How a Septic System Works: A Plain Guide for Rural Homeowners 

How A Septic System Works

Wastewater leaves your house through one main pipe, drops into a buried tank, and separates into layers. The liquid in the middle flows out to a drainfield, where soil finishes cleaning it before it reaches groundwater.

Here’s the sequence:

1. All house drains are connected to one main drain which drains to the tank.
2. Heavy solids settle in the tank creating sludge inside. Light material and grease are floaters and scum.
3. The bacteria that come with the waste begin to break down some of the sludge.
4. The clarified liquid in the center is called effluent, which is removed through an effluent outlet that prevents the sludge and scum from leaving.
5. The effluent runs through perforated pipes into the drainfield.
6. Soil filters it, and microbes remove bacteria, viruses and nutrients as it moves down.

That’s the whole loop. Once you see how septic systems work at this level, most maintenance advice stops sounding like rules and starts sounding like common sense.

The Main Parts Of A Septic System

A conventional setup has four parts: the pipe from the house, the tank, a distribution box, and the drainfield. These septic system components work in a fixed order, so a failure in one backs up into everything before it.

If the tank lid is buried, every service visit starts with a probe rod and a shovel. In frozen ground that’s a rough morning. Risers bring access up to the surface, and they’re one of the cheaper upgrades you can make.

What Happens Inside A Septic Tank?

Solids sink, grease floats, and bacteria digest part of what settles. Only the clearer liquid in the middle is allowed to leave.

That’s how a septic tank works, and the septic tank process depends on time: the tank has to hold each load long enough for the layers to separate. The EPA says that these tanks, made of concrete, fiberglass or polyethylene, are watertight, have compartments and have an outlet tee to keep sludge and scum from going into the drainfield. The residential tanks are usually 1,000 to 1,500 gallons. These larger or a garbage disposal will shorten the time the waste settles with each load.

The bacteria do real work, but not all of it. They can’t digest everything in the sludge, so solids build up year after year. That buildup is the only reason pumping exists. You don’t need to feed the tank anything, because the bacteria arrive with the waste. Additives that stir up settled sludge can push solids toward the drainfield, which is the last place you want them.

How A Drainfield Treats Wastewater

The drainfield uses soil, air and microbes to finish cleaning the liquid from the tank. Effluent trickles out of perforated pipes, moves down through the soil, and by the time it reaches groundwater the bacteria, viruses and much of the nutrient load should be gone.

This last stage of septic wastewater treatment only works in soil that isn’t saturated. If the drainfield takes on too much liquid, it can flood, sending sewage to the surface or backing it up into toilets and sinks. Over time a bacterial layer called a biomat forms along the trench bottoms. A thin one is normal. A thick one, usually from too many solids escaping the tank, slows absorption.

Montana adds two wrinkles. Spring snowmelt raises the water table, so a drainfield that behaves in August can struggle in April. And in winter, leave the snow over the field alone. Snow insulates the pipes, while a packed track from a snowmobile or plow doesn’t.

Types Of Septic Systems

Most homes use gravity systems, but shallow soil, high groundwater, or small lots may require alternatives. Alternative systems pass tank effluent through sand, peat, constructed wetlands or other media, and some evaporate or disinfect it. Montana’s Circular DEQ 4 standards cover several of these, including sand filters and aerobic systems.

These are planning estimates – not quotes. After a site visit, real Montana numbers are pushed up or down by the distance and frost depth of the soil; therefore, get written prices. Understanding how septic systems work also explains why the mound will cost more, because it requires a pump, sand fill and engineered design, rather than trenches.

How Often Septic Tanks Need Pumping

Most household tanks need pumping every three to five years, and a professional should inspect the system at least every three. EPA says to pump sooner if the scum layer is within 6 inches of the bottom of the outlet, the sludge is within 12 inches of the outlet, or solids take up more than 25 percent of the liquid depth.

The EPA care guide lists the same four factors we look at: household size, total wastewater, volume of solids, and tank size. Day to day septic system operation comes down to water volume.

A typical home can use as much as 70 gallons per person per day, and one running toilet can add up to 200 gallons. Fix the leaky flapper first. Spread laundry across the week, too, because five loads on a Saturday can flood a drainfield that handles the same five loads easily over seven days.

Notice the gap. A skipped pump-out is how a $300 job becomes a $10,000 one.

Not sure when your tank was last pumped? Call us at 406.890.3507 and we’ll help you work out where to start.

Items That Should Never Go Into A Septic System

Only human waste and toilet paper belong in the toilet, and grease, chemicals and coffee grounds should stay out of every drain. The bacteria in your tank are alive, and what kills them or can’t be digested ends up in the drainfield.

EPA’s never flush list includes cooking grease, wet wipes, feminine products, dental floss, diapers, cigarette butts, coffee grounds, cat litter, paper towels, pharmaceuticals, and household chemicals like paint thinner and antifreeze.

A wipe labelled flushable is still a wipe. Skip chemical drain openers, too. Boiling water or a drain snake clears most clogs without harming the tank. Since you know how septic systems work, the rule is easy to remember: if bacteria can’t digest it, it doesn’t go in.

The Warning Signs Of A Failing Septic System

Slow drains throughout the house, gurgling pipes, a sewage smell outdoors, and wet ground over the drainfield are the clearest signs. One slow sink is usually a local clog. Several at once point to the tank or the field.

Montana Homeowner Guide To Septic System Installation

You need a local septic permit before installation, and the design has to meet DEQ standards. DEQ writes the technical rules in Circular DEQ 4, and local health requirements can be stricter than the state’s.

Where you are matters. In Yellowstone County, for example, parcels under 20 acres go through DEQ review and parcels of 20 acres or more go through local review. Some counties require the site evaluation to be done by a county sanitarian or a Montana licensed professional engineer. Ask before you buy a tank, because a design done before the site evaluation often gets redone.

Three local details decide most projects:

• Soil. Gravel drains fast and clay doesn’t, and the site evaluation will show which you have.
• Groundwater and runoff. Spring high water can rule out a spot that looks dry in September.
• Setbacks. Codes set minimum distances from wells, streams and property lines, so ask your sanitarian for the exact numbers before staking anything.

Septic System Records To Keep On File

Keep a one page record: it saves a service crew an hour and shows you exactly what has happened to the system. Copy this into your notes:

Septic Record Card

• Install year, tank size and material
• Lid location, measured from two fixed points like the corner of the house and a fence post
• Permit number and installer
• Date of every pump-out, with sludge and scum readings
• Date of every inspection and any repair
• Yearly walk: soggy patches, odors, vehicles parked on the field, new trees or downspouts near it

Frequently asked questions