A new ES&T Letters paper has just been published out of a collaboration between Tanju Karanfil‘s research group and the Ladner group. Mahmut Ersan is the first author. His research quest is to determine how well various membrane processes remove precursors of N‑Nitrosodimethylamine (NDMA), which is a potent disinfection byproduct. NDMA is important whenever chloramine is used as a disinfectant to treat waters with nitrogen-containing compounds. Municipal wastewater is one of the leading sources of these compounds, which we call NDMA precursors.
During his early experiments, Mahmut discovered that the membranes themselves were sources of these precursors. He had a very difficult time cleaning the membranes so that he could have a low background concentration for his precursor removal experiments. We felt that this was an important topic to tell others about, as nobody had previously discussed in the literature the fact that membranes are sources of these compounds. Figure 1 and Figure 2 show some of the data. The full paper can be found at the ES&T Letters website.
I should add that the NDMA precursor problem appears to only occur in new membranes. After a long period of operation (several days to weeks) the precursors are exhausted. So the main risk to public health would be during a startup period, or a membrane replacement period. Also, the concentrations are fairly low and in our tests we evaluated the worst-case scenario where membrane permeates were exposed to high concentrations of chloramine. Another key consideration is that there are relatively few operating plants (that we know of) where NF membranes are followed by chloramine. In short, there seems to be little risk to the public at this time. But we think the leaching problem may not be isolated to nanofiltration membranes, so as new membrane installations of various types (reverse osmosis, ultrafiltration, microfiltration) continue to be built, the potential for NDMA precursor leaching is something that should be considered by engineers.