An emerging process to control bromate formation is to add chloramines upstream of the ozone contactor (Buffle et al., 2004). As part of this process, ammonia can be transformed?via nitrification?on a biologically active filter (Yun et al., 2009). In previous demonstration-scale testing, a nitrified biofilter resulted in an increase in the formation of N-nitrosodimethylamine (NDMA) during post-chloramination (Krasner et al., 2010). It was hypothesized that soluble microbial products (SMPs) from the nitrifiers were a source of NDMA precursors. In other research, laboratory-generated SMPs from activated sludge were found to contribute to the formation of NDMA (Krasner et al., 2008). In addition, in the demonstration-scale tests, there was more formation in the biofilter effluent at pH 9 at 5°C than at 25°C. However, it was not understand why this phenomenon occurred.