The organic and mineral matter in suspension and solution in the water before the addition of sewage matters will of course be included in that found in the resultant sewage, and it is desirable to learn what this amount is. The Naugatuck River at Union City, Conn., contained, as extremes, in September, 1897, 60.5 parts per 1,000,000 of mineral and 22.0 of organic matter, 20.0 parts being lime and 4.2 chlorine; and in April, 1896, but 16.0 of mineral and 15.5 of organic matter; these being fairly average results for New England in a thickly populated district.

The above illustrates in a general way the constitution of sewage; but to understand the methods and processes involved in the purification of sewage, it is necessary to study the chemical conditions and forms in which these matters exist in sewage, as well as those in which they generally appear in chemical analyses. Average American sewage contains about 400 to 700 parts per million of solids when the water consumption is 60 to 80 gallons per capita. Of these about 250 to 450 will be in suspension and the remainder in solution. The older the sewage and the more it has been agitated, the greater will be the proportion of solid matter in solution. Of those in suspension 100 to 150 parts are mineral and 150 to 300 are organic; of those in solution 80 to 150 are mineral, 70 to 100 are organic. Owing to causes already mentioned, as well as to the great variations in per capita water consumption in different places, any individual sewage may vary greatly from the above figures; but they serve to give a general idea of the relative proportions. The average amounts of these constituents per capita in a number of American cities is given in Table No. 22.

The above figures are from New England cities. In cities where the water consumption is much greater the sewage will be proportionately weaker. In comparatively strong sewage the amount of solid and organic matter is fully twice as great in the day flow as in the night flow.

The proportions of the various constituents are stated by some chemists in parts per hundred thousand; but the standard method at present is in parts per million, or, which is practically

the same thing, in milligrams per liter. In this work, parts per million will be used unless otherwise stated.

About 40 ounces per day of human urine is excreted per capita, on an average, and 3 ounces of wet fæces (see Table No. 21). Of the urine, about 0.337 grain is common salt, o.2 being chlorine. In the excrements occurs the great bulk of the nitrogen found in sewage, mostly as albuminous compounds. This leaves the body NH2 in the form of urea, of which the composition is CO

NH2

It

is quickly attacked by either the bacillus ureæ or micrococcus ureæ, or both. Each of these, breaking down the urea, convert it into carbonate of ammonia thus:

"If the sewage is kept without undergoing purification for a day or so, it undergoes putrefaction and begins to give off foul emanations; in the course of two or three days the albuminous matters begin to split up, and the sewage, particularly when the water contains sulphates, yields sulphuretted hydrogen, which is known by its characteristic odor of rotten eggs. When this gas is formed the sewage becomes black. As the above changes take place, more and more of the solid matter enters into solution." (Barwise, "Purification of Sewage.")

Vegetable refuse occasions much of the foulness of stale sewage, largely because of the sulphur it contains. Putrefaction is preceded by the combination of part of the nitrogen and carbon with all the free oxygen and with part of that contained in the nitratcs.

It is evident that the form under which the nitrogen is found will depend to a considerable degree upon the amount of decomposition which the organic matter has undergone. This decomposition is facilitated by comminution of the particles in suspension, such as occurs in pumping, and increases with time, and its character is determined by the amount of oxygen contained in the sewage water. In a short time after entering the sewers,

sewage ordinarily contains very little dissolved oxygen and nitrogen in the form of nitrates; although when fresh it contains some free oxygen and generally nitrates and nitrites.

Sewage contains countless numbers of bacteria of many varieties, as many as 30,000,000 in a cubic centimeter having been estimated, of 200 or more varieties. One of the most important is the Bacillus coli communis, which originates in the animal intestine. Most of these bacteria are harmless; many are beneficial in breaking down complex organic compounds and assisting in the oxidation of the sewage; but a few are the cause of disease if taken into the human system. Among the last are the bacterium of cholera (Spirillum cholera asiatica) and that of typhoid fever (Bacillus typhosus). B. coli communis and B. enteritidis sporogenes are the bacteria most easily identified as directly derived from excrement. The former is most abundant in sewage-polluted water; the latter is not so abundant, but is much more probably pathogenic, being a possible cause of acute diarrhoea. There are also present in sewage large numbers of enzymes, lifeless organic substances which exert chemical action in breaking down complicated organic molecules. Such are pepsin, pancreatin, and other digestive ferments. Their mode of action is not well understood, but they play a very important part in sewage treatment.

ART. 67. ANALYSES OF SEWAGE AND EFFLUENTS

In the majority of cases the purpose of analyzing sewage is to determine the amount and condition of the putrescible matter therein and of other substances that may affect purification processes; and the analyses of sewage effluents are for the purpose of determining the degree to which putrescible matter and bacteria have been removed and the amount of these that remain in the effluent.

If sewage be heated in a platinum dish until evaporated, a solid residue is left consisting of mineral and organic matter. If this be heated to a low red heat, the organic matter will be almost entirely burned off while the mineral will be changed little

if at all. The loss in weight caused by burning will be the amount of organic matter in the quantity of sewage analyzed. The amount after evaporation is called "total solids" or or "residue on evaporation "; the part removed by burning is called "loss by ignition or "volatile or "organic residue," and the part remaining after burning is the "fixed solids" or "mineral residue." If a sample of raw sewage be filtered through a Gooch crucible, the matter in suspension will be intercepted, and the difference between this and the total solids determined as above will be the amount of solids in solution. If the suspended solids be burned as above, thus removing the organic matter, the amount remaining will be the fixed solids in suspension, and the difference between that and the total fixed solids will be the fixed solids in the solution.*

As organic matter decays, it gives off carbonic acid, part of which remains in the solution and part escapes. Ammonia also results from the decay and is taken into solution. Other organic matter which is about ready to decay gives up ammonia if the sewage be boiled. The ammonia in solution and that set free by boiling are known as "free ammonia," which, being the product of decay, is the most characteristic ingredient of stale sewage. "Free ammonia " is not chemically free but is generally in combination with carbonic and organic acids, or even appears as chloride or sulphate of ammonia.

In addition to the nitrogen contained in the ammonia, there is a quantity of combined nitrogen in the organic matter called organic nitrogen," a part of which can be made to pass off as ammonia by use of permanganate of potash. The ammonia thus obtained is called "albuminoid ammonia." Albuminoid ammonia is being constantly changed by decomposition into free ammonia, and hence the older the sewage is the greater the proportion of free to albuminoid ammonia. When comparing two samples of sewage by their ammonias, we must remember that free ammonia is largely the result of decomposition of that previously, but not now, existing as organic matter. A number

* For laboratory methods of making these and the other determinations described on the following pages, see Appendix, "Testing Sewage and Effluents."