Chapter 11 WATER QUALITY CONTROL  I. H. SIDDIQUE

 

 

11.1 Objective:

To provide a review of water quality control. 

11.2 Introduction

The need for water treatment is obvious, not only from a public health standpoint, but also from the standpoint of providing a water free of pathogenic organisms, as well as color, turbidity, tastes and odors. However from an engineer's view, a water is desired which is non-corrosive and non-precipitating, i. e., inert to plumbing and equipment. It is therefore, the purpose of water treatment to provide all of the above.

Water is obtained from three sources:

• (1) surface water;
• (2) ground water; and
• (3) the sea (desalination).

Of these, water from surface supplies is the most common. The initial source of all water supplies is rainfall or precipitation, some of which remains on the surface and some of which percolates through the soil into underground aquifers. Surface water is that which is supplied from a river, lake, natural or artificial reservoir on the surface of the earth. Ground water is usually that which is found in underground aquifers. Water from the sea is obtained by the distillation of sea water in which the dissolved salts are removed.

Surface and ground water have different characteristics as shown in

Table 1.

11.3 Methods of treatment of surface and ground water differ as follows: 

11.3.1 Surface Water

• 1. Clarification by coagulation and sedimentation
• 2. Treatment to remove manganese and algae by oxidation of manganese to insoluble form and by use of activated carbon for absorption of algae
• 3. Filtration to remove suspended material from steps 1 and 2 above
• 4. Additives or adjustment of pH of water to form stable water
• 5. Disinfection, usually by chlorination to kill pathogenic bacterial

11.3.2 Ground Water (Dug, bored, driven, drilled wells)-

sewage system is not to be close by.

• 1. Aeration - expose water to atmosphere to remove dissolved gases and to oxidize iron to insoluble form
• 2. Sedimentation to remove iron floc formed in step no. 1
• 3. Softening - precipitation of calcium carbonate and magnesium hydroxide (includes sedimentation and filtration)
• 4. Adjust to produce stable water
• 5. Disinfection - chlorination

11.3.3 Common impurities of water

Common impurities of water and their effects are given in Table 3. 

11.4 Waterborne Diseases

Water offers a suitable environment for the growth of microorganisms. In the absence of proper care, many disease causing organisms (pathogens) may enter the water and be carried to the users, causing the spread of disease. A communicable disease is one that can be transmitted through water and is called a water-borne disease. Some of the water-borne diseases are cholera, typhoid, paratyphoid, jaundice, bacillary dysentery, amebic dysentery, gastroenteritis and schistosomiasis (caused by an animal parasite living in water snails). 

11.5 Classification of Water and its Treatment-

From a hygienic point of view, water may be classified as follows:

11.5.1 Classification of Water:

11.5.1.1 Potable Water.

A potable water is one which is satisfactory for drinking purposes.

11.5.1.2 Polluted Water.

A polluted water is one which does not have pleasant appearance, taste or odor. The water may or may not contain the pathogenic organisms; may contain wastes from agriculture and industry.

11.5.1.3 Contaminated Water.

A contaminated water is one that contains dangerous chemicals or microbial agents. It's taste, odor and appearance may still be good. The term polluted water is synonymous with contaminated water.

11.5.2 Major water treatment steps

which are employed to make water potable are described herein and in Table 4:

11.5.2.1 Clarification (includes sedimentation).

A coagulant, such as aluminum or iron sulfate, is added to the raw water in the concentration range of 10 to 80 parts per million (ppm). The pH is adjusted by adding lime, (calcium oxide). The coagulant forms a flock of very light precipitate which under gentle agitation swirls through the water trapping or otherwise causing the suspended matter, including some bacteria, to agglomerate, forming large, dense masses which then settle out.

11.5.2.2 Filtration.

The clarified water is directed to rapid sand filters - which consist of graded sand supported on a underdrain system of graded stone or gravel and collection pipes. Activated carbon may be added on top of filters which are periodically cleaned by "backwashing", i.e. pumping water up through the filters.

11.5.2.3 Aeration.

Ground water may contain dissolved gases which are offensive to the taste and odor or otherwise harmful. A large surface of the water is exposed to the atmosphere by spraying or discharging into a forced draft aeration tower. This allows the dissolved gases to escape. Also iron in ground water exists in the soluble ferrous state; which on exposure to oxygen, is oxidized to the insoluble ferric state which may subsequently be removed by sedimentation or filtration.

11.5.2.4 Softening

. Hardness in water is caused by concentrations of calcium and magnesium which form insoluble precipitates with soaps. Lime and sodium carbonate are added to precipitate these elements which are removed by sedimentation and filtration (Ca++ cause hardness). Highly polluted waters may be made potable, crystal clear & almost sterile by these procedures.

11.5.2.5 Additives and pH adjustment.

An acid water will be corrosive i.e. it will attack the iron pipe in the distribution system. On the other hand, too basic a water will precipitate calcium carbonate to the point where the pipe is completely blocked. A delicate balance must be maintained so as to produce a water which is neither corrosive or precipitating (scale forming). The additives and pH adjustment depend on the type of water and previous treatment.

11.5.2.6 Disinfection.

Chlorine is usually added as gas to the water just before entering the distribution system so as to maintain a residual concentration of 0.3 to 1.0 ppm in the system. This concentration is sufficient to kill pathogenic bacteria but the effect on water-borne viruses is largely unknown. Chlorine reacts with water to form aqueous chlorine, Cl2, hypochlorous acid, HOCL, and hypochlorite ion, OCL, depending on the total concentration of chlorine present and the pH of the solution.

Water Quality Cl2 (aq.) + H2O ========== HOCL + Cl- + H+ (1) HOCL ========== H+ + OCL- (2)

Reactions (1) and (2) occur simultaneously and the double arrows indicate that all of the products are present at any given instant of time. For a given total concentration of chlorine, the concentration of the H+ determines the relative proportion of each of the products present.

The higher the ratio of hypochlorous acid to the hypochlorite ions, the better the disinfection efficiency (HOCL). Therefore, as pH goes up, disinfection efficiency goes down. OCL

Other disinfectants, including ozone, O3, and chlorine dioxide, CLO2, have been evaluated but have not found acceptance because of increased costs and difficulty of preparation and handling. Recent work has indicated that iodine may be used for disinfection. No adverse physiological effects of Iodine on human population have been found. One possible advantage of iodine is that it has been found to be more effective against viruses than chlorine. However, under certain circumstances, iodine may result in undesirable tastes and odor.

To remove certain chemicals or taste or odor from water, the following methods are applied:

• (1) Adsorption For the removal of taste and odor
• (2) Chemical Precipitation For the removal of dissolved substances, such as iron, manganese, and hardness
• (3) Ion Exchange Exchange of ions of certain salts present in water

Fluoridation has not been listed as a standard water treatment process as it is not practiced on all water supplies. Fluorine, or rather fluoride ion, is added in the form of sodium fluoride or fluorosilic acid to produce a concentration of 1.0 on teeth. 

11.6 Types of Bacteria Present in Water

Many bacteria are to be found in water. Most of them are of no sanitary significance, some are indicators of pollution but are harmless; others are few in numbers but are pathogenic. These will include bacteria causing typhoid fever, dysentery, gastroenteritis, and Asiatic cholera.

Ground waters normally do not contain many bacteria since the effects of filtration, exposure to unfavorable environment, and the time element will eliminate most of them, including those of sanitary significance. Some shallow wells may show considerable numbers, but these are frequently due to lack of safeguards in well construction. Soils or aquifers having cracks or crevices may allow insufficiently filtered waters to enter wells or springs. The waters of deep wells may have very few bacteria, but rarely or never are they completely sterile.

A. The Crenothrix are a group of bacteria found in ground waters and may be troublesome. They thrive in waters containing minerals in solution. One type grows in iron-bearing waters and precipitates iron oxide or rust. It may occur in water mains and the rust produced will stain clothes and plumbing fixtures. After death, the Crenothrix may cause unpleasant tastes and odors.

Also, there are two other species of bacteria, one that precipitates manganese as a dark-brown deposit and the other aluminum as a yellowish-white deposit. Growth of Crenothrix can be prevented by iron and manganese removal.

11.6.2 Other Bacteria

Untreated surface waters contain many bacteria. However, sanitary-engineer is not concerned with most of them.

1. Coliforms are of great importance. The most important is Aerobacter aerogenes, which is widely distributed in nature, normally inhabits the intestinal tract of man and animals, and is excreted with the feces. It is considered non-pathogenic but may cause infections of the genito-urinary tract. The coliforms, therefore, are useful as indicators of pollution, since they show that the water

• (a) has been in contact with soils or plants or
• (b) has been polluted by sewage and so recently that the bacteria has not died out naturally or been removed by natural filtration or artificial treatment. Coliforms do not increase in numbers in water. They die at a logarithmic rate which may leave a few individuals existing for weeks or months in fresh water. The death rate in salt water in much higher.

Coliforms are of importance not only because they indicate pollution but also because their absence or presence and in what numbers can be determined by routine tests. Tests for pathogens are not adapted to such routine work and are made only in special investigations. The routine test does not differentiate between Aerobacter, and E. coli although special procedures are available to determine as which is present. It is apparent from the foregoing that Aerobacter represents an undesirable condition although possibly not as dangerous as the presence of E. coli.

11.6.3Relationship between Pathogens and Coliforms.

E. coli are excreted in enormous numbers. Fresh feces may contain from 5 million to 500 million per gram, and the average amount of feces is about 82 gms. daily per person. Coliforms in sewage will vary widely according to its concentration, from 25,000 to 500,00 per ml., depending upon time of day of collection and also upon temperature; greater numbers being found in summer than in winter. Pathogens are far less numerous and where exposed to light, unfavorable temperatures, effects of other bacteria, and other unfavorable environmental conditions will die out at least as rapidly as the coliforms. Water treatment methods will have the same effect. Chlorination, which is widely used in water treatment is probably somewhat more efficient in killing pathogens than coliforms in the usual range of dosages, water temperature and pH. The procedure for determination of most probable number (MPN) of coliform present in 100 ml of water has been reviewed earlier. The U.S. Public Health Service Drinking Water Standards states that a water is acceptable if the MPN is less than 2.2 coliform per 100 ml. of water. The presence of coliform in excess of these numbers indicates immediate remedial action until such time as two consecutive samples meet the required standards.

11.7 Chemical and physical tests

Certain chemical and physical characters of waters are determined to make water potable which are:

11.7.1 Chemical Tests

The U.S. Public Health Standards for chemicals in water are based on allowed toxicity levels or on allowed levels for tastes and odors. A few of the chemicals and their allowable maximum concentrations are presented below: 

The low concentration of phenolic compounds is 1 part per billion is allowed as concentration in excess of this causes taste and other problems.

11.7.2 Physical Tests

The physical tests are mostly concerned with turbidity and color. See Table 2.

Even with high quality water leaving the water plant, a substandard distribution system may adversely affect the quality. In addition, an improperly designed or overloaded distribution system may result in insufficient quantity of water for firefighting purposes or even domestic use. Therefore, a water distribution system should:

• - Provide and adequate supply of water at ample pressure in all portions of the system;
• - Sufficient valves should be provided to permit repairs and to allow flushing of system;
• - No cross connection with a source of inferior water should exist;
• - The system should be as leak tight as possible and not be submerged;
• - Provide for effective circulation of water with a minimum of dead ends;
• - When new mains are installed or old mains repaired, they should be thoroughly disinfected with a strong chlorine solution (40-60 mg/L) and followed by a bacteriological examination.

Table 1. Surface and Ground Water Characteristics