Installing such a system is neither expensive nor complicated. The tank itself should be large enough to hold the sewage of a household for twenty-four hours. It can be bought ready to install, or built of brick or concrete. Ready-made tanks are to be had of steel, concrete, or vitrified tile. We installed one of steel (which is the cheapest) some ten years ago and have found it most satisfactory. When it was delivered, two husky truck-men placed it at the edge of the pit prepared for it by the waiting plumber. They exhibited some curiosity and the plumber explained briefly about the bacteria and its action.
“You mean one of these here bugs is into it already?” asked one of them as he applied an awe-struck eye to the aperture in the top. He apparently expected to find an insect akin to a full-size cockroach running around inside, and either decided the light was poor or that the plumber was a first-class liar, for he went off shaking his head doubtfully.
The size of tank and length of disposal field is entirely a matter of size of household. On an average, the daily volume can be reckoned on the basis of fifty gallons per person and, for every fifty gallons of tank capacity, there should be thirty feet of disposal field. Thus, for a family of eight, a tank of five hundred gallons’ capacity connected with a disposal field of three hundred feet will be ample, allowing for guests as well.
In installing this system, the tank itself can be as near the house as ten or fifteen feet but the piping connecting it with the soil line of the plumbing should be water tight. The best way is to use four-inch cast iron pipe, calking all joints with oakum and lead. At a convenient point between house and tank, this line of pipe should have a “clean-out” fitting so that rags, solidified grease, or other substances that might block it can be removed. Sometimes vitrified tile with cemented joints is used instead of cast-iron pipe; but it has the distinct disadvantage that, if the rootlets of trees or large shrubs, attracted by the water, find so much as a pin hole in the cement, they will grow through and finally clog the pipe.
From the tank to the disposal field, the first three or four lengths of pipe should be glazed tile with tight cement joints. From these on, three or four inch porous land tile laid in shallow trenches is used. For proper action, the trenches of the field should be not over eighteen inches deep so that the warmth and evaporation of the sun may be effective. Also in digging these trenches, there should be a slight grade away from the outlet of the tank. An inch to every ten feet is adequate.
The bottom of the trenches is covered with a two-inch layer of medium-sized crushed stone or clean gravel. On this rest the land tile, and the joints are covered with roofing paper to prevent bits of stone or gravel from lodging within the pipe. The latter is covered two inches deep with more stone or gravel and over all go lengths of roofing paper cut slightly wider than the trench so that, when in place, the paper arches and fits tightly to the sides. The purpose of the stone or gravel is to facilitate water seepage from tile to ground while the roofing paper cover prevents silt from reducing the seepage.