Disinfectant change: change from chlorine dioxide to chlorine gas
I have already made the transition in both directions (ClO2 to Cl2 and Cl2 to ClO2). Each time, changes in water taste were observed in the network.
Going from Cl2 to ClO2, it lasted less than a month.
Conversely going from ClO2 to Cl2, we had to backtrack because we had not taken into account the history of our plant: chlorine dioxide treatment had been adopted in the past precisely because of a taste problem. Be careful, therefore, to ensure you are well informed about the history and even possibly get help from the experts to determine if the passage of chlorine gas is not likely to generate tastes.
For the commissioning of chlorine gas, what I advise you in terms of protocol:
- Total stoppage of chlorine dioxide
- Commissioning of chlorine gas with a residual set point of 0.05 mg/L
- Gradually Increase the residual by increments of 0.05 mg/L / week.
Chlorine dioxide is a more powerful oxidizing agent than chlorine gas and reacts with iron and manganese.
The standard of 0.2 mg/l is below the WHO recommendation of 0.7 mg/l.
The switch to chlorine gas may require additional re-chlorination on the network.
Have you considered generating sodium hypochlorite by salt electrolysis?
As @Pateau01 indicates, the switch to chlorine (gaseous or liquid in the form of sodium hypochlorite) may imply, depending on the size and operation of the network (contact time, number of branches, etc.), a need for re-chlorination during the distribution process. This is explained by the persistence of chlorine which is lower than the one of chlorine dioxide. However, if the configuration involves little or no re-chlorination, using chlorine disinfection will be easier and probably cheaper. The issue of taste problems is relevant, because when reacting with organic matter in the water, chlorine gas generates disinfection by-products, some of which give off a characteristic odour perceptible at faucet outlet but which subsides after a rest period in the open air (taste and smell disappear). The term disinfection by-product may cause concern, but it is to be considered in the same way as by-products for the production of chlorine dioxide (chlorite and chlorate in particular).
Advantages of chlorine: easy to use, often the cheapest method, most commonly used disinfectant, sodium hypochlorite solution easier to use and less dangerous.
Disadvantages of chlorine: formation of halogenated by-products, possibility of unpleasant taste and odour, pH-dependent effectiveness, over time degradation of hypochlorite solutions.
Advantages of chlorine dioxide: longer residual time, no halogenated by-products if dosage is correct, no unpleasant taste and odour, no influence of pH.
Disadvantages: on-site production, formation of chlorite and chlorate, precision settings required (to ensure effectiveness and limit by-products), higher installation and operating costs than chlorine.
Overall, I advise you to consult specialists who can evaluate or even carry out a study if necessary to determine the most appropriate disinfectant for your water quality and the configuration of your distribution system.