AWWA WQTC64005

MWH's Applied Research Department (ARD) and SFPUC developed a bench-scale protocolwith 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 atarget chlorine residual. To address the study objectives, an unfiltered water sample was collected from the Moccasinlocation and utilized to conduct bench-scale testing, according to a protocol that identified thefollowing three major tasks. Task 1 consisted of monitoring ClO₂ demand and decay kinetics over a 48-hour period afterthe 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 anysignificant trends. At t=5.5 hours, the sample was pH adjusted to 9.2 using lime addition tosimulate the process occurring to the source water at the Rock River location. The effect oflime addition on ClO₂ kinetics was also addressed.Task 2 focused on determining the stoichiometric amount of a quenching agent required toeliminate any residual ClO₂ at t=16 hr (simulation of Tesla Portal location where sodiumhypochlorite is added). The quenching agent initially utilized was calcium thiosulfate(CaS₂O₃) and the required quenching dose was established via titration following a proceduresimilar 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 thesample.Subsequently, a comparative evaluation of quenching kinetics was performed utilizingsodium sulfite (Na₂SO₃). Since the kinetics of the reaction of ClO₂ and Na₂SO₃ wereexpected to be slower than that with CaS₂O₃, the system was monitored for 8 hours after theaddition of Na<sub>2SO₃. Residual ClO₂, chlorite, and chlorate formation were monitored duringthis period.Task 3 investigated the effects of utilizing extended ClO₂ oxidation (and further quenchingwith CaS₂O₃) on post-treatment chlorine demand and decay. At t=16 hours (i.e., 16 hoursafter ClO₂ addition), a sample was quenched with a 10% excess of the CaS₂O₃ quenchingdose 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 finalchlorine demand is of particular interest, as literature on the utilization of ClO₂ followed bychlorine have reported mixed results. While some studies report a synergy in terms ofreduced formation of chlorinated DBPs and no significant reduction in disinfectioncapabilities from this combined process (Narkis et al., 1995), other studies report reduceddisinfection ability, particularly for Cryptosporidium control (Corona-Vasquez et al., 2002). Includes 11 references, table, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2006

Number of Pages:

11

File Size:

1 file , 420 KB