conveyed with it, and if the sludge contains 95 per cent. moisture, it requires 19 tons of water to be handled. One of the most important considerations, therefore, in handling sludge is the percentage moisture which it contains, as this is a controlling factor in its bulk, as shown in the accompanying diagram. It is highly desirable to obtain sludge with as low a moisture content as possible. Of the dry residue in the sludge, approximately one half is organic and one-half mineral. A large part of the organic matter in freshly deposited sludge is highly putrescent and if improperly handled produces offensive odors. It is, therefore, also of importance that such processes of sludge treatment shall be used that reduce this offensiveness [60] by the destruction of the easily decomposed organic matter.

The final disposal of sludge is accomplished by (a) discharge of wet sludge in the sea; (b) depositing wet sludge on land; (c) the use of partially dried sludge for filling in low waste lands; and (d) where circumstances warrant the use of sludge as a fertilizer, fertilizer base or as fuel. The problem of handling sewage sludge consists in disposing promptly and economically of the watery mass which contains offensive, decomposable matter without menace to the public health or the production of foul odors or other nuisances.

Discharge of Wet Sludge in the Sea

Large cities located near the ocean dispose of the wet sludge most economically by carrying it to sea in specially constructed tank steamers. The sludge is pumped from the sedimentation tanks to reservoirs on the wharf from which the steamers are filled by gravity. When the boat reaches the dumping ground in the open sea the outlet valves are opened and the sludge diffused in the sea water as the boat moves along. This method of sludge disposal is used at London, England, where the sewage is collected by sewers to two works located on the banks of the Thames about 12 miles below London Bridge. Chemicals are added to the sewage to form a flocculent precipitate and free it from most of its suspended matter. The sludge thus produced after settlement contains about 92 per cent. moisture and is carried by 6 tank steamers 55 miles to Black Deep in the open sea, which is 20 miles below Southend. On an average 8,300 tons of sludge are thus disposed of every weekday at a cost of about 9 cents per ton (including interest and sinking fund charges).

At Manchester, England, the sludge from the septic [61] tanks is similarly carried by a tank steamer to the open sea beyond Mersey Bar. During the last fiscal year, 215,082 tons of wet sludge were dumped at sea at a cost of $32,000, or about 15 cents per ton. Among other large cities disposing of their wet sludge in this way may be mentioned Glasgow, Scotland; Salford, England; and Dublin, Ireland. The Metropolitan Sewerage Commission of New York recommended this method of sludge disposal for that city as it appears to be the most economical, due to proximity to the ocean.

Depositing Wet Sludge on Land

For cities situated inland such method of disposal is impracticable on account of the transportation charges, and they are confronted with the problem of reducing the bulk of the sludge by removing the water, either by drainage and evaporation on drying beds or by mechanical processes, such as presses and centrifuges and of handling it so as to minimize offence. The type of sedimentation tank adopted, the use of chemical precipitation or the opportunity afforded for sludge digestion has a marked effect upon the volume of sludge produced on account of the moisture content. Generally speaking, it may be said that chemical precipitation will produce between 20 and 25 cubic yards of wet sludge containing about 92 per cent. moisture from each million gallons of sewage treated; plain sedimentation from 4 to 7 cubic yards between 87 and 93 per cent. moisture; septic tanks from 1.5 to 3.0 cubic yards between 80 and 90 per cent. moisture; and Emscher or Imhoff tanks from 1 to 2.5 cubic yards between 75 and 85 per cent. moisture.

Lagooning

The disposal of wet sludge without prior dewatering may be accomplished by its application to land in several [62] ways. The earliest method used was called lagooning, in which case earth embankments were built enclosing an area of suitable land and the wet sludge run in to a depth of as great as 10 feet. The clogging of the soil preventing free drainage of the moisture, the scum formation upon the surface retarding evaporation, and the frequent great depth of the sludge all tend to prevent it from drying. As an example, there are sludge lagoons at Birmingham, in which the sludge deposited many years ago is practically in the same condition as shortly after being placed except that a heavy crust has formed upon the surface. This method is rapidly being abandoned.

Trenching

To overcome these objections and to dispose of the sludge more quickly, it was run upon the surface of farm land to form a shallow layer which would dry in a reasonable time and could then be plowed in and the field cultivated. But the gross nuisance created by the exposure of such large areas of foul smelling sludge led to the adoption of what is called trenching. As practiced at Birmingham, England, the trenches were dug about 3 feet wide and 18 inches below the surface of the soil, the excavated earth forming banks between the trenches, so that they could be filled to a depth of from 24 to 30 inches with wet sludge, after which the tops of the earth banks were thrown over the sludge to prevent nuisance from smell or flies. The porous earth absorbs the moisture and later the land is plowed across the trenches and placed under cultivation. This process can be repeated at intervals of from 18 months to 2 years.

The cost of trenching at Birmingham amounted to about 8 cents per ton of wet sludge. This is exclusive of the cost of sludging tanks, but includes interest on capital outlay at 5 per cent. and rent of land. This method is not [63] being used in new plants and is being abandoned in old plants on account of the area required, the interference which is caused in times of heavy storms, the increased difficulty of operating caused by winter weather and the general cumbersomeness of the method.

Pressing

Among the early mechanical methods of reducing the bulk of the wet sludge by dewatering was pressing in machines (those used at Bradford, England, being shown in accompanying illustrations) which consist of a number of cast iron plates generally 9 square feet in area, with corrugated faces and surrounded by a machined rim so that when placed together they form water tight cells about 2 inches thick. A central pipe about 6 inches in diameter extends through the middle. Over each plate a canvas cloth is placed and sludge forced into the press and subjected to a pressure of from 60 to 75 pounds per square inch. This squeezes the water out and the resultant cake contains between 50 and 65 per cent. moisture and is about one-fifth the bulk of the original wet sludge. It is necessary to add

At Worcester, Mass., about 18,000,000 gallons of sewage a day are received at the chemical precipitation works. There are added 55.5 pounds of lime to each 1,000 gallons of sludge, which contains on an average about 90 per cent. moisture when pumped from the tanks. The presses reduce [64] the moisture to about 70 per cent. and 0.167 tons of sludge cake are obtained from each cubic yard of the wet sludge. The pressed cake is hauled in electrically propelled cars about a mile and disposed of for filling low waste land.

Centrifuges

Another mechanical method of dewatering sludge is by means of centrifuges which occupy less space than presses and do not require the addition of lime to the sludge. Such machines are continuous in action and the work of extracting the moisture consists of two distinct and constantly repeated periods. During the first period the wet sludge is introduced into the machine and by the action of centrifugal force the moisture content reduced. During the second period the sludge thus partly dried is automatically ejected. The final product contains about 60 per cent. moisture and occupies about one-eighth the volume of the wet sludge.

The largest installation of these machines is in Frankfurt-on-Main, Germany, where the sewage of 400,000 people is subjected to plain sedimentation, and about 310 tons of wet sludge 90 per cent. moisture obtained per diem. The sludge is pumped to overhead reservoirs and kept agitated by revolving paddles. From thence it is fed to 8 centrifugal driers capable of handling 325 cubic yards of wet sludge a day of 10 hours. After drying it is carried by a conveyor through a tunnel heated by the exhaust gases from the power station and, as a 20 per cent. moisture mass resembling soft coal of a gray color, is mixed with garbage and both disposed of by destructors, the steam being used for generating electricity for power and lighting.

Digestion of Sludge

In the methods of sludge handling above described efforts were directed toward preventing the dissemination [65] of the foul odors from the wet mass. Within recent years much thought has been given to devise processes of treatment by the digestion of the putrescent matters to produce an inoffensive sludge both as withdrawn from the tanks and during drying.

Separate Sludge Tanks

One of the methods to accomplish this purpose is to remove the freshly deposited sludge from the sewage sedimentation tanks at intervals and place it in separate tanks. Usually a scum forms upon the surface beneath which more or less active fermentation and decomposition develops. New sludge is added and digested sludge withdrawn from time to time and placed upon under drained sand or cinder beds for drying. On account of the digestion of the sludge it dries more rapidly and is much less offensive than when fresh.

This method is now in use at Baltimore, Md., where the sewage is freed of its settleable solids in large tanks. The accumulated sludge is removed at intervals by centrifugal pumps and discharged into adjacent concrete tanks where considerable digestion occurs, as indicated by the continuous ebullition of gas which is inoffensive. At first, the sludge was withdrawn from the digestion tanks and dried upon underdrained sand beds. It could be removed in a much shorter time than undigested sludge, and but little offense was created. At the present time, part of the wet sludge from the digestion tanks is being sold to farmers for use on truck farms. The same method of separate sludge digestion is in use at Birmingham, England, the dried material being used to fill in a deep ravine between railway fills.

Two story Tanks

The Septic, Travis and Imhoff tanks have been described as devices for settling the sewage. Considering the sludge [66] produced by each, it may be stated that the digestion occurring in the septic tanks reduces the water content of the sludge and somewhat aids in its drainability and reduces its offensiveness; the sludge storage capacity of the Travis tank does not appear to be large enough to provide time for the digestion of the organic matters and the sludge as withdrawn is malodorous.

When an Emscher tank is first put in operation it is entirely filled with sewage; as there is no flow through the sludge compartment, decomposition develops therein and the putrescent matters in solution and as colloids are largely destroyed. The sludge deposited in the lower compartment is, therefore, covered by a water having characteristics different from those of fresh sewage, and it is the opinion of some that this is the cause of the inoffensive sludge digestion, the gases evolved under these conditions being principally methane and carbon-dioxide. These gases while in the sludge, are under a hydrostatic pressure due to the depth of the tank, and when the sludge is withdrawn upon the drying bed the bubbles expand, lightening the mass and aiding in a separation of the water, which flows to the bottom and is carried away by the drains of the drying bed. This is shown in the accompanying illustration. Both cylinders were filled to the same depth with sludge drawn simultaneously from a mature Emscher tank. The sludge with which the left-hand cylinder was filled had been pumped so as to expand and liberate the entrained gas bubbles. The sludge with which the right-hand cylinder was filled was drawn in a normal manner and contained the gas bubbles under pressure. It will be noted that the water stands on top of the sludge in the left-hand cylinder, while in the right-hand cylinder the expansion of the gas bubbles has increased the volume of the sludge, causing it [67] to rise even above the top of the glass, and in the bottom of the cylinder is seen the water which, had the sludge been placed on a sand bed, would have drained away. The digestion of the sludge also reduces its sticky, tenacious nature and facilitates its parting with the water. As is shown in the experiment illustrated in the accompanying half-tone, three wooden boxes of identical dimensions containing sand in the bottom and a drain pipe were filled with three different kinds of sludge. The right-hand box contained sludge resulting from plain sedimentation; the middle box contained sludge withdrawn from a septic tank; the left-hand box contained sludge withdrawn from a mature Emscher tank. The glasses beneath each drain pipe show the amount of water which drained from each sludge in the same time. It will be noted how the level of the Emscher sludge is much lower than the others, due to its shrinking in volume and the much larger quantity of drain water obtained from it.

In normal weather sludge from a matured Emscher tank will dry on sand beds in from 3 to 5 days to a consistency fit to remove, while sludge from septic tanks requires at least two weeks' time, and sludge from plain sedimentation tanks will require about 8 weeks in summer, and at least twice that length of time in winter, to be dry and firm enough to handle. Dried Emscher sludge is suitable for filling lowland or use in agriculture, particularly in lightening heavy soils, as it is very spongy in texture, due to the entrained gas. But experience has demonstrated that the use of air-dried sludge from any

source will not give results comparable with those obtained from the use of artificial fertilizers. The rapid drying of sludge digested in Emscher tanks allows of very much smaller sand beds than for other kinds of sludge; in fact, it is usual to provide only 1 sq. ft. of bed for each three persons tributary to the tank.

[68] The first full-sized installation of Emscher tanks was at Rechlinghausen, Germany, in 1907, for a population of 30,000. The marked improvement in quality of tank effluent and sludge obtained over all previous types of sewage tanks led to the adoption of the principle at first in Germany, and later in this country. The two largest installations are at Bochum, built in 1908 for 145,000 people, and Essen Nord, built in 1911 for a population of 180,000.

In the Pennypack Creek sewage treatment works, Emscher tanks are in use as described in the appendix to this report. They are also being operated successfully at Atlanta, Ga., and in Batavia, N.Y.; they are also being installed at Baltimore to provide increased capacity instead of enlarging the present system of sedimentation and separate sludge digestion.

In the accompanying diagram is shown graphically the relative volumes of wet sludge and of dry residue obtained from 1,000,000 gallons of sewage by different methods of sewage treatment and sludge drying. The outside cubes indicate the total volume of the sludge and the shaded cube shown in the lower left-hand corner of each larger cube represents the volume of dry residue. The difference in volume between the two,

therefore, represents the volume of water in the sludge.

The upper row of cubes represents sludge as deposited in the tank. Considering sludge deposited from plain sedimentation as normal, the increase in volume of wet sludge obtained by chemical precipitation is very evident. The larger amount of dry residue shown by the shaded cube for chemical precipitation is due to the presence of the chemical coagulant and to the fact that more suspended matter is removed by this process than by sedimentation unaided by chemicals. The smaller size of the cubes representing sludge from septic tanks and Emscher tanks is due to their lower moisture content, and the smaller amount of [70] dry residue shown indicates the results of the digestion of the organic matter due to the retention of the sludge in the tank.

In the lower row of cubes is shown the relative volumes of the various kinds of sludge after partial dewatering. It will be noted in each case that the shaded cube representing dry residue is of the same size as the corresponding cube in the wet sludge as deposited, the reduced volume of the dried sludge being entirely due to the extraction of moisture.

When it is borne in mind that the volumes of these figures are in proportion to the cubes of their edge, the great difference between the volume of wet sludge deposited by chemical precipitation and of air-dried Emscher sludge is evident.

The Recovery of Ingredients from Sludge which may have Value

Sludge contains ammonia, phosphoric acid, potash, grease and carbon. Generally speaking, these ingredients are more costly to recover than they are worth. It has been estimated that the manurial value in the excreta of one person in a year is $2.62, but in the dilute sewage of America this would be contained in about 36,000 gallons of water.
If this material is deposited as sludge of 90 per cent. moisture it would weigh about 1,720 pounds per cubic yard, and each cubic yard would contain only about 80 pounds of organic matter, of which only a part has any monetary value. The problem of recovering the valuable ingredients in sewage sludge, therefore, involves the use of economical and efficient processes for drying or pressing to reduce the bulk for transportation. In order to recover the grease in sewage with present methods, it is necessary to have the sludge in a very dry condition.

[71] Dr. MacLean Wilson, Chief Inspector of the West Riding of Yorkshire Rivers Board, who has supervision of the disposal of sewage from 3,000,000 persons located in the center of England, in reporting upon the utilization of sludge, takes a hopeful view of utilizing that part of the valuable ingredients of sewage which can be extracted in the form of sludge. An early solution of this problem lies, in his opinion, in the fact that there are many capable experimenters at work in the effort to prevent the waste at a cost which permits the sludge to be prepared and transported in a condition profitable to agriculturists.

Mr. H. W. Clark, Chemist of the Massachusetts State Board of Health, in charge of the Lawrence experiment station, as a result of his study, expresses the opinion that sludge has some value and as the processes of drying, pressing, and fat separation are improved and as nitrogen advances in price, it seems inevitable that sewage sludge will become of greater agricultural value than it is at present, especially as the basis of fertilizers enriches by the addition of potash, phosphates, etc.

The Metropolitan Sewerage Commission of New York City, after an exhaustive study of the question of utilization, does not encourage the belief that any great profit can be derived, except in cases where the nitrogen or fats are abnormally high. It states that under other conditions, past

For many years the so-called Globe Fertilizer has been manufactured from sewage sludge at Glasgow where the sewage is precipitated with lime and ferrous sulphate. Most of the sludge is sent to sea, but part is pressed and dried by heat, passed through a pug mill and sold as fertilizer. [72] The Royal Commission of England states that it is more economical to sell pressed cake than to make the fertilizer and less is being made each year.

Bradford is the center of the wool industry in England and large quantities of wool washing water are added to the sewers amounting to one-tenth of the total amount of sewage, which, therefore, contains abnormally large amounts of grease. The sludge is first treated with sulphuric acid and then heated to 212 degrees Fahr. It is then pressed hot and grease mostly obtained in the hot press liquor which is then boiled with chemicals and the resultant grease sold at from $35 to $50.50 per ton. Excluding certain fixed charges, it is claimed that the recovery of the grease is done at a profit.

Enlargement in the capacity of the Birmingham Sewage Works for handling sludge is contemplated by the use of the Dickson process as it was worked at the Dublin Sewage Outfall Works. "In this method of handling sludge about 0.5 per cent. of yeast is added to the sludge as removed from the tanks and the mixture pumped through a heater which consists in a number of pipes placed in the path of the hot-air from the furnace to fermenting troughs in the bottom of which are hot-air ducts to keep the fermenting sludge at about 90 degrees Fahr. In 24 hours, as a result of the fermentation, there is a distinct separation of water, the sludge at a density of about 83 per cent. occupying the surface while the water can be readily drained away beneath. A compound of phosphates and potash in about equal proportions by weight of sludge and compound based on dry solid matter is then added and the mixture containing about 73 per cent. water is then pumped to the dryer, which consists of a cylindrical vertical casing containing a series of arms and platforms. Air at a temperature of about 450 degrees Fahr. is blown into the dryer at the bottom and passes out the top The dried mixture falls [73] into a disintegrator which beats it up into a powder, which would be used as fertilizer."

Experiments made in the sewage testing station operated by the Bureau of Surveys in 1909 and 1910 in burning several kinds of dried sludge and in mixing wet sludge and fine coal which was burnt when dried confirm the accepted conclusion that while sludge when dried has a certain value as fuel, which is influenced by the source and amount of moisture which it contains, it is not practicable to recover completely this calorific value in actual amounts of water evaporated by the burning of the sludge.

When refuse disposal plants and sewage treatment works are located in close proximity to each other, an opportunity is offered for the advantageous disposal of sewage sludge by burning it with refuse. This is now being accomplished at Frankfurt-on-Main, Germany, already referred to.

The manipulation of sludge so as to utilize the calorific value has also been the subject of experimentation in Germany, followed by the installation of full-sized plants. Two notable plants are those at Ober-Schoneweide, and at Potsdam, both near Berlin. In the former, layers of sludge of 60 per cent. moisture and culm or dust of brown coal are run into a gas producer, and utilized to run a 60 horse-power gas motor generating electric energy, used for ~power and lighting about the plant and adjacent street lighting. The report of operation, based upon the possible sale of all power at 2-1/2 cents per kilowatt hour, for the plant in question, shows a profit of 50 per cent. on the net cost of production, exclusive of interest on invested capital. The fact of the construction of typical four-story apartments across the street from the plant, after the installation, argues well for the maintenance of sanitary conditions.

In the other case, that of Potsdam, the brown coal is added in the proportion of 1 part coal to 8 of sludge in the [74] sedimentation process, also about 150 grams of sulphate of iron per cubic meter of sewage. These contribute to the separation of the solids from the liquids. The sludge thus produced is pressed into briquettes of 6o per cent. moisture having a heat value of 1,500 units. The briquettes form the only fuel in a city electric plant in an adjoining building. It is said that the cost of briquetting about equals the value of the electric energy, the benefit being the innocuous disposal of sludge without cost.

Experiments are to be made at Birmingham, England, to utilize the air-dried sludge by burning it in specially constructed furnaces in order to obtain heat for the sepa rate digestion of about one-sixth of the wet sludge by the new yeast fermentation process previously mentioned.

Disposal

In order that the purification of the sewage shall be promptly accomplished by the natural processes in the river after the artificial treatment in the proposed works, it is necessary that the effluent of the treatment works shall be diffused with the river water. If the effluent were discharged at the bank of the river it would not readily mingle with the main currents, due to an inadequacy of diluting water. It is, therefore, proposed to discharge the effluent through conduits laid in the bed of the river terminating in the channel in multiple outlets. This will cause the discharge to occur in the deepest water, and when it can be assimilated by the great tidal flow. The sludge at the northeast and southwest works after drying on the sand beds can be utilized for filling in the low lands adjacent to the works and the screenings at the local pumping station, and the southeast works can be incinerated to avoid any offence from handling material containing such a large proportion of organic matter in built-up parts of the City.

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