There are numerous advocates of both systems when discussing them in the abstract. In the adaptability for a city possessing almost 1,300 miles of sewers, for the most part built upon the combined system, the discussion must take into account this important factor. The introduction of the separate system undoubtedly would have an advantage over the combined system in the matter of collecting the sewage at a more uniform rate and less in amount, would exclude all sewage, even at times of storm, from the water courses, and would lessen the load upon treatment works and prevent possible detriment to their operation due to absence of grit or changing sewage characteristics.

[33] On the other hand, the utilization of the present combined sewers and the extension thereof upon the same plans have decided advantages over the separate system that will outweigh advantages claimed for the former system, when applied to the local case.

They may be stated as follows:

• The lifts and consequent pumping costs will be less.
• As the first flush of rainfall contains the deposits on the sides of the sewers and the filth from the street surface, it is generally more polluting than the ordinary dry weather flow of sewage. When the combined system of sewers is used this is intercepted and conveyed to the treatment works. Occasionally the rate of rainfall is sufficient to cause overflow at the point .of interception but the sewage is diluted with rain water to such an extent that the polluting effect upon the water course is no greater than that from the normal runoff from fields and streets.
• The increased fluctuation in flow to the works by the use of the combined system is compensated for by decrease in concentration of the sewage; therefore, it would be susceptible of passage through the works at greater speed.
• While the separate system excludes grit, the cost of grit removal at the works necessitated by the combined system is slight as compared with the amount saved in the first cost of the combined over the separate system.
• The occupation of less space in the streets, by the single conduit of the combined system instead of the two of the separate system and by one set instead of two sets of lateral house connections, with consequent lessening of interference with the sub-surface structures.
• [34] The saving to the property owners alone of approximately $25,000,000, by the use of the combined system, obviating the cost of rearranging plumbing systems to provide double connections required by the separate system. The calculation is based upon 250,000 buildings at $100 per building.
• The utilizing of the present combined system and the consequent saving of the cost of the installation of an additional sewer and possible reconstruction of the existing combined sewer, the length of such sewers approximating 1,300 miles. In addition there would be saved the cost of repaving the streets, all of which work would be attended with inconvenience to traffic and citizens.
• After an examination of the collecting systems of the principal cities of Europe, there is found that in no case has the existing combined sewer system been replaced by a separate system, but, on the contrary, in some cases the existing separate system has been replaced by the combined system in urban areas.
• The difficulty of policing the connections in a double system, involving improper connections, does not guarantee the exclusion of pathogenic germs from the streams. On the other hand, with a single house connection used in the combined system policing is not necessary, and all of the dry weather flow is sure of delivery at the works for treatment.

After observation of the working of the two systems in foreign cities, where the consensus of opinion is given as against the employment of the separate system in closely built-up urban territory, except in the low-Eying areas subject to river overflow, or for the protection of a water supply, and after extensive experience in this City, it has been [35] deemed advisable to adopt the combined system, or the second proposition mentioned at the head of the chapter, for, in addition to avoiding all of the disadvantages of the first, it utilizes the present sewer system of the City with only minor changes at the points of connection with collecting sewers; the first flush, or as much as may be desired, of rain water can be kept out of the water courses and carried to the treatment works; and the protection of the water courses can be accomplished in much shorter time, due to the fact that as fast as the collecting sewers are built the sewage from each drainage area intercepted is collected without the delay of resewering the entire drainage area, and altering and reconnecting all house drains.

Exceptions are of necessity made to the above policy in low lying areas or where required for the protection of a water supply where the separate system will be installed, described in detail at a proper place.

Design of Collectors

As a detail of the policy to be pursued in the design of collecting sewers it may be stated that a principle will be applied which is a radical departure from the design of similar structures heretofore in use in American cities. The design of these structures where such collecting sewers have been in operation for a period of years has been worked out in a scientific manner in a number of European cities and the eminently satisfactory results upon the water courses have been observed. The conditions for constructing collectors abroad were ideal as their grades were made to control and not to be controlled by the grades and therefore the elevations of the tributary sewers at their confluence. In this city, however, the problem as to the design of collectors is more complex since the main tributary sewers [36] have been built with outlets at various elevations, and since it is advisable to utilize in its entirety this system of sewers.

The principle in use abroad may be adopted at the expense of some increase in depth of the collectors. The conditions indicated will admit of placing the invert of a collecting sewer at its up-river end slightly lower than the elevation of the invert of the tributary combined sewer and of carrying the collector through a dam to be used for diverting to the collector the dry weather flow and for excluding tide-water, located at a point in the tributary sewer where the crest of said dam shall not be higher than two-thirds the vertical diameter of the tributary sewer. The declining grade of the collector will carry it at an increasingly lower elevation across the succeeding intercepted tributary sewers, thus admitting of the lowering of the dams progressively until the collector will be completely beneath the inverts of the remaining tributary sewers. The restricted water area in the tributary sewer at a dam would be compensated for by enlarging the section of the sewer at this point sufficiently to accommodate the excess storm flow beyond that which would be taken into the collector and allow the passage of this excess quantity over the dam and into the river. The principle involves the placing of a separate sewer system over low-lying areas subject to tidal inflow between the dam and the river bank.

The principle is adopted for the reason that by the raising of the collecting sewers in ground which is water bearing of from 5 to 8 feet and in some instances 10 feet above the profile which must be followed in case the usual American practice is adopted, reduces the first cost of such construction by a considerable amount. If placed several blocks back from the river, it will become shallower on account of the ascending grade upon the main sewers and will admit of the house drainage from the river front being [37] carried backward into the main interceptors, and reduces to a minimum the area that would be required to be sewered upon the separate or double system. The application of this principle will proportionately lessen the lift required of pumps and consequent permanent maintenance cost and will raise the head on any siphon which may be put into the line.

The most appreciable advantage in the adoption of this principle in the design of collecting sewers is the ability to operate the system without the construction of expensive tide gates, the satisfactory operation of which would be doubtful and the use of which might possibly cause a surcharge of the works or render them ineffective in their operation.

Regulators

The diverting dams also permit the use of a gate regulator without moving parts of complex mechanical arrangement and thus avoid structures liable to break down or requiring constant and expensive maintenance. A steel gate hung upon a trunnion eccentrically as to the axis has been designed to be introduced in the connecting channel between the main and collecting sewers, depending only upon the rise of the water in the main sewer to exclude the excess during storms, shutting positively by impact and opening by the eccentric weight of the gate itself when the water subsides. In the opened condition the gate allows only the first flush of the rain to pass to the works in addition to the dry weather flow and in the closed condition it allows no more to pass through the restricted area under the increased head of water in the main sewer. This will result in a lesser variation of flow to the works under all conditions and will effectively prevent the surcharge of the works or ineffective operation due to the great [38] diluting volume of storm water which would otherwise reach them.

High and Low Level Collectors

Wherever practicable, it is the purpose to construct two levels of collectors, of which the high levels will collect sewage from areas sufficiently high to deliver the sewage to the works at an elevation not requiring pumping, and thus utilize the potential energy in every foot of head, and the low levels to collect sewage from areas too low for the gravity flow and leading either to local pumping stations to lift the sewage into the high levels or to central pumping stations at the treatment works. A further advantage of the two levels of collecting sewers is that the quantity of sewage discharged into the water courses can be reduced by the high level collectors for a term of years at a small annual charge for maintenance due to the postponement of the construction of pumping stations, and thus delay the need for the low level collectors for several years and the accumulated annual savings will largely help to pay for their construction.

The present sewer system of the city, with its future extensions and the projected system of intercepting and collecting sewers to convey the dry weather flow and first flush of rainfall to the treatment works, will constitute the most important part of the entire project for sewage disposal, in that their functions of conveying the liquid wastes of the city inoffensively from the points of origin rank, from a sanitary standpoint, even higher than sewage treatment. Upon the proper design, construction and maintenance of the collecting system will depend also the ability to operate the sewage treatment works without offence. As has been shown, the early sewers were not as well built as those of modem construction, and some of [39] them have considerable deposits within them which should be removed. The sewers constructed in recent years have straight grades and smooth interior surfaces. All sewers and the collectors in the future should be built in this way, so that the sewage flowing at velocities sufficient to maintain solids in suspension, a condition obtained by smooth interiors, may serve to prevent lodgment therein of organic matter.

Maintenance of the sewer system and of the collectors to be built is absolutely necessary. In Europe it is a common practice to have the sewers regularly cleaned of any deposits, which may occur at times even in well constructed sewers. This prevents putrefaction and causes the sewage to be delivered to the treatment works in as fresh a state as possible, thereby forestalling offence.

Amount of Sewage Flow Affecting Design

In the design of sewers for the purposes of carrying sewage only, the factors used are the contributing population, water consumption and the amount of infiltrated ground-water. It is quite common practice in .many cities in designing sewage sewers to calculate that they shall run half full when carrying a water consumption of 150 gallons per capita from the population tributary. To reach a conclusion as to the quantity of sewage to be treated by the City of Philadelphia in the future, and to obtain data for the design of the collecting sewers, gaugings were made of the dry weather flow of a number of main sewers, some of which were located in solidly built-up areas and others in partly built-up districts, and from the factors thus obtained estimates were prepared based upon the probable increase and density of population, of the quantity of sewage that must be cared for in the future, the estimates and population curves being projected to the year 1950.

[40] The amount of sewage flow determined by the gaugings in all cases included the infiltrated ground water, no practical way appearing by which it could be separated from the sewage proper. As a majority of the sewers in which gaugings were taken are of considerable size and length, the variation between the maximum, minimum and average rates of flow is not as great as in smaller sewers. The mean of all the gaugings showed that the maximum flow was 128 per cent. of the average and the minimum 78 per cent. of the average. In the accompanying diagram the results of the gaugings of some of the more important sewers are shown.

In addition to the flow of sewage, it has been decided to admit into the collecting sewers, through automatic regulators, the first flush of the rain, which is usually as polluting as the sewage, and the amount to be admitted has been fixed at 10 per cent. of the maximum dry weather flow of the sewers, but a much larger percentage can be intercepted when the sewage is not flowing at a maximum rate. This additional 10 per cent. makes a storm maximum flow of 141 per cent. of the average flow. In England the authorities are required to receive storm water in the regular treatment works until the flow amounts to three times the normal dry weather flow of sewage. Between three and six times the flow must be partially treated in special storm water works and above six times is allowed to overflow to the water course. This provides for overflowing sewage when diluted with five times its amount of rain water.

When it is considered that the per capita consumption of water in Philadelphia is 178 gallons a day, and that in the towns of England only about 35 gallons are used, it will be seen that by the arrangement proposed the degree of dilution of the sewage, in time of storm, compares well with the English practice of allowing sewage diluted to [41] six times the normal dry weather flow to flow to the streams.

Velocity of Flow

In the design of the collectors, grades and cross sections have been used so that under estimated daily average flows the velocity of the sewage will not be below 2½ feet per second.

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