Septic Systems

Introduction

Approximately 25 % of all Americans live in unsewered areas where they must utilize on-site septic systems for the disposal of wastewater from bathrooms, kitchens, and laundries. A septic system consists of two basic parts:

• A Septic Tank buried in the ground at some distance from the house to which it is connected by a pipe, and
• a soil Absorption Field.

A properly designed, installed and maintained septic system is an approved method of sewage treatment for private residences in rural or urban areas where public sewage treatment are not available. Sewage flows to the septic tank through the house sewer. In the septic tank, the solids are separated from the liquids, undergo anaerobic digestion and are stored as sludge at the bottom of the tank. The liquid, septic tank effluent, flows to the absorption field where it percolates into the soil. The soil acts as a final treatment by removing bacteria, pathogens and fine particles.

Septic Tank

The site of a septic tank is usually determined by the location of a major bathroom and kitchen plumbing within the home and by the slope of the land.

Diagram of a Septic Tank.

Locate septic tanks at least 15 feet from the foundation walls and approximately straight out from the point where the house sewer pipe goes through the wall. Locate the septic tank out of high traffic areas where excessive loads may damage the tank top. Keep clear of areas subject to flooding, pounding or surface drainage from surrounding areas. The location should be accessible to a drive way or other acceptable route so a tank truck can be driven close enough to pump out the sludge and clean out the tank. Do not locate septic tanks under sidewalks or patios where the tank is inaccessible for pumping.

A septic tank is a water tight container made of concrete or fiberglass with a minimum capacity of 1,000 gallons. They should be constructed of sound and durable materials that are not subject to excessive corrosion, frost damage, cracking or buckling due to settlement or backfilling. Minimum liquid depth of the tank should be 4 feet. The inlet should not be less than 2 inches above the liquid level. The figure in the right side illustrates a typical septic tank with sanitary tees used as baffles on the septic tank inlet and outlet.

Sewage entering the septic tank is partially decomposed by bacteria under anaerobic conditions. During this process, sludge settles to the bottom of the tank, while lighter solids and grease, as well as gases from the decomposing sludge, rise to the top to form a floating scum. The major purpose of a septic tank is to slow down the movement of raw sewage and wastes passing trough, so that solids can separate or settle out and be broken down by liquefaction and anaerobic bacterial action. It does not purify the sewage, eliminates odors or destroy all solid matter. The septic tank simply conditions the sewage so that it can be disposed normally to a subsurface absorption system without prematurely clogging the system.

If the septic tank is to receive ground garbage, its capacity should be increase by at least 50 percent. Some authorities recommend a 30% increase, other recommend against garbage disposal to a septic tank. An efficient septic tank should provide for a detention period longer than 24 hours.

Minimum Liquid Tank Capacity
Bedrooms	Gallons
3 or less	1,000
4	1,250
5	1,500
6	1,750

The table on the left shows some minimum capacities for septic tanks serving residential units. A septic tank for a private home will generally require cleaning every 3 to 5 years, but it should be inspected once a year. It is best to clean a septic tank during the dry months of the year.

Absorption Field

The absorption field must be of adequate size and proper soil porosity to ensure that the effluent seeping out of the pipes moves quickly enough to prevent pounding, but no so rapidly as to infiltrate aquifers, wells, or surface water supplies before contaminants in the effluent have been filtered out or oxidized.

Suitable soil is necessary for successful treatment of septic tank effluent. A suitable soil texture has a percolation rate between 5 to 60 minutes per inch. Clay soils are not suitable as sewage will not flow through them at an adequate rate. Very coarse-texture soils, sands or gravel are also not suitable because sewage flows through them too rapidly to be filtered properly. Bacteria may reach the ground water table if sewage is applied to coarse soil. Percolation test will provide information on the required size of the absorption field needed.

The conventional absorption field consists of two or more flat-botttom trenches not more than 100 feet long.

Diagram of a Septic System, including the Septic Tank and the Absorption Field.

Shorter trenches, 60 feet or less, are preferred. Each trench contains a line of 4 inch perforated plastic or sewer pipe, open-jointed drain tile or half-moon tile laid in a gravel bed. This permits the effluent to spread uniformly over the entire length of the trench and percolate into the soil. The amount of absorption area in the field depends on the number of bedrooms in the home and the percolation rate of the soil.

Since more than half of the solids in the wastewater settle out during the retention period in the septic tank, the accumulation of sludge at the bottom, as well as scum layer at the top, must be removed periodically. If this is not done, the sludge build-up reaches the point that solids are discharged in the absorption field, resulting in clogging and pounding. This accumulation of particulates and scum in the pores of soils prevents proper drainage of the effluent and eventually results in failure of the system.

In addition to improper maintenance of the septic tank, other reasons for septic failure are: use of a septic tan which is to small for the householder's needs, too great usage of water in the house, insufficient size of the absorption field. Although a well-located, carefully constructed, and properly maintained septic system can be a perfectly adequate method of sanitary waste disposal, malfunctioning septic systems frequently give rise to serious non-point source water pollution, or leaches containing pathogenic organisms and nutrients seep from absorption fields into water supplies.

Household Tips that Increase the Service Life of Your Septic System

1. How often should the septic tank be pumped?
   Every 1-3 years, when sludge in the tank is approaching the 1/3 full mark.
2. How does the use of household water affect the system?
   Conserve water. The more water used, the water must be disposed in the drain field. When purchasing appliances look for the water conserving features.
3. What are some of the common household habits that create problems in the septic tank?
   Avoid using the drains as a disposal. Grease is the worst thing in the system. Coffee grounds, cooking fats, filter cigarette butts, disposable diapers, paper toweling, tissues, etc. should not enter the system.
4. Can a garbage disposal be used?
   Garbage disposals are not recommended, food particles tend to clog up the system. If a disposal is used, it is recommended that you pump the system twice as often or increase your tank size by one third.
5. What about the use of household cleaning products?
   A moderate use of bleaches, cleaners and other household products will not harm the action of the system.
6. Is a special type of toilet paper required?
   Yes, select white paper specially recommend for septic tank use. There are no dyes to break down in white paper.
7. Absolutely never go into the septic tank!
   Lethal gases build up in the tank that are deadly. Call Professionals if you have a problem.

Groundwater Contamination

In aquifers, contaminants and recharge waters can flow similar pathways. This is the case for nitrates from household septic systems, livestock wastes and excess fertilizer application.

Diagram representing underground water contamination.

Some contaminants, like diesel fuel, are less dense than water and will stay mostly near the top of the aquifer. Other contaminants that are more dense than water and do not dissolve readily will tend to accumulate at the bottom of an aquifer. Such contaminants are referred to as dense, non-aqueous phase liquids.

Groundwater can become contaminated in several ways:

• spills on the ground, e.g., fuel and pesticide spills;
• injection into the ground, e.g., septic leaching beds, disposal of waste in wells, contaminated surface water running into poorly constructed well, poorly maintained wells, improperly plugged wells and back-siphoning from spray tanks into wells;
• improperly handling of industrial solvents and chemicals, e.g., varsol and wood preservatives;
• leakage from wastes, e.g., manure storages, wastewater, septic tanks and landfills;
• leaking underground and aboveground fuel storage tanks;
• movement of ground water between contaminated and clean aquifers;
• overapplication of soil amendments such as manure, commercial fertilizers or pesticides.

Geological formations may remove some contaminants. For example, metals like lead and mercury can become attached to soil particles. Nitrate levels can be reduced in the

Diagram representing underground water contamination caused by a leaking underground tank.

aquifer through denitrification.

The likelihood that groundwater could become contaminated depends on:

Contaminants move most easily through coarse-texture soils (sand and gravel) and fractured bedrock. But even clay soils have fractures that allow the movement of contaminants. Once contaminants have reached an aquifer, they are difficult and expensive to remove. High levels of a contaminant in an aquifer can make the water unfit and unsafe to use.

EPA: Groundwater

EPA: Water Pollutants

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