How a Septic System Works

Most basic septic systems usually consist of at least three parts:

1. The septic tank, into which flows the raw sewage and other liquid waste from the home.
2. The distribution box, which diverts the effluent so that it flows evenly into the drainfield lines.
3. The drainfield, into which flows the liquid effluent from the septic tank. The drainfield can be constructed using the gravel and pipe as illustrated below; when indicated our drainfields are constructed using a technology called the Infiltrator Chamber System, which offers several advantages over the crushed-rock method.


Inside the Septic Tank

The septic tank is basically a storage chamber for the raw sewage. The solids sink to the bottom of the tank, becoming part of the “sludge” layer. The next layer contains the liquid which will flow to the drainfield. On top of the liquid layer floats the “scum” layer composed of lighter liquids such as cooking oil, detergent byproducts and grease. Naturally occurring bacteria break down the solids and reduce the size of the sludge layer. An outlet baffle device with a filter is required by Florida code on all new installations of septic tanks to help keep solids out of the drainfield.


Septic tanks should be watertight. As waste water enters the system, the same amount is expelled into the drainfield. The bacteria that thrive in a septic tank are called “anaerobic bacteria” because they do not require oxygen. These bacteria are essential to the proper functioning of a septic system as they degrade and decompose the solids. It is not necessary to add bacterial additives to your system, no matter what the advertisers say, and some additives may cause harm.

The tanks we install are concrete, multichambered septic tanks with filters. In our experience, concrete is superior to the fiberglass tanks used by some contractors. Some installers use fiberglass due to ease of transport and handling, but in our opinion there is no advantage for the homeowner.

Why get your tank pumped and how often should you have it done? See our page on Taking Care of Your Home Septic System for more information. It is generally suggested that you have your tank pumped every 3 to 5 years on a regular maintenance schedule, which will help prevent premature drainfield failure.

The Distribution Box

The liquid in the septic tank (the effluent) flows by gravity to a distribution box. The distribution box evenly distributes the effluent from the septic tank to the drainfield. If the distribution box is not level more effluent will flow to one area of the drainfield than the others. This will cause an overload on that section and may cause it to fail. Again, concrete distribution boxes are preferable to plastic ones, for the following reasons:

– Sturdier construction
-Can be located with a probe rod from above ground, without digging
– Can be opened for inspection
– Maintains level better

It is our opinion that using a plastic box provides no benefit to the customer and that there is no application in which a plastic box outperforms a concrete box.

The Drainfield or Soil Absorption System

The drainfield or soil absorption field is generally a system of perforated pipes set underground in a bed of crushed rock that allows the effluent to seep slowly into the ground, undergoing further cleansing. The drainfield may be designed as “trenches” or a “bed”. If trenches are used there are usually 2 or more parallel trenches approximately 3 ft. wide. Gravel is placed around the pipe in the trenches and covered by fabric that prevents clogging of the pipe with dirt. Beds are used when space or soil considerations indicate their use instead of a trench system.

The Infiltrator Chamber System is an alternative to the gravel and pipe method of drainfield construction. Infiltrator chambers are specially designed units made of a resin material that are installed in 2 to 3 foot wide trenches or beds. They have a greater effluent filtration capacity than the stone and pipe systems so require as little as half the space to give equal performance. This can be a major advantage when lot size is small and results in less disruption of the landscape. For more information you can access the Infiltrator Chamber System Company website.

The size of the drainfield is determined by the number of square feet that is specified in the Health Department permit. The requirements are based on a number of factors, some of which are the type of soil, the location of the water table, and the number of bedrooms in the house (which is an indicator of future water usage based on number of residents). Some soils are better suited to septic systems than others. Well aerated soil with good permeability is desired. Clay soil will generally have lower permeability and will require a larger drainfield than a system located in sandy soil.

Before a permit is issued a site evaluation must be performed, which identifies these criteria by boring holes at several locations on the lot and examining the soil in these holes. The location of any surface water, such as a lake, and underground water supplies, such as wells, must be identified. Wells should be located at least 75 feet from any part of a septic tank system. Septic tanks and drainfields must be also be located at least 75 feet away from the high water line of natural bodies of water such as ponds, lakes or rivers. Runoff from roof gutters and surface drainage patterns must be taken into consideration when siting a drainfield location so that it will not become saturated by surface water.

Florida Code requires that the bottom of the drainfield be 24 inches above the water table so some systems will need fill to raise the area where the drainfield will be installed. This is often called a “mound” system.

Mounds and Dosing Septic Systems

Mounded septic tank systems are required by Florida code when the water table is shallower than 36 inches below natural grade or when restrictive soils (e.g. limestone rock, pipe clay) are found 42 inches or less below natural grade.

Mounds are more expensive than a standard below grade drainfield and require much more space. Every effort should be made to locate your system in an area that does not require a mound. On some lots there is no other option except to use a mounded system. However, by careful selection of the areas chosen for the site evaluation you may avoid having to install a mound. As a Master Septic Tank Contractor, Mike Sundin can perform the site evaluation and select the best location for your system.

The elements of a mounded system are listed below:

– The drainfield, which can be either trench or bed configuration; the bottom of the drainfield must be 24 inches above the water table.
– The 5 foot buffer of sand which surrounds the drainfield.
– 9 to 12 inches of sand cover which must go over the top of the drainfield. This sand must be of a specific quality, neither too fine nor too coarse. The finished height of a mound must be 21-24 inches above the bottom of the drainfield (12 in. of gravel drainfield + 9-12 in. of sand cover).
– The slope that holds the mound in place, which can be either a 5:1 or 3:1 slope depending on the stabilization method used.
– Stabilization using either seed & mulch for a 5:1 slope or sod for a 3:1 slope.

In some cases, dosing systems are required to pump effluent from a gravity fed septic tank into a raised mound drainfield or to a remote underground drainfield. A dosing system may also be required in some commercial applications when the gallons per day usage or size of the drainfield necessitates its use.

The elements of a dosing system as installed by Apalachee Backhoe and Septic Tank, LLC are:

– A dosing tank which is usually a 250 gallon concrete septic tank.
– A pump with sufficient capacity to pump effluent to the height and distance necessary (usually a 0.3 hp to 0.5 hp pump). Apalachee Backhoe and Septic Tank, LLC. uses Zoeller pumps, which are of the highest quality available for this application to ensure long life and problem free service.
– An inline check valve that prevents backflow of effluent into the pump chamber.
– A high water alarm which signals when effluent rises higher than acceptable in the pump chamber which may indicate a problem with the pump.
– An inline union which allows simple disconnect of the pump from the dosing tank.
– A transmission line which carries the effluent from the pump to the drainfield. We use Schedule 40 PVC pipe which is stronger and more durable than thin wall pipe.

We have experienced service technicians available to attend to any problems with these systems, although the high quality pumps and equipment we use require less service and maintenance than many others. We are able to service pumps and dosing systems installed by other companies.