PURIFICATION OF PUBLIC WATER SUPPLY

PURIFICATION OF PUBLIC WATER SUPPLY

In a paper on the “Purification of public water supplies,” by Gerald McCarthy, biologist of the North Carolina State laboratory of hygiene, occur the following notes:

The standard of quality of the North Carolina State laboratory of hygiene is as follows: Good drinking-water from shallow wells and streams should contain per 1,000,000 parts not more than: Total solids, 250; turbidity, 4; color, 15; chlorine, 5; nitrites, trace; nitrates, .45: phosphates, light trace; free ammonia, .15; albuminoid ammonia, .20; iron, 5; no alum; no heavy metals, except iron; no infusoria or fecal bacteria; no smell. The surest and most trustworthy test of the wholesomeness of any drinking-water is a bacteriological analysis. The bacteriologist identifies the different species or groups of germs present. The disease-producing germs found in water all belong to the fecal group, of which bacillus coli communis is the most common and typical species. Bacillus typhosus, bacillus challenge Asiatic and Bacillus cholerae suis are the three most important pathogenic members of this group. Of these, the first is the direct cause of typhoid fever in humans. All three of these pathogenic germs require for their normal development a temperature at, or above the ordinary temperature of the human body. Hence, they do not multiply very rapidly in natural water supplies, and are likely to be overpowered and killed out by the more vigorous saprophytic germs which are always found in surface waters. The usual length of life of bacillus typhosus in ordinary flowing water is about nine days; in still water it may extend to thirty days, and in wet mud to two or more years. The bacillus of typhoid fever is very rarely isolated by the laboratory method from drinking-water, even in localities where the disease is epidemic and is known from other evidences to have been brought in by water. The explanation of this fact is given in the last sentence—the typhoid germs, after having started the epidemic, die out of the water before the disease has reached its typical stage, or the point of development at which physicians can diagnose it from the symptoms. This development requires fourteen, to twenty days after actual infection. Once started, the disease can be spread by many agencies, among which flies and other insects are the most important. Bacteriologists condemn as unfit to drink any water which contains bacillus coli communis, since such waters are considered as having been polluted bv fecal matter and may contain typhoid germs. Bacteriological analyses of water, to be of real use in preventing epidemics of typhoid, must be made systematically and at frequent intervals. Monthly analyses arc the least frequent that can be depended upon. An occasional examination by an analyst personally unfamiliar with the locality is worse than useless. The annual death-rate from typhoid fever in North Carolina is about seventy-five per 100,000 of population. The usual victims of typhoid are persons in the prime of life. The money value of each such life may be conservatively figured at $2,000. For every death caused by typhoid fever there are about nine recoveries after an average sick period of forty-three days each. The medical and nursing expenses and loss of time caused by an attack of typhoid is on an average $100 per case. ‘The annual money loss to the State directly and indirectly due to typhoid fever caused by polluted drinking-water is probably not less than $2,500,000. At least two-thirds of this loss is an unnecessary and inexcusable waste. Any community having a typhoid death rate exceeding 30 per too.O(X) population can be safely set down as consuming polluted drinking-water. From the records of death-rate in a great number of towns in various countries having different kinds of water supply the following statistics have been compiled: Source of water supply—Mountain streams and springs, average death-rate from typhoid per 100.000 of population. 6; small unland streams, filtered, 12; good shallow wells. 18: large rivers and lakes’ 30; small upland streams, unfiltered.44: polluted wells, ponds and streams. 70 to 300. Statistics indisputably show that for the average town of 3,000 or more inhabitants shallow wells are a constant menace to the public health, and that where artesian wells are impracticable,a filtered water supply is the only safe and economical source of drinking-water. In thickly-settled rural neighborhoods and in regions adjacent to towns of 3,000 or more inhabitants the unfiltered water flowing in streams is not safe to drink, unless previously boiled. The raw or natural water of such streams may be purified by distillation or by filtration. Distillation is the surest and most perfect way of purifying water: but the cost is com monly prohibitive, and the resulting water, though it is chemically pure, is neither as palatable nor as wholesome as water purified by filtration. Water may also be sterilised by chemical treatment; but such treatment is undesirable from an hygienic point of view. As to filtration: bather the slowsand or the American mechanical filtration system may be followed—the socalled household filter, in which the water is strained through a sponge or porous tile, is unreliable and often worse than useless. Of the slow-sand tvpe there are in the United States only about thirty plants, filtering altogether less than ten per cent, of the total production of filtered water. ‘The bacterial efficiency of each kind is practically the same from about 97 to 99.5 per cent. T11 the slow-sand filtration system sedimentation for twenty-four hours in a 10 or 15-ft. deep concrete-lined storage basin, costing about $3,000 per 1,000.000-gal. capacity, will remove two-thirds of the suspended matter and eighty per cent, of the bacteria, thus saving the filter much work and greatl.v decreasing the cost of filtration. The total expense of installing a slow-sand filtration system is about $40,000 per 1.000,000-gal. ordinary daily capacity. A mechani cal or American filter costs to install about one eighth less than the other; its running expenses are about fifty per cent. more. As between the gravity and the pressure type, the latter is not to be trusted for drinking-water, which, although turned out very clear, may be dangerous to use because of the presence of alum. Raw waters which contain as high as 1,000 germs per drop are never desirable sources of a drinking-water supply. When germs are so abundant in the water. they produce, and leave in the water soluble toxins or physiological poisons which the filter does not remove, and such toxins are likely to cause serious disturbance to the health of writer consumers. The efficiency of a mechanical filter in use decreases rapidly bv clogging of the sand. On this account the sand must be washed by a reverse current of water and air-blast every twelve to twenty-four hours. Once cverv three months the sand should be thoroughly washed out with a five per cent, solution of caustic soda. The average small unland stream of North Carolina when sampled at least ten miles below any considerable sewage inflow, and while the water is normally clear, will show not more than 2.000 germs per cubic centimetre, or too germs ner drop. To remove ninetv-nine tier cent, of this germ contamination bv the filter an average of less than onehalf grain of alum ner gallon is sufficient. Tn cultivated agricultural regions heavy rains invariably wash a considerable amount of surface soil into the streams, greatlv increasing turbiditv ‘nd bacterial content. These streams, when turbid, carry three or four times as much iron as when clear. Most unland waters are sufficiently colorless for all ordinary uses, when the silt has b’-en removed bv coagulation arranged for that purpose alone Not so. however with the swamn pond and “branch” waters of the coastal nlain. Such waters invariable carry a comparatively large -mount of discolored vegetable matter and iron. This color ntion is more difficult to remove than either turbidity or bacteria. The limit of color nermissible in good drinking water should lvas in the average pure well water of North Carolina—equivalent to from seventy to ninety or more parts of platinum-cobalt per t.000,000 parts of water. Tn arranging for a public water suoply the amount of water required per head and the decennial in crease of population must be carefully considered and provided for The average number of gallons of water per head consumed bv tbirtv-six mediumsized English towns is 33 For similar sized Ger man towns the average dailv consumption is 28 cals, ner bead. A fair average for medium-sized American cities, excludin’* those notorious for reckless waste, is too gals, per head per day. Where water meters are generally used, and the consumption above a stated minimum icharged for by the gallon, the consumption tends to sink to less than 50 gals, per bead, without endangering either health or comfort of water consumers. The minimum of 50 gals, includes the average requirement of towns for manufacturing uses, street sprinkling, sewer flushing and fires. The reckless waste of water in many towns is to be checked only by meters, as in Atlanta, where these are commonly used, and the per-capita consumption is 38 gals. A necessary and important part of any public water plant is the clear-water basin or standpipe. This is intended to hold in reserve sufficient filtered water to provide for variation in rate of consumption and for emergencies such as a temporary stoppage of filter or the extra demand due to fire. Some water companies arrange to pump raw water directly into the mains in case of a serious fire requiring a large quantity of water. This proceeding is very dangerous and should never be permitted, since it is much easier to get pathogenic germs and troublesome alga? into the mains and j*rvice-pipes than to get rid of them. A disastrous epidemic might easily be caused by polluting the mains in this way. The reserve of filtered water should be ample to take care of any ordinary fire. Usually eight or ten hours’, supply at the usual rate of consumption is considered a sufficient reserve. Tt is not desirable to hold an unnecessarily large reserve, because filtered water deteriorates rapidly when stale. “The reserve basin or tank should be closely covered to exclude light, but must be ventilated in such a manner as to prevent the entrance of atmospheric dust as well as birds and insects. Stored filtered water when exposed to light and air soon becomes contaminated by air-borne spores of algae, molds and bacteria. These soon give the water a disagreeable smell and taste. Where for any reason the basin or tank cannot he covered, the best proven tative of algal growth is treatment of the water with sulphate of copper. I bis is the wellkpown bluestonc used in spraying orchards and gardens Use one part of copper sulphate per 1,000,000 parts w iter about 1 J/to oz. per 10,00) gals.

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