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.