Can you gives us Iron and manganese average at inlet of your treatment system, then at the outlet of your oxydation and filtration. After oxydation and filtration are your reaching the target required by the regulation? What are your iron and manganese residual just before chlorination and after chlorination? You should measure ferrous iron (dissolved part) and total iron and manganese. It would be usefull too to get the same results on the water from a tap in a house far from the water plant for comparison.
When you are taking a sample from a fire hydrant, you increase the speed of water when opening the hydrant and doing this you are doing a cleaning of your network system. A lot of particules attached to the pipe come off and get in the water and that's why you are reaching high level of iron and manganse.
If at the exit of your treatment plant you have iron and manganese in solution, if you add chlorine, you are doing an oxydation of the residual iron and manganese and then a part of it stick to the pipes and probably a part of it goes in the distributed water.
If your water is red the cause is iron and if black it is manganese.
Replacing 2/3 of the sand with Mangagran is simple and effective. I have been using this technique since 1998 in my first plant and on three other since.
Are you using biological or physico-chemical processes? You have to tackle the iron before the manganese.
Oxidation tower, therefore physico-chemical, be careful with the pH.
Whenever i have water with high iron and manganese [bore water] i fill my receiving tank and modify the ph to 8 +5 - 8+6 add a metal filtering agent [phosphonic acid complex] then add a floculant to drop all metals to the bottom of treatment tank ,then draw off perfect water. Empty sludge from tank ready for the next treatment.