SEWAGE DISPOSAL EFFICIENCY OF PROCESSES USED BY AMERICAN CITIES--OPINIONS OF AUTHORITIES--EXPERIMENTS WITH NEW METHODS.

Recognition of the necessity for the proper disposal of sewage is now quite prevalent in most American communities, whether large or small. In many sections the problem has become vital, and as the population increases, it is only a matter of time when all will be compelled to solve the problem, for its importance grows in direct proportion to the rapid increase in inhabitants. The continued concentration of population makes it increasingly difficult and expensive for a municipality to secure and maintain a pure water supply and forces community activity for protection against disease germs. It also causes the demand for the improvement of the esthetic condition of bodies of water within or near a city’s boundaries. Many states have already recognized the conditions due to these nuisances and have enacted strict legislation with a view to preventing the pollution of streams and other bodies of water, for the protection of water supplies, surface and underground, and for the elimination of disease germs accompanying sewage. States and even nations have realized that sewage disposal is more than a local problem. In every case it is an inter-community problem, in some it is inter-state and in a few the question must be settled by national governments.

Even those communities which have not already provided a proper method of disposal of their sewage know that it must be done sooner or later, and many are preparing for it either by making a preliminary study, by preparing tentative plans, by reconstructing their sewerage systems or planning new extensions with that end in view, or by shaping their financial programs so that the community will be prepared to assume the financial burden when the necessity becomes imperative.

The quantity of harmful waste produced by a community is surprisingly small in comparison with the disastrous effects it may produce. All authorities agree that in cities provided with an abundant water supply sewage contains less than one-tenth of one per cent. of foreign substances. This organic matter and the products of its decomposition the Massachusetts State Board of Health has found rarely exceed one-half of one per cent. of the sewage. George W. Fuller, consulting sanitary engineer, says that 99.9 per cent. of sewage is ordinarily pure water and that even much of the remainder is harmless matter of a mineral nature. The experience of George S. Webster, Chief Engineer of the Bureau of Surveys and of the Philadelphia Sewage Testing Station, with sewage works, indicates that on an average 1,000 persons produce per annum forty-five tons of dry sludge matter, or the solid part of the sewage after treatment; and the United States Census Bureau reports that the volume of sewage discharged daily during the year per person is 164 gallons. Yet the small amount of decomposing matter must be properly treated for it is that which gives sewage its offensive character and power to cause disease.

The proper solution of the sewage disposal problem involves first, the construction of a sewerage system that will remove the sewage from the community completely and as rapidly as possible, and secondly, the construction of a disposal plant at which the sewage can be treated in such a way that when it is discharged into the body of water it will not cause a nuisance and disease.

There are two types of sewerage systems in use, the separate and the combined. In the former the storm water is removed in one set of pipes and the domestic sewage in another. The combined system removes both in the same set of pipes. In deciding which system to adopt three factors must be first considered, the cost, the topography of the city and the method of disposal. The general conclusions of sanitary engineers at present regarding the relative merits of the two systems are that either is satisfactory from a sanitary point of view when properly constructed, that the separate system is usually best for suburban districts not closely built up and for all communities where the sanitary sewage requires treatment, and that often a combination of the two systems can be used to advantage. Most engineers point to the advantage of combined sewers in narrow streets and congested districts where only one pipe and one house connection are required.

The belief has been expressed by John H. Gregory, consulting engineer, that as a general proposition the cost of building a combined system is less than that of constructing a separate system, especially where the territory to be served is more or less closely built up and streets paved. In suburban territory, not closely built up and where storm water is easily and quickly diverted into natural water courses, he believes the separate system will in general cost less, for then only sanitary sewers need to be built first, the storm water sewers being deferred for years or only such drains constructed as are immediately required. When there are steep grades and relatively high velocity all authorities agree with Gregory that it is advisable to build combined sewers, even though the development of the territory may hardly be such as to require the removal of the storm water.

Discussing the merits of the two systems so far as they affect the cost of disposal Clark P. Collins, sanitary engineer, concludes that generally speaking “it is unwise to dilute sewage with storm water and to befoul storm water with sewage in the attempt to remove both by the same underground channel.” Gregory has expressed the opinion that if sewage is to be discharged into a body without treatment the combined system will offer the simplest and cheapest solution of the problem.

Among the principal objections to the combined system when the sewage is treated are the increase it causes in the volume of liquid which necessarily requires a larger plant and expenditure, the changes it causes in the character of the sewage which complicates operation of the plant, and the frequency with which it causes the flow of sewage to exceed the maximum of the plant, thereby making it necessary to discharge untreated sewage into the stream. With a combined system all kinds of trade wastes must be run through the disposal plant, whether they are offensive or not; automatic devices, which should be avoided whenever possible, are necessary between the combined and intercepting sewers to limit the amount of flow; a greater amount of grit is deposited at the disposal works unless in the separate system the first wash of the street is intercepted. The New York State Board of Health advocates the separate system.

In constructing, extending or reconstructing a sewerage system it is well to bear in mind that even though a city has not at present a disposal plant, the time will come in all probability when increased population will compel the treatment of its sewage by some process. It may, therefore, be more economical eventually to make present plans so that when disposal does come the sewerage system will make possible the most economical operation of the disposal works. Gregory’s conclusion as recently expressed in an address is that “other things being equal, especially as more and more attention is being given to sewage disposal, the separate system seems to offer greater advantages.”

All engineers advocate good ventilation for sewers and gradients that will develop self-cleansing velocities, so as to reduce gas trouble and to deliver the sewage as fresh as possible to the disposal works. The best practise, according to reports of the State Boards of Health, show that these velocities should be not less than two feet per second in separate systems and two and one-half feet in combined systems. In some instances where it has been necessary to reduce the gradients because of the expense of obtaining steeper ones, a velocity of one foot per second has been found to be satisfactory; but in such instances sewers must be well constructed and flushed. Most trade wastes require a higher velocity to prevent deposits.

The Degree of Purification of Sewage

Before determining the proper method of disposal the first point to be settled by a city is the degree of purification desired or needed for both the present and the future. The decision is dependent upon three factors: the self-purifying capacity of the stream or body of water into which the effluent—liquid portions of the sewage run off after treatment—is to be discharged and its utilization for water supply, bathing, etc., the character and amount of the sewage and the possible future growth not only of the city itself, but also of the communities bordering on the stream. While there have been some demands for the absolute sterilization of sewage, many sanitarians believe that any artificial method of sewage treatment will not esthetically render the final effluent fit for ingestion, and practically all authorities agree that final discharge of sewage need not be in this perfect condition. This seems to be based on logical reasoning when one considers that all waterways are necessarily polluted to some extent. John Duncan Watson, of Birmingham, England, contends that the complete elimination of bacteria is prohibitive inasmuch as it is beyond the limits of the reasonable demands on the purse. Robert Spurr Weston, member of the American Society of Civil Engineers, at one time reminded an audience that the proper place to protect the water consumers against disease is at the water works and not at the sewage disposal plant. Authorities are in general agreed that sewage should be disposed of as the stream demands, and that local conditions should determine degree of purification required. Standards of purity have been studied by many societies and various suggestions have been made. All agree that the sewage after treatment should not deteriorate the stream into which it flows. Watson advocates under certain conditions an effluent that will not putrefy on being kept for seven days at a uniform temperature of 80 degrees F. and that does not contain more than three parts per 100,000 of suspended solid matter.

Generally speaking the suspended matter should be removed, the conditions near the point of discharge be inoffensive and the water be not impaired for purposes of manufacture and pleasure. When a city is located on the seashore or near a large lake or stream the screening out of the heavy particles before the sewage is discharged together with dilution will prevent active decomposition and putrefaction of the sewage the body of water receives and the esthetic senses of the community will not be offended. On small bodies of water and when the water is used for drinking and manufacturing purposes or for bathing or shellfish the conditions usually demand not only a non-putrescible effluent but also one that is free from harmful bacteria or one that is highly purified like that from sand filters.

There seems to be a general agreement among sanitary engineers that the condition of the river below where the effluent joins it is a safe guide and should be the ruling factor in determining the degree of purification desirable. Authorities, however, are not agreed as to whether the standard of cleanliness should be based solely on chemical analysis or on a mixed standard taking into consideration the appearance of the water and its physical, chemical and bacterial conditions, as has been demonstrated by the Metropolitan Sewage Commission of New York. One expert in answer to the question propounded by the Commission based the standard solely on chemical analysis, but none of those whose views were sought was willing to accept the dissolved oxygen test as an all sufficient criterion of the condition of the water. One considered that the oxygen should be regarded as a reliable index of the cleanliness of the water only when dealing with the condition of gross pollution and only when in conjunction with observations of the appearance and physical conditions of the water. One of them would not have a standard of cleanliness based solely upon analysis of any kind and all were agreed that the standard of cleanliness should not rest upon the effect of the polluted water upon health.

After having decided on the degree of purification the next step in the solution of the problem is to select the process of treatment best adapted with local conditions to produce the results at the lowest cost and without nuisance. No specific rules can be laid down for the selection of the best process for all communities. Domestic wastes offer the least difficulty, but they are usually complicated with the presence of trade or street wastes or both. Features difficult to overcome may then be produced. Then also, the character of the sewage varies greatly with the season, days and even hours. This is due to the habits of the people, to climatic conditions and to the amount and character of trade and industrial wastes and to the amount of water used and allowed to infiltrate. A cannery, creamery, tannery, brewery, strawboard factory, wool scouring shop, dyeing and cleaning works may discharge its wastes so that during a certain period the character of the sewage be entirely changed. Knowledge of these conditions and changes are necessary to plan a successful disposal plant. Each community has its own problem, and while there are certain general conditions that should be considered, each case is more or less unique. Charles G. Hyde, consulting engineer of the California State Board of Health, has summed up the situation in this statement: “It is folly to suppose that because one town can dispose of its sewage successfully in some certain fashion, another town can adopt the same method with a certainty of securing equally satisfactory results. Sewage differs widely in character, not only as between towns but in a given town.”

Processes of Treatment

The processes for treating sewage may be divided into three main groups—the preliminary or preparatory, the main or final, and disinfection.

The processes in the preliminary or preparatory group remove more or less of the solids, especially the suspended matter, but the effluent, or liquid that is discharged into the stream, is chemically unstable and will decompose and putrefy. These are the simplest methods of treatment, and, except when sewage is discharged into very large bodies of water where it is desired only to improve the esthetic condition or where the water is capable of rapid self-purification, at least one of these processes is used in combination with some other form of treatment in the next group. The preliminary processes are dilution, screening (coarse or fine), plain sedimentation, straining or roughing filters, chemical precipitation, slate beds, colloidal tanks, septic tank treatment, and single contact beds.

The main or final processes are more complex. These remove a substantial proportion of the dissolved and suspended matter. The effluent is generally stable. When any one of these processes is used it is customary to provide some preliminary treatment. The processes in this group are double contact beds, trickling (also called percolating), sprinkling filters, intermittent sand filtration and broad irrigation or sewage farming.

In the third group is the process of disinfection, either by hypo-chlorite of lime or liquid chlorine. Some authorities call this third group the finishing process and preface two others, secondary settling tanks and secondary filters. The chemical elements of this group destroy the bacteria, especially the disease producing kind, and are used in combination with one or more of the processes in the other two groups to produce a highly purified effluent.

Several other processes have been developed within the last few years. The electrolytic process is now being used in a few American cities, and has been included in almost all of the experiments now being made by municipalities. The activated sludge process has been adopted by two large cities, Milwaukee, Wis., and Houston, Texas, and two small cities, San Marcos, Texas, and Escanaba, Mich., and is being tested in at least eighteen others, among them Baltimore, Cleveland and Brooklyn. Jersey City, N. J., has tentatively adopted the activated sludge process. Another process, known as the Miles Acid Sludge Process, is being experimented with by the city of Boston.

These processes or variations of them may be used singly or in combinations of two or more to yield different degrees of purification that will meet varying local requirements. Which of these or what combination of processes to use according to local requirements is the all important question for a city to answer. Several cities either have adopted or are planning to adopt the plan advocated by John A. Giles, Commissioner of Public Works of Binghamton, New York, to include a number of the different stages of treatment in the original design so that when future installation is necessary on account of increased population, with its increased pollution, or the need for a greater degree of purification becomes imperative, the addition can be made on the site already provided for and each unit will fit into the complete structure at a minimum cost. The consensus of opinion is that a disposal works can be designed and constructed which will produce an effluent that will not deteriorate the water into which it is discharged, that will create no nuisance from odor or from flies and that the cost will be strictly proportionate to the sanitary and esthetic results achieved.

An approximate idea of the efficiency of the various well known processes in the removal of bacteria was given by Professor George G. Whipple, Professor of Sanitary Engineering, Harvard University, before the New York State Conference of Mayors and Other City Officials:

Process	Percentage of Bacteria Removed
Fine screens	10 to 15
Settling tanks	60 to 70
Septic tanks	60 to 70
Chemical precipitation	80 to 90
Contact filters	75 to 85
Percolating filters	85 to 95
Intermittent sand filters	95 to 99
Broad irrigation	95 to 99

Dilution

Comparatively few cities can much longer depend upon large bodies of water to dilute their untreated sewage. Even those cities located on the seacoast and on the banks of large rivers and lakes have either provided some method of treatment, usually one or more of the processes in the preliminary group, or are planning to do so. New York City which has an adjacent large body of water into which it discharges its sewage without treatment of any kind, now finds it necessary to adopt a combination of processes to eliminate the nuisance the waste is causing. In some places where dilution is depended upon, the existing nuisances have been caused by the outlets being extended only to the high water line of the water course, thus preventing a proper mixture of sewage with a sufficient volume of water adequately to dilute it. Other difficulties experienced when untreated or raw sewage is discharged into large volumes of water in excessive quantities are the formation of deposits of sludge, the residue after sewage has been allowed to settle, on the banks and the bottom; turbidity, milkiness and oiliness of the water, bad odors, the formation of scum upon the water and the destruction of shellfish. To overcome these difficulties some cities have resorted to dredging, screening and sedimentation. Others have been compelled to adopt some more complicated process.

The California State Board of Health in one of its bulletins quotes its consulting engineer, Charles G. Hyde, as saying that experience has demonstrated rather definitely that a nuisance will be caused if sewage is diluted with less than about twenty volumes of water while from forty to fifty may in some cases be necessary. Weston believes that in ordinary cases mixtures of sewage and water should be fifty per cent. saturated with oxygen, and when there is an excessive deposit of sludge even seventy per cent. of saturation may be insufficient. Herring and Gregory, in their report on the Albany, New York, system, say: “From observations made in many rivers it has been found that a flow of well oxygenated river water of from three to six cubic feet per second is capable of diluting the sewage from a population of 1,000 to a degree that will allow oxygen in the river water to oxidize the easily putrescible organic matter in the sewage and thereby prevent the water from becoming offensive, provided the velocity of flow is sufficient to prevent accumulations of sewage sludge on the bottom of the stream.”

Screening

The screening process consists of running the sewage through coarse or fine screens, either hand cleaned or mechanically operated, to remove suspended and floating matter. There is almost an unanimity of opinion now in favor of the use of mechanically operated fine screens. The efficiency depends largely although not entirely, upon the size of the mesh or openings through which the sewage passes. Coarse screens, which are cleaned by hand, will remove from two to ten per cent. of the suspended matter and fine screens which are mechanically operated will in some cases remove as much as 25 per cent. Screening will not materially change the turbidity of the liquid or the greasy appearance nor will it remove all of the suspended matter.

Experience has shown that the screening process is valuable in connection with sewage pumping works and inverted siphons, when sewage is disposed of by dilution and when raw sewage is applied without any other preliminary treatment to a final process as it prevents the clogging of machinery and filters.

When the process is used the screenings must ordinarily be disposed of within twenty-four hours on account of fermentation and decomposition which sets in quickly. In some cities the deposits are buried and in others they are burned after having been artificially dried. Robert Spurr Weston says that it seems unwise to attempt to dispose separately of two kinds of sludge, namely that removed before and that remaining after subsidence. “On the other hand,” he continues, “the screening of the effluent from a settling tank in order to reduce the operative charges for cleaning sprinklers is an economical practise. Furthermore, the actual amount of material screened from the effluent is small in comparison with that removed from unsettled sewage and its subsequent disposal is not a serious burden.”

Grit Chambers

If a sewage disposal plant is operated in connection with a combined sewerage system grit chambers are usually necessary for the removal of sand, gravel and dirt before the sewage passes on for further treatment. Where a city has a separate system of sewerage grit chambers are held by some authorities to be unnecessary unless the first wash of the street after a storm is intercepted and the waste is treated. Gregory has expressed the belief that the safest plan under ordinary conditions seems to be to provide a grit chamber. It is generally agreed that the chambers should be so constructed that the sewage will flow through slowly enough for the grit to settle out, but fast enough to carry the organic matter in suspension. To insure proper operation the chamber must be cleaned out frequently. At the Cleveland Sewage Testing Station it has been found that velocities ranging from 30 to 60 feet per minute produce a grit of proper character. The California State Board of Health has advocated chambers with a capacity such that a net period of storage of at least three minutes be allowed and a velocity of not less than five feet per minute.

Straining or Roughing

There are few cities which treat their sewage by the process of straining and roughing. This consists of removing the suspended matter by means of rapid straining through beds of coke or sand arranged like the rapid sand or mechanical water filter. Coke beds, especially in cold climates, have not been a success. The chief objection to the rapid sand filter is the wash water which contains much organic or mineral impurities of the sewage and which requires special treatment which experience has shown to be difficult and expensive. Difficulty has also been found in disposing of the sludge deposited upon the filter surface. Of this process the bulletin of the California State Board of Health says: “The process is an expensive one at best, both as respects construction and operation. The effluent from such works can be made fully equal to, if not better than the effluent of plain sedimentation basins from a sanitary point of view.” The experience of the Cleveland Testing Station with these filters was not favorable. The filters when operated at rates from 30 to 60 gallons per acre per 24 hours removed from 25 to 40 per cent. of suspended matter and their action was simply mechanical, there being no increase in the dissolved oxygen content. The report from the station says that the difficulties encountered in their operation were sufficient to eliminate the process as a method in itself or in combination with other processes.

Treatment in Tanks

The treatment of sewage in tanks, either by chemical or biological processes, has been adopted by many cities, especially as a preliminary treatment. These processes are known as plain sedimentation, chemical precipitation and the septic process. Of these the treatment in the Imhoff tank is the most popular at the present time.

Plain Sedimentation

By allowing the sewage either to flow into properly constructed tanks or through them at a velocity low enough to allow some of the suspended matter to separate from the liquid and to be deposited on the bottom from which the sludge is removed, is another process that has been used by a number of American and European cities. The first tanks were constructed so that they could be filled with sewage and then after the suspended matter had settled the effluent was drawn off. This was known as the fill and draw plan. Later what is now known as the continuous flow principle was used. The velocity of the flowing sewage is reduced sufficiently as it enters and passes through the tank for the suspended matter to settle. The sludge which collects at the bottom of the tank must be removed frequently. The results are affected by the quantity and quality of the sewage, fresh sewage being capable of greater clarification by sedimentation than stale sewage. The range in storage period for American sewages is from four to twelve hours and the removal of suspended matter is from 45 to 75 per cent.

In some cities plain sedimentation has been used in connection with dilution and in others as an aid to filtration. The chief objection to the process is the sludge which is extremely offensive and must be treated separately. It does not dry readily, is difficult to handle and if allowed to accumulate causes serious nuisance. Because of these difficulties and the fact that the sludge from the Cameron and Imhoff tanks can be more easily disposed of the septic process has gradually forced plain sedimentation into the background.

Colloidal tanks were designed to carry the process of clarification further than plain sedimentation, but they have not come into general use. Metcalf and Eddy in their “American Sewerage Practice” say of this process: “There has been a feeling that while under some conditions a portion of the colloidal solids could be removed by such devices, the work accomplished was not likely to be sufficient to offset the expense of construction and some difficulties in operation.”

The Septic Process

In the septic process the raw sewage is conveyed to tanks, and allowed to stand until the solids have settled to the bottom and have been partially destroyed or liquefied by bacterial action. Two types of tanks are used in the septic process, one known as the Cameron type and the other as the Emscher or Imhoff tank.

The best constructed Cameron tanks are not less than 8 feet in depth and are usually large enough to hold about six hours’ maximum flow of sewage. The desirable time of detention depends upon the character of the sewage, both as to strength and freshness, strong and stale sewages demanding a longer period. The tanks are usually built with baffles at the entrance to retard the current and to deflect the suspended matter to the bottom which is so constructed that the sludge, after bacterial action has taken place, can be drawn off from time to time.

H. W. Clark, formerly chemist of the Massachusetts State Board of Health, has expressed the belief that the rate of flow through a septic tank should not be greater than that which will cause passage in twelve hours.

Charles G. Hyde in the California Board of Health Bulletin says that as a rule the period should not be greater than 24 hours nor less than 12 hours, except possibly with weak or stale sewages. He advocates multiple units so that the storage periods may be controlled to give optimum results.

The effluent which is turbid, putrescible and rich in organic matter cannot be discharged into streams with safety without further treatment, unless the volume of water is sufficient to complete the purification by dilution. As the solids settle a scum which forms on top of the tank, keeps out light and air and produces a condition favorable for the bacterial activity caused by minute organisms known as anaerobic bacteria. These germs thrive and functionate best in the absence of oxygen, and their chief function in sewage treatment is the conversion of the solid organic matter into a soluble form, somewhat less complex in chemical composition. The sludge is rotted and when full bacterial action has taken place is humified. In plain sedimentation the solids are simply deposited upon the bottom of the tank and are removed practically unchanged. In the septic tank, however, a part of the solids after settling are broken down or digested, thus somewhat lessening the difficulty of disposing of the sludge.

Reports vary widely as to the amount of suspended matter that can be removed by the septic process. The Iowa State College bulletin says that the amount of purification does not usually exceed 25 to 40 per cent. Professor Whipple places the removal between 60 and 70 per cent., and the State Board of Health of California says it may vary between 35 per cent. and 85 per cent., averaging perhaps 50 to 60 per cent. H. W. Clark places the amount at not less than 40 per cent. and adds that it will vary according to the character of the sewage, the variations being from 30 per cent. with weak sewage to 80 per cent. with strong sewage.

All reports concur that in many cases the Cameron type of tank has failed to produce efficient results. Among the objections raised by authorities are the following:

The sludge is not thoroughly digested and is somewhat offensive. The odor is obnoxious and the effluent is too stale and is treated with difficulty by oxidation processes. Gilbert J. Fowler, Sanitary Expert of England, says the defects which have shown themselves are a nuisance both from the tank effluent and the sludge and an excessive quantity of suspended solids in the tank effluent. Charles G. Hyde believes a review of the principles and results of operation appear to justify the conclusion that “the septic effluents are only less dangerous than crude sewage to the extent of efficiency of removal of organic matter.”

The Imhoff Tank

In an effort to overcome the defects in the Cameron tank, the Imhoff or Emscher tank was developed and this now seems to have the preference among cities making new installations. The tank consists of two compartments, one above the other. It has a smaller area than the ordinary septic tank, but is much deeper. The sewage passes at a low velocity through the upper chamber, which is comparatively shallow and V-shaped, the sides being sufficiently steep to allow the solids to be deposited at the bottom of the V which is equipped with slots. Through these the solids pass into the second chamber below which is much deeper than the other. The inclined partition wall must be cleaned frequently with hose or squeegee in such a way as not to clog the slots. The floating pieces of wood and cork must be skimmed off, but the greater part of the suspended matter that floats will generally sink after a time. Dr. Karl Imhoff, the inventor of the tank, advises spraying with a hose to expedite the sinking. Care must be taken to keep the sides clean and the sludge in the lower tank below the slot level. If neglected suspended matters will rise to the surface behind as well as in front of the scum boards. Dr. Imhoff advises the reversal of the flow of sewage about every three weeks after skimming off the floating matter when one sedimentation chamber feeds more than one sludge chamber. The rate of flow in the upper chamber is sufficiently rapid to prevent any septic action, yet slow enough to allow much of the suspended matter to settle.

The effluent in a comparatively fresh condition passes out of the tank for further treatment or for discharge into water courses. It therefore does not become stale nor does it come in contact with decomposing sludge, thus eliminating in part the objections advanced by authorities against the Cameron tank.

In the lower tank the sludge, after passing through the slots is slowly digested through septic and other actions without any disturbance by the flow of the liquid sewage, above. Before the tank can deliver good, well digested sludge—that is, a black alkaline odorless sludge—it must be inoculated with a proper amount of good sludge, or the raw sludge must be permitted to “ripen.” Dr. Imhoff has found that even without inoculation a tank will discharge good sludge from the beginning if ripe sludge is emptied into the system from cesspools which have been in use a long time.

In some instances cities have had considerable trouble with acid decomposition during the ripening period. This produces a sludge of objectionable odor and one not easily dried. It decomposes very slowly and may rise in a mass to the surface of the sludge chamber. Various remedies have been suggested, among them the addition of lime. “I cannot advise such addition,” Dr. Imhoff has written. “All plants which are known to me and in which acid decomposition has occurred have sooner or later adjusted themselves of their own accord.”

When properly inoculated the particles of sludge rise and fall constantly in the process of giving off the gases. The fresh sludge particles entering the chamber through the slot are covered so that the entire mass becomes thoroughly mixed and the untreated sludge in a short time is inoculated with the proper organisms. The decomposed sludge is discharged from time to time through pipes leading from the bottom of the tank to drying beds.

Dr. Imhoff has advocated the discharge of sludge from each sludge chamber once every two to six weeks, that the optimum of the sludge level should be about three feet below the slot level and if it is desired to promote the early incidence of proper decomposition the sludge should not be allowed to remain quiet at the bottom of the sludge chamber. He advocates constant stirring and a uniform introduction of fresh organic matter and the discharge of the decomposed matter. The scum layer, he says, must be agitated frequently by a jet of water or otherwise and the sludge at the bottom of the chamber should be agitated by a water stirring system. As a substitute, he suggests that the whole body of sludge be pumped out and returned. To determine the elevation of the sludge surface, he advises lowering into the sludge chamber a very thin piece of sheet iron one foot square in area held in a horizontal position. If the level is too high, there will be gas bubbles on the surface of the settling chamber above the slot or there will be floating sludge and in extreme cases foaming sludge. As compared with other tank processes the experience of cities indicates that the Imhoff type has many advantages. Certain inherent difficulties, however, have been pointed out in several reports. Gilbert J. Fowler has expressed the belief that “the comparative short time of settlement means that variations in the character of the sewage must be quickly reflected in the character of the tank effluence and that the filters (when they are used for further treatment) must be called upon rapidly to accommodate themselves to fluctuating conditions.” He believes that this is not conducive to the development of the most efficient bacterial activity. Storm water above moderate dilution, he says, will have to receive separate treatment and he is of the opinion that ordinary stand-by tanks will still be necessary for this purpose, the sludge from which will have to be dealt with. From the results of the operation of an experimental plant in Worcester, Massachusetts, Matthew Gault, Superintendent of Sewers, draws these conclusions: “It appears to be perfectly feasible to treat Worcester sewage by means of Imhoff tanks and sprinkling filters. The results of experimental treatment of the effluent from chemical precipitation tanks indicated that the advantages gained by chemical precipitation as a preliminary treatment were not commensurate with the cost. The Imhoff tank was quite as efficient in sludge digestion as experimental septic tanks have been and much more efficient so far as sedimentation of the sewage is concerned. It was operated without the production of the offensive odors characteristic of the septic tank and the sludge itself was disposed of without creating a nuisance. The effluent from the Imhoff tank was normally as fresh in appearance and odor as the sewage flowing into the tank.”

The experience of the New Jersey State Board of Health with Imhoff tanks has been that if properly designed, constructed and operated, they are a valuable means of sewage clarification. The observation of its engineers has shown that under these conditions the tanks overcome a great deal of trouble due to odors and greatly simplify the sludge problem. “However, their proper operation is a considerable problem,” reads one of its reports. “And the cost of keeping them in working order is several times greater than for septic or sedimentation tanks.” In view of the initial cost of this form of tank as compared with the older single story types the New Jersey engineers believe that “in cases where the works are far removed from a populous community, so that the odor problem is not serious, it is doubtful whether the Imhoff tank has any material advantage over a properly constructed, well baffled sedimentation tank of the old type.”

The Cleveland Sewage Testing Station reports that the most consistent results were obtained from the operation of the Imhoff tank, an average suspended matter removal of 50 per cent. being secured. A recent city report says: “In general it may be said that a detention period of thirty minutes accomplished a removal of suspended matter from 40 to 45 per cent. as compared with a 50 per cent. removal effected by a detention period of two hours and fifteen minutes.”

In a bulletin of the California Board of Health, Charles G. Hyde sums up the importance of the septic process thus: “The septic process as carried out either in the Cameron or Imhoff type, but especially in the latter, has at present two distinct fields of usefulness; first, it constitutes an effective means of preparation for any final process which can be better conducted with a sewage from which the suspended solids are more or less completely removed; secondly, it may be employed when disposal by dilution is permissible if the source of unsightly sludge and scum is removed.” Another advantage may be added, the Imhoff tank produces a sludge that can be disposed of easily.

Chemical Precipitation

By using some coagulant such as copperas, lime, sulphate of alumina or perchloride of iron, the subsidence in basins of between 40 and 55 per cent. of the total organic matter and between 60 and 95 per cent. of the total suspended matter can be obtained. The bacterial removal is between 80 and 90 per cent., depending upon the character of the sewage. The objections to this process are great cost of chemicals and labor required and the difficulty of disposing of a large amount of sludge. There are a few plants of this kind in operation at the present time and there seems to be a general agreement among authorities that the process is now a back number. Fowler says, “It may be doubted whether dilute sewages resulting from the lavish use of water in American cities lend themselves generally to economical treatment by this process.” Metcalf and Eddy in their “American Sewerage Practice” express the opinion that the quantity of chemicals required for results would be a prohibitive expense. The sewerage commission report of New Jersey contains the statement that “on the standpoint of the officials in charge of the experimental station at Lawrence, Massachusetts, chemical precipitation is a process of the past.” The experiments of the Massachusetts State Board of Health showed that it is quite impossible to obtain effluents by chemical precipitation which compare in organic purity with those obtained by intermittent sand filtration. About the only plants of any importance in the United States are those at Worcester, Massachusetts, and Providence, Rhode Island. According to the report of the Superintendent of Sewers of Worcester, the experimental plant in that city has shown that “the cost of operation of Imhoff tanks and sprinkling filters per million gallons of sewage treated would be much less than the cost of operation of chemical precipitation or sand filtration as carried on in Worcester.”

Slate Beds

The equipment for this process consists of tanks with horizontal slabs of slate separated a few inches by stone blocks. The sewage is allowed to stand in the tank for about two hours, during which the suspended matter is deposited on the slabs and is digested by multifarious forms known as aerobic germs, i. e., germs requiring oxygen for the continuance of their proper vital function. The deposits are thereby reduced to harmless and inoffensive humus. Slate beds are dosed and rested alternately so as to give them an opportunity to replenish their supply of oxygen. Multiple units are therefore necessary. The effluent must be treated as a tank effluent. Fowler suggests that when filters are used to purify the effluent, “humus” tanks be provided between the slate and the filter to retain the solids washed away from the beds and somewhat to equalize the composition of the effluent passing into the filter.

Dosing Chambers

After the effluent has passed from a tank after being treated by one or more of the preliminary processes, it usually flows into a compartment known as the dosing chamber where it is admitted to the filter for further purification.

When enough of the liquid has accumulated in the chamber it is automatically emptied by means of a siphon, thus permitting the intermittent application of the sewage to the filter bed. When more than one bed is used the siphons are arranged so that the liquid alternately flows to different filters or parts of filters.

Contact Filters—Single and Double

The treatment of sewage in a single contact filter is classed as a preliminary process and when treated in double contact beds or those arranged in tandem as a final process. A contact filter is a basin filled with broken stone, coke, slag or coarse gravel, thoroughly underdrained. The size of stone or other material to be used depends upon the degree of purification desired, and the manner of operating the beds. The smaller the stone the more brilliant the effluent will be, but all reports agree that the cost of operation will be greater and that there will be a more rapid loss of filter capacity. Experience has taught the superiority of the coarser material because the interstices being so large the bed is not so liable to choke. Watson advises a fine medium bed only when a highly purified effluent is desired, when it would be difficult to get rid of humus from the filtrate, when a high cost of maintenance is not prohibitive and when a temporary stoppage of the whole plant would not be a serious matter. He believes it is not suitable for installations of any magnitude. Beds have been built with various depths, the range being between four and seven feet. Some have been built shallower and have given good results. The method of applying the sewage is important. Some tanks are overfed and others are underfed. Francis E. Daniels, Director of Water and Sewage Inspection of the New Jersey State Board of Health, describes a method which has been found to be successful in plants in this state. At these plants the effluent is applied on the top and at one corner of the contact beds. At the point of application a small area of contact material from 6 inches to one foot deep is removed from the top of the bed, and fine cinders are substituted. An embankment about a foot high is constructed of the same material around this area so that all of the tank effluent applied to the beds strains through the cinders. Mr. Daniels says that a great deal of the suspended matter is thus removed from the tank effluent which reduces clogging and increases the life of the beds. It is Mr. Daniels’ experience that the value of underfed beds is diminishing. If the effluent is very septic this method has the advantage of reducing odors, but as Mr. Daniels has pointed out, the practise of reducing the storage capacity of tanks is becoming prevalent.

In many plants the sewage is distributed by mechanical appliances, some being motor driven and others cable driven. Springfield, Missouri, which uses a motor drive, reports a saving in power, first cost, moving weight, and maintenance, over the cable drive. Another advantage is that the length of the filter can be increased at will. The total cost of the distribution per million gallons according to Springfield’s experience is $1.25 for cable drive and $1.61 for direct motor drive.

After the sewage has been distributed on the beds so that the interstices are filled, it is allowed to stand for a time. The bed is then drained and rested. While standing the sewage comes in contact with a jelly-like film which forms on the surface of the stone, and important changes occur. As with the septic tanks contact beds require a certain period in which to ripen. The time of contact and the period of rest vary in different plants. The rate of filtration varies according to the construction of the beds, the range is between 600,000 and 1,200,000 gallons per acre per day. The effluent from single contact beds is not stable but that from double contact beds is non-putrescible and low in suspended matter, although somewhat turbid. It can be discharged without offense into small streams. Single contact beds have seldom been used for final treatment of sewage and fewer filters of this kind are now being constructed even in conjunction with any preparatory treatment. The general opinion is that this process is on the wane. Watson says, “It may now be assumed that percolating filters are being constructed in England in preference to contact beds wherever the conditions are suitable.” In America they are not being adopted for large installation but they are still considered for small disposal works. In their fifth report the Royal Sewage Commission of England states that 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. Comparing the efficiency of contact beds and percolating filters it is claimed that the latter are better adapted to variations of flow and that the effluent is usually much better aerated; and apart from the suspended solids are of a more uniform character. With percolating filters the likelihood of odors is greater than from contact beds and there may be a greater nuisance from flies.

In the report of the City of Leeds, England, the results of very valuable experiments are given. It says, “Double contact beds give good results with crude sewage and excellent results with partially settled sewage or with septic effluent. Single contact beds are insufficient for dealing with crude sewage but give fair results with settled sewage or with septic effluent. The real difficulty with contact beds is to maintain capacity.”

The principal advantages of this process according to reports are low operating head, and less nuisance from odor and flies, and the disadvantages are large areas required and cost of maintenance.

Trickling, Percolating or Sprinkling Filters

Trickling or percolating filters consist of beds of coarse grained material such as pebbles or crushed stone, one-eighth to four inches in size, from four feet to ten feet deep and well underdrained. The character and strength of the sewage should determine the size of the material, the depth of the bed and the rate of operation. Some engineers give the capacity as about 20,000 persons per acre of stone surface; others say the rate of flow should be from one to two and one-half million gallons per acre. In some designs an auxiliary air supply is inducted into the filter material by tubes connected with the underground system. The Atlanta plant is equipped with ventilator hoods having weather vanes so that the mouth of each hood always points toward the wind. “This form of ventilation is of no particular value and may be detrimental in cold weather,” says Glenn D. Holmes, Chief Engineer of the Syracuse, N. Y., Sewer Board. By means of spray jets and moving sprinklers operated with some device for varying the pressure, such as a butterfly valve, or by means of an intermittent dosing tank operated by a siphon, the sewage is sprinkled or deposited on the surface of the bed in thin films and drops; thus the sewage is freed of objectionable gases and takes up oxygen as it passes through the air and through the filter. Sprinkling filters do not produce the best results when crude sewage is applied. They are most efficient when the suspended matter has been removed by some preparatory treatment. In some cities the screening process is first used, in others the sewage receives a preliminary treatment in tanks. Well designed and efficiently operated filters of this kind produce an effluent that is stable but not clear. Some plants are equipped with secondary settling tanks through which the effluent flows before final discharge and is freed of the humus-like particles it contains after leaving the filter. Reports agree that the effluent is not nearly so good in appearance and has a much higher percentage of bacteria than that produced by good intermittent sand filters. As compared with the double contact process the general opinion is that sprinkling filters are superior in respect to the removal of organic matter and cost less to operate. The chief advantages of a sprinkling filter are the high rate of filtration and the low cost of operation. The disadvantages are a possible nuisance, especially during hot weather, from odor when anything but fresh tank sewage is sprayed; and the development of insect life. Fowler says, “However economical their construction and maintenance it cannot be said that such a process meets all sanitary and Æsthetic requirements.” The experience of Worcester, Massachusetts, at its experimental station was that more than twenty times as much sewage per unit of area was treated by the sprinkler filter as could be treated by intermittent sand filtration, and more than ten times as much per cubic yard of filter. Four times as much sewage was treated by these experimental filters as could be treated satisfactorily by experimental contact beds. In order to obtain equal nitrification with contact beds at least three contacts would be required.

As a final process of purification in sections where land and filter material are available at small cost the intermittent sand filter is superior to any other. This fact has been established by experience and experiments. The filter material may be clean, coarse sand or any other porous soil. If a natural area is available the method of construction is very much simplified and economical. The top soil is removed and used in embankments between the beds. If the water tables are low the beds are not underdrained. In artificial beds the size of the sand is important. While fine sand will give a more brilliant effluent than a coarser material, the sewage has to be applied in small doses with long periods of rest. The rate of purification is higher in coarse sand filters and the effluent while containing more bacteria is non-putrescible. About twenty-four inches of sand should cover the underdrains of tile, placed about five feet apart, and surrounded by small-sized gravel.

In some beds the entire bottom above the underdrain is covered with about six inches of gravel. In others the bottom is ridged, the underdrains being placed at the bottom of the valleys which are then partially or wholly filled with gravel. Risers are constructed at the head of the underdrain and an intercepting drain completes the system. The beds vary in size and number according to the amount of sewage to be treated. The operation of the filter is very important. The sewage must be applied rapidly in rotation to each bed until the surface is covered with about three inches of the liquid. The bed is then slowly drained and allowed to rest. Overdosing and lack of aeration cause clogging. The surface must at all times be kept clean and loose. To maintain this condition it is sometimes necessary to break up the surface to a small depth or periodically to remove the deposit on the surface.

In cold climates the operation of the filters in winter is difficult and the quality of the effluent somewhat impaired. Several methods have been adopted to prevent freezing. Some filter beds are ridged so that when dosed the sewage flows in gutters. The ice which forms at the top of the sewage remains suspended on the ridges, thus permitting succeeding doses to flow underneath the ice. In other plants the surface of the filter is scraped into small piles which form a support for the ice. It is claimed that by this method the cost of subsequent cleaning is less than when the beds are ridged.

The effluent in properly constructed and managed plants is clear and odorless. The bacterial purification is as high as ninety-nine per cent. The Massachusetts State Board of Health in one of its reports says, “When sewage filters slowly and intermittently through five feet of porous earth and sand, an effluent is obtained which is as free from organic matter, from ammonia and from nitrites as many a natural spring water.”

The only drawback noted to this process is the cost of treatment in large quantities where land and filter material are not available. Francis E. Daniels says that under such conditions the cost is almost prohibitive. For many cities sufficient area cannot be obtained at any price, and as population increases the difficulty will become greater.

The New York State Board of Health in general will approve only of the following rates of operation for different types of filters where suitable provision for preliminary treatment is made: Intermittent sand filters, 100,000 gallons per acre per day; contact beds, 100,000 gallons per acre per day per foot of depth; sprinkling filters, 300,000 gallons per acre per day per foot of depth. These rates of operation are based on a sewage contribution of 100 gallons per capita daily and no variation from these rates of filtration is allowed for any other per capita contribution of sewage. The allowable effective depths of said filters will in general range from three to five feet; contact beds from four to seven feet; sprinkling filters, from five to nine feet.

Broad Irrigation

Broad irrigation, or sewage farming, is the oldest process of sewage purification, but the constant increase in population has made it necessary for cities to adopt other methods because of the area of land necessary for such a plant. Two processes are used, surface irrigation and filtration, a greater area of land being required for the former. Sometimes the two are combined into one process. For filtration and irrigation the sewage is generally first subjected to sedimentation or screening and then flows on carefully prepared land on which crops are usually grown. The areas are underdrained and are equipped with distribution systems.

Local conditions determine the method of irrigation, the ridge and furrow system being most generally used. The efficiency of the process depends upon the quality of the soil and proper management. Among the factors which should enter into the selection of the site are the quality of the soil, composition of sewage, method of disposal, kind of crops to be planted, contours and slope of surface, nature of the sub-soil, sub-soil waters, transportation facilities, nature of streams, nature of adjacent property, and availability of water supply. The best lands consist of a fine layer of alluvium overlaying a sub-soil of gravel, chalk or other porous material. Various kinds of crops are grown on sewage farms and the revenues therefrom help to reduce the cost of operation. They also assist in the purification. The principal drawback are heavy transportation cost and a prejudice against sewage-grown produce. During the rainy season when the quantity of sewage requiring treatment is greatest, less sewage can be used for irrigation and the growing of crops of sewage farms. All evidence points to the fact that broad irrigation is on a steady decline, although the efficiency of the treatment under favorable conditions is very high.

Disinfection

When the bacterial efficiency of an effluent from either preparatory or final treatment is low and the effluent is discharged into a body of water from which water supplies are derived or shell fish are taken, disinfection is often found necessary. The purpose is to destroy objectionable bacteria and disease germs. Hypo-chlorite of lime and liquid chlorine are the two chemicals most commonly used. The principal advantages of the liquid chlorine over the hypo-chlorite according to plant supervisors and operators, are less cost of operation and space required for both apparatus and storage of materials, no loss of strength, no lime sludge, and no mixing tanks required. The claim is also made that it can be better controlled. Chlorine, however, is more expensive than hypo-chlorite and the control apparatus usually costs more. There is general agreement among engineers, that except as an emergency measure or under the above stated conditions, disinfection is too expensive a process on account of the amount of chemical required. This varies with the amount, method and degree of previous treatment of the sewage and the degree of bacterial elimination desired. Tests at the Cleveland Testing Station indicated that from five to seven parts per million of available chlorine will effect a bacterial removal of from eighty-five to ninety per cent.

Activated Sludge Process

Sewage treatment by aeration in the presence of sludge is the latest development in sewage disposal, and the process is attracting a great deal of attention in America. Milwaukee has constructed a plant to treat two million gallons of sewage a day. Houston, Texas, is operating a plant to treat the sewage for 160,000 persons, and Escanaba, Michigan, and Jersey City, N. J., have favored the process. Experiments are now being conducted in Milwaukee, Baltimore, Washington, Cleveland, Regina, Chicago, Lawrence, Mass., Brooklyn, New Haven, Conn., the University of Illinois and many other places. The efficiency and economy of the process as compared with others which have long been in use have not been completely established. The chief points in dispute are sludge disposal and cost, but the indications are that these questions will soon be satisfactorily answered.

The process consists of passing raw sewage through tanks from eight to twenty feet deep in which a certain amount of activated sludge is always present. To mix the sewage and the activated sludge air is forced into the bottom of the tank under low pressure of sufficient volume to keep the liquor violently disturbed. From this aerating tank the mixture passes to another or sedimentation tank where the sludge settles and from which the clear effluent passes over a weir to its final destination. In order to maintain the proper volume of activated sludge in the aerating tank a portion of the sludge is pumped back from the sedimentation tank. The balance of the sludge is pressed and used for fertilizer base. The Milwaukee experiments indicate that in order to produce a clear, non-putrescible effluent about four hours aeration is required, twenty per cent. of activated sludge maintained in the aerating tank, and about 1.75 cubic feet of free air supplied per gallon of sewage treated.

The effluent is clear, odorless and practically free from suspended matter. The sludge will begin to decompose after forty-eight hours and must be pressed and dried within that time. Chief Engineer, T. Chalkley Hatton, of the Milwaukee Sewerage Commission, estimates that the sludge can be reduced to a fertilizer basis for about $8.75 per dry ton, including overhead charges. Basing the value of the sludge produced upon a low price per unit, he finds that Milwaukee sludge is worth $12.50 per dry ton, which represents a clear profit of $3.75 a ton. From ten to twelve million gallons can be treated upon one acre of ground, which is about one-fifth the area required for sedimentation tanks and sprinkling filters. The reasons for the adoption of this process by Milwaukee after experimentation by competent engineers for more than a year are given by Mr. Hatton in a recent address before New York State city officials as follows: “It produces a better effluent than any other known process of sewage treatment except land treatment or intermittent sand filtration; it can be built upon a comparatively small area; it produces no objectionable odors or flies; it produces a sludge of sufficient value to meet the cost of its reduction to a fertilizer and therefore relieves the city of the difficult, complicated and wasteful method of sludge disposal common to all other processes; it is subject to complete and satisfactory control throughout its operation; it is not materially influenced by climatic conditions; occupying a small area, its first cost is less than any other known process from which an equal character of effluent can be obtained; its operating cost is not prohibitive.”

In a discussion before the Iowa Section of the American Waterworks Association Dr. Edward Bartow commended activated sludge for its value as a fertilizer. This has been proved, he said, by its chemical composition, by its reaction with various solids and by its effect on the growth of plants. Pot cultures and garden experiments have shown that the nitrogen is in a very available form.

E. E. Sands, City Engineer of Houston, Texas, bases this statement on results of experiments conducted for a year: “Our investigation has demonstrated that sewage can be disposed of anywhere that there is a vacant tract of land in the city without creating a nuisance and without any objectionable feature.” The total estimated cost for treatment will be about $9.14 per million gallons when the plant is run at the rate of 18,900,000 gallons per day. He estimates that the total cost for treatment by the Imhoff tanks and the sprinkling filters would be not less than $11 per million gallons.

After an extended investigation by their sanitary engineers, Armour & Company have concluded that the activated sludge method will satisfactorily purify the industrial wastes from their Packingtown factories. Assistant Superintendent, M. D. Harding, estimates that from data now available the cost per million gallons exclusive of depreciation, interest and repairs, will be $3.

When considering the applicability of this process to conditions in any city consideration should be given to the following points. The process requires competent supervision, which Mr. Hatton claims may be a blessing in disguise in view of the experiences of cities which, after having built disposal plants of various kinds, have left their operation to the kind mercies of Providence with disastrous results. This process also requires the expenditure for constant power. The cheaper the power the more adaptable the process is commercially; but if the unit is small and the power cost high, the operating cost may be too great. The sludge must be constantly treated to avoid nuisance. The process produces a high degree of purification. If the local conditions do not demand this the process might be too expensive in comparison with some other process which will produce a satisfactory effluent.

Other Processes

A few cities, including Oklahoma City and Santa Monica, Cal., have electrolysis treatment plants. The process consists in passing the sewage between a system of electrodes. The change is brought about by chemical reaction from newly formed chemical reagents produced by the decomposition of inorganic compounds already in solution. It is still regarded as an unestablished process.

Boston has within the last year been testing a new process of sewage purification invented and patented by a Boston chemist. By the addition of an acid, an attempt is made to precipitate the bulk of suspended matter and to form a sludge which can be dried and degreased thereby producing a salable and greaseless fertilizer as well as recovering valuable grease. Experiments by E. S. Dorr gave results so full of promise that arrangements were made for a study of the process under the supervision of the Sanitary Research Laboratory of the Massachusetts Institute of Technology. Robert Spurr Weston gives the results of this study in a recent issue of the American Journal of Public Health. His conclusions are that “with facts at hand the process would be very satisfactory for Boston from a sanitary standpoint, and is more promising economically than any other known method.” He includes in his comparison the activated sludge process. An experiment by Boston on a larger scale has been recommended.

Trade Wastes

Industrial trade wastes, such as those coming from canneries, breweries, woolen mills, laundries, dye and cleaning works, paper mills, iron foundries, gas works and packing establishments and others cause nuisances around disposal plants, and the problem of their proper disposal is more difficult of satisfactory solution than the treatment of domestic sewage. Some wastes can be treated with domestic sewage at the disposal works without any difficulty, others require special treatment before being allowed to enter the sewers and often it is desirable to keep certain wastes out of the main sewers and dispose of them independently. Each particular problem must be considered by itself with due regard both to conditions at the factory, the expense burden on the producer of the waste and to the body of water into which the effluent is to be discharged. There are instances where cities have reimbursed certain manufacturers for treating their wastes separately, and others where the manufacturers have reimbursed the city for the additional treatment required.

Sludge Disposal and Value

Authorities are generally agreed that the sludge problem is the center of the entire sewage problem, because it causes more trouble and is the most expensive part of the treatment. The method of handling it is just as important as the treatment of the sewage.

Wet sludge can be pumped out on land or into shallow places or it can be sent to sea in ships and allowed to sink. If pumped on land it must be spread out in very thin layers. If discharged into trenches it is ploughed into the ground after it has dried. In either case a large area of land is necessary and odors cannot be eliminated. Only cities located on or near the seashore can send their sludge to sea, and then the cost of disposal is rather high.

Sludge can be dried by pressing, in centrifugal drying machines, by mixing with some dry matter or by discharging upon drying beds. The cost of pressing is high, depending upon the amount of lime added, the kind of sludge pressed, and the size of the works. George S. Webster states that the average cost in large cities is ten cents per ton of wet sludge. It is especially applicable to chemical precipitation works as it must first be treated with lime or coal powder. When dried in machines the liquid contains much organic matter and is objectionable. The simplest method is to discharge the sludge upon drying beds of porous material and underdrained. The time for drying depends upon sewage treatment. Imhoff tank sludge will dry in less than a week, septic tank sludge in two weeks or more, and sludge from plain sedimentation will require about two months in summer and almost five months in winter. Cleveland, in order to overcome weather conditions at its experimental plant, built a covered sludge bed, modeled after standard greenhouse construction. The report from the Testing Station is that during summer the period of drying is approximately the same as or possibly a little longer than with open beds. Eliminating the three winter months, the station report says, it is possible to operate beds of this type so that one square foot of surface will dry 0.8 cubic feet of sludge per year. Francis E. Daniels suggests that sludge can be handled faster by drying a small portion at one time and removing it from the bed before the next portion is drained off.

Dry sludge can be used for fertilizer or for filling low lands or it can be incinerated. Its fertilizing value is disputed except when produced by the activated sludge method. The filling in method is economical. Authorities advise the consideration of incineration by cities which burn their garbage.

Dr. Imhoff’s recommendations are the use of sludge for agricultural purposes and for filling in low land. “In both cases,” he says, “the sludge must first be dried and this is best effected upon a drying bed after the sludge has been decomposed in an inoffensive, odorless manner, in a separate tank through which sewage does not flow.”

Many unsuccessful efforts have been made to extract the valuable ingredients from sewage, but to date the experience has been that they have been more costly to recover than they are worth. Dr. McLean Wilson, Sanitary Inspector of the West Riding of Yorkshire Rivers Board, believes that the valuable ingredients of sewage will ultimately be recovered and used since many capable experimenters are at work on the problem. H. W. Clark, Chemist of the Massachusetts State Board of Health, is of the opinion that sludge has some value and that “it seems inevitable that as the processes of drying, pressing and fat separation are improved and as nitrogen advances in price sewage sludge will become of greater agricultural value than at present.” Experiments have been made at the Philadelphia Sewage Testing Station by burning dry sludge and wet sludge mixed with fine coal. The results were unsuccessful. Experiments have also been made at the Cleveland station where it was found that the sewage sludge contained about one-half as much nitrogen and one-third as much phosphates as does the garbage tankage.

Management and Supervision

No matter how well a sewage disposal plant is designed or constructed it will not do its work in a satisfactory manner and produce desired results unless it is efficiently managed. Every plant should be in charge of a man who has knowledge of sewage disposal principles, is thoroughly familiar with his plant and who can act intelligently in an emergency. The New Jersey State Sewerage Commission in one of its reports notes the tendency of local authorities to permit the deterioration of disposal plants usually through inattention. “It cannot be too strongly urged on those charged with these, as of other public works, that a competent man in charge is a primary necessity and that the plant should be kept continuously in the highest state of efficiency.” The same condition is complained of by the California State Board of Health and other state organizations. In one of its bulletins the California State Board says that “some of the plants are operating very indifferently well and some very badly. The general situation shows plainly the need of expert advice to municipalities with respect to general methods and necessary efficiencies from some central authority.”

D. C. Faber, Industrial Engineer of the Iowa State College, goes so far as to claim that practically all nuisances in connection with plants can be traced directly to failure to give them attention. He says that even where plants have been found too small increased care in many cases could be made to offset lack of capacity.

In several states, such as New York, Pennsylvania, New Jersey, Kansas, Ohio and Massachusetts, the State Boards of Health have supervision over the designing of new plants and the operation of those established. The good results obtained as a result of this supervision are evidence that similar powers should be granted to all state boards of health.

With a plant designed to meet local conditions, properly constructed and efficiently managed, a city should have no difficulty in disposing of its sewage economically, in a sanitary manner and without creating a nuisance.