How Does a Septic Tank Work?

A septic tank is an essential component of a septic system, which is a small-scale, onsite wastewater treatment system commonly used in rural areas without connection to main sewer lines. Septic tanks provide primary treatment of household wastewater by separating solids from liquids and allowing anaerobic digestion of organic matter before dispersal of the clarified effluent for final treatment and absorption into the soil.

Understanding how a septic tank works helps homeowners ensure proper operation and maintenance of their septic systems. This article provides a comprehensive overview of septic tank function, components, workflow, and upkeep.

How Does a Septic Tank Work: Key Functions

The key functions of a septic tank are:

  • Separation and settling – The septic tank separates solids from liquid waste through gravity settling. Heavy solids sink to the bottom to form sludge while grease and light solids float to the top as scum. This allows the clarified liquid to exit the middle layer of the tank.
  • Anaerobic digestion – Anaerobic bacteria break down and digest settled organic solids and sludge at the bottom of the tank. This reduces total solids volume.
  • Storage – The septic tank provides temporary storage of effluent and retains sludge and scum while they undergo digestion. Storage capacity allows gradual release of clarified liquid to downstream treatment steps.
  • Primary treatment – By retaining solids and through anaerobic digestion, the septic tank provides primary treatment of raw sewage and allows cleaner effluent to discharge for further treatment and dispersal.

How Does a Septic Tank Work: Components

A basic septic tank consists of three main components:

Inlet Pipe

  • The inlet pipe carries wastewater from the home into the septic tank. It has a T-shaped fitting on the end located below the water level. This allows incoming flow to be directed downward which minimizes disruption of the settled contents of the tank.

Outlet Pipe

  • The outlet pipe is connected to a filter and extends below the water level to draw effluent from the clarified liquid zone in the middle layer of the tank. It limits passage of solids, grease, and scum into the drainfield.

Access Points

  • Access points include manholes and inspection pipes with removable caps. These allow periodic pumping of accumulated sludge and access for maintenance.
Diagram of Septic Tank Components

Diagram of the key components of a typical septic tank

How Does a Septic Tank Work: Workflow and Process

The workflow through a septic tank involves four key phases:

1. Inflow

  • All wastewater from household drains flows into the septic tank through the inlet pipe. This includes water from sinks, showers, bathtubs, washing machines, and toilets.

2. Separation and Settling

  • Inside the tank, solids instantly begin to separate from liquids due to differences in density. Heavier solids sink to the bottom to form a sludge layer while grease and light solids float to the top as a scum layer.
  • The scum is held by the tank walls while clarified effluent settles in the middle zone between the sludge and scum levels.
  • Baffles inside the tank facilitate separation by preventing disruption and mixing of the settled contents.

3. Digestion and Storage

  • Settled sludge accumulates in the tank bottom where anaerobic bacteria start breaking it down through digestion.
  • Digestion reduces the total solid mass and odor of the sludge. It allows more accumulation before pumping is needed.
  • The septic tank retains the separated solids, sludge, and scum to provide temporary storage. It also holds a portion of the clarified effluent until it can be gradually released.

4. Outlet Discharge

  • Clarified liquid from the center zone of the tank is discharged through the outlet pipe.
  • Only this partially treated effluent passes to the drainfield or other downstream treatment and dispersal components.
  • The retained sludge and scum continue accumulating until the tank is pumped when needed. Screened solids also remain to undergo further digestion.

How Does a Septic Tank Work: Maintenance

To keep a septic tank functioning properly:

  • Inspect annually – Check sludge and scum levels, inspect baffles, and ensure accessible lids. Look for any signs of damage or clogging issues.
  • Pump regularly – Septic tanks should be pumped every 3-5 years to remove accumulated solids and prevent clogs. Pumping frequency depends on tank size and household occupancy.
  • Avoid additives – Additives are not needed for bacterial digestion and some can contaminate groundwater after leaving the tank.
  • Conserve water – Excess water overwhelms the tank, flushes solids out prematurely, and shortens the retention time needed for proper treatment.
  • Limit solids – Fats, grease, and non-degradable items clog tanks. Scrape plates, collect grease, use a sink strainer, and avoid flushing wipes.

Proper septic tank function relies on appropriate design, installation, and ongoing maintenance. Pumping schedules, usage habits, tank access, and condition checks are essential to optimization. Understanding how the tank works allows homeowners to take better care of their septic system. With routine maintenance and attentive care, a septic tank can provide decades of reliable service.

How Does a Septic Tank Work: Frequently Asked Questions

Here are some common questions homeowners have about how septic tanks work and how to maintain them:

How does a septic tank separate solids from liquids?

Septic tanks separate solids from liquids through a process called sedimentation or settling. Heavier solids sink to the bottom while grease and light solids float to the top. This allows clarified effluent to settle in the middle layer. Baffles and the inlet and outlet placement maximize separation.

What is sludge and why does it need pumping?

Sludge is the layer of settled solids that accumulates at the bottom of the tank. It needs periodic pumping to prevent excessive buildup that can clog pipes and infiltrate the drainfield. Anaerobic bacteria break down sludge but cannot eliminate it entirely.

How often should you pump a septic tank?

Septic tanks should generally be pumped every 3-5 years. Routine pumping frequency depends on tank size, household occupancy, and usage. Tanks may need more frequent pumping if high solids enter the tank or water usage increases.

Can a septic tank get too full?

Yes, septic tanks can become too full. If not pumped out routinely, accumulating solids can get deep enough to enter the outlet pipe or back up through drains. This can clog the system, cause backups and plumbing issues, and contaminate the drainfield.

What helps digestion in a septic tank?

Warm temperatures, bacteria naturally present in wastewater, long retention time, and minimal disturbance help promote anaerobic digestion inside the tank. Household cleaners and chemicals can harm digestion. There is no need to add septic tank digestive aids or enzymes.

How long does wastewater stay in the septic tank?

Wastewater remains in a septic tank for 1-3 days on average. This retention time allows solids to separate and digestion to begin. Flows exceeding the tank’s daily capacity have a shorter retention time and pass through too quickly for proper treatment.

Why is my septic tank overflowing?

Common causes of septic tank overflows include excessive water use, pump failure, blocked outlet screen, and infiltration from groundwater or rain. Overflowing effluent backs up through drains and may surface outside. This requires prompt troubleshooting and repairs.

Can I use additives to clean my septic tank?

Septic tank additives are not recommended. Some advertised additives contain harmful chemicals that contaminate groundwater. Biological treatments are also ineffective since ideal bacteria already exist naturally in wastewater. Pumping is the proper method to clean sludge buildup.

How often should the baffles be inspected?

Baffles should be inspected annually along with other components when the tank is checked and serviced. Cracked, damaged, or missing baffles reduce separation efficiency and should be replaced. Baffles prevent settled sludge and floating scum from leaving the tank with the outgoing effluent.

Why does my septic tank smell?

Septic tank odors are usually caused by improper digestion. This can result from lack of oxygen, cold temperatures, excess solids, or harsh chemicals killing beneficial bacteria. Pumping out excess solids restores proper function. Offensive sulfide gases can also corrode concrete tanks.


In summary, septic tanks provide crucial primary treatment of household wastewater using gravity separation and anaerobic digestion. Understanding the function of inlet and outlet pipes, baffles, and access points allows homeowners to properly operate and maintain their septic systems. Following pumping schedules, avoiding additives, limiting water use, and inspecting annually can prevent costly problems. With regular upkeep, septic tanks can reliably filter and process wastewater onsite and protect the surrounding environment.