The most common process for iron removal from water is to allow water-soluble ferrous iron (FE2+) in water to turn into water-insoluble ferric iron (Fe3+), and then capture FeIII (iron oxide) particles by filtration. For this process, air or some other oxidant (e.g., chlorine) is first added to the raw water before entering a reaction chamber where a slow oxidation reaction (spontaneous oxidation) occurs. The water is then filtered, with the filtration media primarily being a mechanical-physical barrier.
In typical iron removal systems (for all but small flows), the reaction chamber necessary for complete oxidation is very large to permit the flow transit time needed for this slow spontaneous oxidation to occur. Such systems usually require considerable land space and are costly to construct. They are also relatively complex hydraulically and mechanically. Since various wells in a wellfield produce different concentrations and qualities of iron, even on a day-to-day basis, oxidation-filtration plants require frequent attention and adjustment to work properly.
It has been confirmed (by practical experience of several decades and thousands of experiments) that a "new filter media" produced under certain chemical conditions will react immediately on contact with Fe2+ in raw found water to form Fe3+.
4 Fe2+ + O2 + 6 H20 = 4 FeO(OH) + 8 H+
In this "contact oxidation," the rate of oxidation is about 60 times that of spontaneous oxidation. Consequently, the time needed for iron removal is greatly shortened. Since the iron is both oxidized and captured in the media, the process is greatly simplified. There is also no need for a huge oxidation chamber.
The ferric hydroxide (FeO(OH)) formed is a rusty-red membrane attaching to the surface of the filter media. The "new" or "contact oxidation" filter media consists of the media and a coating of a specially structure ferric hydroxide (r-FeO(OH)) with a very strong contact oxidation ability. Anion exchange absorption reactions occur on the surface of the oxide when pH in the water is under the isoelectric point, while cation exchange occurs when the water's pH is over the isoelectric point. The pH of iron-containing groundwater is generally within a range over the isoelectric point of r-FeO(OH), so cation exchange absorption occurs. Ionic exchange absorption of Fe2+ in water occurs first, with equimolar H+ lost from the surface as follows:
Fe2+ + FeO(OH) = FeO(OFe)+ + H+
After the initial exchange absorption, Fe2+ goes on hydrolyzing and oxidizing, and new r-FeO(OH) is continually produced. This autocatalysis reaction protects the new contact oxidation iron filter media against aging:
FeO(OFe)+ + 1?4 O2 + 3?2 H2O = 2FeO(OH) + H+
Chemical Controls on the Fe-Removal Process
Chemical characteristics of natural ground waters are changeable. Various chemical factors may affect the results of reactions (2) and (3). Comparative experiments have been carried out on different chemical factors such as iron concentration, pH, alkalinity, SO4p;, HCO3p;, soluble SiO2, water temperature, types of filter media, filtration rate, and so on. The results have demonstrated that all of the above-mentioned factors influence the results of reactions (2) and (3).
The authors have also discovered that the following functional relation forms:
å [z]r K = [f]
[Fe=]/[Fe2+] [pH] K = [f]
From (4) and (5),
å [z]r = [Fe2+]o/ [Fe2+] [pH] K
[z]r represents various chemical factors
[f] represents effect of various chemical factors on reactions (2) and (3)
[Fe2+] original source water Fe2+ density (gram equivalent)
[Fe2+]o Fe2+ density (gram equivalent) prior to filtration
r index number
Equations (4), (5), and (6) demonstrate that the best process [f] can be obtained simply by the control of [Fe2+]o/ [Fe2+] and pH factors, although the other factors are also effective on r-FeO(OH) formation.
The Manufacture of the "New Filter Media"
Carrier of filter media: The media uses a hard, granular material with the following characteristics as a nucleus.
- specific gravity: 1.4p;2.7
- nonuniformity coefficient: K = d80/ d10 < 2
- particle diameter: 1.0p;1.6 mm
Formation of r-FeO(OH) membrane on the "new media" surface: The membrane is formed by controlling the ratio [Fe2+]o/ [Fe2+] in the solution and the pH of the solution.
Advantages and Applications
The chief advantages of this invented process are that it is
- simple yet effective,
- economical, and
- easy to master in operation.
These characteristics make the media practical for removal of iron, manganese, and hardness in raw ground water to provide an improved product water for many uses.
Simple and effective: In this new, advanced process, the entire filter becomes an r-membrane possessing a high-contact oxidation filtering system. Therefore, it is a high-speed, one-process filtering system.
The well water (with a proper pre-adjusted air mixture) can flow directly from the well to the filtration system, often using well pump pressure only. The iron, manganese and hardness will be removed without other effects on water quality. Once the r-FeO(OH) is formed, it will continue renewing itself. Therefore, the system is self-regenerating. This system is 25 times faster than other oxidation media and 60 times faster than spontaneous oxidation. The system can filter up to 50 mg/L of Fe, Mn and total hardness.
Economical: Since the system is simple, it is economical in construction, process and operational terms. For the same capacity, the system requires much less space and building capacity, and many fewer system components, including repressurization pumps. Operator time is minimized for adjustment, maintenance and repair.
Easy to operate: The system requires no pH adjustment and is very "forgiving" in adjusting to changes in constituent concentrations and ratios of Fe2+/Fe3+ and Mn2+/Mn4+ due to changing wells and variable pumping conditions. There is no longer a need to maintain an aeration mixing chamber and its environment for most applications. There are also no chlorine or permanganate feed systems or ozone generators to maintain.
Some applications of the filter include
Groundwater Source Drinking Water Systems: This media can be used for any size system from very small public or village or commercial (schools, restaurants) systems to large community systems. "New media" systems provide an economical step up from sequestrant-feed treatment when filtration is indicated, and an alternative to complex and cumbersome aeration-filtration or oxidant-feed and filtration plants. There is no need for chlorine, permanganate or other corrosive chemical feed systems.
Groundwater Source Industrial Water Systems: Low maintenance and forgiveness in media operation make this media ideal for industrial systems where low operational costs and consistent product water quality are a must. Sophisticated iron- and manganese-removal system quality with softener-like "use and forget" operating simplicity. This makes the system a real alternative to the cost and difficulty of switching to a regional piped public water system.
High-Quality Bottled Water or Ultra-Pure Systems: The "new media" system provides high quality and low maintenance iron and manganese removal prior to ozonation, ultraviolet irradiation and membrane filtration. Therefore, the process cuts down on iron and manganese interference with ozone and UV, coating and clogging of UV lamps, and clogging and corrosion of membrane filters and elements.
Groundwater Remediation "Pump-and-Treat" Systems: Provides a simple, low-maintenance, small "footprint" alternative to chemical-feed iron removal prior to strippers or carbon filtration. There is no need for chemical use that may encourage bacterial growth, harm receiving waters, or cause clogging in recharge wells, and it drastically cuts maintenance on stripper towers and improves carbon filter service life. It also reduces solid and chemical waste due to system maintenance.
Groundwater Source Horticultural and Agricultural Irrigation Systems: Removes staining and orifice-clogging iron and manganese using a simple system with softener-like low maintenance. There is no need for chemical feed that may harm plants and drive up costs.