The septic tank is the heart of your onsite wastewater system. Heavier solid matter sinks to the bottom of your septic tank and forms the sludge layer, while lighter fats, oils, and greases (FOG) float to the top and are broken down by anaerobic organisms.

Your septic tank is designed to hold your sewage and allow it to settle before flowing into the drain field. 

When you flush a toilet or use your washing machine, wastewater flows down your home’s plumbing lines to a septic tank. This underground, watertight container provides primary wastewater treatment with the help of natural bacteria that break down organic matter and solids. Septic tanks then pipe partially clarified effluent downstream for further purification or dispersal.

All drain pipes in your house connect to the inlet pipe that leads directly to your septic tank. From there, all wastewater flows into the tank. As it does so, fats and oils float to the top to form the scum layer while solid particles sink to the bottom to become the sludge. Bacteria do most of the work of separating these layers, leaving liquid effluent in between.

A septic tank has both an inlet baffle and an outlet baffle, which are essential for the proper functioning of your system. The inlet baffle guides the wastewater in a longer path across the tank so it does not disturb the floating scum layer. Without this baffle, the toilet tissue and other solids that enter the tank could pile up on the scum layer and cause a blockage that redirects sewage back into your home.

An outlet baffle, meanwhile, directs the flow of wastewater from the septic tank to the distribution system and then to your septic drain field. The baffle also helps to prevent sludge from flowing directly into the outlet pipe, which would cause drainfield clogs and premature system failure.

As the septic tank treats wastewater, it produces gases, including hydrogen sulfide. These gases must be released to avoid a build-up of pressure that could stop or reverse the flow of effluent. To prevent this, your septic tank is fitted with a vent. Typically, it has a mushroom shape and can be equipped with a charcoal filter to reduce odors.

A septic tank’s access ports enable you and a septic professional to easily clean out your system’s components. These ports should be large enough for a hose to reach. In addition, a septic professional will install risers to bring the tank up to ground level for cleaning and inspection.

Sludge & Scum Layers

More than one in five households in the United States rely on septic systems to treat wastewater from toilets, showers and drains. These independent, decentralized wastewater treatment structures use natural and technological processes to separate solids from wastewater and drive it through an absorption field. Septic tanks have three distinct layers—a scum layer on top, a sludge layer and clarified effluent layer in the middle—that must be kept in balance for proper function and longevity.

The septic tank must have relatively calm conditions to form the layers and ensure that settleable solids drop to the bottom and floatable solids rise to the surface. In some cases, turbulence causes the layers to mix together. This can occur when there is a high level of water flow into the septic system, large leaks into the tank or a pump that adds sewage into the system. The septic tank should be regularly inspected and pumped to prevent the sludge layer from building up too much, which can block inlet and outlet pipes and cause untreated wastewater to enter the absorption field and soil treatment system.

Regular septic tank pumping is essential for keeping the septic system in good working condition and avoiding costly repairs and replacement of your septic system components. Your septic tank should be pumped when the sludge layer reaches 25% of the tank’s capacity. This is generally every 3-5 years, depending on household usage.

When septic tanks are pumped, the scum layer is removed along with anything else that floats on top of sewage. This includes soap scum, fats, oils and other waste that is lighter than water. If these floatable wastes make their way through the inlet and outlet pipes into your septic system, they will be carried to your absorption field and clog the drain fields, which can lead to wastewater backups.

In addition to routine septic tank pumping, it’s important to avoid using chemicals in your home that can impact stratification. These include phosphate-based cleaners, fabric softeners and degreasers. These chemicals can disrupt the normal bacterial community in your septic tank, which reduces its ability to break down and process wastewater.

Absorption Fields

As wastewater (effluent) leaves your septic tank it goes into an absorption field, also known as a soil drainfield or leach field. The septic drain field disposes of wastewater by percolating it into the groundwater beneath the grass and soil, where it is treated further by filtration and absorption. The soil drainfield is a crucial part of any septic system and must be designed to meet the specific needs of your property and soil type.

The soil of a septic field needs to have the ability to treat liquid waste in a short period of time to avoid excessive water flow and the contamination of groundwater. The soil’s ability to do this is called the percolation rate, which is measured in minutes for each inch of depth. Septic professionals must dig a number of holes to determine the soil’s perc rate, and then they can calculate the size of your absorption field accordingly.

The absorption field consists of a series of gravel-filled trenches with perforated pipes that allow the sewage to seep through them into the soil underneath. The absorption fields are typically separated into different disposal areas, so that the soil can rest between each area. This allows nematodes to work on the fats, oils and other organic material that have been accumulated in the drain field, improving the soil’s percolation rate.

Solids that are allowed to pass from the septic tank may clog absorption fields and lead to expensive repair or replacement costs. To prevent this, septic tanks are designed with effluent filters that trap solids at the outlet of the tank and keep them from entering the absorption field. These filters are included in newer septic tank designs, but can be installed in older tanks as well.

It is important to divert runoff from roofs, sidewalks and patios into a rain garden or other method of drainage, to prevent the absorption field from becoming soaked. Soaking a drain field reduces its percolation rate and can cause problems with the treatment process. Also, the absorption field should be constructed when the soil is dry, rather than wet, to prevent puddling, smearing and soil compaction that reduces the system’s permeability.

Maintenance

Many households across the United States rely on individual septic systems (also known as decentralized sewage systems) to treat wastewater from household plumbing. These are most commonly found in suburban and rural locations not served by municipal centralized sewer systems.

Domestic sewage from toilets and wastewater from bathtubs, showers, sinks, laundry machines, and dishwashers flows into the septic tank. Heavier solid waste settles at the bottom of the tank forming sludge while lighter fats, oils, and greases float to the top of the tank creating scum. Anaerobic bacteria in the septic tank feed on these organic wastewater pollutants converting them to inert matter.

The fully-treated effluent from the septic tank is then discharged into an absorption field. This system consists of pipes with holes in them that are placed in gravel trenches underground. As the wastewater is filtered through the soil and grass above, it is absorbed into the groundwater supply.

In addition to the septic tank, the absorption field, or drainfield is a critical component of your septic system and must be maintained properly for your home septic system to function correctly. A good septic tank installation should include a riser and lid at grade for routine pumping, inspection, and maintenance.