Many other arguments have been advanced on both sides, but the most weighty in favor of the Combined system are: its economy in first cost over two Separate systems, and the ease with which obstructions can be removed and a general examination of its contents made; in favor of the Separate system: its being self-cleansing, its adaptability, as a housesewage system only, to small and poor towns, and its necessity to an economical sanitary treatment of the house-sewage.

ART. 7. SUMMARY.

The proper conclusion in reference to the system to be adopted would seem to be-the water-carriage, where its expense is not prohibitive and the dwellings are abundantly supplied with water. In a few exceptional cases a Pneumatic system might be preferable. But better than the cesspool or privy, if the cost or the water-supply is peremptorily limited,

would be a dry-sewage system-preferably the dry-earth. The last is described to a sufficient length in this chapter, as the proper conduct of it requires little else than cleanliness. and faithful attention. The disposal of sewage thus collected will, however, be referred to in Chapter II.

The water-carriage system is more complicated in design, in construction, and in operation; and to the consideration of this system the remainder of this work will be devoted.

NOTE. The general adoption of the septic tank (see Art. 98), which has been called the "glorified cesspool," can not properly be used as an excuse for the cesspool. In reality the two differ in every essential. In no satisfactory septic tank does the sewage remain longer than twenty-four or at most forty-eight hours. Even then there are given off large quantities of gases which no one would think of piping into his house, as is practically done from most cesspools. A comparison of cesspool with septic tank does not touch upon the objection to the former that its use scatters a large number of centres of soil-pollution throughout a closely populated area.

CHAPTER II.

DISPOSAL BY DILUTION.

ART. 8. "DISPOSAL AND "SEWAGE" DEFINED.

The word disposal is often used where treatment would be more properly employed. As a matter of fact all sewage, dry or water-carried, must be disposed of in some way after having been collected by a sewerage system. But if this disposal consists of anything other than throwing away the sewage this may be properly called a treatment thereof. These words will be thus used in this work-disposal as a general term, treatment as a more specific one.

For a proper consideration of the various methods of disposal it will be necessary to understand the results aimed at and the principles involved. And first we must understand what is implied by the word sewage. In the dry sewage and pneumatic systems it means human excreta and nothing else. In the water-carriage system, however, sewage may be found to contain almost every description of waste matter: fæces, house-" slops," manufacturing waste-waters and acids, drainage of stables, piggeries, and slaughter-houses, waste paper and rags, and frequently "swill," and numberless matters which should never reach the sewer. This is ordinarily called housesewage. Into combined and storm sewers, besides rainwater, not only horse-droppings and vegetable refuse but sand, clay, gravel, and other heavy matters find admission through the street-inlets. These go to make what is called storm-sewage. The common impression is that of these the human excrements alone are dangerous; and this is to a large

extent true so far as concerns dissemination of the germs of disease. But it is known that, aside from this, kitchen-wastes are fully as objectionable, since they contain practically the same putrescible matter, and in a state less easily rendered innocuous by either natural or artificial means. Where stormwater is admitted to the sewers the large quantities of horsedroppings which are washed in during the first few minutes of each rainstorm render the water nearly as offensive, if not so dangerous, as do human excreta.

Owing to diversity of manufacturing industries, to differences in the characters of the water used by different towns, and to other local peculiarities the sewage of each town varies from that of almost every other. Therefore the question of the proper disposal of this compound is seen to be a problem of no easy solution. The difficulty of treatment is increased by the exceeding dilution of the sewage, since the sewage of an average American town will contain but about I part in 1000 of organic matter, I part of mineral matter, and 998 parts of water.

Difficulty of disposal is frequently considered to be con nected with house-sewage only. But if the separate system be used it is generally desirable to connect with the housesewers, cab-stands, market-places, and other parts of streets. liable to collect considerable filth, small inlets being used, so that only a small amount of water from any storm can enter them, or else special traps, ordinarily closed, but through which the filth can be washed by hose.

ART. 9. AIMS OF DISPOSAL.

The first aim is the getting rid of the sewage; the disposing of it in such a way and such a place that it will not create a nuisance. Communities, being even more selfish than indi

viduals, seldom regard the well-being of other communities, but are satisfied if no nuisance is created within their own limits. It is here that the State, by its laws and through its Board of Heaith, should interfere for the protection of each community against all others. In England this protection is afforded by national laws and a national board. In this country many States afford a certain amount of such protection, varying from that given by the excellent laws of Massachusetts down to the almost total lack of any such protection which exists in many of even the older States. It is a duty which the engineer owes to humanity to educate the people to the importance of this matter; though he will often be compelled to yield, in part at least, to the selfish demands of those for whom he acts, that they be put to no expense for protection of other communities not required by State or national laws.

Where this protection is afforded through adequate laws properly enforced the disposal of the sewage must be such that it will lose permanently its power for evil." How this can best and most economically be done is the question to be solved.

Many attempts have been made at a solution of this question of disposal which shall not only meet the sanitary requirements, but which shall also be financially remunerative. Some reports of success have been heard of, but when investigated the details are found to be disappointing. An English company which used a method of Chemical Precipitation was reported as paying dividends, but inquiry showed that these were but a part of the sum paid to the company by the district for disposing of its sewage, and the taxpayers were but little benefited in pocket by the method employed. Investigations of other cases have resulted somewhat similarly. The author knows of no case where the disposal of sewage is accomplished at a profit to the city or town. In the case of