How a septic system works (with diagrams)

Every drain in the house — toilets, showers, sinks, the washing machine — joins one main pipe that leaves the house and runs to the tank. There is no separate path for "grey" water in most homes: shower water goes to the septic tank too.

1. Wastewater leaves the house through the main drain and enters the tank through the inlet baffle, which slows it and directs it downward.
2. In the tank, heavy solids sink to form sludge and lighter material — grease, oils, paper fibres — floats to form scum. The tank is designed to hold wastewater for at least a day so this can happen.
3. Anaerobic bacteria in the tank digest part of the solids. The rest accumulates, which is why tanks need pumping every few years.
4. The liquid between the layers — effluent — flows out through the outlet baffle and, in newer tanks, an effluent filter that catches suspended solids.
5. A distribution box, or a pump on some systems, sends the effluent evenly into the drain field's trenches.
6. Perforated pipes in gravel or plastic chambers let the effluent seep into the soil.
7. The soil does the real treatment: a thin biomat and the soil beneath filter out bacteria and viruses and break down nutrients before the water reaches groundwater.

Most modern tanks are concrete, plastic or fiberglass, hold 750 to 1,500 gallons for a typical house, and have two compartments that improve settling. The three layers — scum, effluent and sludge — are always there; a tank is always full of liquid. What changes over time is how much of the tank the solids occupy, and when they take up about a third of it, it's time to pump.

How often to pump→

The drain field (also called a leach field or absorption field) is the part most people never see and the part that costs the most to replace. It's usually two to six trenches, each a couple of feet deep and a few feet wide, sized from the house's bedrooms and how quickly the soil absorbs water — which is what a perc test measures.

The field needs unsaturated soil beneath it to treat effluent properly, commonly two to four feet above the water table or bedrock depending on the state. When that isn't available, the field is lifted (a mound), the effluent is pre-treated (an aerobic unit or sand filter), or it's dosed in small amounts under pressure.

Types of septic systems→

• Skipping pumping, so solids flow into the field and clog it.
• Flushing things that don't break down: wipes (including "flushable" ones), paper towels, feminine products, cat litter, cooking grease.
• Pouring in chemicals that kill the tank's bacteria: solvents, paint, large amounts of bleach, drain openers.
• Sending too much water at once — several loads of laundry in a row — or leaking fixtures that run all day.
• Driving or parking on the field, building over it, or planting trees nearby.
• Letting gutters, sump pumps or surface water drain onto the field.

What's safe to put down the drain→

Check your water bill for a sewer charge — if there isn't one and you're not on a municipal system, you're almost certainly on septic. The county health department or property records will show a septic permit. In the yard, look for tank lids or risers, a patch of green grass, or a vent pipe near the ground; inside, a main drain that leaves the house toward the back or side rather than toward the street is another hint.

Septic vs sewer compared→