A septic tank is an important component of a septic system, which is a small-scale, onsite wastewater treatment system commonly used in rural and suburban areas without access to centralized sewer systems. The septic tank is a watertight container usually made of concrete, fiberglass, or polyethylene. It holds the wastewater long enough to allow solids to settle out and separate from the liquid, allows some breakdown of organic matter to occur, and provides preliminary treatment of wastewater before it flows out into the drain field.
There are two main pipes that transport wastewater into the septic tank:
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Inlet Pipe
The inlet pipe carries wastewater into the septic tank from the home. This pipe is connected to the plumbing drain system of the house and may include fixtures such as toilets, sinks, showers, washing machines, etc. All the wastewater from the home’s plumbing system flows through the inlet pipe into the first compartment of the septic tank.
Outlet Pipe
The outlet pipe transports the partially treated wastewater out of the septic tank and into the drain field or soil absorption field. The outlet pipe is connected to the second or last compartment of the septic tank on the opposite end from the inlet. As the wastewater works its way through the septic tank, the clarified effluent exits the septic tank through the outlet pipe and travels to the drain field for further treatment and dispersal.
What Happens Inside the Septic Tank?
When wastewater enters the septic tank through the inlet pipe, it goes through a process to separate the solids from the liquids. Here is what happens inside the septic tank:
– The wastewater flows into the first compartment where the heavy solids sink to the bottom forming a sludge layer. Lighter solids like oils and grease float to the top forming a scum layer.
– Baffles and tees between compartments slow the flow of water to allow separation to occur and prevent sludge and scum from flowing out of the tank.
– Anaerobic bacteria in the tank start breaking down the organic solids.
– The partially treated and clarified effluent flows through the outlet pipe into the drain field.
– The sludge at the bottom decomposes over time but remains in the tank to provide ongoing treatment.
– Periodic pumping out of the accumulated sludge is needed to maintain proper functioning.
Septic Tank Cross Section
Here is a cross section diagram of a typical two-compartment septic tank showing the inlet and outlet pipes:
| Septic Tank Cross Section |
|---|
|
Inlet Pipe | | V +—————————————————–+ | | | +———————+ +———————+ | | | | | | | | | First Compartment | | Second Compartment | | | | | | | | | +———————+ +———————+ | | Scum Layer ^ | | | | | | Sludge Layer | | +—————————————————–+ | | Outlet Pipe |
Size Requirements for Inlet and Outlet Pipes
Septic tank inlet and outlet pipes should be sized properly to handle the flow from the house without overflowing or backing up. Here are some general guidelines on inlet and outlet pipe sizes:
Inlet Pipe
– Minimum diameter of 3 inches.
– For homes with up to 3 bedrooms, use a 3-inch inlet pipe.
– For 4-5 bedroom homes, use a 4-inch inlet pipe.
– For larger homes, the inlet pipe should be 6 inches or larger.
Outlet Pipe
– Minimum diameter of 4 inches to prevent clogging.
– The outlet pipe is typically 1 inch smaller than the inlet pipe. So a 3-inch inlet would have a 2-inch outlet, and a 4-inch inlet would need a 3-inch outlet.
– For larger tanks, maintain 1 inch reduction between inlet and outlet. A 6-inch inlet would require a 5-inch outlet.
Inlet and Outlet Pipe Material
The inlet and outlet pipes for septic tanks are typically made of plastic materials such as:
– PVC (polyvinyl chloride) – Most common due to low cost and ease of installation. Class 200 PVC is recommended as minimum.
– ABS (acrylonitrile butadiene styrene) – Durable material but more expensive than PVC.
– PEX (crosslinked polyethylene) – Can withstand some settlement better than rigid PVC.
Many installers also use reinforced concrete pipe for both inlet and outlets particularly on larger tanks. Other options include fiberglass or steel pipe. Pipe connections should be sealed properly using primer, solvent cement, gaskets or sealant to prevent leakage.
Inlet and Outlet Pipe Installation Tips
Proper installation of inlet and outlet pipes is critical to ensure the septic system functions as intended. Here are some tips on installing inlet and outlet pipes:
Inlet Pipe
– Position the inlet tee or baffle 6-12 inches below the top of the tank to allow for scum accumulation before effluent enters the second chamber.
– Extend the inlet pipe down to at least the mid depth of the first chamber to disperse inflow over the tank. Use of an inlet tee or baffle helps redirect solid waste to the bottom.
– Install inlet piping at a 1/4 inch per foot slope minimum.
– Support inlet pipe properly with hangers spaced as per code requirements.
Outlet Pipe
– Position outlet 1 inch lower than inlet to ensure the clarified upper zone of effluent exits to drain field.
– Install outlet piping at a 1/8 inch per foot minimum slope.
– Install a gas deflection baffle over the outlet to prevent scum from escaping.
– Use an effluent filter to screen solids exiting tank if required by code.
– Support outlet pipe to prevent sagging and maintain slope.
Conclusion
A properly designed and installed inlet and outlet piping system is crucial for a septic tank to provide efficient primary treatment of household wastewater. The inlet pipe brings all the raw sewage into the first chamber where solids separation occurs. The outlet pipe then conveys the clarified effluent to the drain field for further filtration and processing before the water re-enters the ground. Following the right size recommendations, materials, slope, and installation method for inlet and outlet piping helps ensure a functional and long-lasting septic system. Regular maintenance like pumping out sludge buildup is also essential for any septic tank owner. Understanding the role of the two main pipes, inlet and outlet, provides critical insight into how septic systems operate to treat wastewater onsite and protect human and environmental health.