Fast Response, Early Boost: Eliminate Ammonia-Oxidizing Bacteria

By JOEL BLETH

Historically, attempts to solve water quality problems in chloraminated systems that use water storage tanks have been minimally successful. Many storage tank facility managers have reported problems of nitrite formation and rapid loss of residual disinfectant. But experiments conducted during 2006 and 2007 in more than 50 tanks at two California locations indicate that nitrite formation and loss of residual disinfectant can be minimized or eliminated with a fast response and early chlorine boost. The solution offers numerous benefits and eliminates some complexity in complying with the Stage 2 Disinfection Byproduct Rule. A fast response–early boost plan consists of implementing an aggressive water quality monitoring plan for warm weather conditions, as well as establishing trigger points in tanks that signal situations requiring a boost in chlorine levels.

THE PROBLEM The primary source of trouble in many chloraminated potable water storage tanks is ammonia-oxidizing bacteria (AOB). Chloraminated water generally has five sources of free ammonia that feed AOB:

• Some free ammonia remains in the water when chloramines are manufactured at the treatment plant.
• Free ammonia can remain after the chlorine component of chloramines has been depleted in killing bacteria.
• Free ammonia can be released from bacterial bodies after they are killed by chlorine.
• Free ammonia can form from the autodecomposition of chloramines into chlorine and ammonia, which occurs readily when existing free ammonia is consumed by AOB or when water age exceeds five days.
• Free ammonia can form when monochloramine converts to dichloramine. This process occurs rapidly when the chlorine-to-ammonia ratio exceeds 5:1 in a particular portion of a tank.

Addressing these problems is a two-step process: Managers must be able to spot the problem and then boost chlorine levels. Both of these steps rely on an adequate mixing system.