amount of pollution without the exhaustion of its oxygen, if bacterial life is of equal vigor in each case." Also the odors given off by putrefying sewage in salt water are greater than when in fresh.

Investigations of New York Harbor made by the Metropolitan Sewerage Commission showed that at all points at all distant from sewer outlets most of the sewage was either floating in the top 6 inches or had settled to the bottom; which indicates that surface area is more important than depth in salt-water dilution. From experiments conducted by the Metropolitan Sewerage Commission in 1896 in Boston Harbor, where sewage is stored and discharged on the ebb-tide and in addition about the same amount is discharged continuously, it was found that the area covered by a reservoir-discharge in three-quarters of an hour of 22,000,000 gallons is approximately 750 acres; but when but 11,000,000 gallons is discharged at once, this area is not more than 250 acres. In calm weather the sewage is offensively visible over two-thirds of this area, but the odors are confined to a relatively small portion. By far the greatest amount of sewage is found in the upper 2 or 3 inches of the polluted area, and this largely disappears in two or more hours after the discharge, depending chiefly upon the force of the waves. A thin film of grease some times covers large areas, but is not accompanied by enough sewage to be detected. Within the polluted area, sewage cannot be detected at a greater depth than 5 feet at the outlet and 2 feet near the edges of the area. When 35,cco,cco to 40,000,000 gallons daily was continuously discharged, the dilution was such that, fifteen minutes after leaving the outlet, sewage constituted Lut 20 per cent of the surface water; thirty minutes after, 15 per cent; forty-five minutes after, 5 per cent; and sixty minutes from the outlet but 4 per cent of the surface-water was sewage. The discoloration was evident for about 1 miles, and covered about 350 acres during ebb-tide and 300 acres during flood-tide. The observations indicate that, the greater the quantity of sewage that is discharged the greater is the area covered, the area increasing in more than direct proportion to the sewage discharged. This is additional reason for discharging at a number of outlets.

The two outlets in Boston offer an illustration of the comparative merits of continuous and ebb-tide discharge. "The great advantage of discharging sewage continuously and in comparatively small quantities into a large volume of tide water, as compared with discharging it in large quantities from reservoirs in a limited time, is well illustrated by a comparison of the conditions at the present outlets at Moon Island and at Deer Island. At Moon Island (discharge during two hours of ebb-tide) a large area is covered densely with sewage during a period of several hours at each tide, while at Deer Island (continuous discharge) the sewage flows in different directions at different parts of the day, covers a much smaller area and becomes more readily broken up and mingled with the sea water." (Report of Mass. State Board of Health on Boston Harbor.)

In the case of discharge into large lakes, there are no tides to assist in diffusion, and currents and winds are the chief agents. Sedimentation is less active than in salt water, and sewage pollution has often been traced for 5 to 10 miles from an outlet. As in the case of salt water, the distance reached by unoxidized sewage seems to increase more rapidly than the amount discharged. In general the same principles hold; that numerous outlets at some distance from shore are desirable to prevent a nuisance.

Organic matter in water forms the food of filth infusoria, hydra, rotifera, entomostracan crustacea, fresh-water shrimp, and the larvæ of a number of water insects. Entomostraca sem to be the most efficient in the purification of streams, and thrive on human excrement. A sewage-polluted river may contain 25 to 50 or more per gallon; but when the pollution becomes intense they seem to disappear, probably because of lack of oxygen, but their place is taken by larvæ. Diatcms, desmids, confervoid alga and other vegetable organisms, together with bacteria, act largely upon the dissolved impurities; although the last-named seem to attack organic matter also. These all serve as food for fish; and fish, in turn, for man; and sewage matter disposed of by dilution is therefore not wasted, although it does not serve as fertilizer for plant life.

Seaweed plays an important part in the purification of tidal water. The Royal Commission (England) found that the green seaweeds assimilate nitrogenous compounds such as ammonia and nitrates, and also evolve large quantities of oxygen. They are thus of great value in the purifying of sewage-laden waters. When thrown upon the shore by storms they give off, in decomposing, quantities of sulphuretted hydrogen, which can be avoided by gathering up, drying and burning them.

Certain fish eat organic matters in sewage when this is discharged fresh into running water, which water is not therefore deprived of oxygen at the sewer mouth. But as intermingling takes place and oxygen is taken up by the sewage, conditions become unfavorable to fish life; and few fish can live in highly polluted waters. This is generally because of lack of oxygen; but some trade wastes contain acids and others gelatinous or colloidal matters which collect around the gills and prevent breathing.

The U. S. Public Health Service in 1914 began exhaustive investigations into the pollution and self-purification of the Ohio river, and is still working upon it. Its conclusions up to September, 1915, were stated by Prof. Earle B. Fhelps, of the Hygienic Laboratory of the Service, in a paper before the American Society of Municipal Improvements. In substance these were: That reaëration of a polluted water plays a large part in self-purification. Average American sewage (assumed at 100 gallons per capita per day) has a biological oxygen demand of from 200 to 400 parts per million. Assuming a water at 68° F. and oxygen-saturated, and that 33 per cent of the oxygen is to be left in the water (an extreme minimum if nuisance is to be prevented), the 67 per cent of oxygen would be exhausted by the sewage if the dilution be 7.56 cubic feet per second per 1000 population (equivalent to about 5000 gals. per day per capita). If less dilution suffices, it must be because of continuous absorption of oxygen from the air by the sewage-laden water as rapidly as the sewage depletes it. Reaëration increases as depth of water decreases and as velocity of flow and degree of turbulence increase. Efforts are being made to determine in definite terms the effect of these and other conditions upon rate of reaëration.

CHAPTER XV

REMOVING SUSPENDED MATTER

ART. 71. GENERAL PRINCIPLES

THE methods of treating sewage may, in conformity with the ideas previously stated, be classified as those for effecting physical removal of suspended matter, chemical change in organic matter, physical removal of bacteria, destruction of bacteria by chemicals, and the biological destruction of both organic and bacterial matter.

Physical removal of suspended matter is effected by straining out the coarser matters by screens, by sedimentation, by surface adhesion, and by precipitating by adding a coagulant. Some chemical change generally accompanies coagulation; but such change is not the chief aim of any process which has been adopted in practice; although some have been suggested, such as oxida-. tion of organic matter by permanganates. There is a change which might perhaps be called structural rather than chemical, and which has been termed "modification" of organic matter, which renders this less subject to putrefaction, although little chemical change can be detected except by the most delicate tests. The change may make it possible to discharge the matter into a stream without creating a nuisance (other than such as would be created by an equal amount of surface soil from a field), where otherwise a nuisance would be inevitable.

More or less physical removal of bacteria is effected by any removal of suspended matter, since the bacteria exist largely in and on such matter. Filters also remove bacteria directly by surface adhesion, and partly by straining when covered with a dense "schmutzdecke." Sterilizing agents destroy the life of bacteria; and heat might theoretically be used for this purpose,

but no method has been devised for making this commercially practicable. Bacteria removed from their habitat or deprived of sustenance will in time die; although some spores can retain the germ of life indefinitely under adverse conditions. However, disease germs do not assume the spore condition.

In all lifeless organic matter there exist countless bacteria whose function it is to break down the organic structure and resolve the matter into simpler forms. One class changes the nitrogenous matter into readily oxidizable forms, and the resulting mineral compounds are the final stage of thorough purification; the next stage of which, in nature's cycle, would be their absorption by plant life as food. "Biological disposal " methods are efforts to intensify this action as to both time and space. Such destruction of objectionable bacteria as is effected by biological action is probably due largely to the creation of adverse conditions.

There is probably no method of sewage treatment in use to-day which does not combine two or more of the above processes. But each is best adapted to most economically or effectively maintain one of them, the others being in a measure incidental, or carried on uneconomically. The effort should be to determine just what method or combination of methods would most economically produce the desired results, consideration being had of local conditions and possibilities and the character of the sewage.

In studying the subject, thought must be given to the ultimate disposal of all the objectionable matter. Every method (except mere disinfection) results in an accumulation in the plant of more or less of the suspended matter, which is of varying degrees of offensiveness depending upon the process; and this must be in some way disposed of. Moreover, the oxidized organic matter (nitrates and nitrites) are rich plant food, and although they are harmless they may lead to an undesirable growth of vegetable matter in the water which receives them.

Certain methods and apparatus are best adapted to the coarse work of removal of gross suspended matters; others to the modification of organic matters; others to biological liquefaction,