Part 20

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.

Intermittent Sand Filters

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.