The formation of THM has been extensively researched and the key measures for controlling their formation are organics, chlorine, pH, and time (see . However, the most secure way to ensure compliance is to maximise the removal of precursor material i.e., natural organic matter (NOM). The pre-AMP3 process technology employed at both Oswestry and Watchgate were not capable of removing the required amounts of NOM in order to ensure compliance with the new regulatory requirements. Also, at Watchgate the existing filters could not reliably provide a robust barrier to Cryptosporidium without provision of the pre-treatment.