For over 100 years, the chlorination of drinking water became the most used method of water disinfection, and “despite the negative side effects” is until today the most widely used form of disinfection in water treatment.
But nowadays, due to global water crisis, is growing the use of alternative sources (rainwater, wells, lakes, rivers or reservoirs) outside the conventional urban water supply systems, and the people use homemade methods of disinfection like boiling and/or chlorination.
A very important issue in any chlorination process is the monitoring of chlorine levels to ensure a minimum concentration for an effective disinfection but avoid high concentrations, harmful for consumption.
I believe that ORP (Oxidation Reduction Potential), or REDOX potential, may be a “complementary” indicator for monitoring the “microbiological quality” of water.
I'm not an expert in chlorination, but I am just sharing some information from my survey. Trying to show how to use Free Software and Open Hardware (Arduino) to improve the use of chlorination in a more appropriate and controlled manner.
I don't believe that chlorine is the best alternative for disinfection, but it is one of the most widespread and accessible way to controll waterborne diseases. So while it is still not feasible to replace it on a large scale, we intend, at least, help for it to be used in a more controlled way.
I would like to highlight some conclusions from this survey:
To ensure the “microbiological protection” of water is not enough to monitor and control just the total concentration of residual chlorine but it is also very important to monitor and control the pH.
Above pH 8 almost all the free chlorine is in the form of hypochlorite and therefore with lower disinfectant effect.
According to WHO, ORP values below 720 mV (using reference electrode Ag/AgCl), for swimming pools and the like, indicate low “microbiological protection”.
ORP values over 720 mV (using Ag/AgCl as reference electrode) can be obtained with increasing the concentration of free residual chlorine and/or lowering the pH to below 7.5.
According to WHO, there is no minimum ORP value recommended for drinking water. This limit should be established in a case-by-case basis.
The use of cyanuric acid stabilizer (to reduce photodegradation of chlorine) should be limited to 100 ppm in order not to reduce the concentration of HOCl and reduce the microbiological protection. (Ref.: PPM or ORP: Which Should Be Used?)
The use of alkaline forms of chlorine (sodium hypochlorite solution - bleach) can significantly increase the pH and, to a lesser extent, with the use of calcium hypochlorite. Especially in systems with low “alkalinity” and therefore with little buffering capacity.
The reaction of chlorine with ammonia releases H+ ions which tends to lower the pH (depending on the alkalinity of the medium).
The experimental measurements of the oxidation-reduction (REDOX) potential made with ORP sensors does not allow the comparison of different oxidising disinfectant agents. For example hydrogen peroxide (H2O2) has a relatively high potential in many redox reactions but is a relatively weak disinfecting agent because the enzyme catalase decomposes H2O2 to the extent that it diffuses into the cell. And besides oxidation is not the only mechanism involved in disinfectant action. In the case of chlorine, it is believed that the substitution reactions play an important role to disable the functionality of proteins and nucleic acids.