5.6 Iron & Manganese Removal
Key Takeaways
- Iron has a California secondary MCL of 0.3 mg/L and manganese 0.05 mg/L, both consumer acceptance standards driven by staining, color, and taste.
- Soluble ferrous iron and manganous manganese must be oxidized to insoluble ferric hydroxide and manganese dioxide before they can be filtered out.
- Manganese oxidizes far more slowly than iron with chlorine and generally requires a higher pH, permanganate, ozone, or a catalytic manganese dioxide filter surface.
- Sequestration with polyphosphate or silicate holds low concentrations in solution but does not remove the metals and fails above roughly 1 mg/L iron or 0.1 mg/L manganese, and it is defeated by heating.
- Greensand and manganese dioxide coated media work by catalytic oxidation at the grain surface and require either continuous or intermittent regeneration.
The Regulatory Driver
Iron and manganese are secondary contaminants in California, regulated under 22 CCR 64449 as consumer acceptance contaminant levels:
| Constituent | California Secondary MCL |
|---|---|
| Iron | 0.3 mg/L |
| Manganese | 0.05 mg/L |
| Color | 15 units |
| Odor - threshold | 3 units |
| Turbidity | 5 units |
| Total dissolved solids | Recommended 500, upper 1,000, short term 1,500 mg/L |
Secondary standards protect taste, odor, and appearance rather than health, but they generate the majority of customer complaints and they are enforceable in California - a system exceeding a secondary MCL must notify the State Board and its customers and develop a plan. Manganese in particular has received increased health attention in recent years and carries a health-based notification level well below levels that cause obvious black water.
Why the low thresholds? At 0.3 mg/L iron water begins to stain laundry and fixtures reddish-brown and taste metallic. At 0.05 mg/L manganese the staining is black to purple-black, and manganese deposits accumulate in mains for years before releasing as a slug of black water during a hydrant flow or a velocity change.
The Underlying Chemistry
Both metals are soluble in their reduced states and insoluble in their oxidized states. Removal is therefore always the same two-step sequence: oxidize, then filter.
| Metal | Soluble (reduced) form | Insoluble (oxidized) form |
|---|---|---|
| Iron | Fe²⁺ (ferrous) - clear water at the tap that turns rusty after standing | Fe(OH)₃ (ferric hydroxide) - reddish-brown floc |
| Manganese | Mn²⁺ (manganous) - clear | MnO₂ (manganese dioxide) - black precipitate |
The tell-tale field observation is water that is clear at the tap and colored after sitting in a glass. That is dissolved reduced metal oxidizing on contact with air, and it means the source contains soluble Fe or Mn that treatment did not oxidize.
Manganese is much harder than iron. Ferrous iron oxidizes with chlorine in minutes at pH 7. Manganese oxidation with chlorine is slow below about pH 9.5 and can take an hour or more. That single fact drives almost every design decision in this topic.
Oxidant Stoichiometry
| Oxidant | mg required per mg Fe²⁺ | mg required per mg Mn²⁺ | Notes |
|---|---|---|---|
| Chlorine (Cl₂) | 0.62 | 1.30 | Cheap; manganese needs high pH and long contact |
| Potassium permanganate (KMnO₄) | 0.94 | 1.92 | Works across a wide pH range; overdose turns water pink |
| Ozone (O₃) | 0.43 | 0.88 | Fast and effective; overdose oxidizes Mn to permanganate, producing pink water |
| Oxygen (aeration) | 0.14 | 0.29 | Effective for iron; very slow for manganese below pH 9.5 |
| Chlorine dioxide (ClO₂) | 1.21 | 2.45 | Effective; chlorite byproduct is regulated |
[!IMPORTANT] Two pink-water traps. Overdosing permanganate leaves unreacted MnO₄⁻ and the water arrives pink. Overdosing ozone oxidizes manganese all the way past MnO₂ to permanganate - and the water also arrives pink. In both cases the fix is to reduce the oxidant dose, and in both cases the pink color is a warning that unfiltered manganese is entering the distribution system.
Removal Processes
1. Oxidation and Filtration (the standard train)
Feed oxidant at the wellhead or plant influent, allow a detention/reaction basin, then filter through dual media, greensand, or a manganese dioxide coated medium. Detention requirements differ sharply: iron may need only 5 to 10 minutes, while manganese with chlorine may need 30 to 60 minutes or a raised pH.
2. Aeration and Filtration
Cascade trays, spray aerators, packed towers, or induced-draft aerators transfer oxygen and simultaneously strip hydrogen sulfide and carbon dioxide. Stripping CO₂ raises pH, which helps manganese oxidation. Aeration alone is generally adequate for iron and inadequate for manganese, so aeration plants that also need manganese removal add a chemical oxidant.
3. Manganese Greensand
Glauconite sand coated with manganese dioxide. The MnO₂ coating oxidizes Fe²⁺ and Mn²⁺ on contact - the medium is both a catalyst and an oxidant sink - and is regenerated by permanganate. Two operating modes:
- Intermittent regeneration (batch): the bed is taken offline and soaked with a permanganate solution, then rinsed to waste until the rinse runs clear. Missing a regeneration allows manganese breakthrough.
- Continuous regeneration (CR): permanganate is fed continuously ahead of the filter so the coating is renewed in service. The dose must be trimmed carefully - too little and manganese breaks through, too much and the filtrate is pink.
Greensand is pH sensitive (generally 6.2 to 8.5) and can be damaged by strong oxidants or by chlorine-free operation that strips the coating.
4. Manganese Dioxide Coated Media / Catalytic Filtration
Modern MnO₂-coated anthracite or ceramic media operate on the same catalytic principle but tolerate a wider pH range and higher loading rates, and are regenerated by the free chlorine residual carried through the bed. These media are common retrofits at California groundwater plants that need manganese removal without a permanganate feed system.
5. Biological Filtration
Iron and manganese oxidizing bacteria (Gallionella, Leptothrix, Crenothrix) can be cultivated on a filter bed and will remove both metals at low oxidant demand. Biological beds need careful dissolved oxygen and pH control and no free chlorine ahead of the filter, which conflicts with disinfection strategy at many plants.
6. Sequestration (control, not removal)
Polyphosphate or sodium silicate complexes the reduced metal and keeps it dissolved so it does not precipitate and stain. Critically:
- It does not remove the metal - total iron and manganese leaving the plant are unchanged.
- It is effective only at low concentrations, generally up to about 1.0 mg/L iron and 0.1 mg/L manganese.
- Heating breaks the complex. Water that looked fine at the tap turns rusty or black in a water heater, dishwasher, or coffee maker.
- Chlorine competes with the sequestrant; feeding chlorine before the polyphosphate defeats the treatment. Feed the sequestrant first, then the disinfectant.
- Polyphosphate adds phosphorus, which can support biofilm growth in the distribution system.
[!TIP] Exam framing. When a question offers sequestration for a source at 2.5 mg/L iron, sequestration is the wrong answer - it is above the practical limit. When a question describes clear water that blackens only in the water heater, sequestration that is failing on heating is the right diagnosis.
Distribution System Consequences
Iron and manganese that escape treatment do not disappear; they accumulate on pipe walls as a loose deposit for months or years. Then a velocity change - a hydrant flow, a fire, a pump start, a valve operation, a main break - resuspends the deposit and produces:
- Red water (iron), typically after a velocity increase in unlined cast iron mains
- Black water (manganese), often much more alarming to customers because the color is so dark
- Turbidity and color complaints clustered downstream of the disturbance
- Increased chlorine demand, because reduced metals consume disinfectant
Prevention beats response: keep finished water iron below 0.3 and manganese well below 0.05 mg/L, and run a unidirectional flushing program that mobilizes and removes deposits at controlled velocities on a planned schedule rather than letting a fire flow do it at 3 a.m.
Worked dose example. A groundwater source contains 0.85 mg/L Fe²⁺ and 0.22 mg/L Mn²⁺ and will be treated with potassium permanganate. Iron demand = 0.85 x 0.94 = 0.80 mg/L. Manganese demand = 0.22 x 1.92 = 0.42 mg/L. Theoretical permanganate dose = 0.80 + 0.42 = 1.22 mg/L, before adding any demand exerted by organics or sulfide. Confirm by bench test and trim to the point where filtrate manganese is non-detect without pink color.
A groundwater source contains 1.5 mg/L of ferrous iron and 0.30 mg/L of manganous manganese. Which treatment approach is appropriate?
A plant feeding potassium permanganate for manganese removal begins delivering faintly pink water to the distribution system. What has happened?
A customer reports that water is clear from the tap but turns rusty in the water heater and stains laundry. The plant sequesters 0.7 mg/L of iron with polyphosphate. What is the best explanation?