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.