No Drain Acid Treatment of Pools

1. Field of Invention

This invention relates to the treatment of pools and, in particular, to a treatment for removal of scale deposits and heavy metal salts from any vessel.

2. Brief Statement of the Prior Art

Scale is commonly formed on pools, spas, cooling towers and other confined containers of bodies of water in which the total hardness of the water increases with evaporation, resulting in the precipitation of hardness from the water as scale deposits, typically calcium carbonate.

The problem is most noticeable and objectionable in swimming pools and spas because of the unsightly appearance of the scale deposits. The problem, however, is nonetheless present in other applications such as in cooling towers where the scale deposits can clog equipment and interfere with efficient operation.

A common technique for the removal of the deposits has been to drain the water completely from the pool and treat the pool surfaces with various agents. Acids are used for this application, however, the pool industry generally recommends against the use of acid treatments. The danger is that the acid will damage equipment or severely pit or erode plaster surfaces of pools. When acids are used, caution must be exercised to avoid damaging the pool surfaces and to avoid injury to the workmen. The potential for injuries increases when treating indoor pools because of the toxic nature of the acid fumes generated during the treatment. Accordingly, the acid washing of pools to remove scale deposits is practiced only by skilled professionals and is generally not recommended for swimming pool treatment, and particularly not recommended to be practiced by the pool owner.

It is an objective of this invention to provide a method for removal of scale from swimming pools without the necessity for draining the pool.

It is also an object of this invention to provide a method for removal of scale deposits from swimming pools which can be practiced by the pool owner.

It is a further object of this invention to provide a method for removal of scale from swimming pools which is safe and does not risk damaging the pool and equipment.

It a further object of this invention to provide a method for the acid treatment of swimming pools in which the treatment acid is neutralized after the treatment without increasing the total alkalinity of the pool.

It is a further object of this invention to provide a method for the removal of scale deposits from swimming pools in a manner in which the pool does not immediately rescale with fresh deposits.

This invention comprises a method for the treatment of swimming pools and other confined bodies of water to remove scale deposits from the surfaces of the vessel containing the water, usually from the sides of swimming pools and spas. In the method, the water is acidified to eliminate its total alkalinity, as determined by a standard neutralization test. While various mineral acids such as hydrochloric acid, hypochlorous acid, nitric acid, etc., can be added for this purpose, it is preferred to employ hydrochloric acid and a mixture of sulfamic acid and sodium bisulfate.

The method includes continuous or repeated monitoring of the total alkalinity of the water following the initial addition of acid, to insure that sufficient acid has been added. Additionally, it has been noted that the acidified water has a tendency to gain total alkalinity as the scale deposits are removed during the treatment.

After the treatment with the acid, which can last from several hours to several days, the excess of the treatment acid is neutralized by the addition of a neutralizing agent which is preferably magnesium oxide or magnesium hydroxide. The neutralizing agent is added to neutralize the acid and is added in a quantity which does not increase the pH of the pool to a value above about 7.8 to 8.0. It has been observed that the addition of magnesium oxide or hydroxide to the pool without exceeding the aforementioned pH effectively neutralizes the excess treatment acids without contributing to total alkalinity and thereby avoids the tendency of the pool to rescale immediately following treatment.

Scale deposits are commonly formed on the walls of containers such as swimming pools and the like, which contain water having dissolved salts, usually carbonates, commonly referred to as "hardness", and which are subject to evaporation that concentrates the dissolved solids until they precipitate on the surfaces of the container. The hardness in the water is usually carbonates and bicarbonates which commonly precipitate as the salt of the least soluble metal, usually calcium. The precipitates form a grey to dark scale on the surfaces of the container and equipment, usually resulting in discoloring and unsightly appearance on the surfaces of swimming pools and spas, and interfering with efficient operation of cooling towers and other equipment. This invention comprises the treatment of water to dissolve the scale deposits. The treatment is practiced by acidification of the pool to a predetermined value, monitoring of the total alkalinity of the pool and re-acidification as needed, maintaining the acidified water in contact with the scaled surfaces for sufficient time to dissolve the scale and finally, neutralization of the acid to restore the pool water to an acceptable and usable pH value.

The acidification of the water is accomplished by the addition to the water of a suitably strong acid to dissolve the deposits. Examples of suitable acids include mineral acids, e.g., hydrochloric acid, hypochlorous acid, sulfuric acid, nitric acid, etc., organic acids such as citric acid, acetic acid, etc. Of the aforementioned, sulfamic, sulfuric and hydrochloric acids are preferred. Most preferably, hydrochloric acid is used admixed with sulfamic acid in amounts from 4 to 12 gallons concentrated hydrochloric acid to 10 pounds of sulfamic acid. If desired, sulfuric acid can also be included, preferably by adding sodium bisulfate to form the sulfuric acid in situ.

The hydrochloric acid can be added as a concentrated aqueous solution, typically with a strength from about 25 to about 50 weight percent, preferablyabout 31.5 weight percent, and the sulfamic acid and sodium bisulfate can be added as a dry powder containing the proper proportions of each to provide the aforementioned quantities of sulfuric and sulfamic acids. The acids are used in a sufficient quantity to completely eliminate the total alkalinity of the pool.

The total alkalinity of water is measured as its acid neutralizing capacity by titrating a water sample with acid to a methyl orange end point (pH of 4.6). Total alkalinity includes caustic, carbonate and bicarbonate alkalinity.

The invention is practiced generally by the addition to the water of from 1 to about 5 pounds of sulfamic acid and 2 gallons of 31.5% hydrochloric acid per each five thousand gallons of water, and the total alkalinity is then measured. If a detectable total alkalinity is observed, additional quantities of acid can be added in proportions of about one pound or more of acid per 5,000 gallons of water and the total alkalinity is again measured.

Preferably, the acidification treatment is combined with treatment by an metal sequestering and/or chelating additive. Examples of suitable sequestering and/or chelating additives include oxalic acid, polyphosphates, e.g., pyrophosphates, tripolyphosphates, metaphosphates, etc., soluble ammonium salts which form ammine complexes with certain metals, and organic chelating agents such as ethylene diamine tetraacetic acid, etc. Mixtures of two or more of these additives can be used. A suitable sequestering chelating composition comprises a mixture of from 5 to 80 weight parts oxalic acid, 5 to 80 weight parts ammonium sulfate, and from 5 to 80 weight parts sodium hexametaphosphate. The sequestering additive is used in an effective amount, usually from about 1 to about 6 pounds per 5000 gallons of water. The actual amount required will depend on the amount of scale to be removed, and can be varied as required over a wide concentration range.

After the water has been acidified to no detectable alkalinity, the water is maintained at this condition for an extended period of time. The length or duration of this treatment will vary, depending upon the severity of the scale deposits. Generally, the period will be no less than several hours and may continue for at least several days. The scale removal can be facilitated by periodic scrubbing of the scale deposits, accelerating their breakdown and dissolution in the acidified water.

As the scale is dissolved, the total alkalinity of the water must be repeatedly monitored or determined to ensure that the total alkalinity is not restored to an objectionable level by the treatment. This is practiced by repeated determinations of the alkalinity at increments of several hours to one day after commencement of the treatment.

After the treatment has effectively removed the scale deposits, the pool is restored for normal use. This is accomplished by neutralization of the treatment acid. Any alkaline additive can be used to neutralize the treatment acids, including sodium or potassium hydroxides, and carbonates or bicarbonates of sodium, potassium, calcium and magnesium. The most preferred neutralizing additive is magnesium oxide or magnesium hydroxide. Preferably magnesium oxide is employed. It has been observed that the neutralization of treatment acids with other bases such as sodium carbonate and the like will result in a tendency for the water in the pool to rescale the pool surfaces. This occurs because most neutralizing agents, such as sodium carbonate, restore the total alkalinity, often elevating it to a value such as 100 to 150 ppm total alkalinity or higher. When this occurs, the pool again becomes unstable and deposits are formed quite rapidly.

The use of magnesium oxide or hydroxide for the neutralization of the treatment acid avoids the tendency of the pool to rescale. This tendency is entirely avoided provided the pool is neutralized to a pH value no greater than about 8 and preferably no greater than about 7.8. At these pH values it has been observed that the pool will be quite stable and will not redeposit scale when returned to normal service and use.

The quantity of magnesium oxide required for the neutralization will depend, of course, on the quantity of treatment acid employed during the acidification treatment. Generally the amount of neutralizing agent, expressed as magnesium oxide, will be from about 1 to about 8 pounds per 5,000 gallons of water. Preferably, the magnesium oxide is dissolved in a solution or suspension prior to addition to the main body of the pool water. For this purpose, the magnesium oxide can be dissolved at a concentration of about 1 to 5 pounds for 5 gallons of water and the resulting solution and suspension of magnesium oxide can then be dispersed into the main body of the pool water.

The treatment provides the advantages of a facile removal of scale deposits which can be practiced by the pool owner without requirement of skilled and professional assistance. A major advantage of the treatment is that it can be practiced without the necessity to drain the pool thereby avoiding any potential structural damage by collapse of the pool walls as well as any potential danger of personal injury resulting from falls into an empty pool. Because the pool is not drained there is also a very significant reduction in the costs of treatment as well as a substantial conservation of water. Additionally, the descaling of the walls is accomplished with acid at very dilute concentrations thereby avoiding the potential injury or toxic effects from handling of concentrated acid and breathing of fumes from the treatment with concentrated acids. The neutralization step of the method is quite effective and rapid and provides the treatment with a very precise control over the time or duration of the treatment thereby providing the capability of controlling and curtailing the severity of any damage to the surfaces, e.g., etching of plaster walls of pools, etc.

The invention has been described with reference to the illustrated and presently preferred embodiment. It is not intended that the invention be unduly limited by this disclosure of the presently preferred embodiment. Instead, it is intended that the invention be defined, by the means, and their obvious equivalents, set forth in the following claims: