Residual Maintenance Using Sodium Hypochlorite, Sodium Dichloroisocyanurate, and Chlorine Dioxide in Laboratory Waters of Varying Turbidity

Abstract

Sodium hypochlorite (NaOCl) and sodium dichloroisocyanurate (NaDCC) are commonly used for household water treatment (HWT); chlorine dioxide (ClO 2 ) is a potential new HWT option. We compared the residual maintenance of NaOCl, NaDCC, and ClO 2 over 24 hours using recommended dosages (2 and 4 mg/L) in waters of varying turbidity (0-300 NTU, from kaolin clay or creek-bottom sediments) and total organic carbon (TOC) concentrations (0-100 mg/L), for a total of 324 reactors. NaOCl and NaDCC had similar free chlorine decay rates, and ClO 2 decayed more rapidly across all of the tested conditions. Little variability was observed across clay-based turbidity levels and TOC concentrations. With a dosage of 2 mg/L, a residual ≥0.2 mg/L was maintained at 30 NTU for NaOCl and 100 NTU for NaDCC; for ClO 2 , 4 mg/L were required to maintain ≥0.2 mg/L under all conditions except at zero turbidity. Comparisons with data from the literature suggest that the three compounds would inactivate E. coli, rotavirus, and Giardia cysts within 1 hour under all conditions, except 300 NTU for NaOCl and NaDCC. All three disinfectants are similarly efficacious for this usage; however, differences are seen in decay rates that may influence disinfectant selection depending on water storage time.

Towards a Predictive Model for Initial Chlorine Dose in Humanitarian Emergencies

“…A typical chlorine decay curve (FCR versus time) contains two phases, a rapid decay phase takes place usually within the first 30 min after chlorine dosing and a slow decay phase afterwards [11,20,21]. Based on the chemistry of chlorine reactions, major contributors to the rapid decay phase are easily oxidizable substances, metals, fast reacting NOM, ammonia, etc.…”

“…The first stage of the study involves conducting a series of 24-h chlorine decay tests under various organics and temperature conditions. Figure 2 illustrates the matrix for the tested conditions inspired by the approach adopted by Gallandat et al [20].…”

“…The first stage of the study involves conducting a series of 24-h chlorine decay tests under various organics and temperature conditions. Figure 2 illustrates the matrix for the tested conditions inspired by the approach adopted by Gallandat et al [20]. For chlorine dose selection, the 2 mg/L and 4 mg/L for clear and turbid water, respectively, were tested based on current recommendations [8,28,29].…”

“…Arguably, suspended matter may not be the best indicator of chemical properties of dissolved compounds in water. Gallandat et al [20] suggest that kaolin clay-based turbidity has no impact on chlorine decay kinetics. Thus, it shows that the type of turbidity matters in terms of how it affects chlorine demand.…”

“…Matrix for the test conditions.These 24-h decay tests were conducted in 125-mL brown bottles with synthetic water, incubated at the designed temperature. Chlorine residual readings were performed at5,10,15,20,25,30, 35, 40, and 45 min and 1, 2, 4, 20, and 24 hr after dosing. These frequent readings increased the resolution of the chlorine decay curve, especially for the rapid decay phase in the first 30 min.…”