SEWAGE DISPOSALEFFICIENCY 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, bypreparing 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.
The Sewerage System
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 theyaffect 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 moreand 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.
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 eliminationof 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 entirelychanged. 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.”
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 thisgroup 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 madeon 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:
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 aproper 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.”
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 favorof 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.”
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 bysome 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.
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 per24 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.
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.
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.”
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 lessthan 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.”
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 ahose 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 decompositionhas 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 thatthe 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 withthe 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.
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 greatcost 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.”
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 depositsare 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.
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.
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 thebed 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. Thetotal 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 filtersthe 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 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 depositedon 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.