How Does A Septic Tank Work

Last Updated On June 11, 2024

Updated on January 11, 2023



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How Does A Septic Tank Work

Do you want a better understanding of how a septic tank works? A septic system is your private sewage treatment plant, which can save homeowners money and reduce their dependence on municipal services.

Most people don’t choose septic tanks; septic tanks choose them. 🙂 This is due to moving to a house with an existing septic tank, or building outside the city limits. Regardless of how you found yourself on a private system, you should know how your septic tank works. 

In this PlumbingNav guide, we will cover:

  • How does a septic tank system work?
  • How does a septic tank leach field work?
  • Concrete septic tank vs plastic septic tank

What's In This Guide?

      How does a Septic Tank System Work?

      Your septic tank receives the waste from your home and performs wastewater treatment on-site. Waste travels through your home’s drain pipes into the main sewer line that transports the waste into the septic tank.

      The waste material gets separated into three sections:

      • Top layer – scum (scum usually consists of grease, oil, and fat)
      • Middle layer – liquid waste (liquid waste usually consists of waste water that scapes down the toilet or another drain)
      • Bottom layer – solid waste (solids generally refer to human waste and toilet paper)

      What happens to the stuff in a septic tank?

      The liquid wastewater, also known as effluent, drains from the outlet pipe through perforated pipes into a leach field. This is also known as a drainage field or absorption field. The soil in the leach field absorbs the liquid waste. Soil offers natural filtering properties, and also contains aerobic bacteria. The filtered water then returns to the groundwater.

      Natural enzymes and septic tank bacteria eat at the solid waste. Enzymes eat organic manner, while bacteria reduce harmful bacteria numbers. Aerobic bacteria numbers require oxygen, while anaerobic bacteria numbers don’t require oxygen. 

      The bacteria and enzymes may also consume some of the scum on top, consisting of fat, oil, and grease, but the scum may be more difficult to remove. This is why it’s important not to pour excess grease or cooking oil down your drain.

      After 3 – 5 years, your septic tank will often require professional septic tank pumping, or you put your home at risk of septic system failure. There are exceptions to this rule.

      You can maintain your system with the right septic tank treatment products, and often prolong the need to pump. Locate the lid to your septic tank, and check it every 1 to 2 years.

      How a Septic Tank Works: 3 Steps

      Watch the video below for more detail on how septic systems work:

      Step One: You dispose of waste down the toilet or drain 

      Everything you allow down your toilet or drain finds its way into your septic tank through the inlet baffle. This includes toilet paper, food waste, and materials from your washing machine and dishwasher. 

      For this reason, you need to be especially careful what you allow to exit through your drains. Grease and fat, in particular, do not belong down the drain. You should never flush wipes, even if they say it’s okay.

      Step Two: Waste is collected in the septic tank where the solids, liquid sewage, and scum are naturally separated and broken down. 

      Your septic tank is the location anaerobic bacteria break down the waste, including any harmful bacteria and pathogens. The liquid sewage, called effluent, then makes its way out toward the drain/leach field.

      Note: This is where problems develop. Items such as baby wipes or grease collect as “scum,” and cannot be broken down by bacteria. Household cleaners may also hinder productive bacteria from doing their job. All these combined mean that pumping might be required to “restore order.”

      Step Three: Liquid waste effluent continues through the septic tank to the drain field

      Liquid waste water (effluent) makes its way out through the outlet in a perforated pipe into the distribution box of the drainfield, where it is then sent equally down various “lanes.” 

      The water enters the leach field, which consists of soil. As it seeps down through the soil, it is naturally filtered by sediment and aerobic bacteria. The “cleaned” water then mixes into the groundwater beneath the soil. By the way, the EPA has a super handy beginners guide to septic systems, here.

      See this related post for a detailed account of how a septic drain field actually works.

      Septic Pro Tips: 

      1. Don’t put off inspections or pumpings. 

      Septic tank pumping removes excess material solids and scum that either can’t be broken down, or have exceeded a safe capacity. You can have your tank inspected every 1 to 2 years, keeping a record of both solids and scum levels. This will signal when you should actually have your tank pumped. See this post for details on when to have it pumped.

      It’s likely that you will need to hire professionals to pump the excess material out of your septic tank every three to five years. Without regular pumping, a full septic tank can lead to plumbing backflow along with damage to the septic tank system or leach field.

      The plumber or septic service will access the pump through a manhole at the top of the tank, usually easily identified by a septic riser that protrudes up from the ground. These may stand alone, or be found in a decorative landscape area.

      1. Be proactive in maintaining your septic tank.

      Regular use, even annually, can greatly reduce your need to have your septic system pumped. While not all septic treatment products are equal, those that work can save you lots of money and headache when combined with not flushing the wrong things down the toilet. See our complete review, here.

      How Does a Two Chamber Septic Tank Work?

      Most septic tanks use one chamber. However, you may have a septic tank with two chambers.

      In a two-chamber septic tank, the first tank collects sludge that won’t dissolve naturally, allowing other material to move to the second chamber. 

      Organic material in the second chamber will dissolve more quickly thanks to the segregation of materials in the two different chambers before moving out to the leach field.

      How Does a 3-Tank Septic System Work?

      A three-chamber septic system works similarly to traditional septic systems. However, there are three chambers instead of only one or two chambers. The third chamber is often where a septic pump is housed. This is required if effluent needs to travel to a drain field located further away from the tank itself, beyond the reach or performance of a gravity system.

      How Does a Pump System Work On Septic Tank?

      Some septic tank systems contain a subversive water pump in the second or third chamber. A float switch activates when the water reaches a certain level in the tank. At this point, the pump sends water out to the drain field. 

      Without a pump, septic systems move water at a rate of 2 ft. per second using gravity. When a septic system sits below the drain field, or when it is located much further away, a pump helps move the wastewater more quickly. 

      Septic pumps require an additional maintenance step. Homeowners should change or clean the filter leading to the pump every 6 months, or according to the manufacturer’s instructions.

      How Does A Concrete vs. Plastic Septic Tank Work?

      Concrete tanks cost more due to the increased weight, but they last longer. Concrete tanks require specialized vehicles to transport them. However, they stay in place underground rather well.

      Plastic septic tanks cost less, but they don’t last as long and aren’t as sturdy. Plastic septic tanks must have gravel or sand to reinforce the tank so that it doesn’t move, making them more vulnerable to code violations.

      How Does A Septic Tank Work: Visual Diagram

      See the following diagram for a visual representation of how septic tanks work:

      septic system diagram

      Alternative Septic Systems

      In some cases, the property doesn’t allow for a conventional septic tank system due to a lack of space for a drainfield or soil that won’t filter (“perc”) the material properly. 

      To prevent unsanitary conditions in the home or contamination of groundwater, you can use a septic system with alternative drainage methods, such as a mound system or a sand filtration system. This will be determined and permitted on a county basis.

      FAQ’s: How Does a Septic Tank Work

      How a septic tank works step by step?

      Step One: You dispose of waste down the toilet and drains 

      Step Two: Wastewater travels through the main drain pipe to the septic tank

      Step Three: Waste is collected in the septic tank where the solids, liquid sewage, and scum are naturally separated and broken down by bacteria.

      Step Four: Liquid effluent (wastewater) continues to the drain field and is filtered as it enters the soil.

      What happens to the stuff in a septic tank?

      Waste and harmful bacteria are “broken down” in the septic tank. The separated liquid waste called effluent then flows out of the septic tank into the drain field. The soil naturally filters the wastewater until it finds its way back into the groundwater supply.

      The “stuff” in the septic tank, also called solids and scum, should be pumped if they reach certain levels. 

      Are septic tanks always full of water?

      Your septic tank should always have a certain level of water in it. However, the amount will be determined by a few factors such as the level of solids, the household size/water usage, and the flow rate to the leach field.

      How is a septic tank powered?

      A septic tank system typically runs on gravity so no “power” is required. In some cases, a pump will contribute to the processes and this will be powered by electricity.

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      We write about “all things plumbing,” helping you navigate common questions, repairs, and the best plumbing products on the market.

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