AWWA QTC97084

With the increasing need to control disinfection byproducts (DBPs) and bacterial regrowth in distribution systems, utilities are considering the use of monochloramine as a secondary disinfectant. Although it is considered a less effective disinfectant than chlorine, it is gaining favor as a control measure against biofilms in distribution systems. This is due in part to the fact that simply raising the chlorine concentration in drinking water often does not control biofilms or the presence of indicator organisms. In fact, there are situations where biofilms grown in the presence of low residuals of free chlorine contain higher numbers of coliforms and heterotrophs than parallel unchlorinated systems. The purpose of this paper is to describe results from several recently completed studies where the ability to disinfect biofilms with free chlorine or monochloramine was compared. The test systems discussed include pipe loop reactors made of mild steel and annular reactors of mild steel, ductile iron and polycarbonate.