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How Does a Septic System Work?

By June 26, 2024Plumbing, Septic
How does a septic system work?

Ever wondered what happens to the water that goes down your drains? If you live in a rural or suburban area, chances are you have a septic system managing your household wastewater – knowing how it does its work can really come in handy when you least expect it.

This guide breaks down the components of a septic system and provides tips for maintaining one. When we wrap up, you’ll have a clear picture of what goes on beneath your yard and how to keep everything running smoothly.

What is a Septic System?

A septic system is an underground wastewater treatment structure commonly used in areas without centralized sewer systems. It consists of two main parts – the septic tank and the drain field (also called the leach field).

The septic tank is a watertight container made of concrete, fiberglass, or plastic, buried underground. Its primary function is to hold wastewater long enough to allow solids to settle at the bottom, forming sludge, while oils and grease float to the top, forming scum. The clarified liquid, known as effluent, then exits the tank into the drain field.

The drain field is a shallow, covered excavation made in unsaturated soil. It receives the effluent from the septic tank and disperses it through pipes into the soil, which naturally removes harmful bacteria, viruses, and nutrients.

Diagram of how the typical septic system will work.

How Does a Septic System Work?

1) Wastewater flows from the house to the septic tank through a network of pipes into the septic tank every time you use water in your home.

2) Once inside the septic tank, the wastewater separates into three layers. The heavier solids settle at the bottom, forming sludge. The lighter oils and grease float to the top, forming a scum layer. The middle layer consists of clarified liquid effluent.

3) Bacteria naturally present in the septic tank break down solids, reducing sludge – but some solids are not biodegradable and will eventually require removal.

4) The effluent flows from the septic tank into the drain field, facilitated by gravity. In some cases, it’s done with a pump if the drain field is located uphill from the tank.

5) Once in the drain field, the effluent is dispersed through a network of perforated pipes buried in gravel-filled trenches. The soil acts as a natural filter, removing harmful bacteria, viruses, and nutrients as the effluent percolates through it and eventually recharges groundwater.

Types of Septic Systems

There are various forms of a septic system, each tailored to different work, conditions, and needs. Knowing the types can help you select the best systems, products, or routines for your home.

  • Conventional Septic Systems – The most common type consists of a septic tank and a drain field. These systems rely on gravity to move effluent from the tank to the drain field, where the soil treats it.
  • Gravity-Fed Systems – In gravity-fed systems, effluent flows naturally from the septic tank to the drain field due to gravity. These systems are simple, cost-effective, and require a proper slope for effective operation.
  • Pressure Distribution Systems – These systems use a pump to distribute effluent evenly across the drain field. This ensures uniform treatment, making it suitable for sites with shallow soil or uneven terrain.
  • Aerobic Treatment Units (ATUs) – ATUs introduce air into the treatment process, promoting aerobic bacteria growth, which breaks down waste more efficiently than the anaerobic bacteria in conventional systems. These are ideal for areas with high groundwater tables or poor soil conditions.
  • Sand Filter Systems – Sand filter systems add an extra layer of treatment by filtering effluent through sand before it reaches the drain field. This provides additional filtration, making it effective in areas with poor soil drainage.
  • Mound Systems – Mound systems are drain fields above the natural soil surface. They are used in areas with high groundwater, shallow soil, or bedrock, ensuring effective treatment while preventing groundwater contamination.

Common Septic System Issues and Troubleshooting

Even with proper maintenance, septic systems can experience problems. Recognizing early signs can prevent minor issues from becoming major problems.

  • Slow Drains and Backups – If sinks, showers, or toilets drain slowly, it could indicate a blockage in the pipes leading to the septic tank or a full tank that needs pumping.
  • Foul Odors – Unpleasant smells around your septic system or inside your home can signal a problem. This could be due to a full tank, a failing drain field, or improper ventilation.
  • Lush, Green Patches in the Yard – Unusually green and lush areas in your yard may indicate effluent surfacing due to a saturated drain field or a leak in the system.
  • Pooling Water – Standing water or soggy areas around your septic system can indicate a failing drain field. This happens when the soil can no longer absorb the effluent.
  • Gurgling Sounds – Gurgling noises in your plumbing system can indicate a blockage or a problem with the septic system’s venting.
Signs that there's an issue not allowing the septic system to work properly.

Recommended Septic Maintenance

One of the most important tasks is having your septic tank pumped regularly, typically every 3 to 5 years, depending on the tank size and household usage. Regular pumping prevents solids from building up and clogging the drain field.

Water conservation is also essential. Reducing the amount of water entering your septic system can help extend its life. Fix leaks promptly, install high-efficiency toilets and showerheads, and spread out laundry loads throughout the week.

Be mindful of what you flush or pour down the drains. Avoid flushing items like sanitary products, diapers, wipes, and chemicals that can harm the bacteria in your septic tank. Only flush human waste and toilet paper. In the kitchen, avoid pouring grease or oil down the drain and use a garbage disposal sparingly.

When to Call a Professional

If slow drains or backups persist despite regular pumping and maintenance, it may indicate a more serious issue like a blockage in the main pipe or a failing drain field. Similarly, unpleasant odors around your septic system or inside your home are signs that something isn’t right that a professional can diagnose.

Standing water or consistently soggy areas around your septic system are red flags that suggest the drain field is failing and needs professional attention. Gurgling noises in your plumbing system can indicate a blockage or venting problem, which requires a thorough inspection to pinpoint and fix the issue.


Knowing how your septic system functions is crucial for keeping it doing good work. Regular maintenance, smart water use, and proper waste disposal go a long way in preventing issues. If problems do come up, recognizing the signs and calling a professional promptly can save you time and money.

At Alpha Building Inspections, we’re here to help you with all your septic system needs. If you have questions or need assistance in NH, MA, or ME, don’t hesitate to reach out.