U.S. Inspect Home Inspection:

Advisory Report

December, 2003

Septic Systems Overview
Part One of Three: Traditional Systems

With so many types of septic systems out there it is easy to become confused when distinguishing between them. In the next couple issues we’ll go through several of the most common types of septic systems. This article will cover traditional septic systems.

The most common septic system is referred to as the “traditional system.” It’s nothing fancy; just a tank, some pipes, and a leaching field.

The Tank

It all starts with the septic tank, which serves several functions. Most important, it retains the solid waste. The heavy solids from the waste settle to the bottom, forming “sludge” and the oils and light solids rise to the top, forming “scum” or a “scum mat.” By separating the solids, the tank allows only the liquid in between the sludge and scum, known as effluent, to pass through to the absorption area.

The septic tank also allows naturally occurring bacteria to break down the retained solids. In a traditional system the bacteria acts without the aid of airflow (i.e. anaerobic) and only breaks down approximately 65-70% of the solids. In a system where air is introduced (i.e. an aerobic system), the breakdown rate is much higher, at approximately 90-95%. Once broken down, the solids become part of the effluent, which is passed into the absorption area.

Buried at variable depths below the ground, the tank is made of concrete, fiberglass, polyethylene, or metal (metal tanks are no longer used, but can still be found in some older systems). Some tanks are separated into two compartments, which allow more solids to settle out before the effluent leaves the tank. At either end of the tank are baffles (or sanitary tees), one is an inlet baffle and one is an outlet baffle. Last, there are two or three access holes into the tank used to inspect the baffles and pump out the tank.

Barring any problems with the tank, the water level in the tank will remain constant with the outlet baffle. As new water is introduced, it forces an equal amount of effluent through the outlet baffle into the absorption area. The baffles are “T” shaped, allowing the vertical piece of the pipe to start above and protrude well below the level of the scum. The inlet baffle slows the fast-moving incoming water from disturbing the solids in the tank. The outlet baffle prevents solids from leaving the tank before being broken down.

The Absorption Area

In a traditional system, the absorption area is a leaching field--also known as a tile bed. The leaching field is the section of the ground that absorbs effluent discharged from the septic tank. The field is comprised of several lines, or “fingers,” that stem from the distribution box, allowing the effluent to spread across all areas of the field.

Once the effluent leaves the septic tank, it enters a distribution box. The distribution box evenly distributes the effluent into each of the leaching field lines. The box must be level. If it is not, a disproportionate amount of effluent will enter one line, which will cause that part of the field to fail prematurely.

When the leaching field is initially installed, trenches are dug, a bed of gravel is laid, the lines are installed over the gravel bed, more gravel is placed over the lines, and the trenches are back-filled with soil. The gravel acts to diffuse the incoming effluent, allowing it to come into contact with more soil and absorb faster. Ultimately the absorption capacity (or percolation rate) of the soil dictates how fast the effluent will be absorbed. The “quality” of the soil varies by geographical location and will be a major factor in the design of the system (or the type of system used).

Maintenance

With proper maintenance, the septic system will last for a long time. This is a good thing, as it can cost quite a bit to replace and anyone would want to avoid the experience having a system back up into the house. Maintenance means more than having the tank pumped regularly. There are several additional things that will lengthen the life of the system.

Pump Tank. Pumping out the septic tank regularly is the most important part of maintaining the system. The average tank should be pumped every 2-3 years. However, this can vary greatly depending on the size of the tank in relation to the number of bedrooms and occupants in the house, the use of a dishwasher, garbage disposals, washing machines, etc. Because of the number of variables involved, it is best to have a local septic company create a regular maintenance schedule.
Appliances. Regular use of garbage disposals, dishwashers and washing machines that drain into the system require the tank to be pumped more frequently. This is also true with water softeners or other water treatment devices. When factoring the use of these appliances into a regular pump schedule, it is common to subtract six months to account for each item.
Chemicals. Chemicals or harmful materials should not be flushed into the tank. Chemicals can kill the naturally occurring bacteria, preventing solids from breaking down properly. The water from a hot tub may have this same effect. Grease, paper (other than toilet paper, cigarettes, coffee grounds, sanitary napkins and other similar solids will not break down and will eventually clog the tank or lines. Also, biological and chemical tank additives should be avoided. These do not improve the performance of the tank and some of these additives can be harmful to the environment.
Water Usage. Water should be used wisely. If water equal to or greater than the maximum amount that the system can handle is routinely used in one day (known as the “daily designed flow”), it will develop problems much faster. Try to keep the amount of water used under the daily designed flow. The daily designed flow varies depending on the size of the tank and a local septic company can help determine this. Employing the use of water saving devices (low-flow toilets, low-flow shower heads, and low-flow sink faucets) will make it much easier to reduce water the amount of water used.
Structures. Do not build over the tank or a leaching field. The weight from a structure, or even driving over the system can cause damage to it. Landscaping and/or other structures over the leaching field can prevent water from evaporating from the soil, decreasing the efficiency of the system and leading to premature failure. Any structures or landscaping located over the system will also prevent access to the system making it very difficult to maintain. Also, do not allow trees or shrubs to grow over the tank or field. The roots from trees and shrubs can infiltrate the lines and tank, clogging them.
Ground Water. Roof water run-off and ground water drainage should be prevented from accumulating in the leaching field area or over the tank. Additional water from outside sources can drastically reduce the efficiency and life span of a septic system. Proper soil grading and the installation of gutters, downspouts, and extensions can help with this.