Types of Septic Systems: Which One Fits Your Property?

What Are the Main Types of Septic Systems?

Soil-dependent systems and alternative systems are septic system types that treat wastewater before it contacts the soil. Understanding how septic systems work helps property owners compare the available options. You will be assigned to a group based upon your site evaluation.

Here’s how the common types of septic systems are sorted out:

• Conventional: gravel trench drainfield and chamber drainfield
• Alternative: aerobic treatment unit and mound and recirculating sand filter
• Specialty: drip distribution and evapotranspiration bed and constructed wetland
• Shared: cluster or community systems for subdivisions

Every one of them starts with a septic tank. What changes is where the liquid goes next and how much treatment it gets on the way.

Conventional Septic System Installation and Function

A conventional septic system filters solids to a buried septic tank and allows the liquid to drain by gravity into gravel trenches. The treatment is completed by bacteria in the soil beneath.

1. Wastewater enters the tank. Toilets and sinks and laundry drain to one buried tank.
2. Solids separate. Heavy sludge sinks to the bottom while grease and scum float to the top.
3. Effluent moves to the drainfield. The clearer middle layer flows out through an outlet pipe into perforated lines laid in gravel.
4. Soil does the final cleanup. Microbes in the soil break down what’s left before the water reaches groundwater.

This design has no pump and no electrical parts. That’s why it’s usually the cheapest system to install and the easiest one to own. The catch is space. A gravel drainfield needs a large footprint and deep soil that drains at a steady rate. If your lot is small or the water table sits close to the surface it won’t pass a permit.

What Is a Chamber Septic System?

A chamber system is a traditional system that replaces the gravel trench with open bottom plastic chambers. Can do the same function, but requires fewer gravels and quicker installation. The chambers snap together in a trench and get covered with soil. Effluent from the tank spreads across the open floor of each chamber and soaks into the ground underneath.

There is no stone to haul in it’s a good fit for rural sites where gravel is expensive to truck out. Chambers also hold a surge of water well. That helps a cabin or a vacation home that sits empty for weeks and then fills up with guests.

How Does an Aerobic Septic System Work?

An aerobic septic system adds oxygen to the septic tank to accelerate the decomposition of waste by oxygen consuming bacteria in the tank that are more effective than a conventional septic system. The improved water quality that exits it enables it to use small size and poor quality soil.

1. Pretreatment: the wastewater is allowed to settle in a first chamber similar to a normal tank.
2. Aeration: the air is pumped into a second chamber where bacteria break down the organic wastes.
3. Clarifying: Any solids that remain are settled out prior to the discharge of water from the unit.
4. Dispersal: treated effluent to smaller drainfield or spray/drip area.

The cost of the aerobic unit is higher in the beginning. It also requires a constant supply of power and a service call on a regular basis. Once the aerator stops an alarm is triggered and treatment is slowed down until the part is replaced. If you have a lot too small for a full-size drainfield, it may be the only option the health department will allow.

Which Alternative Septic Systems Work When the Soil Can’t?

Alternative septic systems are systems that use sand or pumps or plants in a place where native soil is too deep, too wet or too compact to be used. All solve different site issues.

Mound systems build a raised sand bed above the native ground. A pump sends measured doses of effluent up into the mound. They work over shallow bedrock or a high water table but they take up a lot of yard.

Recirculating sand filters run effluent through a lined box of sand before it goes to the drainfield. The treatment level is high. So is the price.

Drip distribution pushes small timed doses through tubing set 6 to 12 inches deep. It handles slopes and shallow soil well. It needs a dose tank and power to run.

Evapotranspiration beds let water evaporate from a lined bed instead of soaking into the ground. They need a dry and sunny climate. Heavy snow can push them into failure so they’re rarely a fit in the northern plains.

Constructed wetlands move effluent through a lined cell planted with wetland plants. Cluster systems collect wastewater from two or more homes and treat it at one shared site. You’ll mostly see those in rural subdivisions.

Septic System Types Compared by Site and Cost

These systems are the least expensive and require the most space. Aerobic units and mounds are more expensive, but are available for sites a conventional system can’t fit.

Read the table as a starting point. Your soil report narrows these choices down to the two or three your county will permit. It’s common for only one to make the cut.

How Do You Go About Choosing a Septic System?

The first step in choosing a septic system is a soil and site evaluation. Brochures come later. The test results will inform you what septic system option your health department will approve.

Book the site evaluation

A licensed evaluator digs test pits and checks soil type and depth to bedrock or groundwater. In Flathead County the site evaluation alone is $275 and it isn’t a permit fee.

Size it to your house

Bedroom count drives the daily flow and that sets the tank and drainfield size. Plan for the bedrooms you might add later.

Map the setbacks

Wells and property lines and streams each need distance from the drainfield. These lines often decide the layout before soil does.

Get the design approved

The designer submits plans and the health office reviews them. Montana sites may also need a non-degradation analysis to protect groundwater. Flathead County charges $200 for that review.

Install and inspect

The crew digs and sets the system. Then the health office can inspect it before anything gets buried.

Not every slow drain means you need a new system. If the tank hasn’t been pumped in years start with a pump-out and an inspection. A clogged effluent filter is a cheap fix.

Septic Rules in Williston ND and the Flathead Valley

The septic permits for Williston area homes are processed through the Upper Missouri District Health Unit. They pass through Flathead County in Kalispell via the Flathead City-County Health Department. The Upper Missouri District Health Unit handles septic permits for Williams and Divide and McKenzie and Mountrail counties. Its environmental health team issued 165 septic permits in 2021 according to its own legislative testimony.

North Dakota is also writing a statewide technical guide for onsite systems that local health units will adopt. Winter adds its own rule of thumb. Lines and tanks need enough depth and cover to stay unfrozen through a hard northwest North Dakota January.The Flathead Valley changed course in 2026. Flathead County had required pressure-dosed systems since 2004. In March 2026 the county commissioners approved new rules that allow gravity systems again where the site supports them.

Health board officials said pressure dosing stays the preferred method. County minutes put the extra cost of a pressure-dosed system at up to $6000. Opponents of the change pointed to the valley’s high groundwater table and impaired streams. That means a Whitefish lot near the lake may still end up with a pressure system while a dry bench lot near Kalispell might qualify for gravity.

Septic Tips From Our Crew

Pro tip: Ask for the soil report in writing before you compare quotes. Two bids for different system types aren’t comparable until both contractors design to the same soil data.

Key takeaways:

• Soil and lot size choose your system more than budget does.
• Conventional gravity systems cost the least when the site allows them.
• Aerobic and mound systems solve tight lots and wet soil but need power and upkeep.
• Permits come from the local health unit in North Dakota and the county health department in Flathead County.

Get a Septic System Designed for Your Lot in Williston and the Flathead Valley

WBP Contracting LLC handles the soil testing and permits and installation for new homes and replacement systems. We’ll tell you which types of septic systems your lot can support before you spend money on a design.

Call (406) 890-3507 or email wilder@wbpcontractingllc.com to set up a site visit.

Frequently asked questions