Types of Septic Tanks
Types of Septic Tanks
Septic tanks are sorted four different ways, and people usually mean one of them when they ask what type they need. Knowing which question you are actually answering makes the choice much faster.
- Material. Plastic, concrete or fiberglass
- Compartments. Single chamber or dual chamber
- Function. A septic tank that discharges, or a holding tank that does not
- Placement. Buried below ground, or sitting above it
A note on wording. A septic tank and a septic system are not the same thing. The tank is one component. The system is the tank plus whatever disperses or treats the effluent afterwards, usually a drainfield. Most of this page is about choosing a tank. The section on system types covers the other question, and explains which tank each system needs.
Requirements are set locally, so treat this as orientation and confirm the specifics with your local health department or a licensed installer.
By Material
Plastic, or polyethylene
Light enough to place without a crane, which matters where site access is tight or the ground will not carry heavy equipment. Polyethylene does not corrode. Bedding and backfill instructions matter more than they do with concrete, and where the water table is high, anchoring or ballast is commonly specified because the tank is light.
Concrete
Heavy, which is an advantage where groundwater is a factor because the weight resists flotation on its own. Made at regional plants, so delivery distances are short. Some counties require it. It needs equipment and ground access to set, and it can be affected by the gases present in a septic environment over time.
Fiberglass
Light like polyethylene and also non corrosive. Less widely stocked than the other two, and availability varies a lot by region. We do not carry fiberglass septic tanks.
Material is usually the first thing people ask about and rarely the thing that decides it. For a fuller comparison of the two most common choices, see concrete vs plastic septic tanks.
By Compartment
Single compartment
One chamber holding everything. Simpler, available in smaller capacities, and adequate where code allows it and the tank sits shallow.
Dual compartment
The same volume divided by an internal baffle. Solids settle in the first chamber and clearer liquid moves to the second before it leaves the tank.
This is the choice most often decided by code rather than preference. Many jurisdictions specify compartment count for a permitted residential system, and it is one of the most common reasons a tank fails inspection after it is already in the ground.
The two also differ on burial depth, backfill and access. See single vs dual compartment septic tanks for the full comparison.
By Function: Septic or Holding
These get confused constantly, and they do different jobs.
A septic tank treats wastewater and discharges the liquid to a drain field. It is part of a permitted onsite wastewater system, which is why approvals and code requirements apply to it.
A holding tank stores everything and gets pumped out. Nothing discharges, so there is no drain field. That makes it the right choice where a drain field is not possible: jobsites, cabins, campsites, temporary facilities and sites where the soil or the water table rules out dispersal.
If you have no drain field and no way to install one, you need a holding tank rather than a septic tank, whatever the property is. Browse septic holding tanks.
Types of Septic Systems
If someone has told you that you need a particular kind of system, that refers to how the effluent is dispersed after it leaves the tank. Soil depth, groundwater level, lot size and local rules decide which one a site can take. The tank itself is often the same across several of them, and the practical question for you is whether the system needs one tank or two.
Conventional, or gravity fed
The most common arrangement. Wastewater flows by gravity from the property to the tank, and effluent flows on to a trench or bed of stone or gravel where soil microbes finish the treatment. No pump and no electricity. It needs enough suitable soil beneath the drainfield, and it takes up more room than most alternatives.
Tank needed: a standard buried septic tank
Chamber, or gravelless
Identical to a conventional system except the drainfield uses connected open bottom chambers instead of stone. Easier to deliver and install, and useful where gravel is scarce, where the groundwater table is high, or where the volume arriving at the system varies, such as a seasonal property.
Tank needed: a standard buried septic tank, plus the chambers themselves
Mound
Used where soil is shallow, groundwater is high or bedrock is close to the surface. A constructed sand mound holds the drainfield above the natural grade, and effluent is pumped up to it in measured doses. It needs a substantial footprint and ongoing maintenance.
Tank needed: a septic tank plus a separate pump chamber
Aerobic treatment unit
Works like a small scale municipal plant. Air is injected into the treatment tank so aerobic bacteria can break down waste more thoroughly than they would in a sealed tank. Suited to small lots, poor soil, high water tables and sites near sensitive surface water. Requires power and regular servicing.
Tank needed: a treatment tank, often with pre treatment or final treatment tanks alongside
Recirculating sand filter
Effluent is pumped from the tank through a sand filter, which can sit above or below ground, before it reaches the drainfield. It gives a high level of treatment and works on sites with high water tables, at the cost of a pump, power and more maintenance.
Tank needed: a septic tank plus a pump chamber
Drip distribution, evapotranspiration and constructed wetland
Three alternative dispersal methods for sites where the usual options do not work. Drip systems place small laterals in the top few inches of soil and need a dose tank. Evapotranspiration beds are sealed underneath so effluent evaporates rather than reaching soil, which only works in dry, warm climates. Constructed wetlands run effluent through a planted, lined cell before it moves on.
Tank needed: varies. Most add a dose or pump tank to the septic tank
Cluster or community
One system serving several dwellings or buildings under shared ownership, with treatment and dispersal on a suitable nearby site. Common in rural subdivisions and on commercial properties.
Tank needed: sized for the whole group, usually engineered rather than selected
The US Environmental Protection Agency publishes descriptions and diagrams of these and other system types at epa.gov. Your local health department is the authority on which are permitted where you are.
By Placement
Below ground. The standard arrangement for a permitted residential system. The tank is buried, and how deep it can go depends on the build. Adding a riser brings the lid up to grade so pumping and inspection do not require digging. See underground septic tanks.
Above ground. Used where burial is impractical or the tank is temporary. Common on jobsites, at campsites and in remote locations where the tank gets pumped on a schedule rather than discharging. See above ground holding tanks.
Narrowing It Down
- Do you have a drain field? No drain field means a holding tank, not a septic tank
- What does your county require? Compartment count, minimum capacity and sometimes material are set locally. Check the approvals we hold by state, then confirm with your local authority
- What size? Usually based on bedroom count or expected daily flow
- What will the site take? Access, water table and burial depth decide material and build more often than preference does
Answer those four and the choice is usually obvious. If it is not, tell us the property, the site conditions and your county, and we will point you to the tanks that fit.