Generation, fate and transport of volatile chlorine compounds following hypochlorite discharges to municipal sewers

Abstract

The presence of volatile chlorine compounds in the headspaces of sewers receiving hypochlorite discharges is of concern because of their potential to cause corrosion of infrastructure and risks associated with human exposures. In the current study their presence was investigated using a commercial gas phase Cl₂ sensor. NCl₃ was identified as the most likely gas phase species when ammonia is present and it was found that the Cl₂ sensor was cross-sensitive to this species. The Cl/N ratio substantially impacted gaseous NCl₃ concentrations with maximum values observed at a Cl:N ratio of 12:1. Gaseous NCl₃ concentrations decreased as pH increased (6.5–7.5) and temperature decreased (20–15^oC). A model that included liquid-gas mass transfer (K_La) and first order decay in liquid (k_dl) and gas (k_dg) phases was calibrated. The value of k_dl decreased as the Cl/N ratio increased (10:1–14:1) but increased as pH and temperature increased. Values of k_dl and K_La were lower in real wastewater than in synthetic wastewater. Simulations with a sewer model revealed that with a fixed hypochlorite loading, peak headspace NCl₃ concentrations decreased as wastewater flow increased due to changes in Cl/N ratio. Increased air flow reduced headspace NCl₃ concentrations immediately downstream of the discharge due to dilution but had little effect further downstream. Wastewater pH impacted headspace NCl₃ concentrations by controlling the concentration of NCl₃ in the wastewater. Temperature influenced peak headspace concentrations through its impact on mass transfer and gas phase decay rates. This study provides insights that can be employed to develop sewer use bylaws that regulate hypochlorite discharges.