Environ. Sci. Technol. 2005, 39, 3811-3818
Influence of the Order of Reagent Addition on NDMA Formation during Chloramination I. MARIE SCHREIBER AND WILLIAM A. MITCH* Department of Chemical Engineering, Yale University, Mason Lab 313b, 9 Hillhouse Avenue, New Haven, Connecticut 06520
The formation of the potent carcinogen, N-nitrosodimethylamine (NDMA), during chlorine disinfection has caused significant concern among drinking water and wastewater recycling utilities practicing intentional or unintentional chloramination. Previous research modeled NDMA formation as arising from a reaction between monochloramine and organic nitrogen precursors, such as dimethylamine, via an unsymmetrical dimethylhydrazine (UDMH) intermediate. Contrary to the importance of monochloramine indicated by previous studies, hypochlorite formed an order of magnitude more NDMA than monochloramine when applied to a secondary municipal wastewater effluent containing excess ammonia. Experiments involving variation of the order that each reagent (i.e., hypochlorite, ammonium chloride, and dimethylamine) was added to solution suggest two factors that may be more important for NDMA formation than the presence of monochloramine: (i) the chlorination state of organic nitrogen precursors and (ii) the partial formation of dichloramine. Although dichloramine formation was most influenced by the pH conditions under which inorganic chloramine formation was performed, mixing effects related to the order of reagent addition may be important at fullscale plants. Chloramination strategies are suggested that may reduce NDMA formation by nearly an order of magnitude.
Introduction A survey of California drinking waters conducted in the late 1990s identified the extremely potent carcinogen N-nitrosodimethylamine (NDMA) as a byproduct of chlorine disinfection (1, 2). While chlorine disinfection of surface waters generally forms less than 10 ng/L NDMA, up to 100 ng/L NDMA may form during chlorination of secondary municipal wastewater effluents (3). High concentrations of NDMA in wastewaters are a major concern, especially for communities that rely on indirect potable reuse of municipal wastewater. Although NDMA is considered a priority pollutant, no federal maximum contaminant level (MCL) has been established for drinking water. However, 10 ng/L has been established as an action level in California (2) and as a guideline in Massachusetts (4). Previous studies (5-7) have modeled NDMA formation during chlorine disinfection as arising from a slow nucleophilic substitution reaction between monochloramine (NH2Cl) and dimethylamine to form an unsymmetrical dimethylhydrazine (UDMH) intermediate. The UDMH intermediate * Corresponding author telephone: (203)432-4386; fax: (203)4324387; e-mail:
[email protected]. 10.1021/es0483286 CCC: $30.25 Published on Web 04/12/2005
2005 American Chemical Society
is rapidly oxidized by NH2Cl to NDMA at 1.5, respectively (21). Even when the molar ratio is
