Ohio State University Extension

Food, Agricultural and Biological Engineering

590 Woody Hayes Dr., Columbus, Ohio 43210

Septic Tank-Soil Absorption Systems

AEX-743-93

Karen Mancl

Households not served by community public sewers or a sewer authority often depend on septic tank-soil absorption systems to treat and dispose of wastewater. The septic tank removes most settleable and floatable solids from the wastewater; the soil absorption system filters and treats the clarified septic tank effluent. By removing most solids, the septic tank protects the soil absorption system from clogging and premature failure. To work properly, the septic tank needs periodic maintenance (see Extension Fact Sheet AEX-740, Septic Tank Maintenance).

In Ohio, soil absorption systems can be used in areas where the percolation rate of the soil is between 3 and 60 minutes per inch (soil permeability between 1 and 20 inches per hour). At least 4 feet of suitable soil is required under the soil absorption system to provide adequate treatment of the septic tank effluent. To accommodate the construction of the system and provide adequate soil cover to grade, a minimum of 5 1/2 to 6 1/2 feet of suitable soil is needed above the limiting layer. A limiting layer may be bedrock, an impervious soil layer (hardpan, fragipan) or a seasonally high water table (gray soil or mottles). The soil absorption system must be at least 8 feet from any drain line on the lot, 50 feet from a water supply, and 10 feet from the property line, right-of-ways and the house. Septic systems cannot be placed on the flood plain and are limited to areas with less than a 15 percent slope.

Septic Tank

A septic tank is a watertight container constructed of a sound, durable material resistant to corrosion or decay. Septic tanks may have one or two compartments. Two compartment tanks or two single tanks in series do a better job of settling solids and are required for homes with four bedrooms or more.

Among the most important components of a septic tank are the baffles. The inlet baffle forces wastewater down into the tank, preventing short-circuiting across the top. The outlet baffle keeps the scum layer from moving into the soil absorption system. Septic tanks have inspection ports for checking the condition of the baffles and a manhole for cleaning the tank.

The capacity of the septic tank is based on the size of the house. In Ohio, a 1000-gallon tank is required for a home with one or two bedrooms. For a three bedroom home, a 1500-gallon tank is required. A 2000-gallon tank is required for 4 to 5 bedrooms and a 2500-gallon tank is required for 6 or more bedrooms.

Figure 1. Cross section of a septic tank.

Soil Absorption System

The soil absorption system receives effluent from the septic tank and filters and treats the effluent before it enters the groundwater. At least 4 feet of unsaturated soil beneath the soil absorption system is needed to renovate wastewater before it reaches a limiting layer. A limiting layer may be bedrock, an impervious soil layer or the seasonal high water table.

The soil absorption system is a set of trenches 18 to 30 inches deep, at least 8 inches wide, and placed at least 6 feet apart. The maximum length of a trench is 150 feet. The bottom of these trenches must be level; construct them to follow the contours of the lot.

The bottom of each trench is filled with 6 inches of clean gravel. A 4-inch perforated pipe is placed on top of the gravel and covered with 2 more inches of gravel. The top of the gravel is covered with synthetic building fabric before the trench is backfilled with native cover soil. This prevents the soil particles from moving down into the gravel. Two inches of straw or a layer of untreated building paper are still sometimes used in place of the fabric. The cover soil should be mounded to account for settling, graded to avoid ponding of rainwater, and seeded with grass to prevent erosion.

The size of a soil absorption system is based on the size of the house and the percolation rate of the soil. For a three-bedroom home the yard area needed for the absorption system could range from 4400 ft² for a soil percolation rate of 3 minutes per inch to 9000 ft² for a soil percolation rate of 60 minutes per inch. This provides only the minimum area for the soil absorption system and the required replacement area. The set-backs from the water supply, lot lines, and drainage lines must also be taken into account.

Figure 2. Trench soil absorption system.

Wastewater Distribution

In Ohio, the soil absorption system is divided into two equal sections. A diversion device is used to alternate the flow of wastewater from one side of the system to the other. The diversion device should be easily accessible for annual switching.

Things to Watch Out For

• Keep heavy equipment off the soil absorption system area both before and after construction. Soil compaction can result in premature failure of the system.
  
• Divert rainwater from building roofs and paved areas away from the soil absorption system. This surface water can increase the amount of water the soil has to absorb and cause premature failure.
  
• Make sure the alternating device and the trench bottoms are level to provide even distribution of the septic tank effluent. If settling and frost action cause shifting, part of the soil absorption system may be overloaded.
  
• Avoid installing the septic tank and soil absorption system when the soil is wet. Construction in wet soil can cause puddling, smearing, and increased soil compaction. This can greatly reduce soil permeability and the life of a system.
  
• Install water-saving devices. Water conservation can greatly reduce the amount of water going into the soil absorption system and extend its useful life.
  
• Have the septic tank pumped and inspected regularly.
  

(See Extension Fact Sheet AEX-740, Septic Tank Maintenance.) For more information on septic tank soil absorption systems contact the local health department or your county office of Ohio State University Extension.

Figure 3. Soil absorption system with diversion box.