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.
| Edition : | Vol. - No. |
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| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Published : | 11/01/2005 |
