AWWA WQTC62533

The impending Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) is focused on limiting disinfection byproducts (DBPs) to acceptable levels throughout the entire distribution system. In order to comply with the impending Stage 2 DBPR and reduce DBP levels at the far ends of their distribution systems, some utilities have already converted and many are considering converting from free chlorine to chloramines for secondary disinfection. For many utilities, the addition of ammonia to water with a free chlorine residual to form chloramines is an automated process. Traditionally, utilities have programmed their control systems so that the amount of ammonia being added depends on flow, free chlorine residual, and a set chlorine to ammonia ratio. Chlorine reacts with ammonia in the stoichiometric mass ratio of 4.2:1, equivalent to chlorine to ammonia-nitrogen mass ratio of 5.1:1. The traditional approach is to add ammonia at a chlorine to ammonia ratio less than 4.2:1 so that some excess free ammonia remains in the water after the formation of chloramines. However, this approach to dose control can lead to fluctuations in finished water free ammonia levels if the free chlorine residual in the finished water fluctuates. This paper provides tables comparing three approaches to dose control: resulting excess free ammonia using traditional ammonia dose control approach; resulting excess free ammonia using new ammonia dose control approach; and, example operator inputs and supervisory control and data acquisition (SCADA) outputs for new chlorine and ammonia feed control approach. Includes tables.