You do not pick a septic system from a catalogue. The soil evaluation picks it for you, and the practical question is which one your site forces.
Here is what each is, when it is required, and what it costs.
Quick comparison
| System | Installed cost | Power | Lifespan | Required when |
|---|---|---|---|---|
| Conventional gravity | $6,000 – $15,000 | None | 20–30 yr | Good soil, adequate depth, field below tank |
| Gravelless chamber | $7,000 – $16,000 | None | 20–30 yr | Same; area reduction helps a tight lot |
| Pressure-dosed (pump) | $9,000 – $20,000 | Intermittent | 20–30 yr | Field above tank, or even distribution needed |
| Sand filter | $10,000 – $22,000 | Intermittent | 20–25 yr | Pre-treatment lets a smaller field work |
| Aerobic treatment unit | $12,000 – $28,000 | Continuous | 15–25 yr | Poor soil, small lot, nitrogen limits |
| Drip dispersal | $14,000 – $30,000 | Intermittent | 20–30 yr | Shallow soil, steep slope, odd-shaped area |
| Mound | $18,000 – $45,000 | Intermittent | 20–30 yr | Shallow bedrock, high water table, very slow soil |
Price your own configuration →
Conventional gravity
Tank, then perforated pipe in gravel-filled trenches, fed by gravity. No pump, no power, no controls.
This is the system everything else is measured against, and it is genuinely the best option when your site allows it. Nothing to break, nothing to service beyond pumping the tank, and the longest service life.
Requires: soil that accepts water at 0.2 gpd/ft² or better, 2 to 4 feet of unsaturated soil below the trench, a field location downhill of the tank, and room for the field plus its reserve.
Gravelless chambers
Open-bottomed plastic arches instead of gravel and pipe. Effluent flows through the chamber interior and infiltrates across the full trench bottom.
Most states grant roughly a 30% area reduction because the open chamber provides a larger effective infiltrative surface than gravel, which occupies space that would otherwise be void. That reduction is often what makes a constrained lot workable.
They also install faster — no gravel hauling, no aggregate delivery — which in remote locations can make them cheaper overall despite costing more per foot.
Pressure-dosed systems
A pump chamber after the tank collects effluent and doses the field in timed batches through small-diameter pressurised pipe.
Two reasons to use one:
Elevation. If the field is uphill of the tank, gravity does not work. Simple.
Even distribution. Gravity trenches overload the near end and leave the far end dry. Pressure dosing loads the whole field at once, then lets it rest and re-aerate between doses. This genuinely extends field life, and some states require it for that reason alone.
Trade-off: you now have a pump, floats, an alarm, and a dedicated electrical circuit. Floats are the most common failure point in the entire system. Test the alarm every six months.
Sand filters
Effluent is dosed over a bed of specified sand, filters down through it, and is collected at the bottom before dispersal. Either recirculating — passing multiple times — or single-pass.
The output is substantially cleaner than septic tank effluent, which earns a drainfield area reduction, so a sand filter can make a small lot work. Common in New England and the Pacific Northwest.
Aerobic treatment units
An ATU injects air into a treatment chamber, supporting aerobic bacteria that work far faster and more completely than the anaerobic bacteria in a septic tank. Output approaches secondary treatment quality.
Why they are used:
- The high-quality effluent clogs soil far more slowly, so states typically allow a 25% drainfield area reduction
- They work where soil is genuinely poor
- They reduce nitrogen, which is mandatory in nitrogen-sensitive watersheds — Maryland’s Chesapeake Bay Critical Area, Suffolk County NY, parts of Florida and Rhode Island
- In Louisiana and much of Mississippi, an ATU with disinfection and surface discharge is the standard system, because the water table makes subsurface dispersal impossible
The ongoing cost is real. Most states mandate a service contract — Texas requires one for at least two years, Louisiana and Oklahoma require them indefinitely — at $200 to $500 a year. Plus $30 to $80 a year in electricity for the blower running continuously.
An ATU is not a way to save money. It is a way to build on a site that otherwise could not be built on.
Drip dispersal
Small-diameter tubing with pressure-compensating emitters, buried 6 to 12 inches deep, dosing tiny volumes over a wide area. Needs pre-treatment and filtration ahead of it, because the emitters clog.
Strong where:
- Soil is shallow — it uses the top layer, which is the most permeable and most biologically active
- The site is steep, where trenches would be difficult
- The available area is an odd shape, since tubing follows contours
- You want minimal disturbance to existing landscape
Typically earns a 40% area reduction where permitted. The controls and filtration are the bulk of the cost premium, and the field needs periodic flushing.
Mound systems
A raised bed of specified sand built on the natural ground surface, with a pressurised distribution network inside it. Effluent is dosed into the sand, treated as it passes through, then enters the natural soil beneath.
Mounds exist for one reason: there is not enough soil depth. Shallow bedrock, a high water table, or very slow soil means the required vertical separation cannot be achieved below grade — so you build it above grade instead.
The costs:
- Sand volume and hauling dominate the budget; distance to a sand source matters enormously
- A pump and dosing chamber are mandatory
- The mound is visible, often 2 to 4 feet high and large
- Basal area is sized on the natural soil beneath, so poor soil means a very large mound
Wisconsin pioneered the mound and still uses them extensively. They are also standard across the Red River Valley in North Dakota and Minnesota, in much of Iowa and Indiana, and anywhere glacial till meets a high water table.
Holding tanks
A sealed tank with no dispersal at all, pumped out on a schedule. Not a treatment system — a storage system.
Most states permit these only for repairs where nothing else is possible, or for seasonal use. Ongoing cost is severe: a family of four fills a 2,000 gallon tank in roughly a week, at $300 to $500 a visit.
Wisconsin requires holding tanks serving a dwelling to be at least 2,000 gallons or five times daily flow, whichever is greater — which gives a sense of how the math works.
Cesspools
A pit that lets raw sewage soak into the ground with no treatment. Banned for new construction essentially everywhere.
Hawaii is the significant case: roughly 83,000 cesspools remain, and Act 125 requires all of them to be converted by 2050. If you own Hawaii property with a cesspool, you are on a clock, and the available tax credit does not cover the full cost.
How the choice is actually made
- Soil evaluation returns a loading rate and a depth to limiting layer
- If the loading rate is adequate and there is 2 to 4 feet of usable soil below the trench → gravity
- If the field must sit above the tank → pressure-dosed
- If area is tight → chambers, or ATU for the pre-treatment reduction
- If soil depth is inadequate → mound, at-grade, or drip
- If the watershed limits nitrogen → ATU or another nitrogen-reducing technology
- If nothing works → holding tank, as a last resort
The order matters. Every step down costs more and adds something that can break.
One thing worth spending on
Whatever system you end up with, the cheapest upgrade available is a larger tank and an effluent filter. Both cost a few hundred dollars at install, both extend the life of the expensive part, and neither can be retrofitted cheaply later.
Common questions
Which septic system is best?
The simplest one your site allows. A conventional gravity system has no moving parts, needs no power, costs the least, and lasts the longest. Everything else exists because a site could not support gravity trenches. Nobody chooses a mound because it is better.
What is the cheapest septic system?
Conventional gravity trenches, at roughly $6,000 to $15,000 installed. Chambers are close and sometimes cheaper overall because the area reduction shrinks the excavation. Every alternative system costs more, and you do not get to choose one to save money.
Do aerobic systems need electricity?
Yes — an aerobic treatment unit runs a blower continuously, costing roughly $30 to $80 a year. During an extended power outage the unit stops treating, and most designs can tolerate a day or two before effluent quality drops. Pump-based systems also need power, but only intermittently.
How long does a mound system last?
Twenty to thirty years, comparable to a conventional field, provided the pump is maintained and the sand is not compacted. The failure mode is the same: the infiltrative surface eventually seals. Mounds are less tolerant of neglect because they depend on the dosing pump working.
Can I convert an aerobic system back to conventional?
Only if the site could support a conventional system in the first place, which it almost certainly could not, or the ATU would not have been required. Where an ATU was installed to earn a drainfield area reduction on a small lot, removing it means the field is now undersized.