albuminous plasm and the oxygen is liberated in a nascent state and becomes valuable foroxidation purposes. The algae, therefore, convert dead organic matter into living organic matter and also liberate oxygen which is necessary for the activity of the bacteria.

The third agency are the protozoa, which utilize as their food supply the bacteria or algae and possibly unconverted dead organic matter.

The fourth agency are the crustacea, which find their food supply in the protozoa and algae.

The economic solution of the sewage disposal problem requires that these natural processes shall be used to the maximum compatible with safety, and the artificial treatment of the sewage reduced to a minimum in order to avoid undue expense for construction and operation of works.

Also in selecting processes for the treatment of the sewage of the city their adaptability to a comprehensive plan is of vital importance in order that the treatment works may be constructed in successive steps as needs arise for more refined treatment. The processes selected should be the most intensive of their kind, so as to secure the maximum of [43] efficiency upon the minimum area of land and at the least expense for operation and maintenance. They should also. be processes which experience in other cities has shown are the least liable to produce nuisance from bad odors.

The solids in sewage exist in four states, i.e., as

1. Floating matter such as faeces, pieces of food, paper, matches, corks, leaves and other objects lighter than water.

2. Solid particles heavier than water, but kept in suspension by the velocity of the current either in the sewer or in the water course. These particles vary in size and composition and are spoken of as settleable matter.

3. Organic matter in a colloidal state, or in such a fine state of sub-division that the particles are microscopic in size and will not settle out even if the sewage be maintained in a quiescent state.

4. Sewage matters in solution, such as the soluble part of faeces, urine, albumen, liquid part of trade wastes, etc.

Available processes of sewage treatment may be divided into classes as regards their function in removing these four states of the sewage matter, such as preparatory or the removal of floating and settleable matter and oxidizing or the conversion of the decomposable organic matter into stable compounds. No clear line separating them can be defined as the functions frequently overlap. The floating debris can be removed by means of screens with openings varying in size from 6 inches to 1/25 inch or less. The finer mesh screens will also intercept a large part of the settleable matter. The settleable matter may also be removed by means of tanks through which the sewage is passed at such a [44] velocity that the solids heavier than water subside to the bottom of the tank and the clarified liquid only flows off. The decomposable matters existing in a colloidal state or in true solution are converted to stable forms by means of oxidation through the agency of beneficent bacteria and the oxygen of the air or water.

Sewage Farms

The earliest method used for the treatment of sewage was its application to land laid out with main and branch distributing channels so that the sewage could be applied to the different parts of the farm in rotation. In some cases underdrains were laid to collect the water which percolated through the soil. This method was very much in vogue in 1860. Paris adopted sewage farms in 1865 and Berlin in 1876. These are the two largest sewage farms in the world, the former amounting to over 12,500 acres and the latter to 43,000 acres. The ideal land for the purpose is alight sandy soil supported by gravel to facilitate rapid percolation. The rates of application of the sewage vary between 3,000 and 25,000 gallons per acre a day, and averaging about 10,000, or one acre of land can receive the sewage from 30 to 250 persons. One of the objects in applying sewage to land was the hope of the recovery of the manurial value of its constituents. But such a large part of the sewage matters are inert, and those having fertilizing value are such a small percentage of the volume of the sewage due to the water carriage of the solids that while some value does exist it is similar to the gold in the clay deposits of Philadelphia, which would cost more to extract than it is worth.

Sir Maurice Fitzmaurice, late Chief Engineer of the London County Council, stated in connection with the utilization of the sewage of the city: "Several enthusiasts on [45] the subject of the useful utilization of sewage have begged for a sample gallon of it at the outfalls, and one even took away a barrelfull, but I never again heard from any of them, and the amount of sewage which they have taken away has made no appreciable diminution in the 280,000,000 gallons a day which are still at the disposal of anyone who will take it in whole or in part." With present knowledge, it is not possible to recover economically the small fertilizing value in sewage. If, in the future, means are devised for its recovery, the concentration of the sewage of Philadelphia to the three proposed treatment works will facilitate such recovery.

The Metropolitan Sewerage Commission of New York City has estimated that 175 square miles of land would be required to treat the sewage of that city, and that the cost of this method of treatment would be $153,000,000, and, therefore, dismissed it as impractical, which view was supported by all the eminent experts who reported on their projects.
The City of Birmingham has abandoned its sewage farm, which was one of the largest and best managed in England, and substituted the more intensive biological processes and the same course will probably be followed in Paris.

Mr. John D. Watson, after years of experience at Birmingham, aptly states that this method of disposing of sewage "may be ideal in theory, but it is difficult, if not impossible, to obtain the ideal on the farm of large size." To treat at the present time the sewage of Philadelphia on farm land would require an area of approximately 60 square miles, and in 1950 of over 100 square miles. To secure this amount of land in Pennsylvania adjacent to the city would be prohibitive on account of cost, would destroy the highly developed suburbs of the city, and would be opposed by citizens and property owners; hence this method of treatment need not be further considered.

Chemical Precipitation

[46] The next process devised for sewage treatment was called chemical precipitation. It was found by experience that the application of crude sewage to the farms of that day was inadvisable, and that higher rates could be obtained if the suspended matter were removed. The sewage was, therefore, passed through large tanks in which the velocity was very low, and the sewage retained many hours. Solutions of lime, salts of iron, alumino-ferric or other chemicals which form a flocculent precipitate were added to the sewage before it entered the tank and aided materially in the rem0val of the suspended matter. The deposit which forms on the bottom of sedimentation tanks is called sludge, and its handling and disposal are described later.

London, the largest city in the world, with a population of 6,000,000 situated on the banks of a river with but little larger flow of upland water than the Schuylkill, disposes of its sewage by removing about 75 per cent. of the suspended matter by chemical precipitation and depends upon the oxidizing power of the great tidal volume of the Thames for the ultimate purification of the dissolved impurities in the tank effluent.

Septic Tanks

These consist of sedimentation tanks of capacity sufficient to hold from 8 to 24 hours' flow of sewage, the purpose of the long retention being the deposition of the suspended matter, the decomposition of the resultant sludge and the breaking down by processes of putrefaction of the organic sewage matters as a preparatory process to their subsequent oxidation. In 1896, when this method of sewage treatment was introduced, it was claim ed that nearly all the organic matter deposited as sludge would be gotten rid of and only the inert mineral substances remain. As the sludge from chemical precipitation had proved to be such a troublesome matter, sanitary engineers hailed this idea as a panacea for the sewage problem, and it was rapidly introduced. Experience has shown that the original claims were very exaggerated, and that the long retention of the settling sewage in contact with the decomposing sludge produces a foul smelling effluent, and that the gas bubbles constantly rising from the sludge seriously interfere with effective sedimentation.

Emscher or Imhoff Tanks

The first attempt to obviate this fouling of the effluent was the digestion of the sludge in a separate tank. Another [49] attempt was made by Dr. Travis at Hampton, England, who built a septic tank divided into upper and lower compartments ; four-fifths of the sewage was passed through the upper part and the sludge settled through slots into the lower part through which the remaining one-fifth of the sewage flowed. Thus four-fifths of the sewage remained fresh, but when the foully contaminated one-fifth was added to the tank effluent it frustrated the purpose. Furthermore, the passage of sewage through the lower part maintained conditions favorable to the development of sulphur bacteria and produced a malodorous sludge.

In the accompanying cut is shown how all the settling sewage in a septic tank is contaminated by the decomposition of the sludge; how one-fifth of the sewage which flows over the decomposing sludge in a Travis tank is fouled; and how the complete separation of the settling sewage from the decomposing sludge in an Imhoff tank maintains the original freshness of the sewage. The separation of the settling sewage from the digesting sludge was adopted by Dr. Imhoff, of Essen, Germany, with the following essential modifications:

• The slot between the upper and lower compartments was so made that the gas bubbles formed in the decomposing sludge could not rise into the settling sewage.
• No sewage was allowed to flow through the lower or sludge chamber.
• The walls of the lower chamber were carried through and above the water surface of the upper compartment so that the gas could have a free vent, the capacity of the lower chamber made sufficient that sludge could be retained as long as six months before withdrawal.

Two-story tanks of this type are known as Emscher or Imhoff tanks. Their extensive introduction in Germany and America is due to the fact that when properly operated they efficiently free the sewage of its settleable solids, yield [50] a fresh inodorous effluent, produce sludge that is inodorous, of low water content and consequent small bulk, and which dries more quickly than any other kind of sewage sludge.

The principles involved in the construction of the Emscher tanks are shown in the accompanying cut. The sewage to be settled flows longitudinally through the tank in the cross section marked A; the solids which settle upon the sloping bottom B slide down through the slots C into the sludge chamber D. The gases of decomposition are prevented from entering the upper chamber by the gas baffle E, but find free exit through the sides at F, in which a scum generally forms. A pipe G extends from the bottom of the sludge compartment to the outside. A quick opening valve at H, located at a distance of over 3 feet below the water surface in the tank, permits the discharge of the digested sludge by hydrostatic pressure without any pumping.

In septic tanks the sewage is retained from 8 to 24 hours; in sedimentation tanks from which the sludge is removed before ebullition of gas from the sludge becomes violent, the retention is from 4 to 6 hours; but tanks built on the two-story principle described above are capable of removing a higher percentage of settleable matter from the sewage in retention periods of from l-1/2 to 3 hours. The maintenance of the settling sewage in a fresh condition, and the greatly reduced superficial area required for such installations more than compensate for the higher cost of construction. The quality of sludge obtained from Emscher tanks will be described later.

Fine Screens

In Germany the preparatory treatment of sewage has been developed to a very high degree not only in the tankage of sewage, but also in removing the suspended solids by means of screens. In many German cities the sewage [52] is collected to central points, passed through grit chambers and screens having sufficiently small openings to remove practically all appreciable sized solids and disposed of by diffusion in the river water through submerged outfalls. Such treatment works are more compact than those using any other processes. No deep excavation for tanks is required and,

where necessary, the screenings can be incinerated at the plant, thereby eliminating the need for sludge drying beds required for tank treatment.

At Dresden, for instance, 26,500,000 gallons a day of sewage are passed through screens, one of which is shown in the accompanying illustration, having openings 1-12 inch wide and 1 1-5 [1.2] inch long before discharge into the river Elbe. The building required is 85 feet x 230 feet in plan, and the foundations are 12.5 feet below the water level in the sewer. A similar capacity treatment works consisting of Emscher tanks and sludge beds would occupy a space of 150 feet x 500 feet and 30 feet below the water level in the sewer. The tank installation would remove a larger percentage of the suspended solids in the crude sewage than the fine screens, but there are circumstances where the oxidizing power of the river water can be utilized for the assimilation of the fine particles passing through the screens. Such a condition will exist at the proposed southeast works. The site selected is in a part of the City anticipated to industrially develop, due to the proximity of the proposed freight terminal railroad yards.

It is estimated that in 1950, 80,000,000 gallons of sewage will be brought by the collecting sewers to this site. The effluent will be discharged into the river below tidal influence upon the river at Torresdale, and it is considered that the more refined treatment of the sewage at the proposed northeast and southwest treatment works will maintain the water of the Delaware River in a fit condition to receive and [53] assimilate without any offence to sight or smell the sewage from the southeast works if it is freed from all appreciable sized suspended matter. The methods of treatment, therefore, proposed at this site will consist of coarse screens, grit chambers and fine screens with disposal of the screenings by incineration and of the effluent by diffusion in the channel of the Delaware River through submerged outfalls.

It is anticipated that the increase in population, the growth of industry, the extension of the sewer system and the increasing standards of hygienic cleanliness of the future will require more refined treatment of the sewage at the northeast and southwest works than the removal of the settleable solids. This would consist in converting the decomposable organic matters in solution in the effluents of the tanks into more stable forms so as to maintain or even raise the ratio between the demands upon the oxygen of the river and its rate of replenishment.

Intermittent Sand Filtration

Based upon the investigations made by Frankland of the principles involved in the purification of sewage by application to farm land, the Massachusetts State Board of Health in 1887 began experiments upon the purification of sewage by filtering it through beds of sand which are naturally found in New England. It found that sewage could be intermittently applied to sand beds at a net rate of from 20,000 to 60,000 gallons per acre per diem and a clear, well-oxidized effluent obtained. The natural glacial sand deposits of New England made this the most economical method of sewage treatment for those States, and "thus doubtless tended to instill into the minds of sanitarians the belief that it was desirable and perhaps necessary to purify sewage very thoroughly in all [54] cases where treatment was required before discharging it into running water."

For communities where such natural sand deposits exist or for small installations where sand beds can be artificially constructed such treatment may be desirable, but for the large quantities of sewage produced by the City of Philadelphia the expense of constructing the enormous areas of artificial sand beds would be unwarranted, for the degree of purification of the sewage by this method is unnecessarily high and the same results can be ultimately accomplished by the artificial treatment to a less degree at a great reduction in cost and utilizing the natural oxidizing power of the river.

Contact Beds

Historically, the next process for the oxidation of sewage was experimentally devised in 1892 by Dubden, at London, and is called the contact bed. It consists of watertight compartments containing a medium such as broken stone, slag, cinders, etc., to a depth of 4 or 5 feet. Sewage at least freed from its grosser suspended matter is introduced, filling the pores or interstices of the medium held in contact with it (hence the name of the process), then withdrawn and the bed allowed to rest and aerate. The biological action in a contact bed is very complex and the resulting effluent of properly operating beds should be fairly stable, showing that the putrescent organic matters have been oxidized. They can handle sewage at an approximate rate of 500,000 gallons per acre per diem. The adherence of sewage matters to the surface of the medium and the disintegration of the latter cause contact beds to lose capacity, so that in large installations some beds are always out of service having their medium washed.

Manchester, England, has the largest installation of contact beds, amounting to about 100 acres. These are found to [55] be expensive to operate and fail to produce an effluent equal to the requirements of the Rivers Board. The consensus of opinion among experts seems to be that contact beds for a large installation are not as efficient as percolating filters.

Percolating Filters

The most modern and intensive method for the oxidation of sewage is the percolating filter which consists of a bed of media such as broken stone, clinker or slag, supported upon a sloping floor with underdrains and enclosed by walls not necessarily water tight. The clarified sewage is applied to the surface of the medium by means of traveling distributors or fixed nozzles. This affords an opportunity for the sewage to absorb oxygen from the air and in this oxygenated condition it percolates over the surfaces of the stones upon which there develops a jelly-like film containing beneficent bacteria which effect the oxidation of the organic sewage matters, the carbon being converted into carbon-dioxide and the nitrogen into nitrate.

Part of the finely divided suspended matter and the colloids pass through the bed and part remain attached to the medium. This same condition is the cause of clogging in contact beds but in properly operated percolating filters the beds automatically unload the accumulated solids usually at the beginning of warm weather There are many installations of percolating filters which after several years of service are in as clean a condition as shortly after being placed in service, due to this automatic unloading of the accumulated solids.

The effluent of a percolating filter does not differ noticeably in appearance from the applied sewage and during the time it is unloading, the former is much more turbid, but the solids which pass through the bed have been subjected to the oxidizing influences and are changed in character. When such effluent is discharged into a small stream [56] or body of water, not having velocity sufficient to maintain the settleable solids in suspension, it is necessary first to pass it through a settling basin having about two hours retention. But when the effluent of a percolating filter is discharged into a stream with a velocity sufficient to maintain the solids in suspension, this expense may be eliminated as the ultimate oxidation will be accomplished in the river water.

The possible objection to large installations of percolating filters is the danger of nuisance from spraying the sewage over the beds. This has been noticed at plants using septic tanks for preliminary treatment. But when the sewage is collected by a properly designed, constructed and maintained sewer system, clarified in two-story tanks so as to maintain its freshness, the danger from nuisance is reduced to a minimum. In comparing the efficiency of the two most extensive methods of sewage oxidation, the Royal Commission of England on sewage disposal states in its fifth report:

"Taking into account the gradual loss of capacity of contact beds, a cubic yard of material arranged in the form of a percolating filter will generally treat satisfactorily nearly twice as much tank liquor as a cubic yard of material in a contact bed. Percolating filters are better adapted to variation of flow than contact beds. The effluents from percolating filters are usually much better aerated than the effluent from contact beds and, apart from suspended solids, are of a more uniform character. On emptying a contact bed, the first flush is usually much more impure than the average effluent from the bed. The risk of nuisance from smell is greater from percolating filters than with contact beds."

The last statement is based on experience with works not [57] using two-story tanks for clarification of the applied sewage. The rate of operation of percolating filters varies between one million and two and one-half millions gallons of clarified sewage per acre per diem which is from two to five times as high as is usual in contact beds.

It is not believed that it will be necessary to oxidize the sewage of Philadelphia when collected at the treatment works for several years in the future, but the treatment works should be so designed as to include oxidation when it becomes necessary. With present knowledge, the percolating filters provide the most efficient method and it has, therefore, been decided to so locate the clarification tanks that their effluent can be applied to percolating filters in the future. Advances in the art of sewage treatment may develop improvements whereby higher rates of filtration may be used and also less head required. Much interest is aroused in the investigations now being carried on in England to obtain a clarified and oxidized effluent in tanks by means of air and "activated" sludge.

When oxidation is resorted to in Philadelphia, it will be required earliest at the Northeast Works as an additional safeguard to the water of the Delaware River used at the Torresdale Water filters.

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