METHODS OF PURIFYING WATER 



59 



rate water into its parts by mearrs of electricity as sug- 

 gested in one of the experiments at the beginning of 

 this topic? This experiment shows that water is a 

 chemical compound made up of two parts of hydro- 

 gen to one part of oxygen. 



water = hydrogen (two parts) + oxygen (one part) 



Pure water has no odor or taste. Drinking water 

 generally has a taste and sometimes an odor because 

 of the presence of dissolved substances or living or- 

 ganisms in it. Small quantities of water are colorless, 

 but large quantities show a distinctly blue color. 



Recently another kind of water known as heavy 

 water has been found mixed in with the molecules of 

 ordinary water. A method of separating heavy water 

 has been devised, but it is costly, thus making the 

 substance expensive to buy. Heavy water is heavier 

 than ordinary water because it contains another kind 

 of hydrogen which is two times as heavy as ordinary 

 hydrogen. Heavy water has some very different prop- 

 erties from ordinary water. Among them is the prop- 

 erty of killing certain types of simple living organisms 

 which live in ordinary water. 



All of us know how readily many substances like 

 sugar and salt disappear in water. We call a liquid 

 which dissolves substances a solvent. Water is some- 

 times called the universal solvent, because it will dis- 

 solve a great many substances and because it is cheap 

 and easy to obtain. 



What impurities are commonly found in water? 

 Absolutely pure water can be obtained only by artifi- 

 cial means. Because water dissolves many materials 

 which occur in nature, it is practically impossible to 

 secure an adequate supply that does not contain dis- 

 solved substances. Then, too, water carries suspended 

 matter, and many tiny plants and animals may also be 

 found in it. 



In general, the impurities found in water may be 

 put into three classes : 



Dissolved matter. Dead plants and animal matter de- 

 cay and dissolve in the water in which they are pres- 

 ent for a long time. Water dissolves mineral sub- 

 stances from rocks and soils through which it passes. 

 Also gases from the air and other sources are some- 

 what soluble in water. 



Suspended matter. If we take some very fine sand 

 and clay, and stir them in water, what seems to be a 

 uniform mixture is obtained. On standing, however, 

 most of the solid matter slowly settles out. Such mix- 

 tures are called suspensions. Small bits of plants, ani- 

 mal matter, clay, sand, and other types of sediment 

 are carried in suspension by most streams of water. 



Living organisms. Water generally contains tiny liv- 

 ing things, many of which are so small that they can- 

 not be seen with the unaided eye. Most of these are 

 not harmful to the body when taken in with the water 



we drink. It is definitely established, however, that 

 some diseases are spread by water containing disease- 

 producing bacteria (tiny plants). The disease typhoid 

 fever is often spread in this way. We cannot tell by 

 looking at, or tasting, water whether it contains dis- 

 ease-producing organisms. It must be examined with 

 the aid. of a microscope and chemicals. Some cities 

 employ bacteriologists to do this work. See Figure 

 129. 



To be satisfactory for drinking purposes water 

 must be free of any substances which have an injuri- 

 ous effect on the human body, and must possess no 

 characteristics which make it offensive to those who 

 drink it. To meet these requirements water should 

 possess the following qualities : be free of poisonous 

 chemicals and living things which produce diseases ; 

 possess no offensive taste or odor ; be clear and reason- 

 ably cool ; have a low content of dissolved substances. 



How are impurities removed from water? Since 

 natural supplies of water generally do not meet all 

 of these requirements, artificial methods of purifica- 

 tion must be used. Four methods of water purification 

 are in use today where large quantities are needed 

 for communities. 



Chemical treatment. Water that contains disease- 

 producing organisms is treated with some disinfecting 

 agent. The most common chemical used for this pur- 

 pose is chlorine. This gas may be applied to the water 

 directly or in one of its chemical compounds. Bleach- 

 ing powder (chloride of lime) is a white solid con- 

 taining chlorine which is commonly used to purify 

 water. The proper amount of it added to the water 

 supply will make the water fit to drink, but it some- 

 times gives the water a slight taste. 



Ozone (a form of oxygen) is being increasingly 

 used to purify water, as it has several marked ad- 

 vantages over chlorine and chloride of lime. It is in 

 no way injurious to the human body, adds no dis- 

 agreeable taste to water, but sterilizes it and removes 

 all taste and color due to organic matter. 1 It has the 

 disadvantages of being more expensive than chlorine 

 and chlorine compounds. 



Chemicals may also be used to remove suspended 

 matter from water. Certain chemical compounds of 

 iron and aluminum when added with lime to water 

 form a jelly-like mass which entraps suspended par- 

 ticles and causes them to settle to the bottom. 



Sedimentation. Water that contains a large amount 

 of suspended matter will lose much of this material 

 if it stands undisturbed long enough. A stream is able 

 to carry a large quantity of suspended matter which 

 it loses by the process of sedimentation as soon as 

 the velocity of its current is decreased. In many water- 

 purification systems, water from natural sources is 

 run into large sedimentation basins or settling tanks 



1 Organic matter, matter pertaining to living things. 



