without doubt resolve a considerable portion of it through the operations of the life process. The Entomostraca, Rotifera, and Infusoria are the principal agents assisting in such reduction. Of these the Entomostraca are probably the most important. This branch of our subject leads to biological studies of great interest and importance, but which for lack of space can not be more than referred to in this place.1

VALUE OF SEWAGE.

From an agricultural point of view the nitrogen, phosphoric acid, and potash are the most useful ingredients of sewage, and accordingly it becomes of interest to establish the quantity of these three elements which may be found in ordinary sewage. Taking that of about average composition,2 a net ton may be expected to contain nitrogen to the amount of from 0.15 to 0.25 pounds; phosphoric acid, from 0.045 to 0.065 pounds; and potash, from 0.025 to 0.040 pounds. With nitrogen at 17 cents per pound, phosphoric acid at 7 cents, and potash at 5 cents, the theoretical value of the fertilizing ingredients of such a sewage would be, per net ton, from about 3.5 cents to 4.5 or 5 cents. Taking into account, however, the various losses of the nitrogen, which is not only the most valuable but also the least stable element, as well as the expense of distribution, we may conclude that the manurial constituents of sewage have an actual value, when applied to good advantage in agricultural utilization, of from 1 to 2 cents or perhaps 3 cents per ton. We should note that this is the manurial value only.

Independent of the manure, the water of sewage has also a distinct value for irrigation. But by reason of the variation in local conditions, it is impossible to make any general statement of value which will apply to all cases, although in a general way we may say, taking into account the manurial constituents as well as the irrigation value of the water, that sewage, when applied to land at the best advantage, may be considered, with the present understanding of things, as worth from 2 to 4 cents per net ton. In some cases, by reason of its value for irrigation, it may be worth several times these figures.

We learn, then, that the irrigation value of sewage may be quite as important as the distinctively manurial value, and it is believed that the recognition of this factor has placed sewage utilization on a somewhat different plane from that formerly occupied. In order to illustrate this point let us consider the matter briefly from an historical point of view.

When sewage disposal as a necessary concomitant of the sewerage

1 For an extended presentation of this phase of the subject see discussion, by the author, of Dr. Charles G. Currier's paper on Self-purification of flowing water and the influence of polluted water in the causation of disease: Trans. Am. Soc. Civil Eng., Vol. XXIV, p. 70.

See extended tabulations from Wolff and Lehmann in the first report of the Rivers Pollution Commission, p. 27. Also see Storer's Agriculture, Vol. II, p. 70.

of cities first grew up in England very extravagant views were entertained as to the commercial benefits to be derived from sewage utilization. Many hundred patents were taken out, mostly, however, on chemical processes, and large investments of capital were made, which have generally proved failures. Following these failures there was a reaction, during the prevalence of which it was held that sewage could not be utilized at a profit. When we examine the whole matter practically we find that the failure, in a commercial way, largely pertained to the chemical processes, although it should not be overlooked that many of the early sewage farms, by reason of poor management, unnecessarily expensive first cost, and other causes, have never been financially successful. We shall also see, farther on, that the best method of managing such farms has only recently been understood, which may be given as another reason why many of the early farms have not been financially successful. The net result of all this has been that many experienced engineers, sanitarians, and agriculturists have held that it was impossible to utilize sewage at a profit.

There is, however, another phase of the question. The main object of sewage purification is to keep streams pure and to preserve the health of the citizens of our cities and of the surrounding country. It has therefore been held, and very properly, that the real object of sewage treatment is purification and not utilization, and that utilization, by introducing commercial considerations, will inevitably tend to lower the degree of purification. It has been held, in short, that sewage purification is a right which one community or individual owes to another, independent of any question of commercial profit. And while this proposition is undoubtedly true, it is believed that, with proper understanding of all the elements of the problem, a satisfactory utilization may be also attained without lowering the standard of purification.

On this point the author's views have undergone some modification since he examined in detail a number of European sewage farms in the fall of 1894. Previous to that time he was disposed to believe that sewage disposal should be placed entirely on the broad plane of purification rather than on that of utilization. At the present time he believes, as the result of seeing what is being done on the best sewage farms of England, Germany, and France, that the agricultural value of sewage may be fairly utilized, and still a high degree of purification attained.

Since the first development of sewage utilization in England the trade in commercial fertilizers has also greatly extended, and many writers have taken the ground that at the prevailing prices of commercial fertilizers agricultural lands can be manured more cheaply by their use than by the use of raw sewage.1 In regions where the distribution of the rainfall is such as to fairly meet the necessities of

See Agriculture in some of its Relations with Chemistry, by F. H. Storer, Vol. II, on this point.

agriculture this is probably true, but it is believed, with the present experience, that in any region where the distribution of the rainfall is such that periods of drought are likely to occur at the critical period of growing crops, provided sewage can be delivered upon agricultural lands by gravity, or even by a moderate pumping lift, a commercial saving will be effected, due to the irrigation value of the sewage, over any gain that can possibly be obtained by the use of commercial fertilizers alone. If pumping is required, its cost may, of course, enter in as a modifying element. At any rate the author wishes to place this thought before the scientific farmers of the United States as one well worthy of their most careful consideration. He desires further to say that from the experience gained in other countries, as well as our own, he believes that frequently lands in the immediate vicinity of our cities and towns can be improved in productiveness more by the general application of sewage irrigation than by any other method at present open to our farmers.

It should be remembered, however, that the quality of soils will enter into the final solution of the problem. Those best suited for sewage irrigation are open, porous, gravelly soils, while heavy clay soils may demand so great an expense of preparation as to render sewage utilization, except in special cases, practically impossible. As already hinted, each location presents its own special problem, which, to some extent, will demand its own special solution independent of all other cases. It can not be too strongly insisted, therefore, that each case must be studied by itself on its own merits; hence the foregoing statements are to be taken as general statements rather than universally true propositions.

METHODS OF SEWAGE DISPOSAL.

As the result of a large amount of experience gained abroad, sewage disposal or purification has resolved itself into three general methods, which are known as (1) chemical precipitation, (2) intermittent filtration, and (3) broad irrigation. In the present paper we are not specially concerned with chemical treatment, except as at times an adjunct of irrigation; nor are we concerned with intermittent filtration, except so far as it may be considered an adjunct of irrigation. For completion of the subject, however, we may briefly describe the process of treatment known as chemical precipitation.1

CHEMICAL PRECIPITATION.

In this process the sewage is allowed to flow into large tanks, in which it is dosed with certain chemicals; these form with the organic constituents an insoluble precipitate, which in its descent to the bottom of the tank may, under favorable circumstances, carry down with

1 More complete descriptions may be found in the several standard treatises recently issued, as, for instance, Crimp's Sewage Disposal, Wardle's Sewage Treatment and Disposal, and Rafter and Baker's Sewage Disposal in the United States.

it the suspended matter of the sewage as well as a portion of the dissolved. To apply this treatment on a large scale, extensive works with a large number of tanks, together with machines for grinding and mixing the chemicals, as well as special mechanical arrangements for mixing the chemicals and sewage and caring for the sludge, are required, the whole including what is commonly called the chemical treatment of sewage, although the complete process is in reality partly chemical and partly mechanical.

The reagents now generally used are common lime, sulphate of alumina, and ferrous sulphite. These reagents are used either singly or in combination, as may be required to fit the case of each particular sewage undergoing treatment. The action of the reagents in producing a precipitation of the organic matter is not fully understood, although in a general way we may say that when lime is used there

FIG. 1.-Mystic Valley Chemical Purification Works, showing sludge beds and effluent channel. is a combination of some of the lime with free carbonic acid gas to form an insoluble carbonate of lime; also, probably, an additional part of the lime combines with a certain portion of the organic matters in solution to form an insoluble precipitate, which in its journey to the bottom carries down with it any portion of the suspended matters which have not entered into combination. The matter settling to the bottom is called sludge.

When sulphate of alumina is used, the precipitating effect is considered as due to a combination of the sulphuric acid of the sulphate of alumina with lime and other mineral bases existing naturally in the sewage. A flocculent alumina hydrate is also formed, which in conjunction with the mineral precipitate further entangles and carries down any suspended organic matter. At the present time, as a general statement, we may say that a combination treatment of lime and

sulphate of alumina is preferable for ordinary sewage to the use of either of them alone, although the composition of the sewage should be taken into account in deciding.

In the case of ferrous sulphate, in order to secure a precipitating action it is necessary either that the sewage be naturally alkaline or, if not naturally so, that an alkali be artificially added. The result of this treatment is the formation of a flocculent hydrated oxide which carries down with it the suspended organic matter as well as a portion of the dissolved.

In order to insure the best results in chemical treatment the sewage should be treated while fresh and the chemicals added to the flowing sewage, in order that they may become fairly incorporated before it passes into the settling tanks. There should also be enough tank space to insure a thorough precipitation. Inasmuch as the sludge must be frequently removed from the bottom of the tanks, the mechanical arrangements should be such as to permit of its removal without interruption of the works.

Methods of chemical treatment may be classified as (1) intermittent treatment in shallow tanks from 5 to 8 feet deep, in which, after the addition and incorporation of the chemicals, the sewage is allowed to remain undisturbed until the completion of the process, when the clarified liquid is drawn off the top, leaving the sludge at the bottom; (2) continuous treatment in a similar series of tanks through which, after the addition and incorporation of the reagents, the sewage is allowed to flow slowly, crude sewage with freshly added chemicals passing in at one end and purified effluent passing out at the other; and (3) vertical tanks through which, after the addition of the chemicals, the sewage rises slowly. At the present time the continuous treatment, in which crude sewage with freshly added chemicals passes into one end and purified effluent passes out at the other continuously, is considered, as the result of experience, to be the preferable method of applying the chemical treatment.

As to the tank capacity required, we may say that in systems which are arranged with reference to receiving a portion of the rainfall the daily capacity should be nearly 50 per cent of the average daily flow, an allowance of this kind giving some leeway for contingencies when required. With the sewage from separate systems of sewers less leeway will be required.

Various methods of disposing of sludge have been used. One is to pump it into basins, from which it is subsequently conveyed to adjacent areas for utilization as fertilizer. It is also frequently deposited in large open basins surrounded by embankments, from which, after the larger portion of the water has drained away, it is removed either for use as a fertilizer or to some other point for filling in low land, etc. The liquid sludge is also sometimes run directly onto agricultural areas, where it is easily disposed of by plowing in. It may also be mixed with combustibles and disposed of by burning. In some cases