AWWA WQTC64005

MWH's Applied Research Department (ARD) and SFPUC developed a bench-scale protocol with the overall objectives of screening the effects of utilizing ClO₂ on the HHA supply (Moccasin Reservoir) not only on disinfection performance, but also on finished water quality, formation of DBPs (chlorite and chlorate), and potential operational issues, such as maintaining a target chlorine residual. To address the study objectives, an unfiltered water sample was collected from the Moccasin location and utilized to conduct bench-scale testing, according to a protocol that identified the following three major tasks. Task 1 consisted of monitoring ClO₂ demand and decay kinetics over a 48-hour period after the addition of a specific ClO₂ dose (t=0). During this task ClO₂ residuals, chlorite, chlorate, apparent color and UV₂₅₄ (unfiltered) levels were monitored to identify any significant trends. At t=5.5 hours, the sample was pH adjusted to 9.2 using lime addition to simulate the process occurring to the source water at the Rock River location. The effect of lime addition on ClO₂ kinetics was also addressed. Task 2 focused on determining the stoichiometric amount of a quenching agent required to eliminate any residual ClO₂ at t=16 hr (simulation of Tesla Portal location where sodium hypochlorite is added). The quenching agent initially utilized was calcium thiosulfate (CaS₂O₃) and the required quenching dose was established via titration following a procedure similar to that described in the Standard Methods (Method 4500-CI B. Iodometric Method) (APHA et al., 1998). This method also allowed estimating ClO₂ residual present in the sample. Subsequently, a comparative evaluation of quenching kinetics was performed utilizing sodium sulfite (Na₂SO₃). Since the kinetics of the reaction of ClO₂ and Na₂SO₃ were expected to be slower than that with CaS₂O₃, the system was monitored for 8 hours after the addition of Na_{2SO3. Residual ClO2, chlorite, and chlorate formation were monitored during this period. Task 3 investigated the effects of utilizing extended ClO2 oxidation (and further quenching with CaS2O3) on post-treatment chlorine demand and decay. At t=16 hours (i.e., 16 hours after ClO2 addition), a sample was quenched with a 10% excess of the CaS2O3 quenching dose established in Task 2. This sample was then dosed with sodium hypochlorite (NaOCl) and the decay was subsequently monitored for 8 hours. The effect of ClO2 treatment on final chlorine demand is of particular interest, as literature on the utilization of ClO2 followed by chlorine have reported mixed results. While some studies report a synergy in terms of reduced formation of chlorinated DBPs and no significant reduction in disinfection capabilities from this combined process (Narkis et al., 1995), other studies report reduced disinfection ability, particularly for Cryptosporidium control (Corona-Vasquez et al., 2002). Includes 11 references, table, figures.}