WO2008025358A1

WO2008025358A1 - Water purification including disinfection, oxidation and arsenic removal

Info

Publication number: WO2008025358A1
Application number: PCT/DK2007/000386
Authority: WO
Inventors: Mikkel Vestergaard Frandsen
Original assignee: Vestergaard Sa
Priority date: 2006-09-01
Filing date: 2007-08-25
Publication date: 2008-03-06
Also published as: WO2008025356A1; CN101522578A; TW200833616A

Abstract

Water purification in a unit with a number of compartments, where water flows successively through these compartments, the unit comprising - a compartment with an iodine releasing resin for killing microbes in water, - a downstream compartment with an iodine scavenger, the iodine scavenger being configured for releasing chlorine during iodine scavenging, the amount of released chlorine being configured for oxidation of trivalent arsenic to pentavalent arsenic, - a further downstream compartment with a arsenic removal resin configured for removal of arsenic from the water.

Description

WATER PURIFICATION INCLUDING DISINFECTION, OXIDATION AND ARSENIC REMOVAL

FIELD OF THE INVENTION

The present invention relates to portable systems and methods for removing arsenic 5 from ground water

BACKGROUND OF THE INVENTION

Arsenic is a naturally occurring contaminant found in a large number of ground wa- 10 ters, for example in Bangladesh and a number of states in the US. Being without odour and taste, no warnings are typically recognised during consumption of arsenic containing water. Especially in Bangladesh, many people are suffering from chronic poisoning appearing with painful, disturbed skin pigmentation and calluses on the palms and the hands. For example, according to information on the Internet site http://www.sos- 15 arsenic.net in India, 48.7% water samples had arsenic concentration above 10 ppb and 23.8% above 50 ppb. hi Bangladesh these values were 43.0% and 31.0% respectively. Almost 9 million people in India were drinking water with more than 10 ppb arsenic and 7 million people with more than 50 ppb arsenic. These facts have resulted in an increased focus on low cost but efficient means for arsenic removal from ground wa- 20 ter.

Typical removal of arsenic from water implies ferric, aluminium, manganese, zirconium and titanium or other heavy metal oxides or anion exchange resins. Companies as Alcan® and Adedge® have developed media containing ferric or aluminium oxides 25 for arsenic removal. The Dow Chemical Co. has developed a titanium based arsenic removal media (Adsorbsia™ GTO™).

Normally, arsenic occurs in water in trivalent form and in pentavalent form, where the trivalent Arsenite As⁺³ form (which can exist as H3AsO3 or AsO2- for example) is

30 regarded as more toxic, whereas the pentavalent Arsenate form As⁺⁵ (exists as

HAsO4— for example) is easier to remove. Therefore, As⁺³ is oxidised to As⁺⁵ in con- ventional processes in order to remove the entire arsenic content to below certain levels, typically to less than 10 micrograms per litre corresponding to 10 ppb (parts per billion).

A system for arsenic removal from ground water is disclosed in US patent No. 6,461,535 by de Esparza. In this case, a clay, a coagulant, such as ferric chloride and aluminium sulphate, and an oxidizer, such as calcium hypochlorite, are used for adsorbing the arsenic onto the coagulated colloidal mixture. In order for the clay to settle from the water before use of the water for drinking, a waiting time of 15-20 minutes is necessary.

A different system is disclosed in European patent application EP 1 568 660 for removing arsenic with a strong base anion exchange resin comprising at least one metal ion or metal-containing ion whose arsenate salt has a K_sp no greater than 10^"5.

hi rural areas, where clean drinking water is scarce, a commercially available water purification suction unit has achieved increased popularity. The unit is marketed under the name LifeStraw® by the Vestergaard Frandsen Group. The unit is used for water filtration by drawing-up water from the water source directly through the unit and into the mouth. The suction unit is compact and has a mouth piece, measures approximately 31 cm in length and 29 mm in width, and acts instantaneously. The drawn-up water is directly safe for human consumption. The unit contains a specially developed halogen-based resin that is extraordinarily effective to kill bacteria (such as Shigella, Salmonella, Enterrococus, Staphylococcus Aureus and E .CoIi), on contact, a textile pre-filters to remove particles equal to or larger than to 6 microns, preferably equal to or larger than 15 microns, and activated carbon to withhold excessive iodine and bad smell & taste. This unit removes efficiently disease causing micro organisms which spread diarrhoea, dysentery, typhoid, and cholera. Though having a number of advantages, namely the ability to almost instantaneously desinfect water, the light weight, a portable construction, the low cost of the device making it suitable for distribution in poor regions, it is not efficient as such for removing arsenic from water.

DESCRIPTION / SUMMARY OF THE INVENTION It is therefore the object of the invention to provide a compact water cleaning device, preferably in the format of LifeStraw®, which is also suitable for removal of arsenic.

This purpose is achieved by a water purification unit having a number of compartments for the water to flow successively through these compartments, the unit comprising:

- a compartment with an iodine releasing resin for killing microbes in water,

- a downstream compartment with an iodine scavenger, the iodine scavenger being configured for releasing chlorine during iodine scavenging, the amount of released chlorine being configured for oxidation of trivalent arsenic (As +III) to pentavalent arsenic (As +V),

- a further downstream compartment with an arsenic removal resin configured for removal of arsenic from the water.

With a purification unit according to the invention, a compact device is provided, for not only cleaning water on a general basis but also for removing arsenic. The compact property is achieved by using the chlorine for successful oxidation of arsenic trivalent in arsenic pentavalent, in order to facilitate removal of arsenic. Thus, the invention utilises a combination of knowledge from entirely different fields, namely the know- how of cleaning water in primarily poor tropical countries with compact, portable units like LifeStraw® and the know-how of arsenic removal in modern household apparatuses or larger facilities.

It should be acknowledged that the invention's rather low cost makes it possible for economically poor regions to not only access biologically cleaned water but also to access arsenic free water at the same time. The LifeStraw® product is already experiencing increased popularity in remote regions with difficult access to clean water, and an extended LifeStraw® product with arsenic removal capabilities would not imply much higher costs for the end user. The fact of providing in remote regions such a compact, low cost product with high quality decontamination properties including removal of arsenic, is in sharp contrast to the statement in US patent No. 6,461,535 col. 1 line 49 to 57 "The removal of chemical elements such as arsenic from water, however, requires resort to more sophisticated processes, hi developed countries, reverse osmosis, ion exchange, and activated carbon are conventional techniques used in purifying water in large agglomerations. However, the above conventional techniques for removing impurities, such as arsenic from ground water, are generally prohibitive or unavailable to small populations living in remote dwellings."

By the invention, both ion exchange and activated carbon can be used, as it will become apparent in the following, at costs and with a compactness that does not prevent access to clean water in remote dwellings and in even very poor regions. Thereby, spreading of diseases following low quality drinking water can be drastically reduced, especially, if governments and non-governmental organisations support the distribution of such compact devices among people in poor regions.

However, it should be noted that application of the invention is not limited to poor and remote regions but may be used in a variety of other applications. For example, due to its compactness, it is suited for general outdoor activities as well. Especially, in US mountainous regions, where water appears clean at first sight and suitable for drinking, but contains the odourless, tasteless and toxic arsenic, the user may be sure that the light weight, portable unit, such as an extended, arsenic removing LifeStraw®, prevents later suffering from arsenic induced illness due to the double function of the invention, where biological and chemical cleaning is performed at the same time at a degree which makes direct drinking through a unit according to the invention possible.

hi a preferred embodiment, the iodine scavenger resin is a strong ion exchange resin, for example a strong base anion exchange resin. Choosing such a resin promotes the compactness of the unit. It is well known to use activated carbon for iodine removal. However, this substance is not as efficient as strong ion exchange resins and rather large quantities are required, hi order to achieve a compact unit, especially in the case of the LifeStraw® product, a strong base anion exchange resin has been investigated instead. The use of this resin, as described above, opens the possibility for arsenic oxidation without loosing compactness. A candidate for an iodine scavenger - chlorine eluting resin is a strong base anion exchange resin partially or completely loaded with a non-covalently bond organic compound that will release chlorine upon passage of water through the resin. Preferably this organic compound is dichloroisocyanurate (from sodium dichloroisocyanurate also called sodium l,3-dichloro-l,3,5-triazine-2,4-dione-6-oxide or "NaDCC") or chloramine-B hydrate or l,3-dichloro-5,5-dimethylhydanthoin or 5-nitroorotic acid potassium salt monohydrate, for example as described in patent application US2005/0029493A1. Another options is a resin of the type as disclosed in US 4382862.

Optionally, the arsenic removing media may comprise activated alumina, for example as known from the commercially available Alcan® media named AAFS50. Alternatively, the arsenic removing media comprises ferric oxide, for example as known from the commercial Adedge® resins named AD33R or AD33L. These commercially avail- able resins contain substances for arsenic adsorption. Thus, in case that the invention is used together with these commercial resins or media, the chlorine oxidation of As(III) to As(V) may be used to reduce the amount of these commercial resins or media, such that primarily the As(V) removal property is utilised. A reduction of the amount of such commercial resins or media is of high interest due to the substantial costs of these materials. For this reason also, a thin layer of ferric oxide, possibly enriched with or substituted by aluminium oxide or by other heavy metal oxides like manganese, zirconium or titanium oxides, is considered as a useful solution.

The iodine needs to be active for a certain time in order to achieve a good result with respect to biological cleaning. The active time depends on the flow from the iodine releasing resin to the iodine scavenger. In the case of LifeStraw®, where water is drawn-up directly through the compact unit by the mouth for drinking from a contaminated water source, the activation time may need to be extended, which can be achieved by including a void space between the iodine releasing resin and the iodine scavenger resin. The volume of the void space should in this case be chosen to provide a substantial extension of the reaction time between the iodine and water contaminants during the typical water flow through said volume when water is drawn-up through the device and into the mouth. The term "substantial extension" covers an extension of the flow time which, typically, is in the order of the flow time through the iodine releasing resin compartment. Thus, the void space may have a volume comparable to the volume of the compartment with the iodine releasing resin. For the LifeStraw product, the flow rate is 100-150 ml/minute approximately, which is also achievable by the inven- tion in the case of a comparable design.

In addition, to remove excess chlorine and improve taste or odour properties of the cleaned water, a compartment or several compartments may, optionally, be provided with activated carbon. Similarly for iodine removal, a compartment or several com- partments containing activated carbon, for example in the form of granular activated carbon (GAC) may optionally be provided. As a further option, the GAC may be silver loaded.

The activated carbon may be used downstream of the iodine scavenging resin. This configuration has the advantage that the scavenging resin primarily takes up the iodine and releases chlorine for the arsenic oxidation, for example in the form of hypochlorite anions. Alternatively activated carbon may be used up-stream of the iodine scavenging resin for at least partly removal of iodine, hi a third configuration, iodine scavenging resin may be split in two or more compartments in order to scavenge all iodine, and activated carbon may be placed between two of these iodine scavenging resin compartments, hi a fourth configuration, activated carbon may be mixed together with the iodine scavenging resin.

The invention can be employed in a number of physical embodiments. However, the preferred solution utilising the potential for high compactness, is a portable water purification unit, for example tubular as the LifeStraw® product. In order to be carried around, the unit is advantageously shorter than 40 cm, or even shorter than 35 cm. For example, LifeStraw® has a length of 31 cm, a width of 29 mm, and a dry weight of 95 grams. Accordingly, the unit in the portable embodiment is preferred to have a diame- ter of less than 50 mm, rather less than 40 mm. Such a tube may be provided with a mouth peace for drawing-up water through the unit, just like LifeStraw®. The amount and efficiency of the chlorine releasing resin should be adjusted to achieve a certain arsenic removal, for example down to a level of less than 10 ppb. The amount of resin necessary to achieve this is dependent on the arsenic content of the influent water, and on the arsenic level of the effluent water that is targeted. Thus, the unit according to the invention may be configured to release a certain amount of iodine in the water; the amount and efficiency of the iodine scavenger resin may then be configured - in dependence on the certain amount of iodine.

The unit according to the invention may also be configured to release a certain amount of active chlorine in the water; this certain amount of active chlorine is configured for oxidation of a substantial amount of trivalent arsenic. For safety reasons, despite possible low amounts of arsenic present in the influent water, the resin may be configured, for secure working, also for high concentrations of arsenic, for example in the region of up to 1000 or 2000 parts per billion. In comparison, it may be mentioned that the level of arsenic in many water sources in Bangladesh is 1200 ppb, exceeding by far the admissible limit of 50 ppb for the Bangladesh drinking water.

The unit according to the invention may use the aforementioned removal of arsenic as a second removal stage, in these multi stage configurations the water may possibly not be drawn-up by the mouth anymore, at the least during the first stage, but with the use of pumps, gravity, siphons, etc. Or it may be that water is treated by batches or continuously during this first stage and then drawn-up by the mouth during the further stage.

A multiple stage arrangement may be useful in the cases where a first media is used for removing the major part of arsenic, for example between 50 and 95%, and the second stage is used to reduce the arsenic content to a very low level (i.e. below the admissible limit of 10 ppb). The reason for using two stage or multi-stage removal could be that the first product is by far cheaper than the second product. Thus, a low cost first stage may be used for removing the first coarse arsenic content, whereas the second, more expensive stage may be used to remove the last part of the arsenic below a predetermined level, such as 10 ppb. For example, it has been disclosed in Shaban W. Al Rmalli et al. "A biomaterial based approach for arsenic removal from water" published in J. Environ. Monit. , 2005, 7, 279-282 that biological material can be used for arsenic removal. Biological material such as dried roots of the water hyacinth plant (Eichhornia crassipes) can remove ar- senic from water. In the article, examples are given for 96% arsenic removal. Though the removal speed was rather slow, namely 30 minutes for 80% removal and 60 minutes for 96% removal of arsenic, the results are promising and have a potential for improvement of the arsenic removal properties. Such low cost, biological material may be considered as a candidate for a first stage for arsenic removal as discussed above.

Further interesting material for arsenic removal is available from the US company VeeTech, P. C. under the commercial names G2 and HIX. These products may be candidates for a single step arsenic removal or in a two or multi-stage arsenic removal system according to the invention.

Whether only one stage is used or whether two or more stages for arsenic removal are used, the aim is to reduce the arsenic to below a very low level, for example the internationally recognised lower level of 10 parts per billion.

To give an idea of the relative amounts of resins in the unit according to the invention, the following typical figures are helpful. Thus, the amount of iodine releasing resin is, typically, between 5 and 30 %, preferably between 10 and 25 %, of the inner volume of the unit. The amount of iodine scavenger - chlorine eluting resin is, typically, be- tween 5 and 60 %, preferably, between 10 and 40 % of the inner volume of the unit. The amount of arsenic removing media is, typically, between 5 and 80 % of the inner volume of the unit. If present, the total amount of activated carbon is, typically, between 10 and 60 % of the inner volume of the unit. Part or all of the activated carbon may be silver loaded. The activated carbon chamber can be split in two or into several compartments and placed strategically inside the unit housing. Alternatively the activated carbon can be mixed with some other ingredients, based on ion exchange resins or arsenic adsorption media, for example resins as also used for the product LifeStraw™. In comparison with the LifeStraw® product, a preferred water purification unit according to the invention is a portable unit with a mouthpiece for drawing-up water through the unit, the length of the unit is less than 40 cm, and the diameter is less than 50 mm. The amount of iodine releasing resin is between 5 and 30 % of the inner volume of the unit, the amount of iodine scavenger - chlorine eluting resin is between 5 and 60 % of the inner volume of the unit, and the amount of arsenic removing media is between 5 and 80 % of the inner volume of the unit.

In a further preferred solution, the water purification unit has a length of around 30 cm and a diameter of around 30 mm. The amount of iodine releasing resin is between 10 and 25 % of the inner volume of the unit, the iodine scavenger - chlorine eluting resin is a strong base anion exchange resin, partially or completely loaded with a non- covalently bond organic compound that will release chlorine upon passage of water through the resin. Preferably this organic compound is dichloroisocyanurate (from sodium dichloroisocyanurate, also called sodium l,3-dichloro-l,3,5-triazine-2,4- dione-6-oxide or "NaDCC") or chloramine-B hydrate or l,3-dichloro-5,5- dimethylhydanthoin or 5-nitroorotic acid potassium salt monohydrate, for example as described in patent US2005/0029493A1, and with a volume between 5 and 60 % of the inner volume of the unit. Optionally, the arsenic removing media is AD33 or AAFS50 or a mixture thereof or a titanium or manganese or zirconium or another heavy metal oxide based media or a mixture of these media at any ratio, with a volume of between 5 and 80 % of the inner volume of the unit, hi addition, the purification unit may comprise a compartment with activated carbon for iodine removal, hi addi- tion, the purification unit may comprise a compartment with activated carbon for chlorine removal. The total amount of activated carbon is between 5 and 60 %, or rather between 20 and 40% of the inner volume of the unit. The carbon may be silver loaded. The activated carbon may be split into two or more chambers and may be placed strategically inside the housing of the unit. Alternatively the activated carbon can be mixed with some other ingredients of the LifeStraw™.

As iodine releasing resin, a number of products are available on the market as well as for the iodine scavenger. Promising results have been achieved by using strong base anion exchange resins (gel type or macroporous type) like Dowex™ Marathon™ A (gel type) produced by Dow Chemical or similar material, or Dowex™ Marathon™ MSA (macroporous type) produced by Dow Chemical or similar material.

All the above mentioned different embodiments may be included in a method for purification of water, the method comprising establishing a flow of water through a number of successive compartments,

- providing a compartment with an iodine releasing resin and killing microbes in water with the released iodine, - providing a downstream compartment with an iodine scavenging resin, the iodine scavenging resin releasing chlorine during iodine scavenging,

- oxidizing trivalent arsenic to pentavalent arsenic with the chlorine released from the resin

- optionally, providing chlorine scavenging with the use of active carbon - providing another compartment with an arsenic removal resin or media; thus removing arsenic from the water by the arsenic removal resin or media.

- optionally, providing iodine scavenging with the use of active carbon, active carbon being possibly silver loaded

A unit according to the invention is preferably dimensioned so as to be a portable filter unit, especially with a mouthpiece and dimensions as the LifeStraw® product. However, other dimensioning is possible. Also, instead of a mouthpiece the unit may be provided with tubing for a controlled water flow and proper connection, such as in a household filter unit.

In a different embodiment, the unit according to the invention may be used as part in a water bag, where extraction of the water from the water bag is through a unit according to the invention. The extraction may occur by actively sucking water out of the bag, by exerting pressure on the bag, or the extraction may occur by gravity, a princi- pie known from the Katadyn Camp® and the Katadyn Siphon® products made by the Swiss company Katadyn Produkte AG. An additional disinfecting or purifying means that may be incorporated in the unit according to the invention is an ultra violet (UV) lamp, for example as it is disclosed US patent application No. 2005/258108. Such a lamp may be used in addition to the above means for disinfecting or purifying the water. For example, the UV LED (Light Emitting Diode) lamp may be used for disinfection under those circumstances, where the chemicals present in the unit are not sufficient. Thus, with relatively little chemistry inside the unit, the unit may still be able to perform satisfactory, even when the contamination suddenly overshoots expectations for contamination levels.

An on-off procedure of an UV LED requires some means for measuring the actual contamination level or means for registering the lack of total removal of contaminants. The latter may be performed with an electronic circuit, the conduction through which is governed by the contamination. In this case, there has to be taken into regard the amount of ions present in the water due to released cleaning agents. However, after a GAC section, the water is believed to should have been cleaned and a high conduction in the water would indicate an unsatisfactory cleaning.

An electronic circuit in the unit, for example at the exit side, may as well be used for indicating, whether the cleaning process is satisfactory within predetermined levels on a general basis. For example, a small electronic circuit and a battery or solar cell may be used to illuminate a lamp or to change colour of an indicator in order to show missing function, for example when the chemical products are exhausted.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates a first embodiment according to the invention, FIG. 2 illustrates a second embodiment according to the invention, FIG. 3 illustrates a LifeStraw® configuration, FIG. 4 illustrates a simplified sketch of an embodiment suitable for the LifeStraw® configuration.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT FIG. 1 illustrates a first embodiment of a unit according to the invention. The unit 1 has a water inlet 2 for inlet of a contaminated water flow 3 containing As and a water outlet 4 for 5 outflow of cleaned, arsenic- free water. The unit 1 comprises a first com- partment 6 with an iodine releasing resin for release of iodine, which is illustrated by arrow 11. The iodine is primarily used for killing microbes. Water with iodine flows into a downstream compartment 7 with an iodine removing resin, where iodine is removed as illustrated by the stopping of arrow 11 and chlorine released, which is illustrated by arrow 12. The chlorine from compartment 7 oxidizes trivalent arsenic As(+III) to pentavalent arsenic As(+V), such that the amount of trivalent arsenic As(+III) is gradually decreased, which is illustrated by the arrow 9. Pentavalent arsenic As(+V) is removed by the arsenic removal resin in compartment 8, which is illustrated by the arrow 10.

The unit in FIG. 1 may be used for water cleaning and arsenic removal, although FIG. 1 illustrates only the basic principles and may be supplemented with other means to optimize the functioning.

An improved system is illustrated in FIG. 2. For example, the unit 1 may in addition have a chlorine removing compartment 13. The resin in this compartment 13 may be activated carbon in the granular form (GAC), optionally silver loaded. This compartment would also remove residual iodine. In order for the iodine to work long enough on the microbes to achieve a proper effect, there may be provided a void space 14 between the iodine resin 6 and the iodine scavenger 7, the size of the void space 14 ad- justed relatively to the water flow and the predetermined necessary reaction time.

In addition, the water inlet 2 may be followed by a mechanical filter 15 in order to filter away larger particles or microbes. For example, the mechanical filter may be textile filter for removing particles or microbes with a size larger than or equal to 6 micrometer, preferably with a size larger than or equal to 15 micrometer, as it is used in the LifeStraw® product. Further illustrated in FIG. 2 is a mouth piece 16 as a water outlet. The built up of the LifeStraw® product is illustrated in FIG. 3, in connection with which the invention may be used in a third design, optionally with small amendments, such as an increased length due to the added arsenic removal function. The present marketed configuration of LifeStraw®, as illustrated in FIG. 1 , comprises a mouth- piece 22 with a removable end cap 21 and another end cap 33 covering an inlet clip 3 in the opposite end of the tubular body 26. A thin polypropylene or polyester filter 15 covered at the water entrance with a thick polypropylene filter 28 just before a compartment filled with iodine releasing resin 29. Another filter arrangement 28, 24 with a polypropylene fibre strainer 27 is found after the iodine compartment. Separated by a void space 20, is a GAC containing compartment 25. This GAC containing compartment of the present LifeStraw® configuration is substituted in accordance with the invention by a multiple compartment stage 25, which also may change the indicated dimensions. With reference to the overview drawing FIG. 4, illustrating a configuration suitable for the LifeStraw® configuration, this multi compartment stage 25 is con- taining a strong base ion exchange resin 25a eluting chlorine during iodine adsorption, followed by an arsenic adsorbent media compartment 25c, followed by a strong base anion exchange resin compartment 25d, followed by a GAC compartment 25e, where GAC can be silver loaded. Optionally, downstream of the strong base ion exchange resin 25a is a GAC containing compartment 25b in order to protect the arsenic adsorb- ent media 25c from chlorine in case that the resin is sensitive in connection with chlorine. The compartments 25a, 25b, 25c, 25d, 25e are separated by a thick polypropylene filter to avoid mixing of the resins.

The void region as mentioned above may, in some cases, dependent on the desired effect, be placed after the chlorine releasing resin.

Claims

1. A water purification unit having a number of compartments for the water to flow successively through these compartments, the unit comprising: - a compartment with an iodine releasing resin for killing microbes in water,

- a downstream compartment with an iodine scavenger, the iodine scavenger being configured for releasing chlorine during iodine scavenging, the amount of released chlorine being configured for oxidation of trivalent arsenic to pentavalent arsenic,

- a further downstream compartment with a arsenic removal resin configured for re- moval of arsenic from the water.

2. A water purification unit according to claim 1, wherein the iodine scavenger resin is a strong ion exchange resin.

3. A water purification unit according to claim 2, wherein the iodine scavenger resin is a strong base anion exchange resin.

4. A water purification unit according to claim 3, wherein the strong base anion exchange resin is loaded with dichloroisocyanurate.

5. A water purification unit according to claim 4, wherein the arsenic removal resin comprises activated alumina.

6. A water purification unit according to any one of the claims 3-5, wherein the strong base anion exchange resin or the arsenic removal resin comprises ferric oxide.

7. A water purification unit according to claim 6, wherein the arsenic removal resin comprises the commercial resin AD33R or AD33L or AAFS50, or a combination of these.

8. A water purification unit according to any preceding claim, wherein a void space is provided between the compartment with the iodine releasing resin and the compart- ment with the iodine scavenging resin, the void space having a volume configured for substantial extension of the reaction time between the iodine and water contaminants.

9. A water purification unit according to claim 8, wherein the void space has a volume comparable to the volume of the compartment with the iodine releasing resin.

10. A water purification unit according to any preceding claim, wherein a compartment is provided with activated carbon for iodine removal.

11. A water purification unit according to claim 10, wherein the activated carbon is silver loaded.

12. A water purification unit according to claim 10 or 11, wherein the compartment with activated carbon is downstream of the iodine scavenging resin.

13. A water purification unit according to claim 10 and 11, wherein the activated carbon is mixed with the iodine scavenger resin.

14. A water purification unit according to any one of the claims 10-13, wherein the activated carbon is located upstream of arsenic removing compartment.

15. A water purification unit according to any preceding claim, wherein the unit comprises a compartment containing a strong base anion exchange resin of a gel or macroporous type.

16. A water purification unit according to any preceding claim, wherein the unit is portable unit.

17. A water purification unit according to claim 16, wherein the unit is tubular.

18. A water purification unit according to claim 17, wherein the unit has length of less than 40 cm.

19. A water purification unit according to claim 18, wherein the unit has length ofless than 35 cm.

20. A water purification unit according to one of the preceding claims 16-19, wherein the unit has diameter of less than 50 mm.

21. A water purification unit according to claim 20, wherein the unit has diameter ofless than 40 mm.

22. A water purification unit according to any one of the preceding claims 16-20, wherein the unit has a mouth peace for drawing-up water through the unit

23. A water purification unit according to any preceding claim, wherein the amount and efficiency of the iodine releasing resin is configured to release a certain amount of iodine in the water, the amount and efficiency of the iodine scavenger resin is configured in dependence on the certain amount of chlorine to be released in the water, the certain amount of chlorine is configured for oxidation of a substantial amount of trivalent arsenic at an arsenic content in the water of the order of up to 2000 parts per billion

24. A water purification unit according to claim 23, wherein the substantial amount of trivalent arsenic is more than 50 %.

25. A water purification unit according to claim 24, wherein the substantial amount of trivalent arsenic is higher than 99%

26. A water purification unit according to claim 25, wherein the substantial amount of trivalent arsenic is higher than 99.9%.

27. A water purification unit according to claim 23, wherein the substantial amount of trivalent arsenic is all arsenic in excess of at most 10 parts per billion.

28. A water purification unit according to any preceding claim, wherein the amount of iodine releasing resin is between 5 and 30 % of the inner volume of the unit.

29. A water purification unit according to any preceding claim, wherein the amount of iodine releasing resin is between 10 and 25 %, preferably between 15 and 25%, of the inner volume of the unit.

30. A water purification unit according to any preceding claim, wherein the amount of iodine scavenger resin is between 5 and 50 %, preferably between 5 and

40%, of the inner volume of the unit.

31. A water purification unit according to any preceding claim, wherein the amount of iodine scavenger resin is between 10 and 40 %, preferably between 20 and 30%, of the inner volume of the unit.

32. A water purification unit according to any preceding claim, wherein the amount of arsenic removing resin is between 5 and 80%, preferably between 5 and 50% of the inner volume of the unit.

33. A water purification unit according to any preceding claim, wherein a compartment is provided with activated carbon for iodine removal and for chlorine removal, the total amount of activated carbon is between 5 and 50 % of the inner volume of the unit.

34. A water purification unit according to claim 33, wherein the total amount of activated carbon is between 10 and 40 %, preferably between 20 and 40%, of the inner volume of the unit.

35. A water purification unit according to any preceding claim, wherein the unit is a portable unit with a mouthpiece for drawing-up water through the unit, the length of the unit is less than 40 cm, the diameter is less than 50 mm, the amount of iodine releasing resin is between 5 and 50 % of the inner volume of the unit, the amount of iodine scavenger resin is between 5 and 50 % of the inner volume of the unit, the amount of arsenic removing resin is between 5 and 80%, preferably between 5 and 50%, of the inner volume of the unit.

36. A water purification unit according to claim 35, wherein the length of the unit is around 31cm, optionally around 25 cm, the diameter is around 30 mm, the amount of iodine releasing resin is between 10 and 30 % of the inner volume of the unit, the iodine scavenger resin is a strong base anion exchange resin with a volume between 10 and 30 % of the inner volume of the unit, the arsenic removing resin is AD33 or AAFS50 or a titanium or manganese or zirconium or another heavy metal oxide based media or a mixture thereof with a volume of between 5 and 80 %, preferably between 5 and 50%, of the inner volume of the unit.

37. A water purification unit according to claim 36 or 37, wherein a compartment is provided with activated carbon for iodine removal and the amount of activated carbon is between 5 and 50 % of the inner volume of the unit.

38. A water purification unit according to claim 38, wherein the amount of activated carbon is between 20 and 40 % of the inner volume of the unit.

39. A water purification unit according to claim 38 or 39, wherein the activated carbon is silver loaded.

40. A method for purification of water, the method comprising establishing a flow of water through a number of successive compartments,

- providing a compartment with an iodine releasing resin and killing microbes in water with the released iodine,

- providing a downstream compartment with an iodine scavenging resin, the iodine scavenging resin releasing chlorine during iodine scavenging, - oxidizing trivalent arsenic to pentavalent arsenic with the chlorine released from the resin

- providing a further downstream compartment with an arsenic removal resin and removing arsenic from the water by the arsenic removal resin.