WO2012110038A1

WO2012110038A1 - A method for producing potable water by enhanced removal of trace species contaminants

Info

Publication number: WO2012110038A1
Application number: PCT/DK2012/050005
Authority: WO
Inventors: Andreas Guldager
Original assignee: Microdrop Aqua Aps
Priority date: 2011-02-15
Filing date: 2012-01-04
Publication date: 2012-08-23
Also published as: CN103476713A; DK201170088A; BR112013020901A2

Abstract

The invention relates to a method for producing potable water by enhanced removal of trace species contaminants, comprising the steps of contacting water with a ferrous material, co-precipitating trace species upon aeration, and recovering drinking water, wherein the efficiency is enhanced by galvanic corrosion working in concert with a layer of green rust and a biofilm of microorganisms. In a second aspect a plant is provided for production of drinking water according to said method.

Description

Method for producing potable water by enhanced removal of trace species contaminants

The present invention relates in a first aspect to a method for pro- ducing drinking water from crude water containing trace species contaminants. In a second aspect, the invention relates to a plant for producing drinking water according to the method of the first aspect of the invention.

Throughout the world, the contamination of groundwater with unwanted substances constitutes a major problem. The natural constitution of the underground may in some regions give rise to serious cases of groundwater contamination, and where ever industrialised farming is carried out, the occurrence of pesticides and their breakdown products in the water is commonplace.

The presence of excess amounts of arsenic in more than three mil- lion groundwater wells of the world is linked to an increased risk of cancer and a range of other diseases and health problems in the affected areas, among which Bangladesh is a paramount example.

To take account of the adverse health effects of arsenic, the WHO has lowered the recommended limit for arsenic in drinking water to 10 pg/L and in many industrialised countries the limit is now set at 5 Mg/L. However, a large number of waterworks using existing methods has failed to comply with this limit and they have had the option of either closing down or investing in new and costly equipment for purification. It has been found a difficult task to reduce the content of arsenic from a frequently encoun- tered level of 20-35 pmg/l to below 5 g/l at a reasonable cost.

Contamination with arsenic is a particularly pronounced problem in waterworks receiving groundwater with a low content of iron. In waterworks endowed with water rich in iron compounds this is less so, since arsenic is often co-precipitated with oxidized iron compounds, when the wa- ter is treated in a conventional way by oxidation, typically aeration, until iron precipitates in sand filters or precipitation basins. However, it is not possible to remove arsenic by conventional oxidation of water, if the iron content of the water is not sufficient to ensure the desired co-precipitation of arsenic and other contaminants, including pesticides. DE 197 45 664 Al discloses a method for treating arsenic- containing water, where the water flows through a reactor filled with an iron-containing granulate, said granulate being produced by mixing sand and iron powder and subsequent firing under exclusion of oxygen. In the reactor, the iron is oxidized by the oxygen dissolved in water generating Fe(III) ions, said ions together with As forming poorly soluble iron arsenate. Excess Fe(IIi) ions are precipitated as iron hydroxide binding As by adsorption. Thus, As binds to the granulate, wherefrom it has to be removed at suitable intervals. When precipitating Fe(HI) compounds, the granulate uses agglomerates comparatively quickly and has to be exchanged frequently. The manufacture of the granulate requires work and energy. Moreover, the method requires the supply of additional oxygen to the reactor prior to treatment, if the treated groundwater is low in oxygen. In conclusion, the known method is work-intensive, complicated and expensive.

US 5 951 869 describes a reactor, where water is treated with iron while simultaneously supplying oxygen. The treatment takes place in a fluid bed with iron particles as the source of iron. The use of a fluid bed, though, is an expensive and cumbersome enterprise.

US 4 525 254 provides the enrichment of water with iron from a dissolving anode in the presence of a non-soluble cathode for treatment of industrial waste water. The anode, the cathode, and the water to be treated are subjected to continuous agitation, so that said dissolution takes place under oxidizing conditions.

The above-mentioned methods share the common feature that the iron treatment takes place concomitant with aeration or require that the water has a substantial content of oxygen from the very start. Accordingly, there is an increased risk that the system is clogged by the precipitated oxidized iron compounds.

U S 2009/0020482 m a rks a g reat ste p fo rwa rd i n t h e development of methods for removing contaminant trace species. Here, the water to be treated is contacted with an iron-containing material prior to oxidation in order to increase the iron content of the water and thus improve co-precipitation of contaminants upon oxidation. Still, even this improved method has been found to be wanting when it comes to the robustness of the release of iron to the water. Even though the release of iron and its binding of arsenic are often satisfactory when a layer of so-called green rust is present on the iron-containing material, said layer is delicate and deteriorates if exposed to even modest levels of oxygen. After service stoppages and other interruptions of the operation resulting in exposure to oxygen, the layer of green rust must build up anew and will only be fully effective after a period of several months.

in view of the above, the object of the present invention is to provide a method a nd a plant for production of drinking water from crude water containing trace species contaminants, wherein a robust, effective and efficient removal of contaminants to a high level is attained, also when starting from crude water low in iron. The method should furthermore be affordable, simple and environmentally friendly.

To meet th is object, accord i ng to the fi rst aspect of the invention a method is provided for producing drinking water from crude water containing trace species contaminants, said method comprising the steps of contacting the crude water containing the trace species contaminants with an iron-containing material under subatmospheric oxy- gen partial pressure such as to enrich the water with Fe(II) compounds; co-precipitating at least a part of the trace species by treating the iron- enriched water under oxidizing conditions in an aerator; and recovering drinking water by separation of the precipitate; wherein a layer of green rust and a biofilm of microaerophilic iron-oxidizing microorganisms is provided for on the iron-containing material, and wherein the iron- containing material when contacted with the crude water is subjected to galvanic corrosion by provision of a nearby material having a more negative electrode potential than iron, wherein the iron-containing material and the material having a more negative electrode potential than iron are arranged so that they do not abut each other but communicate by means of an electric lead.

It has surprisingly been found by the inventors that the galvanic corrosion of the iron-containing material when performed according to the invention not only promotes the abiotic release of iron into the wa- ter, but also significantly assures the maintenance or rapid (re)establishment of a layer of green rust within the course of hours or a few days and enhances the growth and effects of microaerophilic iron- oxidizing microorganisms by contributing to a hypoxic environment in the close surroundings of the iron-containing material.

With the findings of the inventors, a simple yet efficient method is provided, wherein there is made the most of the purifying potential of the iron-containing material. By connecting or disconnecting the electric lead, the galvanic corrosion can be regulated. The method requires only a small consumption of energy and no extraneous chemicals besides the iron-containing material and the material of a more negative electrode potential than iron. Said material having a more negative electrode potential than iron may be a metal, an alloy or a non-metal such as graphite. If otherwise unobjectionable from a health perspective, any such material of higher nobility than iron may come into consideration.

In the current text the "iron-containing material" refers to a material consisting wholly or predominantly of iron, while the "material having a more negative electrode potential than iron" may be an alloy as stated in the above, which may itself comprise a certain proportion of iron. Accordingly, it is to be understood that the overall electrode potential of the "material having a more negative electrode potential than iron" should be more negative than the overall electrode potential of the "iron-containing material".

According to a preferred embodiment of the invention, the elec- trie lead is provided with a resistor, optionally a variable resistor. In this way the degree of corrosion and thus dissolution of iron may be further adjusted according to the flow of water, which is treated, the content of contaminants in the crude water and the desired quality of the final drinking water. Preferably, the variable resistor is in the form of a poten- tiometer.

Advantageously, the iron-containing material is in the form of at least one exchangeable rod being suspended from an isolated lead-in into a closed container consisting of, being lined with or containing the material having a more negative electrode potential than iron. Through- out the present text, a "closed container" is to be understood as a container provided with openings for inlet and outlet of the water to be treated but with substantially no further openings during performance of the method according to the invention. By making use of a closed con- tainer the observance of a sub-atmospheric oxygen partial pressure is facilitated, so that premature precipitation of Fe(III) compounds is minimized. In a preferred embodiment, the closed container consists of stainless steel as the material having a more negative electrode potential than iron. In a converse embodiment, the iron-containing material is in the form of a n excha ngeable closed contai ner to be corroded from within, while the material having a more negative electrode potential than iron is in the form of at least one rod being suspended from an isolated lead-in into said container.

Under certain circumstances it is desirable that prior to the con- tacting of the crude water with the iron-containing material, iron compounds and optionally other compounds present in the crude water is separated therefrom, optionally in a sand filter. This may be relevant in cases, where a substantial proportion of the iron contained in the crude water is present in an inactive state, which is unable to co-precipitate trace species contaminants. Also, the occurrence of iron compounds or other compounds that might inhibit the galvanic corrosion and/or the release of iron from the iron-containing material may call for an initial separation step.

In a preferred embodiment, the treating of the iron-enriched water under oxidizing conditions in an aerator is achieved by leading the water enriched with Fe(II) to the top of an aerator, optionally from an iron-containing material being enclosed in a container mounted above the aerator, said aerator comprising a plate or one or more pipes with holes or slots for forming drops by flow of the water through the holes or slots at the initiation of the treatment process, and means for causing division of the drops by contact therewith, said means being arranged below said plate or pipe(s), wherein the means for causing division of the drops comprise a plurality of tubular elements in the form of pipes having reticulate pipe walls, said tubular elements being placed in hori- zontal layers of several parallel tubular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers; and letting the water pass through said aerator to the bottom thereof by the force of gravity.

Preferentially, the aerator is fit up so that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers by an angle of approximately 90°. In this way good condi- tions for drop divisions to occur within the aerator are generated.

Following aeration, the precipitate is preferably separated from the drinking water by settlement in a collection container, optionally followed by further separation by treatment of the water in one or more filters, optionally assisted by one or more magnets. The one or more fil- ters may be sand filters, ceramic filters or yet another type of filters. It may, however, be relevant to return the water one or more times after precipitation and separation of the iron compounds for renewed contact with the iron-containing material, so that the content of trace species may be brought even further down. Alternatively, the water may be re- turned from the bottom of the above-mentioned aerator to its top with a view to enhanced aeration. Furthermore, air, optionally enriched in oxygen, may be led by passive or active flow in a vent pipe to the part of said aerator containing the tubular elements. In this manner, the degree of oxidation achieved in the aerator may be further regulated. The active supply of oxygen to the aerator could be used as an alternative to returning the water from the bottom to the top of said aerator in instances of crude water heavily loaded with substances to be removed.

In an alternative embodiment, the precipitate is separated from the drinking water by direct dripping of water treated under oxidizing conditions onto a sand filter without any intermediate settlement in a collection container, the precipitate being deposited on or near the upper surface of the sand filter. By ensuring a thorough aeration, a fully satisfactory flocculation of undesired compounds may in some instances be achieved, resulting in the formation of floe, which accumulates on the surface of the sand filter without substantially infiltrating this, so that it can be easily removed.

Preferably, the co-precipitated trace species comprise arsenic and/or pesticides and/or non-volatile organic carbon (NVOC) such as humus. However, also other trace species such as chromium, mercury, other heavy metals, MTBE (methyl t-butyl ether), and a range of non- pesticide chlorinated hydrocarbons may be co-precipitated.

Advantageously, the biofilm of microaerophilic iron-oxidizing microorganisms comprises iron bacteria representing one or more of the genera Gallionella, Sphaerotilus, and Leptothrix. A number of members of these genera thrive at interfaces between anaerobic and aerobic conditions and perform the oxidation of ferrous iron in environments of neutral pH. A layer of mucus made up of exopolymers from the microorganisms offers enhanced adsorption of hydrophilic substances.

According to a preferred embodiment, the biofilm is dominated by Gallionella ferruginea. This species gives off Fe oxides with a high ability to adsorb contaminant trace species, including arsenic.

Preferentially, the content of oxygen in the crude water when contacting the iron-containing material is within the range of 0.1 to 1.6 mg/L, preferably 0.3 to 1.5 mg/L, as Gallionella ferruginea is found to thrive at said oxygen levels. For the same reason, the pH of the crude water when contacting the iron-containing material preferably is within the range of 6.0 to 8.0.

According to one embodiment, the corrosion of the iron- containing material is further intensified by applying an electric current from an external power source to the iron-containing material and the material having a more negative electrode potential than iron, respectively. Generally, however, no external power supply is necessary to effect the required corrosion.

To meet the aforementioned object, according to the second aspect of the invention a plant for producing drinking water from crude water containing trace species contaminants is provided, said plant comprising a closed container consisting of, being lined with or containing a material having a more negative electrode potential than iron, said con- tainer holding at least one exchangeable rod of an iron-containing material being suspended from an isolated lead-in, wherein said materials do not abut each other but are connected by means of an electric lead, said container being connected to a supply of crude water; an aerator capable of supplying oxygen to water received from said container; and a separator unit for separation of precipitate of oxidized iron and co- precipitated trace species from the water received from said aerator, wherein a layer of green rust and a biofilm of microorganisms is present on the iron-containing material.

Said plant provides the same or similar advantages as the first aspect of the invention and due to its modest requirements for energy and materials is suitable for installation and maintenance under harsh or primitive conditions.

In one embodiment, the electric lead is provided with a resistor, optionally a variable resistor.

According to a specific embodiment, the container is mounted above the aerator, said aerator comprising a plate or one or more pipes with holes or slots for forming drops by flow of the water through the holes or slots at the initiation of the treatment process, and means for causing division of the drops by contact therewith, said means being arranged below said plate or pipe(s), wherein the means for causing division of the drops comprise a plurality of tubular elements in the form of pipes having reticulate pipe walls, said tubular elements being placed in horizontal layers of several parallel tubular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers, said elements allowing divided drops to pass through the aerator to the bottom thereof by the force of gravity. In this way, a very robust and simple construction for thorough aeration is provided.

In one embodiment, the plant further comprises a return pipe for recycling treated water to the top of the aerator in order to effect an even more intense aeration. Alternatively, the plant may comprise a return pipe for recycling treated water to the closed container holding the iron-containing material with a view to repeated enrichment with iron. This pipe proves useful in case of a desired reduction from a high to a very low level of a trace species contaminant in the water to be treated.

In the following, a preferred embodiment of the invention will be illustrated by reference to the non-limiting figures.

Figure 1 illustrates an embodiment of a plant for carrying out the method according to the first aspect of the invention.

Figure 2 illustrates an embodiment of the closed container of the plant according to the second aspect of the invention, said plant be- ing for carrying out the method according to the first aspect of the invention.

Referring to figure 1, the main features of the illustrated plant are referenced by numbers as follows: 1 is a pump for pumping crude water to the top of the plant to a closed container 2; 3 is an iron- containing material; 4 is the top of an aerator; 5 is an aeration chamber of said aerator; 6 is a collection container; 7 is a pump for leading the water to a sand filter 8; 9 is an outlet for pure drinking water; 10 is a pump for pumping treated water from the collection container 6 to the top of the plant for repeated treatment.

On figure 2, the main features of the illustrated closed container are referenced by letters as follows: A is a tank of stainless steel, constituting a closed container as earlier defined; B is a rod of iron; C is an isolated lead-in from which an electrical lead (not shown) runs to the exterior of tank A such as to connect the latter galvanically to the rod of iron B via a variable resistor in the form of a potentiometer (not shown); D is an inlet for receiving crude water; E is an outlet for the exit of iron- enriched water from the closed container.

An overall description of a preferred embodiment of the method according to the invention will now be given.

An amount of crude water to be treated is led through the inlet

D to the interior of the tank of stainless steel A to be contacted with the rod of iron B. An electric lead is attached to the exterior of the tank of stainless steel and, through a potentiometer, to the isolated lead-in C, such as to galvanically connect the rod of iron and the tank of stainless steel. Due to their different electrode potentials, a galvanic couple is established and an electric current will run. The rod of iron now acts as an anode and the tank of stainless steel as a cathode, while the water to be treated serves as an electrolyte owing to its conductivity. The electrolyte provides a means for migration of ions released from the iron rod towards the more noble material, i.e. the surface of the tank of stainless steel. At the same time oxygen present in the water will be consumed according to the following reaction :

2 Fe° + 0₂ + 2 H₂0 -> 2 Fe²⁺ + 4 OH^".

The ferrous iron formed in this way may in turn contribute to the formation of green rust, containing iron in its divalent as well as its trivalent form :

10 (Fe3ⁿFeⁱⁿ(OH)₈CI * 2 H₂0) + 2 Fe²⁺ + 7 (S0₄ ^2~ or C0₃ ^2~) + 0₂ -> 7 (Fe₄ ⁿFe₂ ⁱⁿ(OH)i₂(S0₄ or C0₃) + 2 H₂0) + 4 H₂0 + 10 CI^"

Green rust is a transitional compound, which can be maintained only at very low levels of oxygen. It is extremely effective in binding arsenic and apparently also other trace species contaminants such as heavy metals. In groundwater arsenic is present as arsenite (H₂Asⁱⁿ0₃ ^") and/or arsenate (HAs^v0₄ ^2~). Ions of arsenate adsorb to groups of -OH₂ ⁺ in the layer of green rust, while arsenic as arsenite apparently is not able to do so before being oxidized itself to arsenate. However, the green rust often contains the carbonate anion C0₃ ^2" and there is evidence to suggest that said carbonate ions may be exchanged by arsenite, which is then catalytically converted into arsenate by the content of Fe^m in the layer of green rust. This may explain the very effective removal of arsenic found when making use of green rust.

Due to the galvanic corrosion of iron in the closed container under sub-atmospheric oxygen conditions, the green rust is actively supported and may more easily be restored after any interruption of the wa- ter supply or unintended exposure to oxygen.

As the natural level of oxygen in the water to be treated is reduced as a result of the galvanic corrosion of the iron rod, the growth of a biofilm of microaerophilic iron-oxidizing bacteria, notably Gallionella feruginea, on the iron is also promoted. Said species has proven very useful in the removal of contaminant trace species as it precipitates Fe oxide in the form of ferrihydrite, which is a nanoporous hydrous ferric oxyhydroxide mineral presenting a large surface area of several hundred square meters per gram. In addition to its high ratio of surface area to volume, ferrihydrite also has a high density of local defects, such as dangling bonds and vacancies, which all confer to it a high ability to adsorb many environmentally important chemical species, including arsenic.

By the combined action of the green rust and the biofilm of mi- croaerophilic iron-oxidizing bacteria, which both depend on the presence of an oxic-anoxic interface close by the iron-containing material, a far more effective clearing of trace species contaminants from the water is achieved in the present method than if the galvanic corrosion and iron- enrichment was not continuously shielded from exposure to atmospheric levels of oxygen. By virtue of this shielding off, the galvanic corrosion, which gives rise to oxygen-consuming reactions, is allowed to contribute to the generation and maintenance of the required steep oxygen gradient near the surface of the iron rod.

In principle, a substantial surface area of iron-containing mate- rial is desirable in order to provide an ample expanse for release of ferrous iron, establishment of green rust and housing of micoaerophilic iron-oxidizing bacteria. On the other hand, the corrosion rate per unit area of the anode will be highest when observing a large cathode area relative to the anode area. A balance thus must be struck.

In the present embodiment, the precise extent of galvanic corrosion and release of iron to the water is adjusted by means of the potentiometer inserted into the electric connection provided by the lead that connects the exterior of the tank of stainless steel A to the isolated lead-in C. The potentiometer is controlled by a motor, which receives a signal according to the flow of water into the tank. The motor can also be set up to react to varying concentrations of contaminants in the water to be treated in view of the desired quality of the final drinking water.

From the tank A the water is led through the outlet E to the top of the aerator 4 to be received in a pipe provided with slots. By the force of gravity, the water passes through said slots and arrives as drops in the top of the aeration chamber 5. During the course of their fall through the aeration chamber, the drops impinge on a multitude of alternating layers of reticulate tubular elements, mutually displaced by 90°, so that the drops are divided into droplets. The formation of droplets results in a substantially larger drop surface area relative to drop volume, so that enhanced enrichment with oxygen can take place. The height of the stack of layers of tubular elements is adjusted so that the initial drops are divided at least 50-60 times and preferably 60-80 times when falling through the aeration unit, in which case a satisfactory oxygen saturation of up to 95% is assured. Alternatively, the water might have been aerated in a conventional device such as a splasher, a drip-type sheet, a cascade aerator or by blowing in air or oxygen.

The aerated droplets of water is directed to the collection con- tainer 6, where oxidized iron compounds settle together with co- precipitated trace species contaminants. The settled material may be removed from the collection container as necessary by light flushing. Further, the water is fed to the sand filter 8 by means of the pump 7 to effect further precipitation of iron and trace species and to ensure reduc- tion of co-precipitated arsenic, and finally drinking water is taken from the outlet 9. In other cases, however, separation in the collection container would have been sufficient, so that sand filtration might have been dispensed with.

The concentration of arsenic and other trace species in the final drinking water product is monitored on a regular basis and when increasing towards the stipulated limit, the potentiometer is tuned to allow an increased galvanic corrosion and release of iron to the water. During its lifetime the rod of iron is continuously eroded by corrosion, so that the surface available for release of iron will at some point diminish. In order to compensate for this and to ensure a uniform release of iron, the potentiometer is regularly adjusted. When the rod of iron is finally consumed, it is replaced by a new one.

Example A plant according to the invention was installed and received crude water presenting a h ig h level of a rsen ic ( 19 Mg/L). As iron- containing material was used a rod of iron suspended from an isolated lead-in into a tank of stainless steel, which served as a material having a more negative electrode potential than iron.

The content of oxygen in the water when contacting the rod of iron was consistently kept below 1 mg/L. On the iron rod a layer of green rust and a biofilm of a species showing the phenological traits of Gallionella feruginea developed.

In the course of a period of less than one week the concentration of arsenic was reduced to 3 pg/L by dissolution of 2,75 mg/L of iron to the treated water.

Claims

P A T E N T C L A I M S

1. A method for producing drinking water from crude water containing trace species contaminants, said method comprising the steps of a. contacting the crude water containing the trace species con- taminants with an iron-containing material under subatmospheric oxygen partial pressure such as to enrich the water with Fe(II) compounds;

b. co-precipitating at least a part of the trace species by treating the iron-enriched water under oxidizing conditions in an aerator; and c. recovering drinking water by separation of the precipitate; wherein a layer of green rust and a biofilm of microaerophilic iron- oxidizing microorganisms is provided for on the iron-containing material, and wherein in step a.) the iron-containing material is subjected to galvanic corrosion by provision of a nearby material having a more negative electrode potential than iron, wherein the iron-containing material and the material having a more negative electrode potential than iron are arranged so that they do not abut each other but communicate by means of an electric lead.

2. The method according to claim 1, wherein the electric lead is provided with a resistor.

3. The method according to claim 2, wherein the resistor is a variable resistor.

4. The method according to any one of the preceding claims, wherein the iron-containing material is in the form of at least one exchangeable rod being suspended from an isolated lead-in into a closed container consisting of, being lined with or containing the material having a more negative electrode potential than iron.

5. The method according to any one of the preceding claims, wherein prior to step a.) iron compounds and optionally other compounds present in the crude water is separated therefrom, optionally in a sand filter.

6. The method according to any one of the preceding claims, wherein in step b.) said treating under oxidizing conditions in an aerator is achieved by leading the water enriched with Fe(II) to the top of an aerator, optionally from an iron-containing material being enclosed in a container mounted above the aerator, said aerator comprising a plate or one or more pipes with holes or slots for forming drops by flow of the water through the holes or slots at the initiation of the treatment process, and means for causing division of the drops by contact therewith, said means being arranged below said plate or pipe(s), wherein the means for causing division of the drops comprise a plurality of tubular elements in the form of pipes having reticulate pipe walls, said tubular elements being placed in horizontal layers of several parallel tubular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers; and letting the water pass through said aerator to the bottom thereof by the force of gravity.

7. The method according any one of the preceding claims, wherein in step c.) the precipitate is separated from the drinking water by settlement in a collection container, optionally followed by further separation by treatment of the water in one or more filters, optionally assisted by one or more magnets.

8. The method according to any one of the claims 1 to 6, wherein in step c.) the precipitate is separated from the drinking water by direct dripping of water treated under oxidizing conditions onto a sand filter without any intermediate settlement in a collection container, the precipitate being deposited on or near the upper surface of the sand filter.

9. The method according to any one of the preceding claims, wherein the co-precipitated trace species is/are selected from the group consisting of arsenic, pesticides, non-volatile organic carbon (NVOC), chromium, mercury, other heavy metals, MTBE (methyl t-butyl ether), and non-pesticide chlorinated hydrocarbons.

10. The method according to claim 9, wherein the co- precipitated trace species comprise arsenic and/or pesticides and/or non-volatile organic carbon (NVOC).

11. The method according to any one of the previous claims, wherein the biofilm of microaerophilic iron-oxidizing microorganisms comprises iron bacteria representing one or more of the genera Gallio- nella, Sphaerotilus, and Leptothrix.

12. The method according to claim 11, wherein the biofilm is dominated by Gallionella ferruginea.

13. The method according to claim 12, wherein the content of oxygen in the crude water when contacting the iron-containing material is within the range of 0.1 to 1.6 mg/L, preferably 0.3 to 1.5 mg/L.

14. The method according to claim 12 or 13, wherein the pH of the crude water when contacting the iron-containing material is within the range of 6.0 to 8.0.

15. The method according to any one of the preceding claims, wherein in step a.) the corrosion of the iron-containing material is further intensified by applying an electric current from an external power source to the iron-containing material and the material having a more negative electrode potential than iron, respectively.

16. A plant for producing drinking water from crude water containing trace species contaminants, said plant comprising

i. a closed container consisting of, being lined with or containing a material having a more negative electrode potential than iron, said container holding at least one exchangeable rod of an iron-containing material being suspended from an isolated lead-in, wherein said materials do not abut each other but are connected by means of an electric lead, said container being connected to a supply of crude water;

ii. an aerator capable of supplying oxygen to water received from said container; and

iii. a separator unit for separation of precipitate of oxidized iron and co-precipitated trace species from the water received from said aerator,

wherein a layer of green rust and a biofilm of microorganisms is present on the iron-containing material.

17. The plant according to claim 16, wherein the electric lead is provided with a resistor.

18. The plant according to claim 17, wherein the resistor is a variable resistor.

19. The plant according to any one of claims 16 to 18, wherein the container is mounted above the aerator, said aerator comprising a plate or one or more pipes with holes or slots for forming drops by flow of the water through the holes or slots at the initiation of the treatment process, a nd mea ns for ca usi ng d ivision of the d rops by contact therewith, said means being arranged below said plate or pipe(s), wherein the means for causing division of the drops comprise a plurality of tubular elements in the form of pipes having reticulate pipe walls, said tubular elements being placed in horizontal layers of several parallel tu- bular elements stacked in such a way that the longitudinal axes of the tubular elements in one layer are angularly displaced in relation to the longitudinal axes of the tubular elements in the one or more adjacent layers, said elements allowing divided drops to pass through the aerator to the bottom thereof by the force of gravity.

20. The plant according to any one of claims 16 to 19, further comprising a return pipe for recycling treated water to the top of the aerator.