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the storage cistern. These come from the air and other contaminating objects, and illustrate the great growth of the common water bacteria in water stored under these circumstances. The slight increase of the ammonias and nitrites in the cistern water, as compared with the water direct from the well, indicates organic pollution and bacterial activity. The diminution in the residue results largely from separation of the iron. This water is pure and wholesome, despite the fact it contains many more bacteria than that usually allowed. It has been used for some years by about 3,000 persons, who are singularly free from typhoid fever and other water-borne diseases.

Analysis No. 12 is a good illustration of the bacteriological and chemical character of a river water, and illustrates the changes that occur during short storage (3 to 5 days) and after filtration through a slow sand filter.

It will be seen from this table that there is a gradual diminution in the amount of free ammonia and a more marked diminution in the amount of albuminoid ammonia. The amount of organic matter as represented by the ammonias is diminished just one-third. The nitrites show an increase during storage of the water, indicating active oxidation, but a marked decrease after it is filtered, showing the rapid completion of the oxidation of the organic matter in the filter. The nitrates show a tendency to increase in amount, which would be expected as the nitrites diminish. It is evident that storage and filtration have little effect upon the chlorin content of the water. The total residue diminishes as the result of storage, sedimentation, and filtration. It will be noted, however, that this diminution is more marked with the fixed residue than with the volatile residue.

The number of bacteria decrease as the result of storage, but the most marked decrease occurs as the result of filtration. It should be remembered that all the bacteria in the filtered water do not represent those which have passed the filter. The effect of the few days' storage upon this water does not very materially affect the number of B. coli, but there is a marked diminution in their number as the result of filtration.

ANALYSIS No. 13-Typical Analysis of Surface Waters

The analyses of surface waters, shown in the table on page 774, with diagram showing the locations from which samples were obtained, will repay careful study. This table and diagram were furnished through the kindness of Professor Whipple.

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20. 13 OF Bolt ACE WATERS

HARVARD UNIVERSITY LABORATORY OF SANITARY ENGINEERING.

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The diagram above (Fig. 105) shows the location of the samples

employed in Analysis No. 13.

CHAPTER V

THE PURIFICATION OF WATER

The ways in which water may be purified for practical purposes are not many. It is worth noting that most of the advances in water purification come from the development of old empiric processes. It is only at long intervals that a new method or principle of treatment is discovered that is important enough to find a permanent place in the art of water purification.

The principal methods at present serviceable for the purification of water upon a large scale are: (1) storage, (2) filtration, (3) chemicals, such as ozone, hypochlorite of lime, sulphate of aluminium or iron.

No method of purifying water can be considered to approach a satisfactory hygienic standard that does not first of all practically eliminate water-borne diseases. The process must also reduce the turbidity and color to inappreciable amounts and remove something like 99 per cent. of the bacteria, when these organisms result from sewage pollution and are fairly numerous: there is perhaps no final reason for the bacterial standard. It has been adopted by consent because it represents a purification that is reasonably satisfactory and that can be accomplished at the small cost of about $10.00 per million gallons of water treated. With the further awakening of the sanitary conscience of the community the standards will inevitably tend higher, and it is probable that in time our standards will approach an ideal that is now not regarded as necessary. At present there is no evidence that the few microorganisms left in the water after a satisfactory method of purification, such as slow sand filtration, are injurious. Certainly, if injurious influence is exercised, it is too small to be determined or measured by any methods now at our disposal.

NATURE'S METHOD OF PURIFYING WATER

In nature, water is purified by various methods, the chief of which are: (a) evaporation and condensation, which makes rain water the purest of natural waters; (b) the self-purification of running streams, which is a variable and uncertain quantity; (c) storage in lakes and

ponds which clarifies water and in time eliminates danger; and (d) the physical, chemical, and biologic action of the soil upon water that filters into the earth, which is one of nature's greatest purifying agencies.

Evaporation and Condensation.-The purifying action of the distilling and condensing process through which all meteoric water passes is one of nature's beneficent processes. Enormous quantities of sea water, marsh water, and polluted waters of all kinds are thus returned to us suitable for domestic use. Somerville estimates that "186,240 cubic miles of water are annually raised from the surface of the globe in the form of vapor, chiefly in the intertropical seas." Water is thus constantly being purified in nature. The ocean has been compared to a boiler, the sun to a furnace, and the atmosphere to a vast still. The cooler air of the higher atmosphere and of colder zones acts as a condenser, causing the precipitation of the distilled water as rain. About three-fourths of the earth's surface (145,000,000 square miles) is covered with water, much of which is in the tropical belt.

Self-purification of Streams.-The self-purification of streams needs special discussion. Streams become purer during the course of their flow. Of this there can be no doubt. This half-truth based upon chemical data has in the past suffered sanitarians to permit the use of water that now we know was responsible for much sickness and many deaths. Streams become purer, but not pure. Some impurities always remain, that is, the process is not complete and final. All surface supplies are now regarded with suspicion and are either stored, filtered, or other wise purified before they are used by educated communities.

It was formerly said that a stream purifies itself in seven miles Such a generalization is absurd. We now know that it is not the distance so much as the time and opportunity for the various factors involved to become effective. Thus, Buffalo's sewage flows to Niagara's intake, a distance of about 16 miles, in a few hours. There is little chance for self-purification to take place, and despite the great dilution the danger is very great. Niagara's average typhoid rate for 10 years, from 1899 to 1908, was 132.9 per 100,000, the highest in the country. On the other hand, we have the following facts:

A good instance of the self-purification of streams resulted from the studies of the Potomac River and its relation to typhoid fever in the District of Columbia. The Potomac River drains an area of about 11,400 square miles which, in 1900, contained a population estimated to be about half a million, or about 44 per square mile. The velocity of the flow of the Potomac is extremely variable. It takes from 4 to 7 days for the water to travel from Cumberland to Great Falls (where the Washington intake is located), a distance of about 176 miles. The waters of the Potomac are directly polluted by sewage at numerous

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