Iron Removal from Water
Iron may be found in both surface and ground waters, but is more commonly a problem in the latter ( Wells, Wellpoints and Boreholes). In water exposed to the atmosphere, ferrous iron (Fe++) is readily oxidized to ferric iron (Fe+++) by oxygen, and various relatively insoluble precipitates are formed.
Surface waters containing sufficient soluble iron to cause significant problems are fairly rare in the uSA but common in South Africa.In an environment like this molecular oxygen is not readily available and insoluble ferric iron may be reduced to the soluble ferrous form.
Fe +++ is is much less soluble and tends to form precipitates that vary in color from yellow to brown to red. These particles make the water unsightly and can cause staining of plumbing fixtures, interfere with cleaning and washing activities, and impart an unpleasant taste to the water.
In most natural waters, the ferrous form is readily oxidized to the ferric form by contact with molecular oxygen. Certain attached filamentous bacteria (Crenothrix and Leptothrix) derive energy by oxidizing iron and storing the oxidized form in their structure. These organisms are especially troublesome to water systems because they take up residence in piping systems. Occasionally clumps of organisms, in the form of gelatinous masses, break away and are entrained in the flow.
Periodic severe iron problems may result. The diversity of iron problems are such that control techniques must be tailored to fit specific situations. An important first step is to determine what form the iron is in and what concentration is present. ( Water Test is imperitive)
Secondly, the possibility that the iron is present in the water as a result of corrosion should be investigated if applicable. It is generally better to control the corrosion process than remove the iron after corrosion occurs.
Iron can be sequestered / chelated by various “corrosion inhibitors” such as polyphosphates, or may be removed from water by ion exchange/adsorption or by a combination of oxidation, sedimentation, and filtration. The latter process is widely used, with oxygen, chlorine, and potassium permanganate all finding substantial usage as the oxidant.
Iron Removal Technology
Iron removal from water and the technology used is specified according to the iron concentration , required flow rate and pressure. For low pressure systems below 2 bars swimming pool filters can be used with Birm Sand or Greensand.
Birm sand for Iron Removal
For higher pressure , above 2.5 bars to 8 bars , vertical 25, 50 or 75 liter fiber tanks with manual or autobackwash facilities are usually the norm.
Polyphosphates. When the iron is in the ferrous ( Fe++ dissolved, colorless) state and the concentration is no more than about 3 mg/L, the most convenient approach may be to simply mask the problem by adding polyphosphates such as sodium hexamethaphosphate. ( Clarophos, Microphos and Siliphos )
These compounds act as chelating agents to sequester the iron prior to precipitation.
Therefore, they should be applied before oxidation occurs. The sequestering process may be thought of simply as preventing the iron from entering into the reactions that produce precipitates but the iron is not removed from the water.