There are two imperative essentials to sanitary sewerage: I. That all of the sewage be removed without delay to a point where it may be disposed of properly.

II. That it be so disposed of as to lose permanently its power for evil.

The removal of sewage is accomplished by means of systems of conduits called sewers. Such removal is called sewerage, and the system of conduits by which it is effected is called a sewerage system.

*

In some cases house sewage is removed in one set of sewers and storm water in another set, the former being called seperate sewers, and the latter storm sewers.* In other cases the same sewer receives both kinds of sewage, in which case the sewer is called a combined sewer and the mixture of house and storm sewage is called combined sewage. When only house sewage is flowing in a combined sewer, this is often referred to as the dry-weather flow.

Where a sewerage system consists of combined sewers only, it is known as a combined system; where the house sewage is removed by a system of separate sewers and the storm water is removed by another system of sewers (if removed in sewers at all), this is called a separate system of sewerage. In some cities the separate system is used in part of the area and the combined system in the remaining part and the sewerage system of such a city is called a mixed system.

After it has been removed to a greater or less distance, the sewage must be discharged into a stream, the ocean, or other large body of water. Such discharge is always necessary, because the enormous volumes of water flowing in sewerage systems cannot practically be destroyed or harmlessly dissipated.

In some cases it is necessary to partially purify or otherwise alter the nature of sewage before so discharging it, and this is called sewage treatment. Sewage disposal is the final getting

Some use the term "sanitary sewers" instead of "separate sewers." Others call these "sewers," and call storm sewers "drains"-a very confusing use of terms, since drainage is the removal of subsurface water, and the use of commercial "drain tile" for such sewers would not be permitted. The terms used here are those recommended by the American Society of Municipal Improvements.

rid of the sewage that reaches the outlet of the sewerage system, and may or may not include sewage treatment.

The prime object of a sewerage system is to conduct to an appointed outlet the sewage that is received at numerous points along its route, and this should be done as rapidly and continuously as possible. No part of the sewage should be retained in any portion of the system for any considerable time, either in its liquid form or in the shape of deposits upon the bottoms or walls of the conduits or their appurtenances, for such retention may permit of putrescence of the organic matter carried by the sewage before it reaches the place assigned for disposal.

ART. 2. ADVANTAGES OF SEWERAGE

The water-carriage system has now been adopted almost universally for the removal of excreta where this service is performed by the municipality, and its use is always understood when the term sewerage" is used. There are still a number of cities which are using, generally in limited areas in the outskirts, what is known as the "dry sewage" method, by which excreta are removed in pails or boxes, generally mixed with powdered soil or other absorbent. More common is the use of cesspools-pits into which excreta and polluted household waters are discharged. These should be cleaned out periodically, the material removed generally being called “night soil." Such cleaning is not ordinarily done by the municipality. Cesspools, unless watertight (which they seldom are), pollute the soil under and around dwellings. The material in them putrefies and gives off foul gases which, if house plumbing is connected with them, may enter such plumbing and pollute the air of the dwellings. Deposits of excreta on the surface of the ground in privies may pollute the soil and give off offensive odors; but the most serious objection is that flies have access to them and are known to have spread typhoid fever and possibly other diseases because of this; and the same danger is attached to cesspools or privy vaults that are not covered.

(The typhoid rate in certain sections of large cities has been reduced more than 50 per cent by sewering and abolishing surface privies and open vaults.) The use of the out-door privy is inconvenient and in stormy and cold weather even unhealthful. The removal of night-soil involves an expense which, if capitalized and added to the construction-cost of the cesspool, would in almost every case more than suffice for the construction and maintenance of a house sewerage system in a city or any well-populated community.

The water-carriage system meets the principal requirement of a sanitary system-it removes all the house wastes and removes them immediately. It also performs the by no means unimportant service of removing surface water and generally drains wet ground also. In convenience it is excelled by no other system. Where the territory is quite thickly populated, as in the average town, it is in the end cheaper than cesspools or any other method.

Arguments advanced against it are that a large amount of water is needed for its efficient working, that it pollutes streams, and that it causes the waste of the valuable fertilizing properties in the sewage. As to the first, the water entering a separate sewer consists almost exclusively of that which is used in the household, drawn from the water mains, and the amount so drawn need be only very slightly increased for any purposes connected with the sewerage. The matter of pollution of streams is a serious one, and in many cases large sums are spent in reducing such pollution to a minimum. The loss of fertilizing materials by discharging them into the streams is less important than was at one time believed, both because the value of such materials is less than was assumed and because the apparent loss is probably not ultimately a total one. The consensus of opinion is that the two latter objections are more than balanced by the advantages which it offers in comparison with any other method yet tried for the removal of excreta and liquid wastes. It cannot, of course, be stated with certainty that no better system will ever be devised, but up to the present time none has been proposed which promises

to prove anywhere near as satisfactory for the great majority of cities.

As to whether the combined or the separate system of sewerage shall be adopted in a given case, or even a mixed system, decision should be based on considerations to be discussed later. These generally have to do with relative cost of construction and efficiency of service rendered, relative importance of removal of house and of storm sewage, effect upon the body of water receiving the sewage, whether or not the sewage will be treated, and what existing sewers (if any) it is desirable to incorporate into the proposed system.

ART. 3. CONSTITUENTS OF A SEWERAGE SYSTEM

A modern sewerage system comprises several or all of the following:

A system of conduits or sewers, or several systems, all converging toward a common outlet. Each system of sewers consists of a main or trunk sewer, which receives the flow of several sub-main or branch sewers, which sub-mains in turn receive the flow from the lateral sewers, sometimes called "collecting sewers" (although, of course, the main and sub-main sewers also collect as well as carry the sewage brought to them by laterals). A system of sub-mains and laterals is sometimes called a reticulation system." A sewer that extends from the lower end of the collecting system to the point of final discharge is called an outfall sewer.

Manholes at intervals along each line of conduits. These are to give access to the sewer, and are generally in the form of a shaft or well extending vertically from the sewer to the ground surface, where it is provided with a removable cover.

Lampholes (now seldom used), or small shafts extending from sewer to ground surface, down which a lamp can be lowered to facilitate inspecting the sewer from a manhole.

Branches (called T-branches or Y-branches, or simply T's or Y's) built into the sewer by which to join to it house connections, inlet connections or other pipes.

House connections-pipes connecting individual buildings with the sewers.

Inlets openings in the street surface through which storm water enters the sewer system.

Inlet connections-the pipes connecting individual inlets with the sewer.

Flush tanks contrivances, generally placed at the upper ends of the collecting sewers and underground, by which water is discharged into the sewers for flushing out any deposits.

Pumps, ejectors," lifts"-appliances for raising the sewage from a low sewer to a higher one, an outlet, or a treatment plant.

Treatment plants, for removing or rendering less objectionable putrescible and other matters in sewage so that it can be discharged into a given body of water without so polluting it as to violate laws, either federal, state, or local, or produce obnoxious or dangerous conditions.

In addition, there may be many other features, such as appliances for forced ventilation of the sewers, tide gates at the outlets, overflows by which storm water may leave combined sewers for neighboring streams, etc., which will be referred to in the following chapters.

ART. 4. DESIGNING A SEWERAGE SYSTEM

The designing of a sewerage system involves a determination of where the sewage shall be discharged, whether it shall be treated and how and where, whether it shall be collected by the separate or the combined system, the proper size and shape of conduit to be used at each point, the location, depth, and grade at which it is to be laid, the materials of which it may be constructed; also what appurtenances (such as manholes, inlets, etc.) are necessary, and where, and their details.

The amount of polluting matter in the water which flows in sewers is so small that the laws of hydraulics apply to the flow of sewage in sewers, and consequently these laws are employed in designing sewerage systems.