SECRETS OF THE SEPTIC SYSTEM

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So efficient are they that within three to five days, up to 75% of a given load of solids is digested—leaving a bottom of heavy sludge and creating bubbles of methane gas which rise to the surface and further aid the decomposition of the lighter fats and greases. Excess gas is "burped" to the atmosphere through a vent in the household plumbing.

The idea is to allow relatively clear liquid, or effluent, to pass from the tank. By far the simplest way of accomplishing this is to slow the rate of How through it. Older tanks have a single large compartment, so management is limited. Newer designs use two chambers, with a fixed passage between them. In theory, this layout traps grease and sludge in the primary chamber, keeping the secondary one clear. Three-compartment tanks, with deep chambers and a reverse-flow pattern, are believed to function better yet.

Upon leaving the tank, the clarified effluent contains organic and inorganic substances, plus pathogens and bacteria that can pose a health hazard. The absorption field, containing a series of buried distribution pipes, disperses this liquid evenly into the earth, where it's filtered and processed by movement through the soil (see sidebar).

Not all that long ago, it was considered perfectly acceptable to send this waste water to a seepage pit or dry well—an unmortared brick-lined excavation, or simply a hole in the ground filled with gravel. Because such a pit allows so little control over where the effluent travels (and it may well migrate through fissures to the water table and contaminate a drinking supply), most counties now require leach fields or beds, engineered to suit local soil conditions and terrain.

The absorption-field trenches should be carefully planned and laid out to suit the site. Each line is buried in a bed of gravel.

A typical network begins with a concrete distribution box, which allows the single pipe from the septic tank to branch equally into several capped lines, usually four-inch-diameter perforated ABS plastic or bituminous paper pipe (and sometimes vitrified clay drain tile) arranged in rows of equal length, and set in a bed of gravel about two feet below the ground. Normally, several parallel trenches are dug about 10 feet apart to accept the pipe—each 24 inches wide, up to three feet in depth and no more than 60 feet in length.

The particular dimensions depend on the size of the septic tank, the permeability of the soil and, ultimately, the decision of the local building inspector or sanitarian. The goal is to achieve enough soil contact area to safely absorb the expected flow. That's determined, sometimes with the help of a soils engineer, by percolation tests and site analysis.

Perk tests indicate the rate, in inches per minute and under set conditions, at which the ground can absorb water. Sandy soils are particularly porous and may drain in several minutes—great for your plumbing but risky for nearby water supplies, since seeping effluent may not have time to be purified. Clay, on the other hand, may retain water for hours, inevitably causing a back-up in the system and a polluted bog on the surface.

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