Determination of Chloramine Decay Rates at Pipe Surfaces and in Bulk Water in a Simulated Distribution System Environment
Creators: name: Westbrook, J. Andrew
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2006

description: Many utilities have switched from free chlorine to chloramine as a residual disinfectant to avoid formation of disinfection by-products in the distribution system (DS). While not generally appreciated, chloramines undergo autodecomposition, decaying to inert ionic species, thereby losing their disinfecting capacity. The decay process may be accelerated in the distribution system both in the bulk water phase and at the pipe wall. In the bulk phase, autodecomposition may be catalyzed by and chloramines may be reduced by natural organic matter as well as metallic species. At the pipe wall, chloramine decay may be accelerated by a complex set of catalyzing interactions and redox reactions with corrosion deposits, fresh metal surfaces, cement linings and biofilm. These facts suggest that chloramine is not a stable disinfectant and loss of residual within the DS can be problematic. Little is known about the rate of chloramine decay in the DS. Existing water quality models such as EPANET incorporate rate data of free chlorine decay to predict disinfectant residuals. The analogous rate data for chloramine decay rates in bulk water and at the pipe wall are needed for these models to predict chloramine residual throughout the DS. A general rate model has been derived for free chlorine decay at the pipe wall in which an intrinsic wall reaction rate may be input into DS water quality models. Research thus far on free chlorine decay has shown that the wall reaction rate is much greater than the decay rate in the bulk water. It is reasonable to expect similar behavior for chloramine decay although no quantitative assessments are available in the literature. Moreover, the wall reaction rate for chloramine may depend upon several important system characteristics: 1) pipe material because corrosion releases Fe(II) that chemically reduces chloramines, 2) water velocity because it may control mass transfer of chloramines to the pipe wall, 3) pH because many decay pathways are expected to be pH dependent and 4) temperature because reaction rates generally increase with temperature but the extent will depend on activation energies. The purpose of this research was to measure the rate of decay of chloramines in bulk water and at the pipe wall using water and pipe samples obtained from the City of Raleigh. The rate of chloramine decay in the bulk phase was measured in batch rate tests with both finished water and water obtained from various locations in the Raleigh DS. A pipe Section Reactor (PSR) that was developed at the University of North Carolina (UNC) for quantifying the rate of wall decay for free chlorine was used for the analogous measurement of chloramine decay rate. The PSR requires a small section of pipe (<2ft) that is fitted with an apparatus to control water velocity thus enabling measurement of the disinfectant decay rate as a function of water velocity. Batch samples of water from the Raleigh DS were placed into the PSR and the chloramine decay rate was measured. The effects of pipe material, pH and temperature on decay rate were measured. The two pipe materials were old tuberculated cast iron (CIP) and new cement lined ductile iron pipe (DIP). The wall reaction rate was more than one order of magnitude faster on CIP than DIP; this is expected because the presence of Fe(II) from CIP should acceleratethe decay rate. Decay in both of the pipe sections, however, was significantly faster than in chloramine demand-free containers indicating that chloramine decay is greatly accelerated by either pipe wall material. As an example, 3600 hours was required for chloramine to decay from 3 to 1 mg/L in bulk water compared to only 65 hours in the presence of the DIP and only 3 hours in the presence of the CIP wall. Increasing the water velocity from 0.26 ft/s to 1.36 ft/s caused the chloramine decay rate to double for CIP. Thus, mass transfer of chloramine between the bulk water and the wall limits the reaction rate. In contrast, the decay rate was independent of water velocity for DIP. The lack of a mass transfer limitation in DIP is because the rate of chloramine decay at the wall is slow relative to the rate of mass transfer. Chloramine decay rates in bulk water and at the pipe were described with either first- or second-order rate models at three pH values, three temperatures and four water velocities. These decay rates were also much lower than those for free chlorine as obtained in early research at UNC. Chloramine is a weaker oxidant than free chlorine. Therefore, the decay rate both in bulk water and at the wall was expected to be lower. Nonetheless, chloramine decay especially in the presence of CI pipe material can lead to significant loss of residual in the DS. The experimental and modeling approach in this work is general and thus may be used by any water utility. Specific experiments, however, may be required because of chloramine decay rates are influenced by system specific characteristics such as natural organic matter. The rate constants for chloramine decay in this particular research will be adapted by Hazen and Sawyer as input rate constants for use in the Raleigh DS water quality model to predict chloramine residuals as part of compliance with the EPA's Initial Distribution System Evaluation requirement.