms. Because ClOsub2/subsup-/sup is a byproductof ClOsub2/sub the data presented in the literature has not been clear as to whichchemical provides long-term benefit as a secondary disinfectant. Thus the primaryobjective of this project was to determine the extent of biocidal control on heterotrophicbiofilms provided by ClOsub2/subsup-/sup, relative to ClOsub2/sub, under controlled laboratory experiments andin the field. Annular Reactors (ARs), whichare widely used drinking water research, were used to represent model distributionsystems. The AR model used for this experiment was the 1120 LS (Laboratory ModelRegrowth Monitor and Annular Reactor, BioSurface Technologies Corporation, Bozeman, MT). The influent water flowedthrough an annular gap and was mixed by the rotating drum, which contains draft tubes toensure sufficient vertical and horizontal mixing. The hydraulic retention time wascontrolled by the volumetric flow rate of the influents entering the AR. The total workingvolume in the annular gap is approximately 950 mL. Each AR was set at a rotationalspeed that creates the same shear stress at the outer wall of the ARs' cylinders as thatwhich would be seen at the outer wall within the aqueduct. Using a friction factor for alarge pipe diameter of 0.01, the shear stress in the pipe was determined to be 0.68 N/msup2/sup.By estimating the Taylor vortices in the AR, as described by Camper (1996), therotational speed in the AR was determined to be 160 rpm. All non-opaque exposed surfaces were covered to reduce the potential of phototrophic growth in the bench-scalesystem.

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Edition: Vol. - No. Published: 11/01/2005 File Size: 1 file , 250 KB