A common question asked by homeowners, about treating iron, manganese, and hydrogen sulfide issues, before iron filters, is how much chlorine should be injected? There is a short answer to this question and some additional considerations for those that want to treat raw water, let’s examine these options. 
The Shorter Answer:
The method for determining the amount of reactants that are needed for a given reaction is known as stoichiometry. The quantitative relationships that occur between substances that participate in a chemical reaction are known as reaction stoichiometry. The reactant, in this case, is the chlorine and the stoichiometric equivalent for chlorine that would be needed to oxidize iron, manganese, and sulfide is as follows:
Iron: Use 0.62mg/L of chlorine per mg/l of iron
Manganese: Use 1.29 mg/L of chlorine per mg/l of manganese
Sulfide: Use 8.33mg/L of chlorine per mg/l of sulfide
Injecting these quantities of chlorine before the iron filter can be very effective. Hydrogen peroxide can also be used, but it will not work very well if there is any manganese present. When oxidizing iron that is present with manganese, using chlorine or ozone would work well. To avoid any confusion, it should be noted, that milligrams per litre (mg/L) is exactly the same as parts per million (PPM).
Chlorinating Raw Water:
If you want to treat raw water from a well, a chlorine pre treatment and Pro-OX filtration will be effective. This is an efficient and rapid method for removing, iron, manganese, and hydrogen sulfide, from the raw water. The raw water is chlorinated, a 30 to 60 second retention time is required, and then the water is filtered using Pro-OX manganese dioxide filter media. The oxidant (in this case chlorine), should be injected into the distribution system and storage tanks, at a level greater than 0.5mg/L. This will inhibit the growth of sulfur and iron bacteria in your water supply system.
The Method in Detail:
The water is received from the well source and then treated with the chlorine, This oxidizes the iron, manganese, and sulfides, that are present. This produces oxidants, iron is oxidized into ferric iron, manganese is oxidized into a manganic form, and the sulfide is oxidized into sulfate. It should be noted that sulfate is common in many water supplies and it does not add any particular foul taste or odor. A sufficient quantity of chlorine should be added to the raw water, to meet the chemical demand, with the ultimate goal of reaching the chlorine breakpoint. As mentioned earlier, a minimum free chlorine residual of 0.5mg/L is needed for the water distribution. However, it may be necessary to inject a higher residual of chlorine depending upon your specific circumstances.
If you’re worried about iron, manganese, and hydrogen sulfide in your water supply, contact a local water professional. There are many ways to improve the quality of your water by using modern filtration and treatment techniques. Always ensure that you chose a fully WQA certified professional, then you can be sure that they will meet and exceed water industry standards.
With more than 25 years experience in the residential and commercial water treatment space, Terry is a WQA (Water Quality Association) certified water specialist, LEVEL 3, as well as a WQA certified sales representative. Terry currently sits on EcoWater Systems (a Berkshire Hathaway Company) national Peers committee, as a water treatment expert advising other water professionals with less experience on best trade and technology practices. EcoWater Systems of Nebraska is one of the biggest water treatment and water delivery businesses in the state.