Rabu, 20 April 2011

Iron Removal in water



Iron removal is technique used to remove excessive iron and manganese from water. The iron and manganese cause unwanted precipitation and coloring of the water.

Iron removal is based on the controlled precipitation of iron and manganese. It is normally done by mixing of the water with air followed by sand filtration.
Of importance is the pH value of the water. The removal of the iron is normally less difficult than the removal of manganese. High iron concentration should be treated with two or more systems in series.
Complex bound iron and manganese, e.g. complex bound with humic acids, can be very difficult to remove. In this case oxidation with ozone can be a solution.
In order to remove iron, it is also possible to precipitate iron in carbonate or with an iron exchanger system. An ion exchanger system is only used to deal with low Fe 2+ concentration. In addition, ion exchanger resin can remove others cations such as Ca 2+. That is why it is advisable to apply it only with low iron and manganese concentrations

Iron is one of the most abundant metals of the Earth's crust. It occurs naturally in water in soluble form as the ferrous iron (bivalent iron in dissolved form Fe2+ or Fe(OH)+) or complexed form like the ferric iron (trivalent iron: Fe3+ or precipitated as Fe(OH)3). The occurrence of iron in water can also have an industrial origin ; mining, iron and steel industry, metals corrosion, etc.
In general, iron does not present a danger to human health or the environment, but it brings unpleasantness of an aesthetic and organoleptic nature. Indeed, iron gives a rust color to the water, which can stain linen, sanitary facilities or even food industry products. Iron also gives a metallic taste to water, making it unpleasant for consumption. It can also be at the origin of corrosion in drains sewers, due to the development of microorganisms, the ferrobacteries.
In aerated water, the redox potential of the water is such as it allows an oxidation of the ferrous iron in ferric iron which precipitates then in iron hydroxide, Fe(OH)3, thus allowing a natural removal of dissolved iron.

However ground waters are naturally anaerobic: so iron remains in solution and therefore it is important to remove it for a water use.

The elimination of the ferrous iron, by physical-chemical way, is obtained by raising the water redox potential by oxidation thanks to oxygen of the air and this by simple ventilation. In the case of acid water, the treatment could be supplemented by a correction of the pH. Thus, the ferrous iron is oxidized in ferric iron, which precipitates in iron hydroxide, Fe(OH)3. The precipitate is then separated from water by filtration on sand or decantation. The stage of precipitation by chemical oxidation can also be carried out with the stronger oxidants such as the chlorine dioxide (ClO2), ozone (O3) or the potassium permanganate (KMnO4).
This elimination can be carried out by cascade or spraying open-air systems (for an acceptable maximum content of Fe2+ of 7mg.L-1) known as gravitating systems. Those systems require a significant place on the ground, but, in addition to an easy and a cheap exploitation cost, they also make possible aggressive CO2 and hydrogen sulfide (H2S) removal. There are also pressure systems, which in addition to their compactness, make possible to treat water whose Fe2+ concentrations between 7 and 10mg.L-1.

Iron is often found in water in complexed forms. In order to be eliminated, iron complexed requests a coagulation stage, which comes in between oxidation and filtration.

Remark : Thanks to microorganisms, it is possible to remove iron from water by biological way. Indeed, there are many bacteria, whose metabolism and thus their survival, are related to the oxidation of iron. However this biological removal requires conditions specific for the pH, the temperature, the redox potential, etc

Advanced Oxidation
Advanced chemical oxidation processes make use of (chemical) oxidants to reduce COD/BOD levels, and to remove both organic and oxidisable inorganic components. The processes can completely oxidise organic materials to carbon dioxide and water, although it is often not necessary to operate the processes to this level of treatment
A wide variety of advanced oxidation processes are available:
  • chemical oxidation processes using hydrogen peroxide, ozone, combined ozone & peroxide,  hypochlorite, Fenton's reagent etc.
  • ultra-violet enhanced oxidation such as UV/ozone, UV/hydrogen peroxide, UV/air
  • wet air oxidation and catalytic wet air oxidation (where air is used as the oxidant)
Advanced oxidation processes are particularly appropriate for effluents containing refractory, toxic or non-biodegradable materials. The processes offer several advantages over biological or physical processes, including:
 - process operability
 - unattended operation
 - the absence of secondary wastes
 - the ability to handle fluctuating flow rates and compositions

CWAO process
However, advanced oxidation processes often have higher capital and operating costs compared with biological treatment.
The most suitable variant for each application is chosen on the basis of the chemical properties of the effluent.

Manganese Removal by physical-chemical way

As for iron, the origin of manganese, in water, is at the same time natural (dissolution of the reduced form Mn2+) and industrial (mining, the iron and steel industry, etc). The same goes for its removal from water. Manganese does not present a danger to human health, nor for the environment but it is unpleasant. In fact, the water gets a black color and a metallic taste.

Similar to iron, the manganese removal by physical-chemical way, can be carried out by the oxidation of Mn2+ in Mn4+, which precipitates then in manganese dioxide (MnO2). The precipitation is then separated from water by filtration on sand or decantation.
The only difference (with the iron), is in the reagent used. Indeed, oxidation by oxygen is in many cases not sufficient for manganese, which implies the use of stronger oxidants in complement such as dioxide of chlorine (ClO2), chlorine (Cl2), potassium permanganate (KMnO4) or ozone (O3).

Remark : In the same way for iron, manganese can be removed by biological way. There are bacteria which take their energy from the oxidation of manganese and which require a water with specific conditions to have an optimal activity of the micro-organisms. However, even if it is possible to carry out in the same time the iron and manganese removal by physical-chemical treatment, the same doesn’t go for the biological way. In fact, the iron and the manganese specific bacteria need different environmental conditions.

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