Septic tanks treat wastewater when a city sewer system isn’t available. They’re buried, watertight containers of concrete, fiberglass, or polyethylene. Solid waste settles to the bottom of the tank forming a layer of sludge, while fats, oils, and grease float to the top, creating a layer of scum.
Bacteria in the tank break down this sludge. This process releases gases vented through a vent pipe in the roof. Click Here to learn more.
A septic tank is a large, watertight container made of concrete, fiberglass or plastic. It’s buried underground and holds wastewater until solid waste settles and the liquid drains out of the tank to your septic system’s absorption field.
Wastewater from toilets (blackwater) and sinks, tubs and showers (greywater) flows down the drain pipe into your septic tank. When it enters, the wastewater is separated: Heavy solid materials drop to the bottom forming a sludge layer while lighter materials such as FOG float to the top of the tank forming a scum layer. Anaerobic bacteria living in the septic tank break down these organic wastewater pollutants. The septic tank’s baffles and compartments prevent the sludge and scum from leaving the tank and traveling into your septic system’s drain field where they can block the perforated pipes there, or enter the groundwater where they could contaminate soil and cause flooding of your home’s basement.
The septic tank is equipped with inlet and outlet tees that extend from the pipes in and out of the tank down into the water. When you flush your toilets or run water in your house, hydraulic pressure forces wastewater up the tee and out into the septic tank. This same force pushes wastewater out the septic tank’s scum and sludge layers into the water when you’re not using your toilets or running water in your home.
Be sure to only flush human wastes, toilet paper, and safe food waste such as coffee grounds and eggshells down the drain. Putting non-biodegradable items in your septic system’s drain fields or tanks will seriously affect their efficiency. Never park or drive vehicles over a septic system tank or absorption field.
The Drain Field
The septic drain field is the final destination of wastewater that leaves your home, toilets and washing machines. It consists of a network of perforated pipes laid in gravel-filled trenches. The effluent trickles out of these pipes and into the surrounding soil, where it is naturally filtered by microorganisms. This process removes harmful bacteria, viruses, and excessive nutrients from wastewater before it seeps into groundwater. Without a properly functioning drain field, managing household waste would be a much messier and smellier affair.
As the wastewater leaves your septic tank, solid materials sink to the bottom and form sludge, while liquids like oils and grease float to the top and are broken down by aerobic bacteria. The septic tank is designed with baffles to ensure that all of this waste is dispersed evenly in the tank and that it doesn’t clog the drainage pipe leading to the drain field.
Depending on the design of your septic system, the wastewater will flow either by gravity or pressure into the drain field. Gravity discharge systems will flow into a distribution box that evenly disperses the wastewater into the drain field, while pressure discharge systems will push the wastewater directly into the drain field.
A proper septic system requires soil that is permeable and can easily absorb and filter wastewater. The soil must also be deep enough to ensure that the wastewater doesn’t contaminate groundwater.
It’s important to check with your local environmental health department to see what the requirements are for your area. It’s also a good idea to contact a professional to test your soil and conduct a percolation test before installing or upgrading your septic system.
The Inlet Pipe
Incoming wastewater enters your septic tank through the inlet pipe, where it is separated into two layers. The heavier solid particles sink to the bottom, and the lighter scum floats to the top. A septic system needs a baffle in both the inlet and outlet pipes to reduce the amount of solid materials that reach the absorption field and drain lines.
Inlet and outlet baffles protect the septic tank from being overloaded by solid wastes, which can cause premature system failure. The inlet baffle keeps the floating scum layer from entering the pipe leading to the septic tank and blocking it, while the outlet baffle prevents grease and solids from passing directly out of the tank into the drainfield.
Both baffles are made of concrete, acid-resistant fiberglass or cast iron and should be at least 4-inches in diameter. They should also be protected with sanitary or flushable tees, and have 6-inch diameter inspection pipes that extend from them to the ground surface. These allow you to check for clogs or the thickness of the scum and sludge layers in your tank without opening the lid.
It is important to check the sludge depth regularly. One way to do this is to measure the sludge level with a probe that is attached to a long stick. Push the probe through the layers until it meets resistance, and then mark it. Another method is to drive a long, narrow board through the sludge and scum layers. If you do this, you can see the different layers clearly.
The Outlet Pipe
When wastewater exits your home, it first enters a septic tank, a large, watertight container made of concrete, fiberglass or plastic and buried underground. This storage device is mainly meant to contain waste solids and somewhat clarify the wastewater prior to sending it on to the leach field for further cleansing.
The septic tank works through a process called anaerobic decomposition or digestion, where bacteria break down organic waste materials. The result is a sludge at the bottom of the tank and a liquid effluent that flows to the outlet pipe.
This pipe, also known as the drain field distribution box or just the seepage field, is a series of perforated pipes that are buried in the ground and connect to septic tank inlet and outlet structures. The pipe distributes partially clarified wastewater evenly throughout the soil and watercourses, preventing it from pooling in areas of your property that may damage or flood.
A septic tank has a variety of internal components to help manage sewage waste, including inlet and outlet structures, baffle walls, access ports, and a ventilation point. The inlet baffle slows the flow of sewage to the tank, blocking solids from entering. It also prevents odorous gasses from entering the home, and it can keep solid sludge from clogging inlet pipes.
The outlet baffle draws liquid from the middle of the tank, directing it to the outlet pipe. This prevents sludge and scum from leaving the tank, where it could clog downstream components. It can also include a gas deflector and effluent filter to trap solids that might rise with the sewage. If any of these are missing or damaged, it is important to have a septic system professional repair them immediately.
The Vent
Venting pipes work together with drain pipes to ensure your plumbing system works properly. While drain pipes carry water and sewage to and from the structure, venting pipes bring fresh air into the drainage pipe system to prevent vacuum formation that could slow or even stop drain water flow.
Specifically, the venting system helps prevent trap seals from siphoning water and sewage out of the drain pipe. Essentially, without the right amount of fresh air in the system, the pressure variations that occur during drainage would cause water in the P-trap to siphon into the upstream fixture and out of the drain pipe. This is a major problem because hazardous sewer gases can enter your living spaces and make you sick.
To avoid this, each drain pipe is connected to a vent that lets fresh air in through the roof and out through the vent stack. Ideally, the vent pipe should be at least as tall as the drain pipe it’s attached to. The height of the vent pipe also depends on local building codes and space limitations.
Another important function of a vent pipe is to disperse foul waste odors. If you smell a rotting scent coming from your sink or toilet drain, that’s a sign that there may be an issue with the vent.
Since the vent pipes exit through your roof, they’re also prone to being blocked by birds and other animals. If you notice any issues with your vents, call a professional plumber right away to inspect the situation. If you suspect you have a vent blockage, they can install an air admittance valve or “studor” to replace the open-air vent and prevent sewer gasses from entering your home.