How Septic Systems Work

Households that are not served by public sewers usually depend on septic systems to treat and dispose of wastewater. Septic systems represent a significant financial investment. If cared for properly, a well designed, installed, and maintained system will provide years of reliable, low-cost service.​ ​ A failing system can become a source of pollution and public health concern, causing property damage, ground and surface water pollution (such as well water—both yours and your neighbors), and disease outbreaks. Once your septic system fails to operate effectively, you may need to replace it, costing you thousands of dollars. Plus, if you sell your home, your septic system must be in good working order. Therefore, it makes good sense to understand and care for your septic system. There are many different types of septic systems that fit a wide range of soil and site conditions. The following will help you understand the main components of a standard (gravity fed) septic system and how to keep it operating safely at the lowest possible cost.

Septic Tank

The typical septic tank is a large buried rectangular or cylindrical container made of concrete, fiberglass, or polyethylene. A septic tank’s purpose is to separate solids from the wastewater, store and partially decompose as much solid material as possible, while allowing the liquid (or effluent) to go to the drainfield. ​ Wastewater from your toilet, bath, kitchen, and laundry flows into the tank and remains there for up to 24 hours (known as the retention time) before it passes to the drainfield. This helps prevent clogging of the drainfield, which can lead to failure and costly repairs.

The Septic Tank

A septic tank’s purpose is to separate solids from the wastewater, store and partially decompose as much solid material as possible, while allowing the liquid (or effluent) to go to the drainfield.

The Drainfield

After solids settle in the septic tank, the liquid wastewater (or effluent) is discharged to the drainfield, also known as an absorption or leach field.

The Soil

The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the wastewater has passed into the soil, organisms in the soil treat the effluent before it percolates downward and outward, eventually entering ground or surface water. The type of soil also impacts the effectiveness of the drainfield; for instance, clay soils may be too tight to allow much wastewater to pass through and gravelly soil may be too coarse to provide much treatment.

Give It Time to Sink and Float

The retention time is necessary to allow the solids to properly separate from the liquids—heavy solids settle to the bottom as sludge and the lighter particles rise to the top, forming a scum layer. Although bacterial action partially decomposes some of the solids, up to 50 percent remain in the tank. Until the mid-1970’s, septic tanks had one compartment; however, current regulations require two chambers, which do a better job of settling solids. In residential systems, the tank size is determined by the number of bedrooms and should be enough to handle approximately three years worth of sludge and scum. In commercial establishments, the tank size is determined by the amount of daily flow.

As wastewater flows into the tank, a tee (or baffle) at the tank’s inlet pipe slows the incoming wastes and reduces the disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank. In tanks installed since 1995, an effluent filter is attached to the outlet baffle (going to the drainfield) to keep solids in the tank instead of entering the drainfield. Effluent filters are an excellent addition to an older tank, and can be installed by a pumper or other septic system professional.

As the volume of sludge and scum builds up, there is less space and time for the solids to separate before the wastewater leaves the tank, which causes the system to be less effective. With not enough time for solids to settle, they can pass into the drainfield with the wastewater (or clog the effluent filter, if there is one). This causes the drainfield to gradually plug and eventually fail, causing sewage to back up into the house or effluent to surface outside. Also, the closer the thickening scum and sludge layers come to the outlet tees, the greater the risk that they can plug the tank inlet or pass into the drainfield.

Consequently, it is important that solids be removed by periodic pumping, so they do not overflow into the drainfield. Most septic tanks need to be pumped every 3 to 5 years, depending on the tank size and the amount and type of solids entering the tank.

All tanks should have accessible covers for checking the condition of the tees and for pumping both compartments. To eliminate the time and nuisance of digging down to the access covers, risers can be installed. The riser(s) should be secure to prevent accidental entry into the tank, and should also be watertight to prevent groundwater from entering the riser cavity, which may cause the tank to flood.

Drainfield

The drainfield is a network of perforated pipes (or “laterals”) laid in gravel-filled trenches or beds. After solids settle in the septic tank, the liquid wastewater (or effluent) is discharged, either by gravity or pressure, to an absorption field, also known as a drainfield or leachfield. NOTE: In most gravity systems the wastewater first flows into a distribution box (d-box) or tee, which then disburses the effluent equally among the trenches in the drainfield, which is where the final treatment takes place. ​ Effluent trickles out of the pipes, through the gravel layer, and into the soil where further treatment occurs. The soil filters the wastewater as it passes (or “percolates) through the pore spaces and the soil microbes treat it before it eventually enters the groundwater. These processes work best where the soil is somewhat dry, permeable, and contains plenty of oxygen for several feet below the drainfield. The drainfield is generally located in a stretch of lawn in the back or side yard of a property. The size and type of drainfield depends on the estimated daily wastewater flow and local soil conditions.

The Soil

The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the wastewater has passed into the soil, organisms in the soil treat the effluent before it percolates downward and outward, eventually entering ground or surface water. The type of soil also impacts the effectiveness of the drainfield; for instance, clay soils may be too tight to allow much wastewater to pass through and gravelly soil may be too coarse to provide much treatment.

Replacement (Reserve) Area

Every new home or building served by a septic system is required to have a designated replacement or reserve area. This is a designated area suitable for a new drainfield and must be treated in the same manner as your existing drainfield. (A reserve area should have been designated as part of the permit process for any sewage system installed since 1980.) Once a septic system has failed, it is too late to solve the problem by pumping your tank. A new drainfield will have to be installed at a different location. This is why it is important to know where the replacement area is located and how to protect it (see “Drainfield Do’s and Don’ts” for replacement area care).

Do These Things

• Know where your drainfield and replacement area are located. When you know where it is located, it is easier to protect.
• Keep heavy equipment off your drainfield. Cars and heavy equipment should not park or drive over the drainfield; doing so can crack pipes. Create a barrier if accessible to cars, livestock, or heavy equipment.
  Keep water usage to a minimum. Drainfields do not have an unlimited capacity. When there is more water than it can absorb, the system is unable to drain and filter effluent before it reaches groundwater.
  Divert water away from the drainfield. Water runoff from roofs and drainage ditches can saturate the soil. Drainfields are most efficient when the soil beneath the drainfield is not saturated.
• Keep trees and shrubs at least 30 feet away from the drainfield. (NOTE: Some soil conditions may require that plantings be kept an even greater distance from the drainfield.) Trees and shrubs generally have extensive root systems that seek out and grow into wet areas, such as drainfields. This can lead to clogged and damaged drain lines.
• Plant only grass or shallow-rooted plants over the drainfield. This will prevent soil erosion.
  Protect your replacement area. It may be the only area with acceptable soil conditions in case you need to replace, repair, or add on to the drainfield. All of the above suggestions apply to the replacement area as well.

Don’t Do These Things

• Don’t build over your drainfield. This includes patios, carports, and other structures. You may damage the drainfield.
• Don’t pave over the drainfield. Drainfields need air to function properly. Oxygen is needed by bacteria to break down and treat sewage.
• Don’t dig in your drainfield. Damage to the pipes can occur.
• Keep large animals and livestock off the drainfield. Soil compaction prevents oxygen from getting into the soil and prevents water from flowing away from the drainfield.
• Don’t use landscaping plastic over the drainfield. Air is necessary for the drainfield to function efficiently.
• Don’t plant a vegetable garden over a drainfield. You risk the possibility of food contamination.
• Don’t install an irrigation system in the drainfield. Neither should the irrigation system drain toward the drainfield.

Is Your System Failing? Warning Signs:

If you notice any of the following signs of a potential failure or if you suspect your septic system may be having problems, contact Master Rooter right away to discuss and schedule an inspection:

• Odors, surfacing sewage, or wet spots in the drainfield area.
• Plumbing or septic tank backups (often a black liquid with a disagreeable odor).
• Slow draining fixtures.
• Gurgling sounds in the plumbing system.
• If you have a well and tests show the presence of coliform (bacteria) or nitrates, your drainfield may be failing.
• Standing liquid over the drainfield, even during dry weather. This may indicate an excessive amount of effluent is moving up through the soil, instead of downward. ​