CWQA Position Statement
Hydrogen Sulphide, Fluorides and Other Water Problems
Alkalinity
Alkalinity of water may be due to the presence of one or more of a number of ions. These include hydroxides, carbonates and bicarbonates. As you know from earlier lessons, hydroxide ions are always present in water even if the concentration is extremely small. However, significant concentrations of hydroxides are usual in natural water supplies but, may be present after certain types of treatment. Large amounts of carbonates are found in natural water supplies in certain sections of the country. They may also be found in water after treatments such as limesoda ash softening. Bicarbonates are the most common sources of alkalinity. Almost all natural supplies have a measurable amount of this ion.
Phosphates and silicates are rarely found in natural supplies in concentrations significant in the home. Compounds containing these ions may be used in a variety of water treatment processes. Moderate concentrations of alkalinity are desirable in most water supplies to balance the corrosive effects of acidity. However, excessive quantities cause a number of problems. These ions are, of course, free in the water but have their counterpart in cations such as calcium magnesium and sodium or potassium.
You will not probably notice an alkaline condition due to bicarbonate ions except when present in large amounts. In contrast, you should readily detect alkalinity due even to fairly small amounts of carbonate and hydroxide ions. Strongly alkaline waters have an objectionable "soda" taste. The U.S. Public Health Service Drinking Water Standards limit alkalinity only in terms of total alkalinity and the pH of the treated water. Highly mineralized alkaline waters also cause excessive drying of the skin due to the fact that they tend to remove normal skin oils.
Although troublesome amounts of alkalinity can be removed in several ways, none of these methods are normally satisfactory for household needs. These include:
(1) Lime softening removes hardness. At the same time, this process will precipitate an equivalent amount of alkalinity. Lime softening is usually restricted to industrial and municipal installations.
(2) An anion resin regenerated with sodium chloride does the job. This process removes substantially all the anions (carbonates, bicarbonates and sulphates). It replaces these anions with a chemically equivalent amount of chloride ions. The disadvantage of this process is that in almost all cases a high chloride ion concentration results. For household purposes such results are almost as undesirable as the original alkalinity.
(3) The feed of a mineral acid will neutralize the alkalinity of a water. Hydrochloric acid, sulphuric acid or a combination of these can be used. This process converts the bicarbonates and carbonates present into carbonic acid. At this point, it is advisable to provide some method to permit the gas to escape into the atmosphere. The disadvantages of this acid feed technique are obvious. There are needs for precise control of the process and caution in handling the strong acid. Distillation and demineralization are omitted here as they are not normally suitable for home needs.
Free Carbon Dioxide
Almost all natural waters contain some carbon dioxide which they gain in several ways. Carbon dioxide gas (CO2) is present in the air to the extent of 0.03% by volume and 0.05% by weight. As rain falls through the air, it absorbs some of this gas. On reaching the earth, the rain water - now slightly acid - will absorb additional amounts of carbon dioxide if it flows through decaying vegetation. At the same time, the carbon dioxide becomes carbonic acid. If the water now passes through limestone formations, its carbonic acid content will react with the limestone to form soluble calcium bicarbonate. In this process, the carbonic acid is partially neutralized.
On the other hand, if water passes through rock formations such as granite, no such reaction occurs. The carbonic acid is not neutralized. It continues as carbonic acid until drawn to the surface where it can then cause corrosion if not neutralized. If nature or chemical agents do not neutralize carbonic acid, it will cause corrosion of both copper and galvanized plumbing systems. In those parts of the country where the problem is prevalent, it is serious for it can lead to serious damaging of plumbing equipment. Carbon dioxide together with carbonic acid is primarily a problem in water containing relatively low concentrations of minerals. In such waters, there are not sufficient alkaline salts to buffer the effect of carbonic acid.
The simplest removal method of carbonic acid is to pass the water through a tank containing limestone chips. A neutralizing filter of this type affects the carbonic acid just as does the underground limestone formation. The limestone in the filter reacts with the carbonic acid to produce calcium bicarbonate. In the same way, lesser amounts of magnesium bicarbonate are formed.
NOTE: Not all forms of limestone are suitable for this purpose. Excessively soft material may break down to form a solid mass and block the filter. The best types are hard, strong granules which retain their physical structure even as they are dissolved.
Another type of material used in this neutralizing process is magnesium oxide. Although this procedure does add hardness and alkaline salts to the water, it effectively neutralizes a considerable amount of carbonic acid at a relatively low cost.
Where high carbon dioxide concentrations are encountered, a solution of soda ash - sodium bicarbonate (Na2CO3) - may be fed into the water. The carbonic acid and the sodium carbonate react directly to form sodium bicarbonate. This method of treatment offers the advantage of not adding hardness to the water. Also, it is especially effective where it is necessary to remove carbonic acid from large volumes of water. This method, as we have seen, has the disadvantage of requiring more attention in the preparation and maintenance of proper feeds.
Where water is obtained from a private well, a small positive displacement pump can be used to feed the soda ash solution into the water. Normally such pumps are wired to act in conjunction with the operation of the well pump. This permits the proportioning of the solution with a good degree of accuracy. Where a private water system is not used to draw water to the household lines, some other type of feeding device is necessary. However, the design of such devices is limited only by the ingenuity of pump manufacturers and installation men.
It is important to feed soda ash solutions into the water in advance of some type of tank or mixing device. This is necessary to provide for reasonably consistent concentrations in the water to be treated. The type of pressure tank utilized in connection with most private water systems is adequate for this purpose.
Chlorides and Sulphates
Almost all natural waters contain chloride and sulphate ions. Their concentrations vary considerably according to the mineral content of the soil in any given area. In small amounts, they are not significant. In large concentrations, they present problems. Usually chloride concentrations are light. Sulphates can be more troublesome because they generally occur in greater concentrations. Low to moderate concentrations of both chloride and sulphate ions add palatability to water. In fact, they are desirable for this reason. Excessive concentrations of either, of course, can make water unpleasant to drink.
The U.S. Public Health Service Drinking Water Standards recommend the same maximum concentration - 250 ppm - for each of the chloride and sulphate ions. (Expressed as Cl and SO4= , not as CaCO3). Water containing calcium sulphate ions is likely to have a characteristic taste...somewhat bitter and astringent. In fact, it has been compared to the way dissolved gypsum might taste in water. If equal amounts of magnesium sulphate or sodium sulphate are dissolved in water, the taste would not be noticeable. Both possess definite laxative effects in concentrations above 30 grains per gallon. In this way, they can be troublesome, especially to people not accustomed to such water. In addition to their laxative properties and possible medicinal taste, sulphate water can mean extreme hardness, large amounts of sodium salts or acidity. Alone or together these can pose special problems in the conditioning of water.
Chlorides give water a salty taste. At what concentrations this becomes noticeable again depends on the individual. In large concentrations chlorides cause a brackish, briny taste that definitely is undesirable. Although chlorides are extremely soluble, they possess marked stability. This enables them to resist change and to remain fairly constant in any given water unless the supply is altered by dilution or by industrial or human wastes. Both chlorides and sulphates contribute to the total mineral content of water. As indicated above, the total concentration of minerals may have a variety of effects in the home. High concentrations of either sulphate or chloride ions add to the electrical conductivity of water. Chlorides and sulphates cannot be removed from water with the aid of regular home treatment methods. Normally demineralization or distillation is necessary for their removal.
Fluorides
Fluorides in water can be detrimental or beneficial. It all depends on the concentration. Surface water supplies are normally low in fluorides (less than 0.5 ppm). Some have no fluoride at all. Well waters may contain excessive amounts of fluoride (over 1.5 ppm). There are some wells which contain the recommended amount (about 1 ppm) for drinking water. Fluorides are important because they have a definite relation to dental health. Research has shown that a concentration of 1 ppm of fluoride in drinking water reduces tooth decay.
On the other hand, when water contains over 1.5 ppm of fluorides, it causes a condition known as "endemic dental fluorosis". Sometimes called "Colorado Brown Stain", this condition appears as dark brown mottling or spotting of the teeth. In certain cases, the teeth become chalky white in appearance.
Research studies in the 1930's indicated that fluoride concentrations of 1 ppm are optimum. Authorities generally agree: (1) Where concentrations are greater, the excess fluorides should be removed from water. (2) Where concentrations are less, fluorides should be added. As a result of these earlier studies, a number of cities now add fluorides to produce a concentration of 1 ppm fluoride in municipal water supplies. The results are still under study. However, a large number of medical and public health groups have endorsed the principle of fluoridation of public water supplies. Among these are: The American Medical Association; American Dental Association; State and Territorial Dental Health Directors; American Association of Public Health Dentists; Public Health Service; American Public Health Association and others.
Up to the present time, the addition of fluorides to private water systems has not been considered feasible. The toxicity of bulk fluoride materials and the need for rigorous control has led to this cautious attitude. However at present, several manufacturers are in the process of marketing fail-safe home fluoridation systems. Where the fluoride concentration is too great, it is necessary to reduce the amount to acceptable limits.
Various methods have been suggested for reducing fluorides. These can be classified broadly into two groups:
1. Those involving treatment with chemicals, such as aluminum sulphate, magnesium or calcium phosphate and others.
2. Those involving percolation through a bed of material such as activated carbon, activated alumina, granular tricalcium phosphate or ion exchange resins.
Methods in the first category have distinct disadvantages. They require use of elaborate treatment plants, careful control of chemical dosage and pH. In some cases, further treatment is necessary to restore the pH of the treated water to normal. Methods in the second category do not require such elaborate control. Of these, the only widely used method of reducing fluoride content involves the use of a tricalcium phosphate filter. Such a filter functions in much the same way as a carbon filter. As the water flows through a tricalcium phosphate filter, the fluorides are absorbed. Theoretically, a tricalcium phosphate filter can be regenerated by flushing with caustic soda. As a general rule, however, it is more economical and safer to discard the exhausted filter material and replace it with a fresh supply.
Hydrogen Sulphide
Hydrogen sulphide is a gas present in some waters. There is never any doubt as to when it is present due to its offensive "rotten egg" odour. This characteristic odour is apparent in concentrations as low as 1 ppm. As obnoxious as the taste and odour of hydrogen sulphide is, these are the only two problems it presents. Hydrogen sulphide promotes corrosion due to its activity as a weak acid. Further, its presence in the air causes silver to tarnish in a matter of seconds.
High concentrations of hydrogen sulphide gas are both inflammable and poisonous. While such concentrations are rare, their presence in drinking water has been known to cause nausea, illness and in extreme cases, death. High concentrations of dissolved hydrogen sulphide can also foul the bed of an ion exchange softener. Their continued presence will lead to lower and lower capacity and may finally necessitate replacement of the resin bed. Generally, hydrogen sulphide occurs in concentrations of less than 10 ppm. Occasionally, the amount goes as high as 50 to 75 ppm. Hydrogen sulphide is more common to well waters than to surface water supplies. There are several methods for removing hydrogen sulphide from water. Most of them involve converting the gas into elemental sulphur. This insoluble yellow powder can then be removed by filtration. Low to moderate concentrations of hydrogen sulphide can be eliminated through use of an oxidizing filter of the same type satisfactory for iron removal. Because the elemental sulphur precipitate tends to clog the filter material, it is usually necessary to replace this material from time to time. Chemical treatment is recommended for medium to high concentrations of hydrogen sulphide. In such cases, solutions of household bleach or potassium permanganate serve as satisfactory oxidizing agents. When these oxidizing agents, such as household bleach and permanganate solution are used, a small chemical feed pump will serve to feed the agent into the water.
A ratio of 2 ppm chlorine per 1 ppm H2S is suggested as a starting dosage. This level will normally provide a high enough chlorine residue to ensure complete oxidation of the sulphide to sulphur. The feeding rate of the chlorine solution may be adjusted from the original settings to provide the most efficient operation. As in the case of iron, the chlorine solution should enter the water upstream from the mixing or storage tank to provide sufficient contact time. A contact time of at least 20 minutes should be allowed for complete reaction. After this contact time, the water should pass through an activated carbon filter to remove the now insoluble sulphur and excess chlorine.
If potassium permanganate can be used as the oxidizing agent, an iron filter is recommended to remove the insoluble products from the water. (Theoretically, 9.3 ppm of pure KMnO4 are necessary to oxidize 1 ppm H2S). However, a slight excess of permanganate, as shown by a light pink colour, should be fed to keep the filter in a "regenerated" state. In this way, it acts as a reserve to protect against any unexpected increase in the hydrogen sulphide content of the water. An activated carbon filter alone will remove trace amounts of hydrogen sulphide. In this process, the carbon simply absorbs the gas on its surface areas. The use of an activated carbon filter can be economical when extremely small amounts of the gas are present. Regeneration of the activated carbon filter is not usually practical. Period replacement is necessary. With moderate to high concentrations of hydrogen sulphide, this becomes impractical from an economic standpoint.
Some large users of water depend on aeration to remove hydrogen sulphide from water. Although this is the simplest method, it is not normally used for household applications. It has the disadvantage of high initial costs and results of no complete removal of the gas. There has been some use of the ion exchange process for removal of hydrogen sulphide. The ion exchange material for this purpose is a strong base anion substance which can be regenerated with salt or a mixture of salt and sodium bicarbonate. This technique has the advantage of simplicity in operation. On the other hand, it offers relatively low capacity, a low flow rate and an effluent water that has all chloride anions.
Nitrates (Nitrate Nitrogen)
Many ground waters contain small amounts of nitrate nitrogen. Concentrations range from 0.1 ppm to 3 or 4 ppm in most areas. However, amounts as high as 100 ppm have been found. Nitrates may occur in both shallow and deep well supplies but, they are most common in water from shallow wells. The presence of nitrate nitrogen can result from the seepage of water through soil containing nitrate-bearing minerals. They may also occur as the result of using certain fertilizers in the soil. However, nitrates are one of the products of decomposition of animal and human wastes. Thus, the presence of nitrates in a water supply indicates possible pollution of the water. Nitrate nitrogen has been much publicized in recent years in relation to the problem of "blue babies".
In concentrations as low as 10 to 20 ppm, nitrate nitrogen has caused illness and even death among infants under six months of age. If such water is used for supplemental or for complete bottle feeding, it may affect the ability of the blood to carry oxygen. This oxygen starvation is called "methaemoglobinaemia" or more commonly, the "blue baby" condition. Older children and adults are able to throw off the effects of nitrates in the blood.
As serious as this problem is, it is not the major concern of public health officials in regard to nitrate nitrogen. Rather, they feel, it is a strong indicator of water pollution.
In the process of decomposition, raw sewage undergoes a chemical change. Among the end products is nitrate nitrogen. When nitrate nitrogen occurs, it is considered evidence of pollution either from septic tank fields, cesspools or other sewage sources. Where a ground water is known to contain little or no nitrate nitrogen naturally, the appearance of any significant increase is a probable indication of pollution. Because of these factors, well waters containing nitrate nitrogen should be checked periodically by local or state health authorities. The best method for treatment of large nitrate nitrogen concentrations due to human or animal wastes is prevention. Wells should be properly located and constructed in order to prevent sewage contamination.
Nitrates can be removed either through distillation or demineralization.
Today, such processes are not economically feasible for treating household supplies. Bottled water is usually the most practical source of nitrate-free water for infants.
In commercial and industrial water supplies, nitrates do not usually present serious problems.
Oxygen
As meteoric water falls through the atmosphere, it collects oxygen gas. This dissolved oxygen gas is not the same oxygen in the water molecule. Amounts of dissolved oxygen are present in all rain waters and surface supplies due to contact with the atmosphere. Just how much dissolved oxygen a water supply will contain depends on several factors.
(1) Under high pressure, relatively large quantities of oxygen dissolve in water. When the pressure is proportionate weight of the gas escapes (Henry's Law).
(2) The amount of minerals in a water affect its ability to dissolve oxygen. Distilled water can absorb more oxygen than will waters with higher mineral content. Obviously, sea water for this same reason holds less dissolved oxygen than fresh water.
Well waters usually contain smaller amounts of dissolved oxygen than surface supplies. In deep wells, there may be a total absence of the gas.
Oxygen adds to the taste of water. For this reason, a small amount of it is desirable in drinking water.
We are all familiar with the "flat" taste which water often possessed after it has been standing in an open container for some time. The taste can be improved simply by shaking the water in a partially filled bottle. This re-introduces oxygen into the water to give it a more appealing taste. Despite this desirable feature, dissolved oxygen can be a source of serious trouble in a household water supply. this fact is that oxygen causes corrosion. In cold water, oxygen normally has little corrosive effect. In contrast, when the water is heated, the oxygen can cause serious corrosion effects.
A number of chemicals are used in industry to remove oxygen from a water supply. Sodium sulphite (Na2SO3) is probably most widely used for this purpose. It reacts with oxygen at high temperatures to form sodium sulphate (Na2SO4), in this way reducing the oxygen. There are a number of chemicals that react similarly with oxygen to effect its removal. The degree of success varies.
In general, however, none of these chemicals are used for home control of corrosion due to oxygen. For household purposes, treatment is normally limited to the use of polyphosphates to coat the insides of water lines to protect the metal from contact with the oxygen.
Silica
Many water supplies contain silica. This is not surprising since silicon is the second most abundant chemical element in the earth. The solid crust of the earth contains 80% to 90% silicates or other compounds of silicon. Water passing through or over the earth dissolves silica from sands, rocks and minerals as one of the impurities it collects. The silica content of water ranges from a few parts per million in surface supplies to well over 100 ppm in certain well waters. Silica exists in water in two forms as both a colloid and a crystalloid.
In its colloid form, it consists of very fine particles in suspension. These can usually be removed by coagulation and settling or filtering.
In its crystalloid form, silica is slightly soluble and extremely difficult to remove by either chemical or physical means. It produces scale formations that can cause extreme loss of heat transmission in high pressure industrial boilers.
Silica is not too significant a factor, however, in water for household uses.
Sodium
Sodium salts are present to a greater or lesser degree in all natural waters. Their concentrations vary from a few parts per million in some surface supplies to several hundred grains per gallon in certain well supplies. Sodium is extremely soluble and increase in solubility as the temperature of water rises. Because of this characteristic, sodium salts do not form scale when water is heated. Likewise, sodium salts do not produce curd when combined with soap. In fact, ordinary soap is an organic sodium compound. As such, it does not react with the sodium in water. High concentrations, on the other hand, mean high total minerals and tend to increase the corrosive action of water. In concentrations over 30 to 40 grains per gallon, sodium salts may give water an unpleasant taste.
Further, sodium ions in large amounts hamper the operation of ion exchange softeners in the removal of hardness. Where water contains appreciable amounts of both hardness minerals and sodium, several grains of hardness may continue to appear in the softened water. This occurs because of the regenerative action of the sodium ions on the ion exchange material. Sodium salts have a lesser effect on the resinous exchange materials than on those of gel zeolite type. Distillation or demineralization are the only effective ways to remove sodium salts from water.
Methane
Wells that contain methane are generally located in areas where gas and oil wells are common sights. Amounts run from 0.1 to 11.6 cubic feet per 1,000 gallons. This is roughly equivalent to 0.8 to 87 millilitres of methane per litre of water. Methane is objectionable in drinking water because of the odour and flammability. Where water contains methane gas, it is advisable to aerate it prior to use for either industrial or household purposes. This is necessary to avoid the dangers of fire or explosion. The aerator must be vented to the open air to permit the gas to escape into the atmosphere.
Phenol
There is a growing trend by both government and private groups to control pollution of water due to the discharge of industrial waste materials. One of the offensive wastes is phenol. Phenol (C6H5OH) imparts a medicinal taste and odour to water when the latter is chlorinated. This objectionable taste in a chlorinated water occurs in concentrations as low as one part per billion.
