What Is a Gravity Septic System?

By Geoffrey Sperring

November 02, 2024

Does your septic tank keep you awake at night because you don't understand how it works? You've heard horror stories of people who have had gravity septic system failures, and you don't want that to happen to you.

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The first step is arming yourself with a little bit of septic system knowledge.

How Does a Gravity Septic System Work?

A gravity septic system is one of the most common waste disposal systems. Relying on gravity, they need a slight slope to let effluent flow to the tank and drainfield. The three main components are the septic tank, the drainfield and the soil under the drainfield.

Septic Tank

This is a two-compartment waterproof tank buried in the ground. It's made from concrete, polyethylene or fiberglass, and each compartment has a lid. Effluent flows through pipes from the home into the first compartment, where the solids settle into a layer of sludge that's slowly decomposed by bacteria.

Liquid, scum and fat flow into the second part of the tank through a pipe. The tank is kept full, and as new waste enters the tank, it drives effluent into the drainfield. The sludge must be pumped out every three to five years.

Drainfield Design

The septic drainfield consists of a series of perforated pipes laid in gravel-filled trenches. The pipes are fed via a distribution box from the septic tank. The wastewater trickles from the holes in the pipes and flows through the gravel.

Soil Beneath the Drainfield

The soil beneath the drainfield plays a critical role in the treatment of wastewater. The effluent still has harmful bacteria and nutrients that must be removed by the oxygen and microorganisms in the soil as the water percolates downwards.

What Is the Difference Between Gravity and Typical Septic Systems?

The primary difference between a gravity septic system and other types of septic systems is the design of the effluent distribution system and drainfield. All septic solutions use a septic tank.

Chamber System

This is a type of gravity-fed septic system that uses a series of chambers covered in soil for a drainfield. The chambers are relatively large, are open at the bottom and have holes in the sides. These chambers hold the wastewater until it filters into the soil.

Pressure Distribution System

A pressure distribution system uses a pump installed in a chamber next to the septic tank. The pump distributes the wastewater flow into a series of perforated pipes. Applications include soils with limited depth and drainfields on a steep slope.

Mound System

A mound septic system is used for shallow soil or locations with high groundwater. The mound is covered in topsoil. It has a sand fill covered by a layer of gravel containing the distribution pipes. A pump is used to distribute wastewater into the pipes.

Sand Filter

This system has a large chamber filled with sand covered by a layer of gravel. Wastewater is pumped into distribution pipes in the gravel and percolates through the sand. Treated wastewater is pumped to a drainfield. Sand filters are good for sites close to water or with a high water table.

Aerobic System

Aerobic septic systems have a blower that pumps air into the septic tank. The oxygen in the air increases bacterial activity, improving the quality of effluent treatment. This simplifies the drainfield and is suitable for small lots, poor soil conditions and sites near water bodies.

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What Type of Soil Is Needed for a Gravity-Fed Septic System?

The best soil types for a gravity septic system allow the wastewater and air to percolate slowly. These soils have numerous interconnected pores and are typically dry, loamy soils with a mixture of sand, clay and silt to a depth of around 3 feet. It's best to have the soil evaluated before constructing a drainfield.

What Are the Pros and Cons of a Gravity Septic System?

A gravity flow septic system is a robust and reliable solution with many pros and a few cons.

Pros

The cost of a gravity drainfield is lower than other types, and gravity septic system maintenance requirements are minimal. The main requisite is monitoring the septic tank and having it emptied when the sludge level exceeds 30% of the tank capacity.

Cons

Gravity systems require ground with a reasonable slope, and they aren't suitable for sites with rocky terrain, limited soil cover or high water tables. They're susceptible to soil shifts and vehicular traffic and can be saturated during heavy rain.

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