Septic Systems

From the Series: "Home Construction & Maintenance in Mexico"

By J. Brad Grieve, P.Eng. MBA Copyright© 2006
His Web Site

For the average new homebuyer, there is nothing more horrifying than the thought of buying a home with a septic system. This hidden, underground system seems to conjure up thoughts of backyards flooded with human waste or worse; backed up toilets.

A septic system is typically comprised of a tank that collects all the “black water” from the home. Black water is the common term for wastewater from the toilets that needs to be treated in the septic tank. The other wastewater from sinks, laundry and showers (tubs), is referred to as “gray water," which, in general, will not contain waste material (you know what I mean) that needs to be treated in the septic tank. In some homes, the gray water is separated and bypasses the septic tank to drain separately.

The septic tank itself is typically a two-stage tank that has a baffle wall between the two sides that allows transfer between the two sides via an opening near the base of the subdividing baffle wall. By making the tank a two-stage tank, it minimizes the amount of scum, solids or sludge that could drain out of the septic tank, prolonging the lifespan of the entire system. The effluent in the first stage of the tank usually has three distinct layers; at the top is the scum layer, at the bottom is the sludge layer and in-between is the layer of liquid effluent. The bacteria activity occurs in all three layers however, primarily in the liquid layer where natural anaerobic bacteria break down, degrade and decompose the waste products and convert many of the solids into liquid. This natural process converts the nasty aspects of the black water into more benign liquid that is drained from the septic tank.

In general, septic systems are similar to the rest of North America however; they do not use a drain field but rather an absorption pit. The absorption pit, which is also called a seepage pit or drywell, is an underground cavity, that is typically has an open bottom, walls with courses of staggered bricks (with openings between the bricks) and a top made of boveda (steel beams with the bricks arched between the beams, like ceilings of many homes here at Lake Chapala). The treated effluent leaves the septic tank to be absorbed into the ground surrounding the absorption pit.

Most areas here at Lake Chapala have soils with very good percolation (the ability of the soil to absorb liquid) and the systems remain active (bacteria and enzyme activity which breaks down the waste materials) year round because the underground soils layers remain relatively warm. However, there are areas with soils that have little ability to absorb or the natural water table is high, and limits the amount of treated effluent to be absorbed into the soil. Typically homes in these areas will have larger or various, absorption pits to provide more area of absorption, which will compensate for the low absorption rate of the soil. . .

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