Drinking water regulators also required that the Water Bureau conduct pilot plant work to evaluate potential treatment process changes. The study included various filtration and enhanced disinfection processes. As a result of the study, Portland modified its disinfection system in 1991. The Water Bureau built a new facility to store and feed ammonia so that existing conduits could be used to provide the required chlorine residual concentration and contact time. Chlorine is added to the water at the Headworks .
Ammonia is added three hours after the chlorine, in a process called chloramination .
The Bull Run water source is of very high quality. Turbidity of the water entering the conduits has exceeded 5 NTU only twice since 1981. Regulations to maintain filtration exemption also require alternative operating plans to address high turbidities and other water quality issues. The Enhanced Surface Water Treatment Rule will, when adopted, make changes to the Surface Water Treatment Rule . Changes may include requiring water systems using surface water sources to monitor raw water for Giardia lamblia, Cryptosporidium , total coliforms , fecal coliforms or E. coli and enteroviruses, and to provide levels of treatment based on the raw water quality measurements.