Control of N-Nitrosamines, a class of potent carcinogenic disinfection byproducts, is a critical challenge during wastewater reuse. Wastewater reuse is a worst-case scenario for nitrosamine formation: The two key reactants are wastewater-derived amines and dichloramine, both in abundance in chlorinated wastewater effluent. Reuse plants apply reverse osmosis (RO) and advanced oxidation processes (AOP), at considerable expense, to minimize nitrosamine concentrations in final effluent. At two major Southern California wastewater reuse facilities sampled during this study, concentrations of NDMA (the most abundant nitrosamine, subject to a 10 ng/L notification level in California) were quite high (e.g., 100 ng/L) before advanced treatment, but generally dropped to zero after AOP. Subsequent monitoring downstream of AOP (1 – 3 hr hydraulic residence time) mysteriously showed detectable NDMA, sometimes exceeding the notification level. These exceedances are of grave concern, particularly to utilities exploring direct potable reuse in the future. We hypothesized that RO-induced shifts in the speciation of chloramines caused the NDMA regrowth. Computer simulations of chloramine speciation suggested that RO stripping out charged species, including NH4+ and CO32-, and dropping the pH to 5.5, caused a shift from monochloramine to dichloramine as the predominant chlorine species. Bench-scale RO tests with chlorine and ammonia confirmed that chloramine speciation shifts from mono- to dichloramine over time in RO permeate, leading to elevated NDMA formation. This suggested a treatment modification to eliminate this problem: adding lime (Ca(OH)2) to raise pH back to neutral immediately after RO. This treatment modification was evaluated at two wastewater reuse plants, and found to reduce NDMA regrowth by 75 – 100%, both to below the notification level. This treatment intervention, while not reliant on sophisticated materials, is nonetheless an important contribution to the safe reuse of wastewater, particularly as direct potable reuse expands in California and elsewhere.

Preventing regrowth of nitrosamines during wastewater reuse by manipulating chloramine chemistry

253rd National Meeting (2017)

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