Monochloramine or chlorine is used by a municipality as a disinfectant to ensure that the whole drinking water delivery system, from the municipality to the tap, is free of illness-causing bacteria. Municipalities started to utilise monochloramine as opposed to chlorine due to the discovery of disinfection byproducts (DBPs) in the 1970s. DBPs are formed from the interaction of chlorine with organic matter in the source water that can be introduced by a variety of contaminants such as industrial pollution (factory/road runoff) or natural contamination (leaves biodegrading, animals living near or in source water). According to the U.S. EPA, these DBPs can cause harm to humans when ingested in levels above safe limits for an extended period, causing liver, kidney, or central nervous system damage as well as an increased chance of cancer. To limit the public’s exposure to these DBPs, the EPA implemented Stage 1 and Stage 2 disinfection byproduct rules, triggering many municipalities to transition from chlorine to monochloramine. Monochloramine is more stable than chlorine, making it less likely to form DBPs. This decreases the risk of exposure to the public without requiring pre-treatment of the water for organics prior to dosing disinfectant. Currently, the U.S. EPA indicates that approximately 113 million Americans are drinking water that is disinfected using monochloramine.
Municipalities started to utilise monochloramine as opposed to chlorine due to the discovery of disinfection byproducts (DBPs) in the 1970s
Around the same time as discovering DBPs and the increased study and use of monochloramine, an unidentified byproduct was found in water that had been treated with monochloramine. Because of the analytical limitations at the time, the EPA determined it could not be speciated. However, with advancements in analytical capabilities and the work done by Julian Fairey et al., this byproduct was recently identified as the chloronitramide anion. The chloronitramide anion is a chemical compound with the structure Cl-N-NO2-. Because of its recent speciation, it has an unknown health risk and is therefore treated with an abundance of safety as potentially hazardous in drinking water. If testing determines that the chloronitramide anion is a health hazard, current and future studies will be required to determine specific health risks and how to remove this compound from the water.
Since chlorine produces more DBPs than monochloramine and further research is needed on DBPs from monochloramine, it's important to focus on minimizing DBP formation overall. This can be managed by minimising the organic concentration flowing into the system with a disinfection agent prior to treatment. This limiting of total organic carbon (TOC) is commonly accomplished through the application of powder activated carbon (PAC) or granular activated carbon (GAC). PAC is introduced into the municipality’s system like a chemical dosage, allowed contact time for adsorption, and removed in the coagulation, flocculation, and sedimentation step of a conventional treatment train. PAC is commonly applied to systems that have transient spikes in organic pollutants such as 2-methylisoborneol (MIB) and geosmin. GAC, on the other hand, is more commonly used for consistent organic contamination and applied in a fixed bed system. Due to the fixed nature of GAC application, it requires more capital expenditure than PAC but allows for better utilisation of the carbon’s capacity. This higher utilisation of the media commonly results in lower media replacement costs over the long run if continual treatment of the water is required.
Both PAC and GAC are commonly used to limit organic concentrations in drinking water. The benefits of each — GAC utilises more of the carbon’s capacity while PAC can be applied intermittently to treat spikes in contamination — must be weighed against both the performance of the material and cost. With further studies being conducted on DBP formation, organics removal in advance of disinfection could become an increasingly more viable option for DBP treatment for regulatory compliance.