There are several types of septic systems, each with a different design. A conventional system consists of three main parts:  the septic tank, the drain field, and the soil beneath the drain field.


Learn about septic systems and how septic systems work.

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There are several types of septic systems, each with a different design. A conventional system consists of three main parts:  the septic tank, the drain field, and the soil beneath the drain field.

The Septic Tank

The septic tank is a watertight container made of concrete, fiberglass or polyethylene.  Most single family homes have a 1,000 – 1,500 gallon tank, which is buried in the ground just outside the home. The tank outlet is connected to the drain field by a buried pipe.

Wastewater and solids exit the home through toilets and drains and end up in the septic tank.

The function of the septic tank is to separate solid material from the wastewater. The solids are broken down by naturally occurring bacteria that inhabit the tank.

Septic Tank cutaway

Septic tanks may contain one or two chambers. Scum is the material, which floats on the top of the water in the tank where aerobic (air breathing) bacteria digest it. Sludge is undigested solid matter, which settles to the bottom of the tank. Here anaerobic (without air) bacteria digest some of the matter, the undigested part must be removed periodically by a licensed septic system pumping service every 2 - 5 years depending on use.

The Distribution Box

A distribution box (D Box) can be made of concrete or plastic.

Fiberglass D-box
Concrete D-box
Plastic D-box

In many systems, a distribution box helps move wastewater to each trench.


The Drain Field or Leach Field

...absorbs wastewater and delivers it to the surrounding soil. A typical drain field consists of one to five trenches filled with a layer of washed gravel or stone placed around a 4-inch-diameter perforated drain line. After the trenches are covered with soil, the area is landscaped to keep surface waters from reaching the drain field. Beneficial bacteria in the soil purify wastewater by removing pathogens and chemicals before it reaches the groundwater or any adjacent surface waters such as rivers or lakes.

Gravity fed septic system drain field

A septic tank that is not pumped on a regular basis can send undigested solids and organic matter to the drain field. This material will eventually clog the soil in the drain field and cause flooding and total failure of the system. When soil floods natural bacteria colonies will die off and no longer keep the system in balance. To restore beneficial bacteria and help keep the drain field working properly try our industrial-grade bacteria treatment Mega Bio.

A typical drain field

is gravity fed. It usually consists of several gravel-filled trenches with perforated drainpipe buried just below the surface of the gravel running the length of the trenches. The effluent is distributed, usually through a distribution box, to the various lateral drainpipes exiting through the perforations into the surrounding gravel bed. Secondary processing begins in these beds as the effluent fills the trench then percolates through the bio mat and into the surrounding soil.

Septic System Drain Field

Infiltrator® systems

are more recent technology. Infiltrator chambers are hollow structures that attach end-to-end. They are installed in trenches or beds without gravel (except where local codes require the use of gravel). The entire bottom of the trench is open for unobstructed infiltration of water. The large storage volume within the hollow chambers accommodates peak flows of effluent from the home. Infiltrator chambers also feature patented sidewall louvers that allow lateral leaching of effluent into the soil.

infiltrator system

Mound systems

On problem sites, mound systems may be an alternative. In a mound system septic tank effluent is delivered to the mound through the use of a pump in a dosing tank placed after the septic tank.

Mound system

Seepage Pit

The mound itself is carefully constructed above ground using specially selected sand placed on top of the natural soil to help treat and dispose of septic tank effluent. The depth of sand is determined by the depth of the natural soil above a limiting layer. A limiting layer can be bedrock, a soil layer with a very low percolation rate, such as clay, or seasonally high groundwater. The depth of sand added to the depth of the natural soil must equal the minimum treatment depth prescribed by local authority.

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A seepage pit may be used instead of a lateral drain field or leach field.  It is usually a hole in the ground about 4 to 8 feet deep lined with perforated stone on sides and bottom.  The bottom may not come within a certain distance, usually about three feet, from the local water table. A seepage pit is not a drywell or a cesspool. The main difference is what is being put in the pit. A drywell receives graywater and other liquid that comes from sources other than septic, such as clothes washer, or water softener discharge.  A seepage pit receives liquid from a septic tank, the solids remain in the tank and the liquid effluent goes to the seepage pit to be processed before returning to the water table. A cesspool receives raw sewage, liquid and solids, and holds and treats it before returning it to the local water table.

Because of the depth of most seepage pits little or no oxygen exists in this environment, aerobic bacteria, which make up the bio mat, cannot survive therefore the effluent, which is processed as it passes through the bio mat will not be properly treated.

When trying to determine if a seepage pit is working properly observe the water level in the pit, it should be low and maintain a somewhat constant level over time. If a seepage pit is full it is not properly functioning.

The main cause of seepage pit failure is failure of the soil. This occurs for various reasons, as with all septic system leachfields and drainfields, the biomat can become overgrown when active, naturally occurring aerobic bacteria in the system die off. When the system floods anaerobic bacteria take over. This causes the rate of digestion to be greatly reduced and the slow working anaerobes increase the tar-like layer known as the biomat. When the biomat becomes too thick it prevents passage of water to the surrounding soil where it is further processed before returning to the local water table.

The second reason is sodium bonding, which occurs especially in clay bearing soil. Studies have shown clay particles become hardpan in the presence of sodium, which is a common mineral in cleaning supplies, diet and water softener discharge water.

Seepage pits are difficult to treat effectively and some jurisdictions are not allowing new seepage pits to be installed. We have had some success treating failing seepage pits but the success rate is lower than that of conventional drainfields.

Mega Bio is a combination of industrial-grade bacteria that can exist in extremely low oxygen environments. It can bring the biomat back into balance and colonize within the system to maintain that balance.

Septic Perc® is an eco-friendly chemical formula that can release the bond in hardpan soil and restore drainage.

Have the pit pumped to remove most of the water. Start with 6 gallons of Septic Perc. Apply 3 gallons directly to the pit for first treatment, then one gallon a week flushed down a toilet in the home.

Where the bio mat is overgrown an application of 8 ounces of Mega Bio applied directly to the pit may help restore drainage. In extreme cases pressure washing of the walls of the pit may increase the chance of restoring drainage. We suggest contacting your local septic system professional.

Recommended Treatment for a Failing Seepage Pit

The Cause of Seepage Pit Failure

The Biomat

Typical bacteria

The purpose of an on-site septic system is to treat sewage before it returns to the local water table by reducing the number of pathogens and nutrients it contains. This is accomplished by beneficial bacteria in the septic drainfield that digest organic material present in wastewater.

Treatment occurs in 2 phases, anaerobic (without oxygen) digestion in the septic tank & aerobic (with oxygen) and anaerobic digestion in the drainfield area and filtration in the surrounding soils. About half the pollutants are removed by anaerobic digestion or retained in the septic tank, and the remaining constituents are digested by bacteria in the drainfield and the surrounding soil.

First, wastewater runs from the house to the septic tank where solids are separated from the liquid effluent. This is also where anaerobic digestion begins, the resulting byproducts include methane, hydrogen sulfide, sulfur dioxide and sludge.

When the partially treated effluent leaves the septic tank it is distributed to the various gravel filled trenches by a distribution box, which channels the effluent through a series of drainpipes or chambers in the septic system drainfield.  


A biological mat (biomat) develops on the sides and bottoms of the trenches. This biomat consists of solids from the effluent, some minerals, microorganisms and sulfites, which are the product of decomposition. The biomat is a necessary part of a healthy septic system drainfield as it provides a home for anaerobic bacteria that digest organic matter in the effluent before it reaches the surrounding soil. A problem occurs when these bacteria die off due to the introduction of chemicals, strong medications and overuse of cleaning agents. Left untreated this could result in drainfield failure due to an overgrown biomat.

The surrounding soil is an integral part of the working septic system drainfield. This is where digestion of dissolved components occurs by aerobic bacteria. The aerobic bacteria’s digestive action is the final treatment before the processed effluent is returned to the water table.

Facultative bacteria which can exist both with and without oxygen, or random seed bacteria which are everywhere in the soil can colonize a drainfield, as long as food is present. A well working septic system should never need bacteria added to the septic tank as a maintenance routine.


Mega Bio is an industrial-grade bioenzymatic formula containing 10 billion CFU (colonizing facultative units) per gram. A single 8 ounce treatment provides more than 2,240,000,000,000 (two trillion-two hundred-forty billion, compare to others on the market) microbes to your drainfield.

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Mega Bio bacteria septic treatment