Rusty water? You're right - it's iron!
It often happens that the water coming from the well is clear and clean, but after a little time, it becomes turbid and rusty. This is an indicator that the liquid contains a lot of dissolved iron (Fe2). The situation can be corrected by using water filters to purify water from iron.
Iron is one of the most common natural elements. Iron is present in most volcanic rocks, it is also part of the rocks that cement sandstones. Iron is found in significant amounts in various clays, and in sedimentary carbonate rocks (e.g., limestone) it occurs only as minor impurities.
Why should you install water filters for iron removal?
Not surprisingly, the problem of iron in natural water is one of the most common. A number of problems arise with such water, both at home and in commercial and industrial operations. Already at concentrations of iron over 0.3 mg/l such water in domestic use in cottages and apartments causes the formation of rusty streaks, can change the color of fabrics when washing them, etc. At high concentrations, water has a characteristic metallic taste, which negatively affects the quality of drinks (tea, coffee, etc.). In some cases, even the quality of food cooked in water with high iron content may suffer. All this makes the task of iron removal very urgent both for drinking and domestic use as well as for industrial use.
There are the following types of organic iron:
- Bacterial iron. Some species of bacteria are able to utilize the energy of dissolved iron during their life activity. This converts divalent iron into trivalent iron, which is stored in a jelly-like shell around the bacterium.
- Colloidal iron. Colloids are insoluble particles of very small size (less than 1 micron), making them difficult to filter on granular filter media. Large organic molecules (such as tannins and lignins) also fall into this category. Colloidal particles, because of their small size and high surface charge (which repels particles from each other, preventing them from becoming larger), create suspensions in water and do not settle when suspended.
- Soluble organic iron. Just as polyphosphates, for example, are able to bind and retain calcium and other metals in solution, some organic molecules are able to bind iron into complex soluble complexes called chelates. Examples of such binding are the iron-holding porphyrin group of blood hemoglobin or the magnesium-holding chlorophyll of plants. Thus, humic acid, which plays an important role in soil ion exchange, is an excellent chelating agent. All of the above iron species "behave" differently in water.
It should only be noted that "trouble never walks alone" and in practice almost always there is a combination of several or even all types of iron. Given that there are no unified approved methods of determining organic, colloidal and bacterial iron, in selecting an effective method (or rather a set of methods) of deferrization much depends on the practical experience of the specialist involved in providing water filter systems for water treatment.
Filtering water from iron, removing it from the water - without exaggeration, one of the most difficult tasks in water filters for water purification. Even a cursory review of existing methods of deferrization leads to the reasonable conclusion that there is currently no universal economically viable method applicable in all instances of life. Some methods and water filters are quite effective at home in an urban environment, but they may be powerless in the process of purifying water from iron in cottages or production - much depends on the quality of the filtered water. Each of the existing methods is applicable only within certain limits and has both advantages and significant disadvantages.
Methods of iron removal
So, the existing methods of iron removal include:
1. Oxidation (by air oxygen or aeration, chlorine, potassium permanganate, hydrogen peroxide, ozone) followed by precipitation (with or without coagulation) and filtration.
The traditional method has been used for many decades. Since the iron oxidation reaction takes quite a long time, using only air for oxidation requires large tanks in which the required contact time can be ensured. This is the oldest method and is only used on large municipal systems. Adding special oxidizing agents, on the other hand, speeds up the process. Chlorination is the most widely used method, as it simultaneously allows solving the disinfection problem.
The most advanced and strongest oxidizing agent today is ozone. However, installations for its production are rather complicated, expensive and require significant power consumption, which limits its application. It should also be noted that in concentrated form (e.g. at the point of injection into water) ozone is poisonous (as, in fact, are many other oxidizing agents) and requires very careful attention.
Oxidized iron particles are small enough (1-3 microns) and therefore take quite a long time to precipitate, so special coagulant chemicals are used to promote particle enlargement and accelerated precipitation. The use of coagulants is also necessary because filtration at municipal sewage treatment plants is mostly carried out on obsolete sand or anthracite clarifiers (not capable of retaining fine particles). However, even the use of more modern filter fillings (e.g., aluminosilicates) does not allow filtering particles smaller than 20 microns. The problem of purifying water from iron could be solved by using special ceramics, but they are quite expensive (since they are not produced in Russia).
All the above methods of oxidation have a number of disadvantages.
- if coagulants are not used, the process of precipitation of oxidized iron takes a long time; otherwise, filtration of uncoagulated particles is very difficult because of their small size.
- these oxidation methods (to a lesser extent, this applies to ozone) are of little help in combating organic iron.
- the presence of iron in water is often (almost always) accompanied by the presence of manganese. Manganese is much more difficult to oxidize than iron and, in addition, at much higher pH levels, making it naturally difficult to purify water of iron
All of the above disadvantages have made it impossible to use this method in relatively small domestic and commercial-industrial systems operating at high rates.
2.Catalytic oxidation followed by filtration is the most common method of iron removal today and is used in high capacity compact systems.
The essence of the method is that the oxidation reaction of iron occurs on the surface of pellets of a special filtering medium, which has the properties of a catalyst (gas pedal of the chemical reaction of oxidation). The filtering "backfills" differ from each other both by their physical characteristics and the content of manganese dioxide, and therefore work effectively in different ranges of values of the parameters characterizing water.
However, the principle of their work is the same. Iron (and to a lesser extent manganese) in the presence of manganese dioxide quickly oxidizes and settles on the surface of the filter media granules. Subsequently, most of the oxidized iron is washed down the drain during backwashing. Thus, the layer of granular catalyst is at the same time the filtering medium.
Additional chemical oxidizing agents can be added to the water to improve the oxidation process. Potassium permanganate is the most common because its use not only activates the oxidation reaction, but also compensates the "washout" of manganese from the surface of the filter medium granules, i.e. regenerates it. Both periodic and continuous regeneration are used.
All systems based on catalytic oxidation with manganese dioxide, in addition to specific (not all of them work on manganese, almost all of them have a high specific weight and require high water consumption during backwashing) have a number of general drawbacks.
- First, they are ineffective against organic iron. Moreover, in the presence of any form of organic iron in the water on the surface of the filter material granules over time, an organic film is formed, isolating the catalyst - manganese dioxide from the water. Thus, the whole catalytic capacity of the filtering backfill is reduced to zero. The ability of the filter media to remove iron is practically "zero", as there is simply not enough time for the natural course of the oxidation reaction in filters of this type.
- Secondly, systems of this type still cannot cope with cases where the iron content in water exceeds 10-15 mg/l, which is not at all uncommon. The presence of manganese in the water only aggravates the situation.
From the point of view of removing iron from water, the fact that cation exchangers can remove not only calcium and magnesium ions from water, but also other divalent metals, and therefore dissolved divalent iron, is important. And theoretically, concentrations of iron, which ion exchange resins can handle, are very high. Another major advantage of ion exchange is the ability to soften water simultaneously with deironing and removal of manganese.
Regeneration of the filter media (regeneration) is an automated process using a salt and water solution. The salt solution is passed through the filtering material and the removed impurities are flushed down the drain.
Another advantage of ion exchange is that it is not "afraid" of iron - manganese, which makes the work of systems based on the use of oxidation methods very difficult. The main advantage of ion exchange is that iron and manganese in dissolved state can be removed from water. That is, there is no need for such a capricious and "dirty" (because of the need to wash out rust) stage as oxidation.