What happens to wastewater in a septic tank and drainfield, the main system types, and why newer rules favor advanced treatment. From EPA and extension.
By the SepticRated team · Facts checked against the sources listed below on November 1, 2026. General information, not professional advice.
A septic system treats a home's wastewater on its own lot instead of piping it to a sewer plant. The US Environmental Protection Agency (EPA) describes a typical system as two main parts: a septic tank and a drainfield, also called a soil absorption field. The University of Georgia's extension guide counts three: the tank, the drain lines that carry liquid away from it, and the soil treatment area. Either way, the soil is part of the treatment, not just a place for the water to go.
Septic is common in several of the areas we cover. University of Florida (UF/IFAS) researchers estimate that more than 2.5 million systems are in use across Florida, and UF/IFAS says nearly one in every three Florida households depends on one. Gwinnett County, citing the Georgia Department of Public Health, says an estimated 30 percent of its population uses septic. EPA does not keep a national database of septic systems and points to the Census Bureau's American Housing Survey for national figures.
Every drain in the house, from the toilets to the washing machine, empties through the main drain line into the tank. EPA describes the tank as a buried, watertight container, usually made of concrete, fiberglass or polyethylene. Its job is to hold the wastewater long enough for it to separate:
EPA says compartments inside the tank and a T-shaped outlet keep the sludge and scum from leaving the tank and reaching the drainfield. A UF/IFAS fact sheet adds that a filter in the outlet pipe keeps most solids in the tank. Bacteria break down part of the solids, but not all of them, which is why the sludge layer grows and the tank has to be pumped from time to time. UGA notes that no additives are needed for this, because new microbes arrive with every flush.
Sizes vary. UGA's Georgia bulletin says the average three-bedroom, two-bath home will often have a 1,000-gallon tank, and that new tanks must have two chambers, while older ones may have only one.
From the tank, effluent flows to the drainfield. UGA says there may be a distribution box between the tank and the drain lines, sending the effluent to several lines, and UF/IFAS says the box ensures it is distributed evenly. The effluent seeps out of perforated pipes along their length; in a pipe-and-gravel system it trickles through a layer of gravel into the soil. EPA describes the drainfield as a shallow, covered excavation made in unsaturated soil.
The soil does most of the cleaning. As effluent percolates down, EPA says, the soil removes harmful coliform bacteria, viruses and nutrients before the water eventually reaches groundwater, and UF/IFAS says most of the treatment takes place in the drainfield soil. That only works if the soil under the pipes is unsaturated: UF/IFAS says Florida law requires the bottom of a drainfield to be at least 24 inches above the wet-season high water table.
Nitrogen is the weak spot of a conventional system. UF/IFAS says a conventional septic tank removes only about 30 percent of the nitrogen that flows into it, and that even a well-maintained system will be a source of nutrients, especially nitrate, to the surrounding soil.
If the drainfield receives more liquid than it can absorb, EPA says it can flood, and sewage can surface in the yard or back up into toilets and sinks.
The tank-and-trench design is only one option. EPA says system design varies with factors such as household size, soil type, site slope, lot size, closeness to sensitive water bodies and local rules, and it describes these common types:
Florida and Suffolk County now require advanced treatment for many new systems. In Florida, a 2023 law (House Bill 1379) requires new septic systems on lots of one acre or less to be enhanced nutrient-reducing systems from July 1, 2023, in areas where waterbodies fail water quality standards and a restoration plan is in place, according to the Florida Department of Environmental Protection (DEP). DEP's qualifying types include certified nitrogen-reducing aerobic treatment units and in-ground nitrogen-reducing biofilters, which it says meet a 65 percent nitrogen-reduction standard. DEP has a mapping tool for checking whether a property is in one of these areas.
In Suffolk County, NY, the Peconic Estuary Partnership says innovative and alternative onsite wastewater treatment systems (I/A OWTS) have been required since July 1, 2021 for new or expanded single-family homes. Our guide to aerobic and advanced systems covers the upkeep these systems need.
EPA's SepticSmart advice for homeowners comes down to a few habits: have the system inspected, in general every three years, by a licensed contractor; have the tank pumped when necessary, generally every three to five years; don't park or drive on the drainfield; keep trees and shrubs away from it; and avoid running the dishwasher, shower and washing machine all at once. There is more detail in our guides to pumping and the drainfield.
Only partly. EPA says the tank separates solids, grease and liquid and digests some organic matter. The soil in the drainfield then removes bacteria, viruses and nutrients, and UF/IFAS says most of the treatment happens there.
Ask the local permitting authority, usually the county health or environmental department. EPA says it can provide the as-built record drawing and the permits, which may also show the installation date and soil details.
Some do. EPA says drip distribution needs electrical power, and that mound systems and sand filters pump effluent from a pump chamber. Ask your service provider which parts of your own system rely on power.
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