Septic systems: how they work and the main types

What's under the lawn, and why your neighbour's system may be nothing like yours.

The short answer

Every septic system has a tank that separates solids from liquid and a way to return the liquid to the ground. Conventional systems let the soil do the treating; mound, aerobic, drip and sand filter systems add treatment or lift the field where the soil can't. The soil evaluation decides which type a lot can use.

In this section

How a septic system worksTank, drain field and soil, step by step, with diagrams.Types of septic systemsSeven system types compared on cost, lifespan and upkeep.Aerobic septic systemsHow ATUs work, what they cost to run, and the downsides.Mound septic systemsWhy a site needs one, what it costs, and how to care for it.Chamber systemsGravelless chamber drain fields — cost, pros, cons and sizing.Holding tank vs cesspoolThree systems that look alike and cost very different amounts to run.Septic vs sewerRunning costs, risks, switching costs, and how to tell which you have.

ToolSeptic system cost calculatorA planning range for a new or replacement system from type, bedrooms, soil and region — then check a quote for price and scope red flags.

A septic system in one picture

The tank separates, the field disperses, and the soil treats. Everything else is a variation on those three jobs.

A conventional septic system. Everything from the house flows to the tank, where solids settle and scum floats. The clearer liquid in the middle flows on to a distribution box, which splits it between perforated pipes in gravel trenches. The soil under the trenches does most of the treatment.

Step by step

Which type suits which site

The soil evaluation decides, but the pattern is predictable enough to know what to expect before the report arrives.

Site condition	Usually permitted
Deep, well-drained soil	Conventional gravity or chamber
Field uphill from the house	Conventional or chamber with a pump, or a pressure-dosed field
Marginal soil, shallow depth	Low-pressure dosed field or drip
High water table or shallow rock	Mound, or aerobic unit with shallow dispersal
Small lot, tight setbacks	Aerobic unit or drip
Near a lake, stream or sensitive coast	Aerobic unit, sand filter or nitrogen-reducing system

Working out which type you already have

The county's permit record names the system type. Without it, look in the yard: a raised grassy bed points to a mound; a humming unit, a control box and spray heads to an aerobic system; a control panel with an alarm light to a pumped system of some kind; and nothing visible but tank lids to a conventional or chamber system.