How Does a Septic System Work?

A septic system is a small wastewater treatment plant that sits in your yard and runs mostly on gravity and bacteria. Nothing about it is complicated, but each part has a specific job, and a problem in one part shows up somewhere else. Knowing how the pieces fit together makes it much easier to look after the system, understand a contractor’s diagnosis and judge whether a quote makes sense.

From the house to the tank

Every drain in the house, from toilets and showers to the kitchen sink and the washing machine, joins a single main drain that leaves through the foundation. Outside the wall that pipe is called the building sewer. It runs at a steady downward slope to the septic tank, usually a few metres from the house. A cleanout, a capped access pipe, lets a plumber or drain contractor clear a blockage without digging.

The slope matters. Too flat, and solids settle in the pipe. Too steep, and liquid runs ahead of the solids and leaves them stranded. Roots, settling soil and frost can all disturb the line over time, which is why camera inspections are a common first step when drains are slow.

Inside the septic tank

The tank is a watertight box of concrete, polyethylene or fibreglass, buried just below the surface. Its job is to hold wastewater still long enough for it to separate. In a healthy tank you would find three layers:

• Scum on top: fats, oils, soap residue and light material that floats.
• A clear zone in the middle: partly treated liquid, called effluent, which is what leaves the tank.
• Sludge on the bottom: heavier solids that sink.

Bacteria that live without oxygen, known as anaerobic bacteria, slowly digest part of the sludge and scum. They reduce the volume of solids but never eliminate them, so the tank still needs pumping on a schedule.

Baffles or sanitary tees at the inlet and outlet keep the layers in place. The inlet baffle directs incoming flow downward so it does not stir up the scum. The outlet baffle draws effluent from the clear middle zone so floating scum stays behind. Many tanks are split into two compartments for better settling, and most newer tanks also have an effluent filter in the outlet, a slotted cartridge that catches fine solids before they can reach the bed. That filter needs cleaning, usually at each pump-out.

Moving effluent to the bed

Effluent leaves the tank through a solid pipe. On a level or sloping lot where the bed is lower than the tank, gravity carries it to a header or distribution box, which splits the flow between several distribution pipes.

Where the bed is higher than the tank, which is normal with raised beds, effluent first flows into a pump chamber. A float switch turns on a submersible pump when the chamber fills to a set level, and the pump sends a measured dose out to the bed. Dosing spreads effluent more evenly and gives the soil time to rest between doses. A high-level alarm warns if the pump fails or the chamber overfills.

The leaching bed: where treatment really happens

Effluent leaving the tank is cloudier and cleaner than raw sewage, but it still carries bacteria, viruses and nutrients. The leaching bed, also called a tile bed or drain field, is where it is actually treated. Perforated distribution pipes lie in trenches of clear stone. Effluent trickles out of the holes, spreads through the stone and seeps down into the soil.

The soil does three jobs at once. It filters out particles. Bacteria living in the air spaces of unsaturated soil consume organic matter and many disease-causing organisms. And soil particles hold on to some nutrients and other substances. Over time a thin biological layer, called a biomat, forms where stone meets soil, slowing the flow and helping the treatment.

All of this depends on air in the soil. That is why the Building Code requires a minimum depth of unsaturated soil between the bottom of the bed and the high groundwater level, bedrock or very tight soil. When a bed floods from groundwater, gets compacted under a vehicle or clogs with solids from an overdue tank, the soil loses its air, treatment stops and effluent rises to the surface instead.

Raised beds, filter beds and treatment units

Not every lot has the right soil at the right depth, so the Building Code allows several variations on the basic design:

• Raised or fill-based beds are built up with imported sand where native soil is too shallow, too wet or too tight. A sand mantle around the bed helps effluent spread into the ground below.
• Filter beds use a layer of specified filter sand to treat effluent in a smaller footprint than a conventional bed, with their own sizing rules.
• Tertiary treatment units add oxygen and media inside a manufactured unit to treat effluent much further before it reaches the soil. Treated effluent can then go to a smaller dispersal bed. These units need regular servicing under an agreement with the maker or an authorized agent.

Which one you have depends on the soil tests and site conditions at the time it was designed. The septic installation page covers how a designer makes that choice on a new build.

Holding tanks and the five classes of sewage systems

Ontario’s Building Code sorts on-site sewage systems into five classes:

• Class 1: systems that do not use water to carry waste, such as privies, composting toilets and chemical toilets.
• Class 2: a leaching pit for greywater only.
• Class 3: a cesspool, allowed only in narrow circumstances.
• Class 4: a septic tank or treatment unit with a leaching bed. Almost every house on septic has one.
• Class 5: a holding tank, which stores all sewage until it is pumped and hauled away.

A holding tank does no treatment at all. It is simply a large sealed tank with an alarm. Because nothing leaves except by truck, it is used only where a Class 4 system will not fit. The trade-offs are covered in holding tank vs septic tank.

What keeps it all working

The system’s weak point is the soil, and almost every rule of septic care is about protecting it: pump the tank so solids stay out of the bed, keep heavy water use spread out so the soil is not flooded, keep vehicles and buildings off the bed, and keep anything that kills bacteria out of the drains. The septic care guide turns that into a practical list, and the rules guide explains who approves systems and how far they must sit from wells and water.

If you are not sure what kind of system you have or what shape it is in, a septic inspection will map it, open it and report on each part. You can ask for one here and we will pass your request to one licensed local contractor.