Iron removal and manganese removal

Aeration basins for iron removal and manganese removal

Iron and manganese is in the water chemistry , the removal of 2-valent compounds of iron and manganese called from a water. Surface water usually contains no or only small amounts of these metal compounds. In contrast, higher amounts of iron (II) and manganese (II) compounds can be found in groundwater .

Low levels of iron (II) and manganese (II) compounds have no toxic properties. However, since these iron and manganese compounds form sparingly soluble red-brown to black oxide hydrates in the presence of oxygen, they are removed from the water before use.

According to the legal regulations, drinking water may contain no or only very small amounts. According to the international guidelines of the WHO, the limit values ​​for drinking water are 0.2 mg / l for iron and 0.05 mg / l for manganese. For drinking water in Germany, the legal requirements of the German Drinking Water Ordinance ( DIN 2000 ) must be met. These correspond to the limit values ​​specified by the WHO. In addition to drinking water, service water is also mostly treated before use so that it is free from higher levels of these compounds.

Iron and manganese compounds in water

Groundwater is often poor in oxygen and has somewhat reducing properties. This leads to the leaching of iron and manganese from the layers of the earth, insofar as they contain such metal compounds. Often hydrogen carbonates are present and that

• Iron (II) hydrogen carbonate = Fe (HCO ₃ ) ₂ and
• Manganese (II) hydrogen carbonate = Mn (HCO ₃ ) ₂

Sulphidic (for example iron (II) hydrogen sulphide) and humic acid compounds are more rarely detectable in the water. Mine water can contain particularly high amounts of these metal compounds . Iron contents of up to 15 mg / l are given in the literature. In a study carried out in the GDR in 1971, the following frequencies for iron and manganese levels were given for raw water there:

Metal type	Content in mg / l	Number of waters
iron	≤ 0.4 mg / l 0.4 to 1.0 mg / l 1.0 to 5.0 mg / l > 5.0 mg / l	62 raw water 55 raw water 278 raw water 99 raw water
manganese	≤ 0.4 mg / l 0.4 to 0.8 mg / l > 0.8 mg / l	379 raw water 74 raw water 41 raw water

Processing method

Most treatment systems for the extensive removal of the dissolved iron and manganese compounds consist of a ventilation device and a filter stage. Quartz sand is mainly used as the filter medium. Layered bed filters with 2 different filter media are also used for higher iron contents .

Even gravel filters that have not been flooded, so-called "dry filters" (the water level in the filter is kept below the filter bottom), have proven themselves here as the first filter stage, especially with high Fe contents (approx. 10 to 25 mg / l). The oxidation air can be supplied in cocurrent and countercurrent. A manganese removal stage is usually connected downstream.

The oxidized metal oxide hydrates are sparingly soluble and preferentially deposit on the surface of the gravel grains. The layers formed on the grains catalytically intensify the oxidation. New filters and new filter fillings therefore require a training period of around one to two weeks to achieve the best possible cleaning effect, especially the removal of manganese, which can take even longer for a hydrate oxide layer to build up on the filter medium.

The following processes are suitable for processing:

• Aerobic and microaerobic treatment
• Anoxic treatment
• Treatment with ozone

Aerobic and microaerobic treatment

Aeration increases the oxygen content of the raw water. This supply of oxygen enables the conversion of the metals from the 2-valent oxidation stage to the 3-valent for iron and to the 4-valent for manganese.

During aerobic treatment , an oxygen content of ≥ 4.0 mg / l O _{2 is set} in the raw water and bacterial reactions are only slightly detectable, if at all.

However, if only a small amount of oxygen is added to raw water, the oxidation takes place largely with the help of bacteria. One then speaks of a microaerobic treatment . However, higher oxygen contents hinder bacterial growth and are therefore not permitted upstream of the filter stage. However, a further ventilation stage is then required after the filter stage. In this, the oxygen content is increased to such an extent that a protective layer of lime corrosion can be formed in iron pipes. If it is also necessary to adjust the lime-carbonic acid balance , this is also only done after the filter stage in the microaerobic treatment.

The reaction equations for iron and manganese are:

${\displaystyle \mathrm {2\ Fe(HCO_{3})_{2}+{\tfrac {1}{2}}\ O_{2}+\ H_{2}O\longrightarrow } }$ ${\displaystyle \mathrm {{2\ FeO(OH)\cdot H_{2}O\downarrow }+\ 4\ CO_{2}} }$

Iron (II) hydrogen carbonate reacts with oxygen and water to form undissolved iron (III) oxide hydrate and carbon dioxide

Note : for iron (III) oxide hydrates different formulas are possible, often also known as formula: Fe ₂ O ₃ .xH ₂ O indicated

${\displaystyle \mathrm {Mn(HCO_{3})_{2}+{\tfrac {1}{2}}\ O_{2}\longrightarrow } }$ ${\displaystyle \mathrm {{\ MnO_{2}\cdot H_{2}O\downarrow }+2\ CO_{2}} }$

Manganese (II) hydrogen carbonate reacts with oxygen to form undissolved manganese (IV) oxide hydrate and carbon dioxide

The oxidation of the metals is accelerated both by catalytic reactions on the surface of the precipitation products and by bacterial-induced reactions. Depending on which process is used, either the catalytic or the bacterial reaction predominates. The influence of bacteria has been demonstrated especially in the case of iron. Scanning electron microscopic investigations showed that the oxide hydrate sludge in an experimental filter consisted mainly of ribbon-shaped deposits of the bacterial species Gallionella feruginea . This bacterium gets its metabolic energy from iron oxidation. In addition to iron, a bacterial increase in oxidation was also observed in manganese. Hyphomicrobium and Pedomicrobium were identified as bacteria .

In contrast to iron removal, which is largely unproblematic as long as the iron is not bound to humic acids , extensive manganese removal can be more difficult. This is especially the case when the iron compound content is low and the pH value of the water to be treated is above 7.8. At these higher pH values ​​than 7.8, in most cases the demanganization (formation of MnO ₂ ) must be supported by adding a strong oxidizing agent, for example potassium permanganate . To shorten the training time for new filters, which are mainly intended to remove manganese from the water, it is advantageous to let a weak permanganate solution act on the filter material. A thin film of manganese dioxide is deposited on the surface of the filter grains, which catalytically accelerates the oxidation of the divalent manganese. You can also apply so-called "incorporated" gravel from a filter that has been in operation for a long time, which shortens the training period.

Anoxic treatment

If there is a lack of oxygen, nitrate can also react as an electron acceptor and thus as an oxidizing agent with the metals in oxidation state 2. An extensive removal of the metal content could be demonstrated in a test facility. Here the nitrate is broken down to nitrogen, whereby nitrite was not detectable. However, it is at this nitrate oxidation by a method known in the art as a single process is not economical because of their susceptibility to interference and longer reaction times and is hardly used.

The reaction equation for nitrate oxidation follows:

${\displaystyle \mathrm {5\ Fe(HCO_{3})_{2}+NaNO_{3}+3\ H_{2}O\longrightarrow } }$ ${\displaystyle \mathrm {{5\ FeO(OH)\cdot H_{2}O\downarrow }+\ 10\ CO_{2}+NaOH+0,5\ N_{2}} }$

Iron (II) hydrogen carbonate reacts with sodium nitrate and water to form undissolved iron (III) oxide hydrate, carbon dioxide , sodium hydroxide and nitrogen

Treatment with ozone

Groundwater, particularly near the surface, can also contain increased amounts of organic pollution. With normal ventilation, flocculation and filtering, these org. Compounds, also with regard to the smell and taste of the pure water, are often not largely removed. Such raw water, for example bank filtrates , are treated with stronger oxidizing agents and adsorbers as filter media. If ozone is used as the oxidizing agent for cleaning , a modified technique for the separation of iron and manganese is used.

The divalent iron reacts with ozone in the same way as when oxygen is used to form iron (III) oxide hydrate. In contrast, the manganese is oxidized up to the 7-valent level. The reaction equation follows:

${\displaystyle \mathrm {2\ Mn(HCO_{3})_{2}+\ 5\ O_{3}+\ 2\ NaHCO_{3}\longrightarrow } }$ ${\displaystyle \mathrm {2\ NaMnO_{4}\ +6\ CO_{2}+\ 5\ O_{2}} }$

Manganese (II) hydrogen carbonate + sodium hydrogen carbonate reacts with ozone to form sodium permanganate, carbon dioxide and oxygen

Since the permanganates are easily soluble, only the precipitated iron is often separated out in a first filter stage. The manganese is still dissolved in the pure water after the filter stage. The reduction to manganese dioxide takes place in a downstream, second filter stage with activated carbon as the reducing agent, adsorber and filter medium. Theoretically, iron and manganese could also be filtered off with just filtering through activated carbon or a bed filter with activated carbon and gravel. At higher iron contents, however, the activated carbon layer would very quickly become silted up. This would make the cycles between 2 backwashes too short. Therefore, for economic reasons, separation usually takes place in 2 separate filter stages.

In water technology, the Düsseldorf process is a process (see below for details) that was developed for cleaning filtrate from the banks of the Rhine . Since the raw water to be treated contains only a small amount of iron and manganese compounds, this process is used for filtering through 2 separate layers of activated carbon.

See also

• De-iced water

Individual evidence

1. ^ Heinrich Sontheimer, Paul Spindler, Ulrich Rohmann: Water chemistry for engineers . DVGW research center at the Engler-Bunte-Institute of the University of Karlsruhe 1980, ZfGW-Verlag Frankfurt, ISBN 3-922671-00-4
2. ↑ International Standards for Drinking Water , World Health Organization, Geneva 1971, Third Edition, Table 3, Page 40
3. ↑ Klaus Hagen , bbr, Technical Water Treatment, Volume 46, 4/95
4. ↑ Dipl.-Ing. Günter Lamm , WWT, 21st year, 1971, issue 4, p. 120
5. ↑ Christoph Czekalla , bbr Fachtechnik Wasseraufbereitung, vol. 48, 1997, issue 4, p. 26
6. ↑ Christoph Czekalla , bbr Fachtechnik Wasseraufbereitung, vol. 48, 1997, issue 4, p. 24
7. ↑ Christoph Czekalla , bbr Fachtechnik Wasseraufbereitung, Vol. 48, 1997, Issue 4, pp. 22-26
8. ^ AF Holleman , E. Wiberg : Textbook of Inorganic Chemistry . 37-39 Edition. Walter de Gruyter, Berlin 1956, p. 536.
9. ↑ Christoph Czekalla, Hubert Kotulla , gfw Wasser · Abwasser, Vol. 131, 1990, Issue 3, pp. 126-132
10. ↑ Christoph Czekalla, Hubert Kotulla , gfw Wasser Abwasser, Vol. 131, 1990, Issue 3, p. 129
11. ↑ Christoph Czekalla , bbr Fachtechnik Wasseraufbereitung, vol. 48, 1997, issue 4, p. 23
12. ↑ Christoph Czekalla, Hubert Kotulla , gfw Wasser Abwasser, Vol. 131, 1990, Issue 3, p. 131
13. ↑ Christoph Czekalla, Hubert Kotulla , gfw Wasser Abwasser, vol. 131, 1990, issue 3, p. 130
14. ^ Report from France , wlb (water, air and operation), Vol. 21, 1977, No. 7, p. 400
15. ^ Stadtwerke Düsseldorf: private customers> drinking water

Remarks

1. ↑ For the consequences of the increased intake of iron-containing compounds by the human organism, see Iron Metabolism