Iron is one of the most abundant metals in the earth’s crust. It is found in natural fresh waters at levels ranging from 0.5 to 50 mg/litre. Iron may also be present in drinking water as a result of the use of iron coagulants or the corrosion of steel and cast iron pipes during water distribution. Excessive iron and rust in tap water can stain fixtures and laundry, and give tap water a rusty tinge and metallic taste. The EPA (Environmental protection agency) limit of iron in the water is 0.3 ppm.
There are two main forms of iron in the water:
) which is dissolved in the water at any pH level.
) which is insoluble in water.
There are several methods for the removal of iron from the water; the most common types of which are described here below:
Birm is an efficient media for the reduction of dissolved iron and manganese compounds from raw water. It may be used in either gravity fed or pressurized water treatment systems. Acting as a catalyst between the oxygen and the soluble iron compounds, birm enhances the oxidation reaction of Fe++
and produces ferric hydroxide which precipitates and may be easily filtered.
Birm may also be used for manganese reduction. In these applications, the water to be treated should have a pH of 8.0 – 9.0 for best results. If the water also contains iron, the pH should be below 8.5. High pH conditions may cause the formation of colloidal iron, which is very difficult to filter out.
Conditions for operation
In some cases, an additional treatment is required in order to improve the efficiency of the birm filter.
- Dissolved oxygen content should be equal to at least 15% of the iron (or iron and manganese).
- Water pH range should be 6.8 – 9.0.
- Alkalinity should be greater than two times the combined sulfate and chloride concentration.
- Bed depth should be 30 – 36 in.
- Free board should be 50% of bed depth (min).
- Service flow rate should be 3.5 – 5 gpm/sq.ft.
- Backwash rate should be 10 – 12 gpm/sq.ft.
- Free chlorine concentration should be less than 0.5 ppm.
- Hydrogen sulfide should be removed prior to the Birm filter.
- Organic matter should not exceed 4-5 ppm.
- If the level of dissolved oxygen in the water is under 15%, the addition of air will be required. Either oxygen will be introduced to the inlet water via an injector, or the water will be aerated.
- If the influent water has a pH of less than 6.8, neutralizing additives such as soda ash may be used prior to the Birm filter to raise the pH.
Manganese Greensand is formulated from a glauconite greensand and is capable of reducing iron, manganese and hydrogen sulfide from water through oxidation and filtration. Soluble iron and manganese are oxidized and precipitated by contact with higher oxides of manganese on the greensand granules. Precipitates are then filtered and removed by backwashing. When the oxidizing capacity power of the manganese greensand bed is exhausted, the bed has to be regenerated with a weak potassium permanganate (KMnO4
Low pH, lack of chlorine oxidant or lack of permanganate oxidant are the most likely conditions which may lead to media destruction.
Conditions for operation
- Water pH range should be 6.2 - 8.5.
- Bed depth should be 30 in.
- Freeboard should be 50% of bed depth (min).
- Service flow rate should be 3 – 5 gpm/sq.ft.
- Backwash flow rate should be 10 – 12 gpm/sq.ft.
- Maximum practical limit of iron (Fe++) or manganese (Mn++) in raw water should be 15 ppm.
- Maximum practical limit of hydrogen sulfide (H2S) should be 5 ppm.
Aeration provides the dissolved oxygen needed to convert the iron (and manganese) from their ferrous (and manganous) forms to their insoluble oxidized ferric and (manganic) forms. The aeration process applies to raw water with a maximum iron level of 5 mg/l. It takes 0.14 ppm of dissolved oxygen to oxidize 1 ppm of iron.
The reaction can be expressed as follows:
4Fe2+ + O2 + 8OH- + 2H2O -> 4Fe (OH)3
There are many ways to provide the aeration. Either the water being treated is dispersed into the air, or air is bubbled into the water. Other aeration methods include cascade trays, cone aerators, and porous air stones.
Operation of the aeration process requires careful control of the flow throughout the process. If the flow becomes too great, not enough air is applied to oxidize the iron. If the flow is too small and the aeration is not cut back, the water can become saturated with dissolved oxygen and, consequently, become corrosive to the distribution system.
The speed at which the bivalent iron is oxidized by the oxygen depends on several factors, particularly temperature, pH, level of iron, and oxygen contents.
After the oxidation of the iron is completed, the water must be filtered to remove the ferric hydroxide. The filtration rate should be between 5 to 15 m/h. The weight of iron retained per unit of filter surface varies from 200 to 1000 g Fe per m2
of sand, depending on the circumstances. In general, dual-media filters (anthracite & sand) are particularly suitable for iron removal applications.
Iron in water can be oxidized by a strong oxidant (Cl2
) converting it to ferric hydroxide. This method is generally used when the level of dissolved iron is high in the water (above 10 ppm - although this process can be used at any level of chlorine). Chlorine solution is injected with a chemical feed pump. The contact time needed to oxidize the ferrous iron depends on two factors: the level of iron in the water, and the quantity of injected chlorine. Once the precipitated iron begins to form, it is removed by the filtration. Backwashing the filter on a regular basis is important, in order to remove the precipitated iron. In some cases, it is recommended to install a carbon filter after the sand (or sand & anthracite) filter, in order to remove the excess of injected chlorine in the water. The oxidation process should occur at a pH of about 8, which is the optimum rate of oxidation. Soda ash injected with the chlorine will increase the pH to the optimum level.
In order to improve the filtration process, it is recommended to inject coagulant.