This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
This method and the broad irrigation already described are the only purification processes in use on a large scale which can remove practically all. the organic matter from sewage without being supplemented by some other method. The process is a simple one and consists in running the sewage out through distributing pipes onto beds of sand 4 or 5 feet in thickness with a system of pipes or drains below for collecting the purified liquid. In operation the sewage is first turned on one bed and then another, thus allowing an opportunity for the liquid portion to filter through. As the surface becomes clogged it is raked over or the sludge may be scraped off together with a thin layer of sand. The best filtering material consists of a clean, sharp sand with grains of uniform size such that the free space between them will equal about one-third the total volume. When the sewage is admitted, to the sand only a part of the air is driven out, so there is a store of oxygen left upon which the bacteria may draw. This is not a mere process of straining but the formation of new compounds by the action of the oxygen in the air, thus changing the organic matter into inorganic. Much depends upon the size and quality of the sand used. The grains that have been found to give the best results range from .1 to .5 of an inch in diameter. The work done by a filter is largely determined by the finer particles of sand and that used should be of fairly uniform quality, and the coarser and finer particles should be well mixed. The area and volume of sand or gravel required are so large that the transportation of material any great distance cannot be considered. Usually the beds are constructed on natural deposits, the top soil or loam being removed. The sewage should be brought into the beds so as to disturb their surface as little as possible, and should be distributed evenly over the whole bed.
The under drains should not be placed more than 50 feet apart, usually much less, and should be provided with manholes at the junctions of the pipes. Before admitting the sewage to the beds it is usually best to screen it sufficiently to take out paper, rags and other floating matter. The size of each bed should be such as to permit an even distribution of sewage over its surface.
Where the filtration area is small, it must be divided so as to permit of intermittent operation; that is, if a bed is to be in use and at rest for equal periods, then two or more beds would be necessary, the number depending on the relative periods of use and rest. Some additional area should also be provided for emergency, or for use while the beds are being scraped. If a large area is laid out, so that the size of the beds is limited only by convenience in use, then an acre may be taken as a good size.
The degree of purification depends upon various circumstances, but with the best material practically all of the organic matter can be removed from sewage by intermittent filtration at a rate of about 100,000 gallons per day.
There is often much opposition to sewage purification by those living or owning property near the plants; but experience has shown that well-conducted plants are inoffensive both within and without their enclosures. The employees about such works are as healthy as similar classes of men in other occupations. The crops raised on sewage farms are as healthful as those of the same kind raised elsewhere, and meat and milk from sewage farms are usually as good as when produced under other conditions. Good design and construction, followed by proper methods of operation, are all that are needed to make sewage purification a success. No one system can be said to be the best for all localities. The special problems of each case must be met and solved by a selection from among the several systems and the combinations of systems, and parts chosen that are best adapted to the conditions at hand.