Iron and manganese are the most common metallic impurities in natural water, and they are usually present together. Although iron is not toxic, its presence in water leads to deterioration of organoleptic and taste properties, appearance of brown sediment on surfaces in contact with water. Manganese belongs to the category of hazardous substances.
Ozonation is especially effective when iron and manganese are present in water as organic complex compounds. Standard iron removal methods (aeration, liming, cation exchange) are ineffective in this case. Ozone oxidizes complex compounds, causing metals to precipitate in insoluble forms.
Iron removal methods comparison
| Parameter | Ozonation | Aeration + filtration |
|---|---|---|
| Fe removal efficiency | > 95% | 80-90% |
| Mn removal efficiency | > 95% | 50-70% |
| Organic complex removal | Yes | No |
| Color reduction | Yes (up to 80%) | Partial |
| Works at any pH | Yes (6-9) | Requires pH > 7.5 |
| Disinfection | Yes | No |
| H₂S removal | Yes | Partial |
| Reagent requirement | No | May be required |
Iron oxidation chemistry with ozone
2Fe²⁺ + O₃ + H₂O → 2Fe³⁺ + O₂ + 2OH⁻
Fe³⁺ + 3H₂O → Fe(OH)₃↓ + 3H⁺
Ozone oxidizes Fe²⁺ to Fe³⁺, which forms insoluble iron hydroxide Fe(OH)₃, easily removed by filtration. The reaction proceeds at pH from 6 to 9.
| Metal | Theoretical ozone dose | Practical ozone dose | Note |
|---|---|---|---|
| Iron (Fe²⁺) | 0.43 mg O₃ / mg Fe | 1 mg O₃ / mg Fe | With organics |
| Manganese (Mn²⁺) | 0.87 mg O₃ / mg Mn | 4 mg O₃ / mg Mn | With organics |
| Hydrogen sulfide (H₂S) | 3.0 mg O₃ / mg H₂S | 3-5 mg O₃ / mg H₂S | To elemental sulfur |
Manganese oxidation chemistry with ozone
Mn²⁺ + O₃ + H₂O → MnO₂↓ + 2H⁺ + O₂
Ozone oxidizes Mn²⁺ to Mn⁴⁺, which forms insoluble manganese dioxide MnO₂. At pH around 8, the reaction proceeds most effectively.
Important: With excess ozone, further oxidation of manganese to Mn⁷⁺ with formation of soluble permanganate MnO₄⁻ (pink water color) is possible. In the presence of organic compounds, permanganate decomposes to MnO₂. For high manganese concentrations, double ozonation is used.
Iron removal process
Source water supply
Water with elevated iron and manganese content is fed into the contact tank. Pre-filtration from coarse impurities.
Ozone generation
Atmospheric air is cleaned by filter, fed into oxygen concentrator (85-95% O₂), then into ozone generator.
Ozonation in contact chamber
Bubbling ozone-air mixture through water layer or injection through Venturi ejector. Contact time: 5-15 minutes.
Precipitation of oxidized metals
Fe(OH)₃ and MnO₂ precipitate in the contact tank. Periodic tank flushing to remove sediment.
Mechanical filtration
Removal of suspended particles of oxidized metals through sand filter. Pore size: 10-50 μm.
Carbon filtration
Final treatment through activated carbon to remove residual ozone and improve organoleptic properties.
Double ozonation (for high Mn concentrations)
At manganese content above 0.5 mg/L, double ozonation scheme may be required: primary ozonation of source water, filtration, then secondary ozonation of filtrate. This prevents formation of soluble permanganate.
| Parameter | Before treatment | After treatment |
|---|---|---|
| Manganese, mg/L | 0.5 | 0.05 |
| Color, degrees | 22 | 5 |
| Turbidity, NTU | 15 | < 2 |
Organic complex problems
In oil-bearing regions where water contains large amounts of organic impurities, iron and manganese are often present as organic complex compounds. This makes standard iron removal methods ineffective.
- Aeration doesn't work: organic complexes are not oxidized by air oxygen
- Liming is ineffective: complex compounds do not precipitate when pH is raised
- Cation exchange is difficult: organic complexes do not exchange on ion exchange resins
- Ozone solves the problem: breaks down organic complexes, releasing metals for precipitation
Recommended system parameters
| Capacity, m³/h | Ozonator power, g/h | Contact chamber volume, m³ |
|---|---|---|
| 1-5 | 5-25 | 0.2-1 |
| 5-20 | 25-100 | 1-4 |
| 20-50 | 100-250 | 4-10 |
| 50-100 | 250-500 | 10-20 |
| 100-500 | 500-2500 | 20-100 |
Note: Ozonator power is calculated for water with Fe content up to 10 mg/L and Mn up to 1 mg/L. For higher concentrations, individual calculation is required.
Benefits of ozonation for iron removal
Versatility
Effectively removes iron and manganese of any forms, including organic complexes.
Wide pH range
Works at pH 6-9 without need for water acidity correction.
Complex action
Simultaneously disinfects water, removes color, odors, oxidizes H₂S.
No reagents
Ozone is produced on-site from air. No need to purchase and store chemicals.
Regulatory requirements
| Metal | Drinking water | Bottled water | Hazard class |
|---|---|---|---|
| Iron (Fe) | 0.3 mg/L | 0.3 mg/L | III |
| Manganese (Mn) | 0.1 mg/L | 0.05 mg/L | III |
- Drinking water standards «Drinking water. Hygienic requirements for centralized drinking water supply systems water quality»
- Bottled water standards «Drinking water. Hygienic requirements for bottled water quality»
- GOST 2874-82 «Drinking water. Hygienic requirements and quality control»