Cover Image
close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 1: Water supply sources
View the document Overview:
View the document Background information
View the document Evaluation of sources
View the document Factors influencing the quality of water
View the document The quantity of water
View the document Types of sources
View the document Development of water sources
View the document The basic requirements of a water supply
View the document Selection of the source of supply
View the document Lesson plans

Evaluation of sources

THE QUALITY OF WATER

Absolutely pure water is never found in nature. The impurities in water vary from dissolved gases and chemical compounds to suspended matter such as disease organisms and dirt. While some of these impurities can be seen by the naked eye and others can be detected by taste or odor, most can be detected only by laboratory test.

Water takes on various characteristics and properties as it passes over and through the earth. These characteristics and properties vary, and are dependent on the materials encountered. They may be classified according to means of detection as physical (detected by one or more of the five senses) and chemical (detected by chemical analysis). The most important physical characteristics are turbidity, color, odor, taste and temperature. The most important chemical characteristics are acidity, alkalinity, hardness, and corrosiveness. Sometimes these two types of characteristics overlap; for example, iron in water is a dissolved mineral detectable by chemical analysis, yet its color and taste are also physical. This section discusses these characteristics and their causes.

BACTERIOLOGICAL QUALITY

The selection of a source of supply may be restricted because of economic or technical limitations involved in the use of normal water-treatment processes for making the water from this source safe for human consumption.

The effectiveness of a water-treatment process can not be established in specific, quantitative values. For instance, the bacteriological quality of filtered chlorinated water is dependent upon the bacterial content of the raw water; its chlorine demand; the coagulating, settling and filterinq characteristics of the treatment plant; the degree of uniformity of the raw water; and, not least, the integrity and ability of the treatment plant operator. Furthermore, the public health significance of the degree of bacterial pollution of raw water, and hence of any bacteria remaining in the treated water produced, depends upon the probable source of contamination of the raw water with coliform organisms, which serve as an indicator of pollution. These organisms may have originated largely from surface drainage, a situation likely to be most noticeable when manured fields are found within the watershed involved. On the other hand, sewage pollution may be the chief source of such organisms, in which case the indidence of intestinal diseases among the population contributing the sewage would have ammarked impact on the water. In this case the probable ratio between the numbers of pathogenic organisms and of coliform bacteria in the polluted water will be considerably increased. Such conditions are often encountered in rural as well as semi-urban areas where intestinal diseases constitute a serious public health problem, the treatment of the sewage is not practicable, and effective water-treatment is beyond the economic and technical resources available.

For these reasons, any bacteriological standards of quality adopted for drinking-water on a country-wide basis generally appear to be too rigid for large areas where, because of economic and social conditions, they are most difficult to apply and enforce. However, it must be reckoned that the adoption under these circumstances of more lenient standards, because they appear to be more realistic, only confuse the issue by lowering the goal of safety and potability without providing a meaningful substitute. Instead, it is preferable to keep the public health objectives of the water supply constantly in mind, but to appraise local situations and review pertinent information in the light of qualified professional judgement. In many cases, therefore, it is best for you to rule out the use of many surface waters which might appear to be suitable and convenient sources of potable water supply, and to throw considerable emphasis upon the use of ground waters whenever feasible.

The above statement should not be interpreted to mean that surface waters, because of bacteriological considerations, are unsuitable sources of supply for rural communities. This would be far from the truth. In fact, the use of surface waters often rakes it possible to provide consumers with ample quantities of water in their own homes, thus fulfilling most of the major health objectives of the systems. In some instances this is achieved by passing surface water through a simple and economical treatment plant. In most rural situations such a system may be considered as a step in the right direction, and is to be preferred to the appalling conditions under which the villagers are forced to carry, or even to purchase, small amounts of raw and polluted water. As time goes on, public pressure, technological advances, and the development through training of local skills, will gradually bring about the improvement of plant efficiency, operation and technical supervision to a point where the enforcement of existing standards of wiser quality may be possible.

It is necessary and desirable to establish some form of control over rural water-supplies. However, in most countries of the world, routine bacteriological control, which is obligatory in urban communities, would be unrealistic under rural situations, as indicated by the above discussion. In the latter, the attention of the local health administration should be concentrated primarily on those mayor elements of location and design of the supplies which will afford natural protection later against outside contamination, and on routine sanitary inspections by qualified sanitarians, to educate the rural population in the application and enforcement of rural sanitation regulations. Periodically tests for physical, chemical, and bacteriological quality should be made for the purpose of detecting mayor health hazards.

Bacteriological Standards for Drinking Water Recommended by the WHO Study Group

Some public drinking-water supplies are chlorinated or otherwise disinfected before being distributed; others are not. Effective chlorination yields a water which is virtually free from coliform organisms i.e. these organisms are absent in 100-ml portions) if communal supplies which are distributed without treatment or disinfection cannot be maintained to the bacteriological standard established for treated and disinfected water, steps should be taken to institute chlorination or disinfection, or other treatment, of these supplies.

A standard demanding that coliform organisms be absent from each 100-ml sample of water entering the distribution system - whether the water be disinfected or naturally pure-and from at least 90% of the samples taken from the distribution system can be applied in many parts of the world. Although there is no doubt that this is a standard that should be aimed at everywhere, there are many areas in which the attainment of such a standard is not economically or technically practicable.

In these circumstances there would appear to be economic and technical reasons for establishing different bacteriological standards for public water-supplies which are treated or disinfected and for those which are not treated. The following bacteriological standards are recommended for treated and untreated supplies of present use throughout the world, with the hope that improvements in economic and technical resources will permit stricter standards to be adopted in the future.

The standards described below are based on the assumption that frequent samples of water will be taken...For each individual sample, coliform density is estimated in terms of the "most probable number (MPN)" in 100-ml of water, or "MPN" index... The use of the MPN index is recommended as the basis of quantitative estimation of coliform density after full recognition of its limitations. However the value of the index is sufficiently enhanced by the use of data from a series of samples to warrant its use in the recommended standards.

Treated Water

In 90% of the samples examined throughout any year, coliform bacteria shall not be detected or the MPN index of coliform microorganisms shall be less than 1.0. None of the samples shall have an MPN index of coliform bacteria in excess of 10.

An MPN index of 8-10 should not occur in consecutive samples. With the examination of five 10-ml portions of a sample this would preclude three of the five 10-ml portions (an MPN index of 9.2) being positive in consecutive samples.

In any instance in which two consecutive samples show an MPN index of coliform bacteria in excess of 8, an additional sample or samples from the same sampling point should be examined without delay. This is the minimum action that should be taken. It may also be desirable to examine samples from several points in the distribution system and to supplement these with samples collected from sources, reservoirs, pumping stations and treatment points. In addition, the operation of all treatment processes should be investigated immediately.

Untreated Water

In 90% of the samples examined throughout any year, the MPN index of coliform micro-organisms should be less than 10. None of the samples should show an MPN index greater than 20.

An MPN index of 15 or more should not be permitted in consecutive samples. With the examination of five 10-ml portions of a sample, this would preclude four of the five 10-ml portions (an MPN index of 16) being positive in consecutive samples. If the MPN index is consistently 20 or greater, application of treatment to the water-supply should be considered.

In any instance in which two consecutive samples show an MPN index of coliform organisms greater than 10, an additional sample or samples from the same sampling point should be examined immediately. It may also be desirable to examine samples from several points in the distribution system and to supplement these with samples collected from sources, reservoirs and pumping stations.

When accurate and complete data concerning the sanitary conditions at the sources of an untreated water-supply, covering all possible points of pollution, are available and indicate that indices higher than the established maximum may bear little relation to potential health hazards, the local health and water-supply authorities should be responsible for ruling that such higher indices do not constitute need for treatment of the water.

CHEMICAL AND PHYSICAL QUALITY

Water of good chemical and physical quality is necessary from the points of view of its acceptability by the people, the protection of the health of the consumer, and the conservation of the water system. Anyone who has drank water from different sources encountered situations in which offending chemical substances have made a water source unacceptable even though its bacteriological quality was excellent.

Palatability of water is a term which describes the characteristic of being pleasing to the sense of taste. Drinking water should be free from color, turbidity, taste, and odor, and should be cool and aerated. At least four human perceptions can be used in judging these qualities. They are the senses of sight (color and turbidity), taste, smell (odor), and touch (temperature). However, palatable water is not always safe to drink or potable.

Turbidity and color are important in rural water-supplies. Depending upon the character of the watershed, turbidity may vary considerably from one season to another because of rainfall. A sudden increase in turbidity may do serious damage, or at least stop the operation, of small water-treatment plants if adequate precautions are not taken in advance in order to allow for rejection of the incoming supplies at such times. Water from slow-moving streams and small lakes is likely to be colored, at least during certain seasons of the year. Both turbidity and color will cause discoloration of clothes and may be responsible for rejection of the supply if removal by simple and economical processes cannot be achieved.

CORROSION AND SCALE

Hydrogen sulfide, dissolved oxygen, and carbon dioxide in water cause acidity and are responsible for corrosion of iron pipes. Hydrogen sulfide, which is sometimes found in deep-well water, is a product of decomposition of organic matter. It attacks cement and concrete and destroys storage tanks built of these materials. Dissolved oxygen combines with ferrous iron, which is sometimes found in solution in well water, and produces ferric hydroxide, which is insoluble and gives the water a rusty color. It may also cause serious corrosion of distribution pipes and house plumbing pipes.

Perhaps the most important and troublesome of the three products mentioned here is carbon dioxide, which is often found in well water and in surface water drawn from heavily wooded watersheds or from the lower layers of deep ponds. Carbon dioxide in water is responsible for heavy and rapid corrosion of unprotected pipes, thus creating increasing difficulties with maintenance and operation of a water system. Various materials, mostly bituminous compounds and cement are used by manufacturers for lining the interior surfaces of pipes against corrosion. These materials are also used to protect outside pipe surfaces against corrosion caused by the contact of pipes with certain soils and, under certain circumstances, by electrolysis.

Natural water containing carbon dioxide will dissolve carbonates from rocks in the ground, thus producing soluble bicarbonates. Depending upon the relationships between the bicarbonate alkalinity and the pH of the water on the one hand, and between the free carbon dioxide and the alkalinity on the other, the water will either be corrosive or, on the contrary, will deposit a film of carbonate on the inner surface of pipes. This film may sometimes develop sufficiently to become a thick scale which obstructs small distribution and service pipes, water meters, etc. The prevention of corrosion and scale rests upon the chemical control and maintenance of the proper equilibrium between the three factors mentioned, i.e., by reducing the content of carbon dioxide or increasing the alkalinity as determined by special tests. Except in rare instances, this type of chemical control is beyond the technical resources of small rural water-supply systems and, therefore, will not be discussed here in greater detail.

TURBIDITY

Turbidity is a muddy or unclear condition of water, caused by particles of sand, silt, clay, or organic ratter being held in suspension. The faster water flows, the more material it picks up and the larger the size of the pieces carried alone. As water shows down, the larger particles settle out. Clay and silt remain suspended in water longest, because of their particle size and specific gravities.

COLOR

Color in water is due to the presence of colored substances in solution such as vegetable matter dissolved from roots and leaves, and to humus and iron and manganese salts. true color is due to substances in true solution; apparent color includes true color and also that due to substances in suspension. Water taken from swamps, weedy lakes, and streams containing vegetation is most likely to be colored. Color may also be caused by industrial wastes and turbidity. The latter is responsible for an apparent color, rather than the true color, and is caused by materials of vegetable origin. Color as such is harmless, but objectionable due to its appearance and to the taste and odors sometimes associated with it.

ODORS AND TASTE

Taste and odors found in water are most commonly caused by alga (minute water plants), decomposing organic matter, dissolved gases, or industrial waste. Mineral substances may also be a cause. Portability is not normally affected by the presence of odors and tastes. On the other hand, palatability is frequently affected, particularly when a substance such as bone or fish oil is present. Water containing one of these substances in noticeable quantities is unpalatable. Tastes and odors which make water unpalatable must be removed. Use of free available chlorine and activated carbon will do much to prevent odorous combinations of chlorine with organic impurities in water.

TEMPERATURE

Warm water tastes flat. lowering the temperature of water suppresses odors and tastes and, therefore, increases its palatability. In the summer the temperature of deep lakes and reservoirs decreases sharply from top to bottom By shifting the depth of intake, it may be possible to draw relatively cool water even during hot weather. Water should be drawn from the lower depths when possible. Cool water is more viscous than warm water and thus is more difficult to coagulate and effectively chlorinate than warm water due to slower reactions. Water treatment times should be increased when water temperatures are less than 45°F.

ACIDITY AND ALKALINITY

Some of the physical impurities mentioned cause water to behave as either an acid or as a base. The degree of acid behavior is called acidity. The degree of basic behavior is called alkalinity. Since either condition has an important hearing on water treatment, the degree of acidity or of alkalinity must be determined.

The pH value is a measure of the acidic or alkaline nature of the water. The pH value ranges from 0 - 14. A value of 7 is neutral. A high pH value indicates a very strong alkaline solution.

The pH influences the corrosiveness of the water, the amount of chemical dosages necessary for proper disinfection, and the ability of an analyst to detect contaminants.

HARDNESS AND OTHER CHARACTERISTICS DUE TO DISSOLVED MINERALS

Hardness is caused by the soluble salts of calcium, magnesium, iron, manganese, sodium, sulfates, chlorides, and nitrates. The degree of hardness depends on the type and on the amount of impurities present in the water. Hardness also depends on the amount of carbon dioxide influences the solubility of the impurities that cause hardness.

The hardness caused by carbonates and bicarbonates is called carbonate hardness. The hardness caused by all others (chlorides, sulfates, nitrates) is called non-carbonate hardness. Alkalinity is usually equivalent to the carbonate hardness. Sodium, however, also causes alkalinity. In natural waters, sodium is not normally present in appreciable amounts. Therefore, in natural waters, the alkalinity is equal to the carbonate hardness. After a water has been softened, however, a large amount of sodium remains in the treated water. In softened water, the total alkalinity is the sum of the carbonate alkalinity plus the sodium alkalinity.

Hardness is undesirable in that it consumes soap, makes water less satisfactory for cooking, and produces scale in boilers and distillation units.

The following minerals cause hardness in ground and surface waters:

Calcium carbonate. Alkaline and only slightly soluble; causes carbonate hardness and alkalinity in water.

Calcium bicarbonate. Contributes to the alkalinity and carbonate hardness of water. Calcium bicarbonate when heated produces carbon dioxide and calcium carbonate. This calcium carbonate precipitates as scale in boilers and distillation units.

Calcium sulfate or gypsum. Causes noncarbonate hardness in water. Being more soluble in cold water than in hot, it separates from the water in boilers and forms scale on the boiler tubes.

Calcium chloride. Causes noncarbonate hardness in water. In steam boilers and distillation units, the presence of calcium chloride can cause chemical reactions which result in pitting of the boiler tubes.