Understanding Your Septic System

Learn about your septic system so you can prolong its life and avoid costly breakdowns.

Septic tanks are concrete, polyethylene, or fiberglass boxes that are buried underground. Image: Delaware Valley Septics Inc./delawarevalleyseptics.com

If you’re installing a septic system—or living with one—it pays to learn the basics of how it works. This will help you take simple steps to prolong its life, understand what you should do if something goes wrong, and evaluate your options if you ever need to expand your system because of a home improvement project.

These one-house-at-a-time sewage treatment systems—used by one in five U.S. households and nearly half of all houses in the South—clean up wastewater just as well as city municipal systems. Septic systems are just better options for sparsely populated rural and spread-out suburban areas, where running sewer lines would be cost-prohibitive.

The typical life expectancy of a septic system is 25 to 30 years.

How septic systems work

All septic systems consist of two main parts: a tank where solids settle to the bottom and a drainfield (also known as a leachfield) where the water disperses. Specifics about the type of system you have should be included in papers you received when you bought your house.

If time has blurred the details, dig out those papers and read up. Your local health department or state environmental agency might also have backup records. If that fails, a plumber can help figure out what you have.

Standard system

In a conventional septic system, gravity carries wastewater from the house into the septic tank and then to the drainfield. The septic tank is an underground box usually made of concrete, polyethylene, or fiberglass. Water pools there long enough for ingredients to separate.

The greases and oils that rise to the top as scum and the solids that sink to the bottom as sludge both get removed by a septic pumping company every few years and carted to an approved disposal site.

The relatively clear water in the middle flows out to the drainfield. This area, which must be kept free of trees and shrubs so their roots don’t damage it, consists of perforated pipes or drain tiles buried in trenches or set on a gravel bed one to three feet below the surface. As water trickles out of the pipes, the soil and its microbes act as natural filters to clean the water.

Installing a standard system costs $5,000 to $10,000, depending on where you live. But it’s not an option for every lot.

A three-bedroom house requires 1,000 to 1,800 square feet or more of open land that’s level with—or downhill from—the house to qualify for a standard-system permit from the local public health department (or possibly state environmental agency, depending on how your locale handles such matters). And that’s only if there’s plenty of well-draining soil above the water table.

Alternative systems

Where the soil type, the property size, or proximity to a wetland prohibits a standard system, you’ll need an alternative system, which is one with an enhanced septic tank, drainfield, or both. These cost more to install than basic systems, but the prices vary widely, depending on your site, your local environmental codes, and what technology you need.

Here are some of the most common types.

1. Treatment alternatives

The following alternative systems help to purify the water more before it gets to the drainfield. That way, you can get by with a smaller drainfield—one in soil that doesn’t drain well—or a site that’s close to a lake or stream and therefore must meet stricter environmental standards.

An aerobic unit(about $6,000) mixes air into the wastewater, which allows oxygen-loving bacteria to flourish. They break down solids much more quickly than the anaerobic bacteria in standard septic tanks, so cleaner water goes into the drainfield.

Some units also disinfect the fluid with chlorine or ultraviolet light, an advantage if you live near water, where you may face tighter environmental standards on what your system releases. An aerobic unit can serve as a substitute for a septic tank—or work in concert with one.

A sand filter ($5,000 to $13,000) works in conjunction with your tank or aerobic treatment unit. The filter consists of a large buried or above-ground box filled with sand. A pump tank releases the partially clarified water to the top of the sand in measured doses. Water then trickles through the sand before going out into the drainfield.

A bottomless sand filter takes things a step farther: It doubles as a drainfield alternative by allowing the fluids to pass into the ground underneath.

2. Drainfield alternatives

The other main category of alternative treatment systems—which also can be used in conjunction with or instead of standard equipment, depending on your situation—focuses on the drainfield end of the process. These systems help water disperse safely even where soil conditions aren’t great or where there isn’t enough open space for a standard drainfield.

A mound system ($9,000 or more) is a pile of trucked-in sand and gravel with a drainfield buried inside. It’s used where the soil is thin or has too much clay, or where the water table is too high. Disguising a mound is a major landscaping challenge, especially since you can’t use trees or shrubs, which might have invasive roots.

Drip irrigation ($2,500 to $15,000) is a shallow drainfield where water trickles out over a wide area in measured doses from a pump chamber. Installers can snake the flexible piping around trees and shrubs, which makes it easier to fit the drainfield into an established landscape.

Because the piping is just 6 to 8 inches belowground, though, you might need to purify the water first, perhaps with an aerobic unit. You’ll also need a filter and regular maintenance to keep the system from clogging.