The Total Residual Chlorine measurement in your effluent hasn’t changed, but disinfection rates have dropped sending E. coli results over the permit limits. If the chlorine residual in the effluent produced good disinfection results one day, why not the next?
Determining the cause of this type of problem will require an understanding of what chlorine residual really is and what it is not. When we measure for Total Residual Chlorine (TRC) in wastewater we are not determining actual chlorine atom concentration in the way we would determine, for instance, copper concentration. The measure we make is more of a reactive form of chlorine concentration than anything specific.
Most treatment plants that disinfect the effluent by chlorination use either chlorine gas or the liquid sodium hypochlorite (a concentrated form of bleach). Chlorine is added and a percentage of that is deactivated by sunlight, reduced (a type of change in the chemical properties), converted to less active forms of chlorine by substances in the water, or is taken up in the disinfection mechanisms. Whatever uses up the chlorine to make it ineffective is called the chlorine demand. The remaining chlorine that has retained its disinfection properties is measured as the TRC.
Ideally once you can measure the TRC, however small the value, disinfection should be complete. But that’s not the way it works in reality. In practice an excess of reactive chlorine must be present in the effluent for disinfection to work to the level of NPDES requirements.
How much extra chlorine must be present? That depends on what chlorine compounds are available for disinfection. Sodium hypochlorite or chlorine gas is added to the effluent to produce hypochlorous acid (HOCl), hypochlorite ion (OCl-), and if gas is used, a small amount of dissolved gas (Cl2). If you measure the free available chlorine you would be measuring the total of these three chemical species. At a pH of 7.3 there are roughly equal amounts of HOCl and OCL-. Less than pH 7.3 and HOCl is favored. Higher and OCl- is favored. However, HOCl works much better as a disinfectant than OCl-.
Free available chlorine will also react with ammonia in wastewater to produce chloramines, collectively called combined available chlorine. Chloramines also have important disinfection properties in treatment plant effluents. And like the chlorine compounds that make up free available chlorine, some of the different chloramines that make up the combined available chlorine work better as disinfectants than others depending on pH.
TRC is a measure of the combined available chlorine and the free available chlorine after the demand has been met. While this TRC value can remain the same, the ratio of all the chlorine compounds that make up this value can vary widely. So, if the effluent pH changes as a result of a process change or from an industrial batch discharge, the disinfection ability of chlorination can change even though the TRC concentration hasn’t changed. It all depends on the type of chlorine species present. That is why disinfection effectiveness, as measured by E. coli or fecal coliform analyses, can change.