WO2005030653A1

WO2005030653A1 - Improvement in fluoridation system

Info

Publication number: WO2005030653A1
Application number: PCT/AU2003/001301
Authority: WO
Inventors: Ian William Hall; Richard James Parker
Original assignee: Orica Australia Pty. Ltd.
Priority date: 2003-10-02
Filing date: 2003-10-02
Publication date: 2005-04-07

Abstract

An improvement in water fluoridation processes which use a dry powder fluoridation agent, the improvement comprising delivering the fluoridation agent into water within a sealed container, the container having at least a portion comprised of a composition which is water soluble or water dispersible, the composition being acceptable for inclusion in a potable water supply.

Description

IMPROVEMENT IN FLUORIDATION SYSTEM

Field of the Invention

This invention relates to an improvement in systems for fluoridation of potable water and method of handling chemicals used in the fluoridation process.

Background of the Invention

Fluoride has been added to the potable water in a number of countries for over 40 years. The fluoride acts to prevent tooth decay by making the enamel of teeth harder and more resistant to damage which can result in cavities. Whilst water may naturally contain fluorine, for the most part there is insufficient fluoride in most potable water supplies to provide the sufficient beneficial effect. For this reason a number of water suppliers will fluoridate the potable water supply. Frequently the fluoridation equipment will be included within an overall water treatment (filtration and chlorination) plant.

Typically fluorosilicic acid, sodium silicofluoride or sodium fluoride is used to fluoridate potable water. The sodium silicofluoride (Na₂SiF₆) and sodium fluoride (NaF) are provided as dry powders or fine crystals. Sodium silicofluoride is also referred to as silicofluoride or sodium fluorosilicate. The fluorosilicic acid (H₂SiF₆) is a liquid form of fluoride and is also known as hexafluorosilicic acid and hydrofluosilicic acid. Other fluoride chemicals have been used for water fluoridation. Such chemicals include ammonium silicofluoride, magnesium silicofluoride, potassium fluoride, hydrofluoric acid and calcium fluoride. Cost, handling difficulties and corrosiveness (with respect of hydrofluoric acid) has hindered the use of these other fluoride chemicals.

The three chemicals have different properties which affect their suitability for use in water fluoridation processes. Fluorosilicic acid is the most commonly used compound in water fluoridation. It is a straw-coloured, transparent, fuming, corrosive liquid with a pungent odour and is extremely irritating to the skin, eyes and respiratory system. This chemical is a class 8 dangerous good, being corrosive and toxic to touch or swallow. Water operators require specialised training before they can work with fluorosilicic acid. The operator must wear personal protective equipment such as breathing apparatus with a full face guard to protect against vapour fumes. Additionally, operators must also wear acid-proof suits, gloves and boots.

The acid is completely soluble in water at all temperatures. As it is provided as a liquid typically in a 20% to 30% solution, then it can be directly fed via a pump into the water supply. Typically these direct acid feed systems utilise a metering pump for accurately delivering a precise flow rate of the acid to the water being treated. The acid is often used for smaller water supply systems or large acid feed systems.

A diluted acid feed system may be used when the acid is provided at a concentration too high for the solution feeder to handle. A transfer pump is used to transfer acid into a dilution tank to provide a diluted solution. The metering pump then delivers the diluted solution into the water to be treated.

The plant safety requirements regarding the use of fluorosilicic acid varies from country to country. In general the chemical should be stored away from all other chemicals, away from sun light and so that it can not freeze. Good ventilation is also required, with high speed exhaust system for use when the fluoride storage room or tank room is occupied. A sump or containment bund capable of holding at least 110% of the volume of the largest size acid storage drum is normally required. The electric panels, switches and other fittings in the tank room should be protected with a grease tape or epoxy coating.

Sodium silicofluoride is a salt made from fluorosilicic acid and is also widely used in water fluoridation. It has a varying solubility over the standard operating temperatures and as such is typically used with a volumetric dry feeder to prevent dosing errors from occurring during the mixing of batches of solution. Only large water plants can accommodate a dry feed system and as such sodium silicofluoride is generally not used in small water treatment plants. Dry feed systems deliver a measured amount of the dry chemical compound into a mixing tank (solution tank) where it is thoroughly mixed before being delivered to the main flow of water. It tends to be used in medium or large sized systems. Sodium fluoride is the least used compound in fluoridation systems. The solubility of sodium fluoride is practically constant at 4 kg/100 L water over typical operating temperatures. Because of this property sodium fluoride can be used with a saturator (effectively large water tank) or dry feed systems. The principle of a saturator is that a saturated fluoride solution will result if water is allowed to pass through a bed of sodium fluoride. In general sodium fluoride powder sits at the bottom of a saturator tank and water is continuously pumped into the tank and powder and a saturated solution is pumped therefrom at a metered rate. The saturated solution is added to the water supply to provide an appropriate concentration of fluoride therein. The tank is refilled with sodium fluoride level by simply pouring more sodium fluoride into the tank. Such tanks have been in use in the USA since at least the 1960's.

Both sodium fluoride and sodium silicofluoride are less hazardous chemicals to handle than fluorosilicic acid. On the other hand, the hazards posed by their dust make good ventilation in the storage area at all times essential even if there is no visible dust. Inhaled dust can lead to acute poisoning. When these chemicals are used it is necessary to include specialised large, reasonably complex and costly dust extraction and air filtration systems.

All of the occupational health and safety issues involved with the three fluoridation chemicals can be overcome but generally at such cost that the use of fluorosilicic acid, sodium silicafluoride or sodium fluoride in water treatment facilities fox smaller communities is not economically viable.

Of the three chemicals it is believed that fluorosilicic acid is the most widely used fluoridation chemical in the United States. It has been suggested that it is used in over 90% of water fluoridation systems in the United States. It is also widely used in other countries. The Fluoridation Forum Report, Ireland 2002 suggested that fluorosilicic acid should be used over sodium fluoride. The report noted the that sodium fluoride was very hygroscopic and as water treatment plants are by nature damp places there was a tendency for the powder to become solid. Objective of the Invention

The object of the invention is to provide an economically viable method for fluoridating potable water in small water treatment facilities.

Summary of the Invention

The invention can be broadly described as an improvement in water fluoridation processes which use a dry powder fluoridation agent, the improvement comprising delivering the fluoridation agent into water within a sealed container, the container having at least a portion comprised of a composition which is water soluble or water dispersible, the composition being acceptable for inclusion in a potable water supply.

In another embodiment of the invention there is provided a sealed container of a fluoridation agent, the container having at least a portion comprised of a water soluble or water dispersible composition which, when dissolved or dispersed in water, is acceptable for inclusion in a potable water supply.

In another embodiment of the invention there is provided a process for the fluoridation of potable water which includes the step of adding the above described sealed container to a tank of water.

In another embodiment of the invention there is provided a process for the fluoridation of potable water which includes the steps of: a) providing a sealed container of a fluoridation agent, the container having at least a portion comprised of a water soluble or water dispersible composition which, when dissolved or dispersed in water, is acceptable for inclusion in a potable water supply; b) locating the sealed container within a tank of water; c) allowing the water soluble or water dispersible composition of the container to dissolve or disperse within water in the tank and at least of a portion of the fluoridation agent to dissolve within the water in the tank to thereby provide a fluoridated water solution; d) adding fluoridated water solution to potable water to fluoridate the potable water.

In another embodiment of the invention there is provided the use of above described sealed container in the fluoridation of a potable water supply.

Preferably the fluoridation agent is sodium fluoride. Preferably the container is a bag or sachet entirely formed from a water soluble or water dispersible composition, preferably a eβld water soluble polyvinylalcohol film.

Detailed Description of the Invention

The present invention is predicated on the finding that it is possible to use sodium fluoride in the fluoridation of water without the need for large, complex and costly dust extraction and air filtration systems. This can be achieved by using sodium fluoride which has been pre-packaged in sealed containers which are made from a composition which is water soluble or water dispersible and suitable for use in potable water. The sealed containers of sodium fluoride are simply placed into the water tanks. The containers dissolve or disperse in the water, thereby avoiding the creation of any clouds of hazardous dust and remove the need for the extraction systems. Furthermore, the sealed containers prevent the contents from producing dust as they are transported to the site, left in storage and

about on site. Thus it is particularly useful with fluoridation agents which are or produce a hazardous dust. It makes it possible to economically and more safely fluoridate water in smaller water treatment facilities than before. It offers health and safety improvements over that used in existing systems and avoids the pitfalls of using fluorosilicic acid and the need to use class 8 dangerous good trained personal.

The fluoridation agent may be any dry powder fluoridation agent, such as ammonium silicofluoride, magnesium silicofluoride, potassium fluoride and calcium fluoride as well as sodium silicofluoride or sodium fluoride as the fluoridation agent. It is preferred to use sodium silicofluoride or sodium fluoride as the fluoridation agent. Both of these chemicals are more commonly in use in water fluoridation processes and both are normally provided in the form of hazardous fine powder.

When sodium silicofluoride is used then it would be necessary to modify the usual process of using a constant volumetric or gravimetric feeder. Instead, it is envisaged that a batchwise method of operation would be used wherein a number of unit dose amounts of sodium silicofluoride based on a standardised weight amount within each container (for example 5 kg) would be delivered into a partially filed solution tank and additional water would be added to the tank to provide the desired final concentration.

Most preferably the fluoridation agent is sodium fluoride as this can be used in a saturator system. Most saturators in use nowadays are generally upflow saturators, wherein a bed of sodium fluoride sits at the bottom of the tank together with a spider-type water distributor. Water is forced through the distributor and upward through the sodium fluoride bed. A metering pump floats at the top of the tank and withdraws a saturated solution from the tank for feeding into the main water supply. An alternative downflow arrangement locates the sodium fluoride on a graded media underbed, such as sand and gravel, which surrounds a slotted collection manifold. The saturated solution is withdrawn from the bottom of the tank via a metering pump. The invention is expected to work with any standard design of the saturator with minimal modification.

In use a saturator is loaded with sodium fluoride powder. The maximum load will depend on the size and design of the tank. By way of example a small tank could be loaded with 400 kg of sodium fluoride. Such a load should be sufficient for saturating approximately 10000 litres of water. This is based on a consistent 4% solution strength, so that every 100 litres drawn from the tank would reduce the amount of sodium fluoride powder by 4 kg.

The tank would be periodically reloaded with sodium fluoride by simply placing a sufficient number of water soluble containers of sodium fluoride into the tank. The weight of the sodium fluoride in the container will result in the container falling into the existing bed and dissolving therein. This could be done in response to a low loading alarm or visual inspection of the tank. An electronic alarm can be triggered by known methods such as by monitoring water inflow to electronically calculate the amount of sodium fluoride .remaining in the tank or measuring conductivity of the supernatant liquid. It is expected that other alternative electronic sensing means could also be used such as an opacity sensor.

It is envisaged that the container may include a combination of water soluble or water dispersible materials and water insoluble materials. By example a combination container could comprise a sheet of water insoluble film onto which the fluoridation agent is placed on one half, the sheet being folded in half on itself and over the agent and sealed together around the agent with a water soluble or water dispersible glue or sealant. Alternatively, the container could be formed from a sheet of water insoluble film having a plurality of water soluble or water dispersible plugs therein.

When using such a container it would be necessary to take care to avoid blocking the water outlets from the tank. It would be necessary to restrain and remove the insoluble portions from the mixing or saturator tank. This could be done by attaching a tether to the insoluble portion or by placing the container in a retrievable restraint such as a cage before lowering the cage and container therein into the tank. This could be used with large containers of the fluoridation agent. For example, the container could comprise a water insoluble sack with one end sealed with a water soluble or water dispersible patch. A hoist could be used to lift the sack, move it above the tank and lower the sack into a water tank with the patched end downward. Once the patch dissolves or disperses in the water, the sack would empty and could be removed using the hoist.

More preferably the container is entirely composed of water soluble or water dispersible composition as this makes it easier to operate the equipment and avoids the need to retrieve the non-soluble portion from the tank.

The sealed water dissolvable or water dispersible containers are preferably stored and transported within a water proof storage container. This can be particularly important in moist or humid local conditions. The water-proof container would act to retain the integrity of the water soluble containers therein during transport and storage. The waterproof container may contain one or more water soluble containers therein. As fluoridation chemicals have a hazardous rating, most countries or regions have limits as to the amount of the fluoridation agent that may be transported in a single container. It is preferred that the total, amount of fluoridation in each storage container is less than those limits for ease of transportation (typically around 40 to 50 kg).

A wide range of water-proof storage containers could be used. For example it may be a sack or bag made from a suitable water-proof polymer, a metal drum or box or a cardboard or wooden box fitted with a water-proof liner. Stackable water-proof containers are preferred. A plurality of water soluble or dispersible containers are preferred be included with each water proof container. The water proof container may also include a desiccant.

As mentioned earlier, it is preferred to provide the fluoridation agent in unitary dosing amounts within the water soluble containers. Preferably, the unitary dosing amount would be in the order of 1 to 10 kg, preferably 5 kg for ease of handling. It may be helpful to provide two different dosing amounts for use with sodium silicafluoride. A standard dose of 5 kg could be used for bulk filling the tank, whilst a smaller dose of 1 kg could be used for fine concentration control.

Preferably the water soluble or water dispersible container will dissolve or disperse in cold water within two hours, more preferably one hour, more preferably 30 minutes, more preferably 20 minutes, more preferably 15 minutes, more preferably 10 minutes and most preferably within 2 minutes when totally immersed in water. It is preferred that the container quickly dissolves or disperses to a sufficient extent so as to minimise the likelihood of blocking the water collection ports. However, it is also important that the container does not open or break whilst being handled. In very humid or moist conditions it can be better to use a material that dissolves from between 10 to 30 minutes, particularly if a plurality of water soluble or dispersible containers are stored with a single water-proof container so to allow for the time taken between opening the water-proof container and placing the first soluble container in the tank and placing the last. As indicated above it is also important that the last water soluble or dispersible container stored with the storage container should not fail or break when handled, preferably after being exposed to humid or moist conditions whilst the other containers are removed from the storage container and placed in a saturator or other water containing tank. Thus the container is preferably made of a composition capable of withstanding a drop of 1.0 metres after being exposed to moist air for 15, more preferably 30 minutes.

The water soluble or dispersible composition must, in the water dissolved state, be suitable for inclusion in potable water as it will become a contaminant present in the water supply.

The acceptability of the composition will largely depend on its final concentration in the potable water. The concentration of the dissolved or dispersed composition will depend on the number of containers used, amount of the water soluble composition in each container and the amount of water. It is expected that when using sodium fluoride, the concentration of dissolved or dispersed composition will be most concentrated shortly after loading the saturator tank. This initial peak concentration of the composition will decrease over time as further water is added to the saturator tank to replace that used in fluoridating the potable water. The concentration of the composition in the potable water supply will be less than that of the water in the saturator as the water from the saturator is further mixed with water to produce the fluoridated potable water, and will decrease over time as the water is replaced in the saturator tank.

The water soluble or dispersible composition is preferably a film formed from entirely water soluble polymers or a blend of soluble and insoluble polymers, provided the overall film disperses within the water. A range of known water soluble polymers are suitable for this purpose and can be prepared by known methods such as those found in "Water Soluble Polymer — synthesis, solution properties and applications", edited by Zahid Amjad, proceedings of a symposium on water soluble polymers, Nevada 1997, and "Water Soluble Polymer - synthesis, solution properties and applications", edited by Shalaby Shalaby et al, developed from a symposium sponsored by the division of polymer chemistry, inc. at the 198^th national meeting of the American Chemical Society, Florida 1989. Suitable polymer materials include high molecular weight polyvinyl alcohol (PVA) film. Other suitable materials may include polyethylene oxide, such as polyethylene glycol; starch and modified starch; alkyl and hydroxyalkylcellulose, such as hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropyl cellulose; carboxymethylcellulose; polyvinylethers such as poly methyl vinyl ether or poly(2- methoxyethoxyethylene); poly(2,4-dimethyl-6-triazinylethylene; poly(3 -morpholinyl ethylene); poly(N-l,2,4-triazolylethylene); poly(vinylsulfonic acid); polyanhydrides; low molecular weight melamine-formaldehyde resins; low molecular weight urea formaldehyde resins; poly(2-hydroxyethyl methacrylate); polyacrylic acid and its homologs.

It is preferred to use high molecular PVA film as the water soluble composition. PVA is well understood and has been extensively tested and ingested by humans. It is commonly used as a binder in pharmaceutical compositions. High molecular PVA films generally have a molecular weight in the order of 25,000 to 300,000. Substances with such high molecular weights are not normally metabolised or absorbed by the human body and therefore do not cause toxicity. It is thought when water from a 1000 litre saturator tank is loaded with 400 kg of sodium fluoride in five kg bags is used to fluoridate potable water, then only trace levels of PVA (1-10 ppb) will be present in the potable water. The reported acute oral LD50 value for PVA with rats indicate that PVA is relatively harmless with an D50 in the order of 20,000 mg/kg or higher, being the highest dose tested — see Sanders JM & Matthews HB (1990), Vaginal Absorption of polyvinyl alcohol in Fischer 344 Rats, Human & Experimental Toxicology, 9(2), 71-77. The Joint FAO/Who Expert Committee On Food Additives (JEFCA 2003) have recently set a tolerable daily intake (TDI) for polyvinyl alcohol for use as a food additive. The TDI is an estimate of the intake of a substance which can occur over a lifetime without appreciable health risk. The TDI for PVA was set at 50 mg/kg bodyweight/day. Importantly, the polymer have.feeen found not to react with other common chemicals present in water include chlorine or fluorine and as such is unlikely to produce any other compounds which may be metabolised by the human body. PVA films can be prepared with a range of desirable properties. Films can be manufactured of sufficient toughness and tensile strength to provide containers able to cope with ordinary manhandling and possible mishaps (eg dropping a filled container). Polyvinyl alcohol films also provide gas resistance, good water solubility and can be sealed using standard sealing equipment.

The PVA film preferable has a thickness of between 20 and 100 microns. The thicker the film the stronger it is but the slower it is to dissolve in water. It is preferred to use a thick film when the container is wholly made from PVA in order to reduce the likelihood of container breakage if a container were to be dropped onto the floor when loading a water tank. When using sodium fluoride in a saturator tank then the time it takes for the film to dissolve when reloading a saturator tank is not particularly important, and as such that thicker, stronger films are preferred. Particularly preferred is the cold water soluble polyvinyl alcohol film produced by Aquafilm Limited cold water "L" series films, although it is expected that films from other suppliers would also be suitable.

PVA film can be obtained in 100 to 500 metre rolls up to 1100 mm in width. It can also be obtained in continuous tubes of up to 500 mm in width. The tubes may be preferred as it reduces the number of heat seals required to form a sealed container and may provide a stronger container.

It is thought that polyethylene oxide polymers may also be particularly suitable for use in the water soluble or dispersible containers. Polyethylene glycols (low molecular weight polymers of an average weight less than 20,000) and ethylene oxide polymers (those with an average molecular weight up to 5 million) are both soluble at room temperature. Cellulose, starch and other natural polymers could also be used or modified for use in forming a water soluble or dispersible films. The use of such films may be more acceptable to the public due to their 'natural' connotation.

It is preferred for economic reasons that the water soluble or water dispersible containers take the form of bags or sachets. Water soluble or dispersible films can be easily machine processed into bags, filled and sealed. PVA film can be obtained as rolls of continuous tube, which can be cut to size, heat sealed with a PVA hot bar sealer at one end, filled with the fluoridation agent and heat sealed at the other end. The film can also be obtained as sheet rolls, which can be formed into sealed bags or sachets around a dose of the fluoridation agent. The film also can be obtained as pre manufactured bags of desired dimension. The bags are filed with a desired amount of fluoridation agent before the mouth of the bag is sealed. The bag may also include handles or other means for facilitating the lifting and handling of the bags.

Whilst the product is sealed, basic precautions should be taken when handling the containers. It would be recommended practice to use a dust mask, gloves and protective clothing when handling the containers or when cleaning up a spillage. The container should not be handled with wet hands or gloves. In the event of a container breakage care should be taken to avoid inhaling any resultant dust.

It is envisaged that the water soluble or dispersible containers of sodium fluoride could be used with standard commercially available saturators. However, it is preferred to use the soluble containers with low maintenance, small footprint saturators and automated delivery systems such as that developed by Hydromet Pty. Limited. The use of such systems with the soluble containers of the present invention can make it economically viable to fluoridate water supplies of small communities.

Examples

The test bags were formed from PVA obtained from Aquafilm Limited. The PVA bag was formed from PVA film 40 microns thick, density 1.25g/cm³, yield 20m²/kg, and with the dimensions of 260 mm x 450 mm. Five kilograms of sodium fluoride was placed inside each bag and the mouth was heat sealed closed. The filled bags were stored within water proof and air sealed plastic drums. Test l.

Drop tests were conducted to simulate a handling accident at a water treatment plant. Five test bags were individually placed within snap lock bags and dropped from a height of 1.0 metre onto a concrete slab. The test bags were then inspected. None of the test bags were found to have ruptured.

Test 2.

Delayed drop tests were conducted to simulate a handling accident near the end of the process of loading a saturator tank. A water proof and air sealed drum containing 10 of the test bags was opened and left in humid conditions of 83% for a period of 30 minutes. Each test bag was then visually inspected for signs of dissolution or damage. There was no sign of dissolution or damage in the bags. Five bags were drop tested in accordance with the method used in Test 1. None of the bags ruptured.

Test 3.

Ten water proof drums, each containing ten 5 kg test bags were shipped from Perth Australia to a small fluoridation test site in Christmas Island. The bags were inspected and none of the bags were found to be damaged during transportation. Forty test bags (200kg) were placed in a saturator tank. Each bag dissolved within 2 minutes. No obstruction of the collection nozzles was observed. The water was determined to contain no more than 10 ppb of the polymer.

Test 4.

A water proof and air sealed drum containing 10 test bags was stored at Christmas Island in a small water fluoridation site for 3 months. The bags were then visually inspected and considered sound. The bags were loaded into a saturator and dissolved in the cold water within 2 minutes. The bag composition did not block the water extractors or metering pump.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form or suggestion that that prior art forms part of the common general knowledge in Australia.

It would be appreciated by a person skilled in the art numerous variations and/or modifications may be made to the invention as shown the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. An improvement in water fluoridation processes which use a dry powder fluoridation agent, the improvement comprising delivering the fluoridation agent into water within a sealed container, the container having at least a portion comprised of a composition which is water soluble or water dispersible, the composition being acceptable for inclusion in a potable water supply.

2. A sealed container of a fluoridation agent, the container having at least a portion comprised of a water soluble or water dispersible composition which, when dissolved or dispersed in water, is acceptable for inclusion in a potable water supply.

3. A process for the fluoridation of potable water which includes the step of adding the sealed container of claim 2 to a tank of water.

4. Use of the sealed container of claim 2 in the fluoridation of a potable water supply.

5. A process for the fluoridation of potable water which includes the steps of: a) providing a sealed container of a fluoridation agent, the container having at least a portion comprised of a water soluble or water dispersible composition which, when dissolved or dispersed in water, is acceptable for inclusion in a potable water supply; b) locating the sealed container within a tank of water; c) allowing the water soluble or water dispersible composition of the container to dissolve or disperse within water in the tank and at least of a portion of the fluoridation agent to dissolve within the water in the tank to thereby provide a fluoridated water solution; d) adding fluoridated water solution to potable water to fluoridate the potable water.

6. The improvement, container, process or use according to any one of claims 1 to 5 wherein the fluoridation agent is sodium fluoride or sodium silicofluoride.

7. The improvement, container process or use according to any one of claim 1 to 5 wherein the fluoridation agent is sodium fluoride.

8. The improvement, container, process or use according to claims 1 to 7 wherein the container is wholly comprises the water soluble or water dispersible composition.

9. The improvement, container, process or use according to any one of claims 1 to 8 wherein the water soluble or water dispersible composition is a high molecular weight polyvinylalcohol polymer film.

10. The improvement, container, process or use according to claims 1 to 9 wherein the container completely dissolves or disperses in cold water within 30 minutes.