Most septic systems start the same way, with a buried tank that settles out solids. The difference is what happens to the liquid next, and that comes down to your soil. A conventional gravity system needs enough good, unsaturated soil under its drain field to treat the wastewater. Where a lot doesn’t have it, the system has to do more: pump the effluent in measured doses, build the field up out of imported sand, or treat the wastewater in a unit before it reaches the ground. Each step away from gravity adds parts, running costs and inspections. Here is why your system is the type it is, how to find out which one you have, and which guide to read next.
This is general information, not professional or legal advice. SepticPlain does no installs, pumping or inspections.
Why your soil decides the type
The soil under the drain field does most of the treatment, so every design rule comes back to one question: how much suitable soil sits between the bottom of the field and the water table, bedrock or a layer water can’t pass through? A perc test and soil evaluation answers that before a system is designed.
Washington’s Department of Health shows the idea clearly (Washington DOH, Types of Septic Systems). With at least 3 feet of native suitable soil under the field, a gravity system works. With about 2 to 3 feet, it names pressure distribution, where a pump doses the field evenly. With 1 or 2 feet, a mound. With about 1.5 feet, a sand filter treats the effluent before the field, and with a minimum of 1 to 1.5 feet, an aerobic treatment unit does that job. Other states set their own numbers. Pennsylvania, for example, requires an elevated sand mound wherever the limiting zone is less than 60 inches down, and denies a standard permit where it is within 20 inches (25 Pa. Code § 73.51).
There is no national rule behind any of this. EPA says septic systems are regulated “by states, tribes and local governments, not the federal government” (EPA, Frequent Questions, updated 11 March 2026). Your state or county decides which designs your lot may use. Soil isn’t the only reason, either. EPA’s Types of Septic Systems page names small lots, nearby water that is sensitive to nutrients, and climate.
Find out which one you have
The permit record is the reliable answer. Ask your local health department or permitting office for the file on your address. University of Georgia Extension says those records should include the type of system, its length and its depth. Ask for the system type as they classify it, because names change from state to state and the local label is what sets your inspection and permit duties.
The yard gives clues, not proof. No alarm or electrical parts points to gravity; an alarm panel points to a pump chamber or treatment unit; a long, raised, grassy bed with small pipes sticking up is usually a mound; sprinkler heads that run on their own can mean an aerobic unit. Look from the surface only; never open a lid to check. Our overview of every type of septic system has the full table of clues and how each design works.
The main families, and the guide for each
Conventional gravity and chamber systems
The baseline: a tank, then perforated pipe in gravel trenches or open-bottom plastic chambers. Water runs downhill, with no pump and no power. If you want the parts explained first, start with how a septic tank works.
Mound systems
A mound is a drain field built above the ground in a bed of specified sand, fed by a pump. EPA calls it “an option in areas of shallow soil depth, high groundwater, or shallow bedrock”. The sand supplies the treatment depth the native soil lacks. In our worked example on Pennsylvania’s code, a three-bedroom house with the limiting zone at 30 inches needs 18 inches of sand under a 600-square-foot bed, about 46 cubic yards in all. Our mound septic system guide covers sizing, costs and care.
Aerobic treatment units
An aerobic unit works, in EPA’s words, like “a municipal sewage plant, but on a smaller scale”: an air pump runs day and night so oxygen-using bacteria break the waste down. Texas A&M AgriLife says these units remove 85 to 98 percent of the organic matter and solids. The trade-off is ongoing cost. Texas and Florida require a service contract, unless the owner qualifies to do the work. In our worked year for a Charlotte County, Florida home, electricity, a published contract range, the operating permit and pumping come to an estimated $553 to $1,188 a year, before repairs. The aerobic septic system guide shows the arithmetic and state rules.
Above-ground systems and holding tanks
“Above ground septic” means two very different things. Usually it is a raised treatment system, such as a mound, an at-grade bed or a sand filter, with the tank still buried. Less often it means a holding tank, which has no outlet and stores everything until a truck hauls it away. In our worked example, a three-bedroom home used full time with a 1,200-gallon holding tank and an alarm at 75% would reach the alarm about every five days, about 71 pump-outs a year. New Mexico, Utah and Texas allow a holding tank only in limited cases. Read above-ground septic systems explained before buying any tank.
Cesspools and other outdated systems
A cesspool has no septic tank ahead of it. It is a pit that lets untreated sewage soak into the ground, and Washington DOH says it may meet the definition of a failure. Federal rules banned new large-capacity cesspools from 5 April 2000, but they “do not apply to single family residential cesspools” (EPA, Large-Capacity Cesspools), so states decide. Hawaii’s 2017 law, Act 125, requires all of its cesspools to be upgraded, converted or connected to sewer by 2050, and Rhode Island requires one to be removed or abandoned within 12 months of a sale. Our cesspool vs septic tank comparison covers the state rules we read and what replacement costs.
What the type means for upkeep and cost
Inspection is the clearest split. EPA advises an inspection at least every three years, and generally once a year for systems with “electrical float switches, pumps, or mechanical components” (EPA, How to Care for Your Septic System, updated 17 August 2026). Washington writes that into law: every three years for a tank with gravity dispersal, every year for all other systems, unless the local health officer sets a shorter interval (WAC 246-272A-0270).
We found no government source that publishes installed prices by system type. These are published estimates from two commercial sites that sell homeowner leads to contractors, shown side by side and not averaged. They are not quotes.
| Type | Angi (updated 14 Oct 2026) | HomeGuide (22 Sep 2025) |
|---|---|---|
| Conventional | $3,000 to $8,000 | $3,500 to $8,500 |
| Mound | $10,000 to $20,000 | $25,000 to $50,000+ |
| Aerobic treatment unit | $10,000 to $20,000 | $20,000 to $40,000+ |
Each step away from gravity costs more to build and to run, and your soil, fill and state rules move the price more than the type label does. For a line-by-line estimate on your own house, use our septic system cost calculator.
Where to start
- You don’t know what you have: get the permit record, then read the overview of system types above to match it to a design.
- A designer says your lot needs a mound or an aerobic unit: read both guides. Their running costs differ, and your permitting office decides which are allowed.
- You’re buying a house: find the type before closing. A septic inspection before you buy should cover every part, not only the tank. An old house with no record could be on a cesspool.
- You want a tank for a cabin or lot: run the holding tank math in the above-ground guide first.
Safety for every type
Never enter or lean into a tank, pump chamber or cesspool. Washington DOH warns that gases inside pump chambers “are poisonous and the lack of oxygen can be fatal”. Keep lids secured and never leave one open unattended. If someone collapses in or near an open tank, call 911 and don’t go in after them. If an alarm sounds, see our guide to a septic alarm going off. If sewage backs up into the house or surfaces in the yard, stay out of it and call your local health department, and if you have a well, ask them about testing your water.
Sources checked on 3 November 2026. Figures come from the guides linked above, where each is cited.
