There are times when a couple of things (loss of chlorination or increased flow volume) allow for fecal coliform bacteria to re-grow in the de-chlorination tank. This happens due to inadequately treated water passing through the chlorine contact tank into the de-chlorination section where the chlorine is usually taken out. If part of the under treated water is allowed to enter an un-chlorinated section of the tank then some of the remaining bacteria will likely live and reproduce.
Many newer operators who have never seen this condition before don’t really know why they are getting high fecal coliform concentrations while the total residual chlorine (TRC) concentrations and contact times are both near normal.
Once the de-chlorination section has been disinfected, the fecal coliform concentrations usually return to normal. With that being said, some or all of the following steps can be taken to correct this problem:
· Suspend de-chlorination
· Monitor TRC at the nearest point to the outfall to ensure compliance and disinfection of the effected de-chlorination tank
· If possible, drain the de-chlorination tank to prohibit the chlorinated water from leaving the plant until the tank is determined to be disinfected
· Resume normal flow through the de-chlorination tank