Iron and Manganese in Well Water: Removal Guide

The 0.3 mg/L iron and 0.05 mg/L manganese figures are aesthetic thresholds, not health limits. They are also the practical limits for downstream equipment: an RO membrane fouled by iron oxide loses flux and cannot be cleaned back to baseline, and a softener resin coated with iron loses capacity. Treat at the lower threshold when the water feeds either device, and treat earlier when the water is heated or stored in a tank.

The trigger is lower when the water is chlorinated downstream. Chlorine oxidises dissolved iron into particles after the filter, so a well at 0.2 mg/L iron that passes the filter can still stain fixtures when the distribution line is chlorinated. In that case, oxidise and filter before the chlorine injection point, or accept the staining.

Oxidation converts soluble metal into an insoluble oxide; filtration then removes the particles. The choice depends on concentration, the presence of manganese, the need to disinfect, and what is downstream. Aeration is the cheapest oxidant but adds no residual and struggles with manganese. Chlorine is effective and leaves a residual, but it must be removed before an RO membrane. Ozone is the strongest option and also removes colour and odour.

Iron oxidises readily at pH 7.0 and above, with a redox potential around 200 mV. Manganese needs roughly pH 8.0–8.5 and a stronger oxidant, because its oxidation kinetics slow sharply below that range. At pH 7.5, a chlorine dose that removes iron will often leave manganese in solution, and the filter appears to work while the black staining continues.

Raise pH with sodium hydroxide or soda ash before oxidation, then filter. Keep the pH inside the range the media can tolerate, and check the treated water rather than the raw water, because oxidation consumes alkalinity and the outlet pH can fall. Cold water slows the reaction further: a 10 °C drop can roughly double the contact time needed for the same removal.

A contact tank is not a substitute for a filter. It gives the oxidant time to react, but the resulting particles still need a bed with enough depth and the right backwash. Systems that add ozone and skip filtration send the oxide straight to the cartridge, which then becomes the filter and blocks within days.

Size on peak flow, not average. Multimedia or catalytic beds usually run at 10–30 m/h service velocity with 0.8–1.2 m of bed depth. Backwash at 20–30 m/h for 10–15 minutes, and add air scour if the media is heavy. When chlorine or ozone is used, provide 10–20 minutes of contact time before filtration so the oxidation reaction completes; a short contact tank passes unreacted metal into the bed and fouls it.

Iron bacteria are not a chemical problem. They oxidise iron and manganese as an energy source and build orange or brown slime that plugs filters, pumps and pipes. Signs are a rotten or swampy smell, slimy growth on the well screen, and orange biofilm in the toilet tank. A new filter will foul within weeks if the well is not rehabilitated first.

Treat the well, not only the water: shock chlorination at 50–200 mg/L with 12–24 hours of contact, brushing or surging the screen, and pumping to waste until the chlorine demand drops. Then disinfect the distribution line. If the well produces iron bacteria repeatedly, continuous low-dose chlorination with dechlorination before the RO is the usual long-term arrangement.

Membrane manufacturers usually specify iron below 0.05–0.1 mg/L at the RO inlet. Iron oxide fouls the feed spacer and the membrane surface, and acid cleaning removes some of it but not the portions that have baked in; repeated events permanently lower flux. Manganese is worse because it needs a stronger acid and can catalyse membrane degradation.

The protection order is oxidation, filtration, then a 5 µm cartridge as a guard. Do not rely on antiscalant for iron: antiscalants disperse hardness salts, not metal oxides. If the feed contains 0.5 mg/L dissolved iron and the RO runs continuously, the iron load is 0.5 g per m³; at 10 m³/h that is 120 g per day, enough to foul a small array within weeks.

Backwash is the main routine. Trigger it on differential pressure, typically 0.5–1.0 bar above the clean bed, or on a fixed interval, and confirm the backwash flow with a meter rather than by sound. Check the media bed annually for loss and compaction, and replace media every 3–5 years depending on iron load. Test treated water for iron and manganese quarterly; a rising outlet value means the oxidant dose, pH or contact time has drifted. Keep a spare set of cartridges and a spare dosing pump on site; these are the two components that stop the train when they fail.