Before taking up this subject in detail, it will be well to consider what sewage is, from a chemical standpoint.

When fresh, sewage appears at the mouth of an outlet sewer as a milky-looking liquid with some large particles of matter in suspension, such as orange peels, rags, paper, and various other articles not easily broken up. It often has a faint, musty odor, and in general appearance is similar to the suds-water from a family laundry. Nearly all of the sewage is water, the total amount of solid matter not being more than 2 parts in 1,000, of which half may be organic matter. It is this 1 part in 1,000 which should be removed, or so changed in character as to render it harmless.

The systems of purification now in most common use are the septic treatment already described, chemical precipitation, and the land treatment. Mechanical straininj, sedimentation, and chemical precipitation are largely removal processes; while land treatment, by the slow process of infiltration or irrigation, changes the decaying organic matter into stable mineral compounds.

Sedimentation. This is effected by allowing the suspended matter to settle in tanks. The partially clarified liquid is then drawn off, leaving the solid matter, called sludge, at the bottom for later disposal. This system requires a good deal of time and large settling-tanks, and until recently has been considered suitable only for small quantities of sewage.

Mechanical Straining. This is accomplished in different ways, with varying degrees of success. Wire screens or filters of various materials may be employed. Straining of itself is of little value except as a step to further purification. Beds of coke from six to eight inches in depth are often used with good results.

Chemical Precipitation. Sedimentation alone removes only such suspended matter as will sink by its own weight during the comparatively short time which can be allowed for the process. By adding certain substances, chemical action is set up, which greatly increases the rapidity with which precipitation takes place. Some of the organic substances are brought together by the formation of new compounds; and, as they fall in flaky masses, they carry with them other suspended matter.

A great number and variety of chemicals have been employed for this purpose; but those which experience has shown to be most useful are lime, sulphate of alumina, and some of the salts of iron. The best chemical to use in any given case depends upon the character of the sewage, and on relative cost in the particular locality where it is to be used. Lime is cheap, but the large quantity required greatly increases the amount of sludge. Sulphate of alumina is more expensive, but it is often used to advantage in connection with lime. Where an acid sewage is to be treated, lime alone should be used.

The chemicals should be added to the sewage, and thoroughly mixed, before it reaches the settling-tank; this may be effected by the use of projections or baffling-plates placed in the conduits leading to the tank. The best results are obtained by means of long, narrow tanks; and they should be operated on the continuous rather than the intermittent plan. The width of the tank should be about one-fourth its length. In the continuous method, the sewage is constantly flowing into one part of the tank and discharging from another, In the intermittent system, a tank is filled and then the flow is turned into another, allowing the sewage in the first tank to come to rest. In the continuous plan, the sewage generally flows through a set of tanks without interruption until one of the compartments needs cleaning. The clear portion is drawn off from the top, the sludge is then removed, and the tank thoroughly disinfected before being put in use again.

The satisfactory disposal of the sludge is a somewhat difficult problem. The most common method is to press it into cakes, which greatly reduces its bulk and makes it more easily handled. These are sometimes burned, but are more often used for fertilizing purposes. In some cases, peat or some other absorbent is mixed with the sludge, and the whole mass removed in bulk. In other instances, the sludge is run out on the surface of coarse gravel beds, and reduced by draining and drying. In wet weather, little drying takes place; and during the cold months, the sludge accumulates in considerable quantities. This process also requires much manual labor, and in many cases suitable land is not available for the purpose. The required capacity of the settling-tanks is the principal item in determining the cost of installing precipitation works.

In the treatment of house sewage, provision must be made for about 1/12 the total daily flow; and in addition to this, allowance must be made for throwing out a portion of the tanks for cleaning and repairs. In general, the tank capacity should not be much less than 1/8 the total daily flow.

In the combined system, it is impossible to provide tanks for the total amount; and the excess due to storm water must discharge into natural watercourses or pass by the works without treatment.

Broad Irrigation or Sewage Farming. Where sewage is applied to the surface of the ground upon which crops are raised, the process is called sewage farming. This varies but little from ordinary irrigation, where clean water is used instead of sewage. The land employed for this purpose should have a rather light and porous soil, and the crops should be such as require a large amount of moisture. The application of from 5,000 to 10,000 gallons of sewage per day per acre is considered a liberal allowance. On the basis of 100 gallons of sewage per head of population, this would mean that one acre would care for a population of from 50 to 100 people.

Sub-Surface Irrigation. This system is employed, as already described, only upon a small scale, and chiefly for private dwellings, public institutions, and small communities where for any reason surface disposal would be objectionable. The sewage is distributed through agricultural drain tiles laid with open joints and placed only a few inches below the surface. Provision should be made for changing the disposal area as often as the soil may require, by turning the sewage into other subdivisions of the distributing pipes.

Intermittent Filtration. This method, and the broad irrigation already described, are the principal purification processes - not considering the septic method - in use on a large scale, which can remove practically all the organic matter from sewage without being supplemented by some other method. The process is a simple one, and consists in running the sewage out through distributing pipes on beds of sand 4 or 5 feet in thickness, with a system of pipes or drains below for collecting the purified liquid. In operation, the sewage is turned first on one bed and then on another, thus allowing an opportunity for the liquid portion to filter through. As the surface becomes clogged, it is raked over, or the sludge may be scraped off together with a thin layer of sand. The best filtering material consists of a clean, sharp sand with grains of uniform size, such that the free space between them will equal about one-third the total volume. When the sewage is admitted to the sand, only a part of the air is driven out, so that there is a, store of oxygen left, upon which the bacteria may draw. This is not a mere process of straining, but the formation of new compounds by the action of the oxygen in the air, thus changing the organic matter into inorganic. Much depends upon the size and quality of the sand used.

The work done by a filter is largely determined by the finer particles of sand, and that used should be of fairly uniform quality, and the coarser and finer particles should be well sized. The area and volume of sand or gravel required are so large that the transportation of material any great distance is out of the question. Usually the beds are constructed on natural deposits, the top soil or loam being removed. The sewage should be brought into the beds so as to disturb their surface as little as possible, and should be distributed evenly over the whole bed.

The underdrains should not be placed more than 50 feet apart, usually much less, and should be provided with manholes at the junctions of the pipes. Before admitting the sewage to the beds, it is usually best to screen it sufficiently to take out paper, rags, and other floating matter. The size and slope of each bed should be such as to permit an even distribution of sewage over its surface.

Where the filtration area is small, it must be divided so as to permit of intermittent operation; that is, if a bed is to be in use and at rest for equal periods, then two or more beds will be necessary, the number depending on the relative periods of use and rest. Some additional area should also be provided for emergency, or for use while the beds are being scraped. If a large area is laid out, so that the size of the beds is limited only by convenience in use, then an acre may be taken as a good size.

The degree of purification depends upon various circumstances; but with the best material, practically all of the organic matter can be removed from sewage by intermittent filtration, at a rate of about 100,000 gallons per day.

There is often much opposition to sewage purification by those living or owning property near the plants; but experience has shown that well-conducted plants are inoffensive, both within and without their enclosures. The employees about such works are as healthy as similar classes of men in other occupations. The crops raised on sewage farms are as healthful as those of the same kind raised elsewhere; and meat and milk from sewage farms are usually as good as when produced under other conditions. Good design and construction, followed by proper methods of operation, are all that are needed to make sewage purification a success. No one system can be said to be the best for all localities. The special problems of each case must be met and solved by a selection from among the several systems and combinations of systems, and parts chosen that are best adapted to the conditions at hand.

Where sewerage and storm water are carried in one system of pipes or conduits, rain-water leaders may ventilate the drain more or less. Trapping a leader drain is often unnecessary, if it opens above the highest windows. Porch and veranda roof drains having windows above them may require trapping, but it should be done in a way to insure the maintenance of the water-seal of the trap. As these drains are small, merely connecting to the main drain inside (on the house side), the main intercepting trap may be deemed sufficient unless closure by hoar frost is likely. Other pipes so connected being higher, the chances are that air will enter the open end to supply a current up the taller lines; and if this is not the case, dilution of what air may be thus brought into the open will render its danger of little consequence. Care must be taken in the design of a system of plumbing, that leader traps are not omitted where such omission would result in the weakening of the flow of air through the principal vent pipes.