Septic Tanks: An Overview

A septic tank is a large, underground, watertight container, typically about 9 feet long, 4-5 feet wide and 5 feet tall that is connected to the home’s sewer line. While typically designed with a 1,000-gallon liquid capacity, the size of the tank is legally determined by the number of bedrooms in the home. (Septic tanks come under the legal supervision of counties in Montana.) Septic tanks may be rectangular or cylindrical and may be made of concrete, fiberglass or polyethylene.

Raw waste water from the bathroom, kitchen and laundry room flows into the tank where the solids separate from the liquid. Light solids, such as soap suds and fat, float to the top and form a scum layer. This layer remains on top and gradually thickens until you have the tank cleaned. The liquid waste goes into the drainfield, while the heavier solids settle to the bottom of the tank where they are gradually decomposed by bacteria. But some non-decomposed solids remain, forming a sludge layer that eventually must be pumped out.

Septic tanks may have one or two compartments. Two-compartment tanks do a better job of settling solids and are required in some areas for new installations. Tees or baffles at the tank’s inlet pipe slow the incoming wastes and reduce disturbance of the settled sludge. A tee or baffle at the outlet keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments.

The Drainfield: Further treatment of wastewater occurs in the soil beneath the drainfield. The drainfield consists of long underground perforated pipes or tiles connected to the septic tank. The network of pipes is laid in gravel-filled trenches (2–3 feet wide), or beds (over 3 feet wide) in the soil. Liquid waste or effluent flows out of the tank and is evenly distributed into the soil through the piping system. The soil below the drain-field provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil.

The soil filters the effluent as it passes through the pore spaces. Chemical and biological processes treat the effluent before it reaches groundwater, or a restrictive layer, such as hardpan, bedrock, or clay soils. These processes work best where the soil is somewhat dry and permeable, and contains plenty of oxygen for several feet below the drain field. The size and type of drainfield depends on the estimated daily wastewater flow and soil conditions.

Tips for using your septic system:

-Even a properly designed and installed septic system cannot treat wastewater if the tank is not used and maintained properly. Here are a few tips for installing and using your septic system:

-For future maintenance and to avoid deep root planting and other damaging activities in the drain-field area, make an accurate diagram showing the location of your

tank, drainfield and replacement area.

-Keep a record of pumping, inspection, and other maintenance. Include name, address and phone numbers for installers and pumpers.

-To simplify tank access for inspection and maintenance, install a watertight concrete riser over the septic tank.

-The area over the drainfield should be left undisturbed, with only a mowed grass cover. Roots from nearby trees or shrubs may clog and damage your drain lines.

Scott Byers