NDMA formation in secondary wastewater effluent

Concern over prospective levels of N-nitrosodimethylamine (NDMA) in waters has increased in recent years due to its disinfection byproduct formation potential from chloramination. It has been mooted that this is promoted by organic precursors from municipal wastewaters, such that there is a more significant risk of excessive levels in water reuse applications.Experiments conducted on chloramination and chlorination of secondary wastewater have confirmed that that significant NDMA formation arises only from chloramination, with its concentration varying with test conditions used. A full factor analysis revealed all parameters studied (temperature, pH, monochloramine dose and contact time), both individually and synergistically, to have a statistically significant impact on NDMA formation with contact time being the most important. At raw water temperatures below 10 °C, the NDMA concentration can be minimised to below the 10 ng L−1 threshold by not exceeding a monochloramine dose of 2 mg L−1 as Cl2. However, at higher water temperatures other measures are required to suppress NDMA formation, such as reducing the contact time (which could prove impractical in most applications) or maintaining a pH below 6. Further trials are required to fully develop the operating envelope to ensure NDMA concentrations do not exceed the 10 ng L−1 threshold, or else to identify effective pretreatment methods for removing the NDMA precursors.

► Impact of disinfectant chemistry and application conditions on NDMA formation studied.
► Order of significance was contact time > pH > temperature > dose within ranges used.
► Significant synergistic effects exist between the three leading parameters studied.
► Temperature sensitivity demands low pH levels/chloramine doses during the summer.