WO2007107434A1

WO2007107434A1 - Water purification tablet and process

Info

Publication number: WO2007107434A1
Application number: PCT/EP2007/051953
Authority: WO
Inventors: Samiran Mahapatra; Suresh Murigeppa Nadakatti; Mahesh Subhash Tendulkar
Original assignee: Unilever N.V.; Unilever Plc; Hindustan Unilever Limited
Priority date: 2006-03-21
Filing date: 2007-03-01
Publication date: 2007-09-27

Abstract

The invention relates to water purification tablet compositions for purification of contaminated water to make it suitable for potable purposes. The invention especially relates to highly stable water purification tablet compositions that also disintegrate easily when dispersed in water. It is thus an object of the present invention to provide for a water purification tablet composition which is more stable as compared to prior art tablet compositions while at the same time having a fast dissolution rate. Accordingly, the present invention there is provided a water purification tablet composition comprising a solid chlorine containing disinfectant and a swellable clay.

Description

WATER PURIFICATION TABLET AND PROCESS

TECHNICAL FIELD

The invention relates to water purification tablet compositions for purification of contaminated water to make it suitable for potable purposes. The invention especially relates to highly stable water purification tablet compositions that also disintegrate easily when dispersed in water.

BACKGROUND AND PRIOR ART

A large population of people in the world lives in developing and underdeveloped countries where there is a severe shortage of hygienic potable water. A high percentage of these people live in rural areas where there are no water purification systems like the ones provided by urban municipal drinking water treatment plants. Many people have to depend directly on ground water sources like wells, ponds and rivers. Often these water sources are contaminated by sewage, industrial and agricultural wastes. The modern methods of water purification for producing potable water have either not been available or have proved to be too expensive, not sufficiently effective or inconvenient for use under these conditions. Therefore the consumption of contaminated water, leading to high mortality and morbidity in these areas, is still a large health concern. Hence there is a constant desire to develop newer water purification methods and compositions which are convenient and inexpensive to use and are consistently more effective in producing healthy potable water. Several types of water purification systems are known and available. They can be broadly classified into thermal, radiation, physical and chemical methods of purification or a combination of one or more of these methods.

Boiling of water is one of the oldest and most convenient ways to kill pathogenic microorganisms in contaminated drinking water. However boiling of water is cost intensive and generally gives the water a flat taste.

Radiation methods include treating water with Ultra-violet (UV) rays which are also effective in killing microorganisms without altering the taste of water, but a majority of the rural areas suffer from lack of supply of consistent electrical power to run the UV purifiers and so these devices have not been popular in these areas.

The physical methods include a variety of filtration methods and media. However filtration methods and media have generally not been able to effectively separate the disease causing microorganisms from water and therefore have not been very successful in providing the rural population with good quality water who seek purified water, free of such pathogenic microorganisms.

Chemical methods of purifying water have been most successful in providing purified water at all levels viz. at the municipal, local and domestic levels. The chemicals most often used are oxidising agents like halogenated chemicals e.g. chlorinated or iodinated chemicals for killing microorganisms.

EP0066421 (1982, Buchan) describes a composition for purification of water that includes a coagulant for coagulating solid impurities dispersed in the water to form floes; and an organic hydrophilic colloid capable, when dispersed in the water, of absorbing large quantities of water to form a sol for aggregating the floes, the proportion of organic hydrophilic colloid in the composition being such that when the composition is used to purify the intended quantity of water, the organic hydrophilic colloid does not interfere with coagulant dispersal in the water or with floe formation. While this publication has examples where clay e.g. bentonite is used as a swelling and disintegration agent for tablets from 0.5 gm to 10 gram size, and also describes the need for use of moisture sink, it does not specifically teach how to solve the problem of water purification tablets to get them to have high stability and fast disintegration.

WO96/32194 (1996, Truetech) describes a water treatment composition in unit dose form capable of batch wise purification of water for drinking purposes comprising basically a disinfecting agent, a coagulant-precipitate agent, a primary and a secondary flocculent, a dispersion-buffer agent, an agglomeration matrix and prefilter and a bulk ion-exchange absorbent. The preferred unit dose form described therein is a tablet.

WO02/00557 (2002, Procter and Gamble) describes a water purification composition, a method and kit for purifying and clarifying drinking water. The composition described therein comprises essentially a primary coagulant, a bridging flocculent and a coagulant aid. Although this publication also describes that clays of the smectite class e.g. that of montmorillonite type as specifically useful for good disintegration agents of such tablets and that it is essential to keep the moisture low e.g. less than 2.5% for good efficacy, it does not teach the specific selective range of the composition required for getting high stability of the disinfecting agent while ensuring good disintegration.

WO2004076364 (2004, Pur Water Purification) essentially is directed to inclusion of citrus flavour in a flocculation composition to mask residual chlorine taste. While this publication describes compositions comprising chlorine based disinfectants and clays of the montmorillonite class, the invention is generally demonstrated using powder mixes and does not teach how to solve the problem of maintaining stability, and ensuring easy disintegration of a tablet.

US 5,320,773 discloses compositions for purifying water, said composition formed by pretreating a dry solid powder with a liquid biocide.

The compositions of the prior art cited above describe water treatment compositions in many forms including tablets. The present inventors have found that many such formulations are effective in killing the microorganisms when the compositions are used shortly after they are prepared. These compositions are observed to be not very stable when stored over an extended period of time especially when stored in hot and humid climates that are usually present in the countries in the tropical areas where the problem of pure drinking water is more acute. Further, many of these tablet compositions do not disintegrate very easily when added to water to be purified. In these prior art tablet compositions containing the disinfectant e.g. a chlorinated compound, it has been seen that they have very high available chlorine at the time of preparation but after a period of time of storage e.g. a few weeks or months, the available chlorine is much lower than desired.

Thus there exists a need for developing a water purification tablet that is more stable when stored over extended period of time, while ensuring that it disintegrates faster when dropped into water as compared to the tablets of the prior art. The present inventors during the course of their extensive research have found that preparing a tablet which meets both of these opposing demands is very difficult. When one tries to prepare highly stable tablets, one finds that they disintegrate poorly. When one tries to prepare tablets which disintegrate fast, they have very poor stability. The present inventors have solved this problem by preparing a tablet composition comprising a halogen containing disinfectant whose by-products are safe for human consumption and a swellable clay in a very specific selective ratio while requiring the moisture content in the tablet composition to be below a specific value. While many of the above cited prior art use chlorine based disinfectants like calcium hypochlorite and sodium dichloro cyanuric acid and clays of the smectite class as disintegrating agents, none of the above cited prior art teach or demonstrate such a selective combination in a tablet composition to achieve the opposing demands of high stability and fast disintegration time.

It is thus an object of the present invention to provide for a water purification tablet composition which is more stable as compared to prior art tablet compositions while at the same time having a fast dissolution rate.

It is another object of the invention to provide for a water purification tablet composition which is more stable and disintegrates faster than prior art tablets while ensuring that the by-products of the disinfecting material when dispersed in water are safe for human consumption.

It is yet another object of the present invention to provide for a water purification tablet composition that is able to deliver at least 6 ppm of active halogen in water after the tablet has been stored in hot and humid condition for at least two months with minimal wastage of active halogen during storage period, at the same time, ensuring that the tablet disintegrates in less than two minutes when dropped in a batch of water. It is yet another object of the present invention to provide for a water purification tablet composition which in addition to being more stable and disintegrating faster than prior art tablets, is also easy to tablet and which maintains integrity during the period of storage and transport of the tablet composition.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a water purification tablet composition comprising: (a) 15 to 85% of a solid halogen containing disinfectant;

(b) 15 to 85% of a swellable clay and

(c ) less than 3% moisture.

All percentages herein are by weight, unless specified otherwise.

It is particularly preferred that the halogen containing disinfectant is calcium hypochlorite [Ca(OCI)₂] or bleaching powder [Ca(OCI)CI]. Particularly preferred swellable clay is sodium bentonite.

According to another aspect of the invention there is provided a process to prepare a water purification tablet composition comprising the steps of

(a) mixing 15 to 85% powder of a halogen containing disinfectant with 15 to 85% of a powder of a swellable clay such that the powder mixture has a moisture content of less than 3% and (b) tableting the powder mixture into tablets of desired size.

DETAILED DESCRIPTION OF THE INVENTION The invention provides for a water purification tablet composition which is very stable even when stored under very harsh conditions of hot temperature and high humidity. The tablet comprises 15 to 85% of a solid halogen containing disinfectant; 85 to 15% of a swellable clay and has less than 3% moisture.

The halogen containing disinfectants are preferably solid compounds whose by-products are safe for human consumption. They are preferably an inorganic compound. Suitable inorganic disinfectants include sodium hypohalite, calcium hypohalite, bleaching powder, or hypoiodous acid. The halogen may be iodine, bromine or chlorine preferably chlorine. Particularly preferred disinfectant according to the invention are calcium hypochlorite having the formula [Ca(OCI)₂] and bleaching powder having the formula [Ca(OCI)CI]. Optimally the halogen containing disinfectant according to the invention is calcium hypochlorite.

The halogen containing disinfectant is present in the water purification tablet of the invention in an amount in the range of 15 -85 %, more preferably in the range of 30-75% by weight of the tablet.

A swellable clay is an essential component of the water purification tablet composition of the invention. The swellable clay is preferably chosen from the smectite class of clays

Smectites constitute a group in the class of natural aluminosilicate minerals known as phyllosilicates or layer silicates. Other groups in this class include micas, kaolins, vermiculites, chlorites, talc and pyrophyllite. The phyllosilicate structure consists of layers in which planes of oxygen atoms coordinate to cations such as Si, Al, Mg and Fe to form two-dimensional sheets. The coordination of cations in adjacent sheets typically alternates between tetrahedral and octahedral. Smectites are characterized by a 2:1 layer structure in which two tetrahedral sheets form on either side of an octahedral sheet through sharing of apical oxygens. As the apical oxygens from the tetrahedral sheet form ditrigonal or hexagonal rings, one oxygen from the octahedral sheet is located on the centre of each ring and is protonated to yield a structural hydroxyl. In 2:1 phyllosilicates, isomorphous substitution of cations having different valencies can lead to charge imbalances within a sheet. These may be partly balanced by the opposite type of charge imbalance in the adjacent sheet (e.g., a positively charged octahedral sheet may offset some of the negative charge associated with a tetrahedral sheet). The net charge imbalance on a 2:1 layer, if it occurs, is negative. This charge is referred to as the layer charge of the mineral and is balanced by larger cations (eg., Na⁺, K⁺, Ca⁺² and Mg⁺²) that coordinate to the basal surfaces of the tetrahedral sheets from the adjacent layers. Since, these charge-balancing cations are located between adjacent 2:1 layers they are referred to as "interlayer cations". The 2:1 phyllosilicates are distinguished chiefly on the basis of their layer charge.

The compounds/clay materials belonging to the class of smectites is quite large, and, especially, the Cs exchanged classes of smectites include hectorite, saponite, and montmorillonite, in addition to vermiculite, and, several non exchangeable phyllosilicates.

The preferred clays of the invention are bentonites. Even more preferred clays as per this invention are sodium and calcium montmorillonite, which are part of the smectite group of natural aluminosilicate minerals and are the most common members. Montmorillonite is the major phase in a type of clays called bentonites. Amongst the calcium and sodium bentonites, sodium bentonite is more preferred. The swellable clay is present in the water purification tablet composition of the invention in an amount in the range of 15 to 85%, more preferably in the range of 30 to 70% by weight of the tablet composition.

It is important that the water purification tablet composition of the invention has less than 3% moisture, more preferably less than 2% moisture. This is achieved by using powder of the halogen containing disinfectant and the swellable clay for tableting the tablet composition of the invention which are low in moisture content. The swellable clays are generally available with moisture content of 5 to 15% (wt). The clay is dried to a moisture level <5%, more preferably <2% before they are used in the tablet composition of the invention.

The water purification tablet composition of the invention is preferably prepared as small tablets i.e. those which weigh 0.1 to 0.6 grams, preferably 0.2 to 0.4 grams. These tablets are usually added to a batch of water in the range of 5 to 20 litres of water, more preferably 8 to 15 litres of water. It is very important that the tablet composition of the invention loses very little active disinfectant during storage under harsh conditions.

The term 'available chlorine' is widely used in water purification industry. This term was originated for comparing the potential bleaching or oxidising or disinfecting power of chlorine compounds. This is accomplished by a quantitative analysis of the chlorine that is available for oxidising, by a method known as iodometric method. The term 'available chlorine' is the calculated weight of chlorine molecule (CI2) that is required to liberate the same amount of iodine molecule (I2). As only half of the chlorine molecule is of positive valence (+1 oxidation state), when in solution, the available chlorine content of any chlorine compound that has Cl+ (e.g., HOCI) will always be twice the amount of this cation present.

HOCI + 2Kl + HAc = I₂ + KCI + KAc + H₂O NaOCI + 2Kl + 2HAc = I₂ + NaCI + 2KAc + H₂O Ca(OCI)₂ + 4Kl + 4HAc = 2I₂ + CaCI₂ + 4KAc + 2H₂O Ca(OCI)CI + 2Kl +2HAc = I₂ + CaCI₂ + 2KAc + H₂O wherein Ac refers to acetate.

In summary, the term 'available chlorine' refers to the oxidising power of the compound tested. It is noted that the available chlorine number relates to the pure components, whereas commercially available products are normally only 60-65% pure.

More information can be found in 'Handbook of chlorine and alternative disinfectants', fourth edition, Geo. Clifford White, (John Wiley & sons Inc.)

For example, when pure calcium hypochlorite (available chlorine level = 99.2%) is the disinfectant used, a 0.4 g tablet with 50% calcium hypochlorite, gives an available active chlorine content of about 19.84 ppm when added to 10 litres of water, when the tablet is freshly prepared. This means that the available chlorine for commercially available calcium hypochlorite is about 12 ppm (i.e. 60-65% of 19.84 ppm).

A highly preferred aspect of the present invention provides for a tablet that is sufficiently stable in providing halogen of at least 80% of the initial amount of available halogen after tablet has been stored in hot and humid conditions (40 ⁰C and 85% humidity) for two months.

The tablets according to the invention are preferably formulated and dosed in such a way that they provide at least 6 ppm of available halogen, preferably chlorine, to the water that is being purified when the tablet is dissolved in said water.

The water purification tablet composition of the invention preferably disintegrates in less than 2 minutes, more preferably in less than one minute in water. This can be achieved by merely dropping the tablet in water or by dropping the tablet in water followed by mild stirring by hand.

The water purification tablet composition of the invention preferably comprises a moisture sink compound. Suitable moisture sink compounds are calcium oxide, silica gel, fused calcium chloride, phosphorous pentaoxide, most preferably calcium oxide. The moisture sink compound is preferably present in an amount in the range of 2 to 10%, more preferably 4 to 6% by weight of the tablet composition.

A highly preferred water purification tablet composition comprises a lubricant. The lubricant is a compound that facilitates free flow of material during processing, and minimizes the frictional forces that the tablet undergoes during the tableting process. Suitable lubricant for the purposes of this invention include talc, magnesium stearate, ascorbyl palmitate, fumed silica, most suitably talc and magnesium stearate. Lubricants are preferably present in an amount in the range of 0.2 to 2% by weight of the tablet composition.

(a) mixing 15 to 85% powder of a halogen containing disinfectant whose byproducts are safe for human consumption with 85 to 15% of a powder of a swellable clay such that the powder mixture has a moisture content of less than 3%; and (b) tableting the powder mixture into tablets of desired size.

Mixing of the powder of the halogen containing disinfectant with the powder of the swellable clay may be carried out in any known powder mixer preferably a mixer which exerts low shear on the powder e.g. a ribbon mixer. The mixing is preferably carried out in closed atmosphere preferably under low relative humidity conditions (-20% RH). The powder composition is converted into a tablet by using either a manual hydraulic press or a tableting machine. The tablet generally having a hardness of 2 to 4 kg/cm² as measured using a hardness tester, are preferred.

According to yet another aspect of the invention, there is provided a kit comprising (a) the water purification tablet composition of the invention; and (b)a flocculation composition.

The flocculation composition of the kit of the invention is preferably a tablet. The flocculation composition preferably comprises a coagulating agent, a flocculating agent and a buffering agent. A coagulating agent is one or more compounds selected from the group consisting of water soluble inorganic metal salt having polyvalent cation preferably a trivalent cation. Suitable examples of coagulating agents include inorganic metal salts like ferric sulfate, aluminium sulfate, copper sulfate, or polyaluminium chloride. Aluminium sulfate and polyaluminium chloride are particularly preferred as coagulating agents as per the flocculation composition of the kit of the present invention.

A flocculating agent is one or more compounds selected from the group consisting of high molecular weight water soluble polymer and mixtures thereof. Examples of flocculating agents are polysaccharides (dextran celluloses), proteins, modified celluloses (hydroxyethyl/hydroxypropyl or carboxymethyl), polyacrylamide. A highly preferred flocculating agent is a high molecular weight polyacrylamide. It is especially preferred that the polyacrylamide is non- ionically modified.

Buffering agent is one or more compounds selected from a group of solid alkaline compounds which cause an effervescent action in presence of acid. It is desirable that when the coagulating agent and the buffering agent present in the flocculation composition of the kit are added to water, a pH in the range of 6 to 7 is obtained. The dispersion-buffering agent is preferably non-polymeric. Especially preferred buffering agents are carbonates, bicarbonates and oxides of alkali metal atoms.

The invention will now be illustrated with reference to the following non-limiting examples by way of the invention.

Examples

Comparative Examples A to F: Tablet as per compositions shown in Table-1 as Comparative Examples A to F were prepared. The powders of calcium hypochlorite and bentonite clay were mixed manually in a beaker under low relative humidity conditions. The powder compositions were converted into a tablet by using a manual hydraulic press to make 10 mm dia. tablets. The tablets had a hardness of ~2 to 4 kg/cm² as measured using a hardness tester. These tablets were packed in a moisture impermeable polymer and stored under Hot & Humid conditions i.e. high temperature of 4O⁰C and a high humidity of 85%. The tablets after storage for 2 months were checked for available chlorine. Available chlorine was analyzed by iodometric method in accordance with Standard Methods to a detectible level of 1 ppm (APHA, AWWA & WEF 2001. Standard Methods for the Examination of Water and Wastewater. 20th ed., Part 2130. Washington, DC: American Public Health Association/American Water Works Association and Water Environment Federation).

The stability was deemed 'passed' if available chlorine at the end of the storage period was greater than 6 ppm or 'failed' if it was less than 6 ppm.

The disintegration time (DT) of the tablets was checked by dropping the tablet into a bucket containing 10 ltr water. The water was stirred manually and time required for disintegration of the tablet was noted. The DT was deemed 'passed' if the DT was less than 2 minutes and deemed 'failed' if the DT was more than 2 minutes.

Table-1a

The results of the tests are summarised in Table-1 b.

Table-1b

Calcium hypochlorite source: a : Calcium hypochlorite with available chlorine -65%. b: Bleaching powder with available chlorine -35%. Examples 1-3

Water purification tablet compositions as per the invention were prepared using similar processes as used to prepare Comparative Examples A to E and the data is summarized in Table-2. The tablets were tested for available chlorine and DT and the results are summarized in Table-2.

Table-2

The data in Table-1 indicates that tablets prepared outside the scope of the invention (Comparative Examples A to F) do not meet the requirement of both stability and dissolution time. The data in Table-2 which are tablets prepared as per the invention meet both the requirement of stability and dissolution time.

The invention thus provides for water purification tablet compositions which are more stable as compared to prior art tablet compositions while at the same time having a fast dissolution rate.

Claims

1. A water purification tablet composition comprising:

(a) 15 to 85% of a solid chlorine containing disinfectant;

(b) 15 to 85% of a swellable clay and

(c) less than 3% moisture.

2. A water purification tablet composition as claimed in claim 1 wherein said disinfectant is an inorganic compound.

3. A water purification tablet composition as claimed in claim 1 or claim 2 wherein said halogen is chlorine.

4. A water purification tablet composition as claimed in any one of the preceding claims wherein said disinfectant is calcium hyprochlorite having the formula [Ca(OCI)₂] or bleaching powder having the formula [Ca(OCI)CI].

5. A water purification tablet composition as claimed in any one of the preceding claims wherein said swellable clay belongs to the smectite class.

6. A water purification tablet composition as claimed in claim 5 wherein said swellable clay is sodium bentonite.

7. A water purification tablet composition as claimed in any one of the preceding claims wherein said disinfectant is present in an amount of 30 to 75% by weight of the composition.

8. A water purification tablet composition as claimed in any one of the preceding claims wherein said swellable clay is present in an amount in the range of 30 to 70% by weight of the composition.

9. A water purification tablet composition as claimed in any one of the preceding claims wherein said tablet has less than 2% moisture..

10. A water purification tablet composition as claimed in any one of the preceding claims wherein weight of the tablet is in the range of 0.2 to 0.4 grams.

11. A water purification tablet composition as claimed in any one of the preceding claims wherein the tablet is sufficiently stable in providing halogen of at least 80% of the initial amount of available halogen after tablet has been stored in hot and humid conditions (40 ⁰C and 85% humidity) for two months.

12. A water purification tablet composition as claimed in any one of the preceding claims wherein the tablet disintegrates in less than two minutes.

13. A water purification tablet composition as claimed in any one of the preceding claims comprising a moisture sink compound.

14. A water purification tablet composition as claimed in claim 13 wherein said moisture sink compound is present in an amount in the range of 2 to 10% by weight of the tablet composition.

15. A water purification tablet composition as claimed in claim 13 or 14 wherein said moisture sink compound is calcium oxide.

16. A water purification tablet composition as claimed in any one of the preceding claims comprising a lubricant.

17. A water purification tablet composition as claimed in claim 16 wherein said lubricant is present in an amount in the range of 0.2 to 2% by weight of the tablet composition.

18. A water purification tablet composition as claimed in claim 16 or 17 wherein said lubricant is chosen from magnesium stearate and talc.

19. A kit comprising:

(a) a water purification tablet composition as claimed in any one of the preceding claims ; and

(b) a flocculation composition.

20. A kit as claimed in claim 19 wherein the flocculation composition comprises a flocculating agent, a coagulating agent and a buffering agent.

21. A process to prepare a water purification tablet composition comprising the steps of

(a) mixing 15 to 85% powder of a halogen containing disinfectant with 15 to 85% of a powder of a swellable clay such that the powder mixture has a moisture content of less than 3%;

(b) tableting the powder mixture into tablets of desired size.

22. A method for cleaning water comprising the steps of:

(a) adding a tablet according to any one of claims 1 to 18 to water; and

(b) leaving the tablet to desintegrate, with the proviso that said tablet provides at least 6 ppm of available halogen to the water that is being purified when the tablet is dissolved in said water.