AWWA WQTC62494

The practice of primary disinfection and the maintenance of a disinfectant residual within the distribution system are important in the control of microbial contaminants and bacterial re-growth. Chlorine dioxide (ClO₂) is a strong disinfectant and oxidant that has demonstrated promise as a secondary disinfectant in full-scale distribution systems (Volk et al., 2002). The formation of organohalogens (e.g., trihalomethanes) with ClO₂ is typically much lower when compared to the use of free chlorine (Cl(sub>2) (Hofmann et al., 1999; Werdehoff and Singer, 1987). Chlorite (ClO₂^-) is a known byproduct of ClO₂ (Gordon, 2001). When applied to drinking water, a portion of the ClO₂ will form ClO₂^- upon reaction with natural organic matter (NOM). ClO₂^- has been suggested to have potential benefits as a biocide for mitigating ammonia oxidizing bacteria (AOB) which are known to cause nitrification in distribution systems. In particular, McGuire et al. (1999) reported that the occurrence of nitrification in full-scale systems could be acutely mitigated by switching from chloramines to chlorite. In that study no information was provided concerning long-term affect of ClO₂^- on suppressing heterotrophic microorganisms. Because ClO₂^- is a byproduct of ClO₂ the data presented in the literature has not been clear as to which chemical provides long-term benefit as a secondary disinfectant. Thus the primary objective of this project was to determine the extent of biocidal control on heterotrophic biofilms provided by ClO₂^-, relative to ClO₂, under controlled laboratory experiments and in the field. Annular Reactors (ARs), which are widely used drinking water research, were used to represent model distribution systems. The AR model used for this experiment was the 1120 LS (Laboratory Model Regrowth Monitor and Annular Reactor, BioSurface Technologies Corporation, Bozeman, MT). The influent water flowed through an annular gap and was mixed by the rotating drum, which contains draft tubes to ensure sufficient vertical and horizontal mixing. The hydraulic retention time was controlled by the volumetric flow rate of the influents entering the AR. The total working volume in the annular gap is approximately 950 mL. Each AR was set at a rotational speed that creates the same shear stress at the outer wall of the ARs' cylinders as that which would be seen at the outer wall within the aqueduct. Using a friction factor for a large pipe diameter of 0.01, the shear stress in the pipe was determined to be 0.68 N/m². By estimating the Taylor vortices in the AR, as described by Camper (1996), the rotational 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-scale system.