Short answer, for anyone in Ontario with a conventional septic system: yes, and it has to. Your sink water, shower water, tub water, dishwasher, and washing machine all drain into the same tank as the toilet. That is not a design compromise. It is what the system is sized for.
The reason people ask is usually one of two things. Either they are wondering whether they can divert grey water out of a septic system that is struggling, or they have read something written for a different country and are now unsure what applies here. Both are worth answering properly, because the Ontario answer is not the same as the answer you will find on most search results.
Why It All Goes to the Same Place
Under the Ontario Building Code, grey water is sanitary sewage. It is a category of sewage, not an exception to it.
The Code defines grey water as sanitary sewage derived from fixtures other than sanitary units, and defines a sanitary unit as a water closet, urinal, bidet, or bedpan washer. So everything that is not a toilet — kitchen sink, bathroom sink, tub, shower, laundry tub, washing machine, dishwasher — produces grey water. And the Code’s definition of sanitary sewage expressly includes shower, tub, culinary, sink, and laundry waste.
Being sewage means it has to go to a permitted sewage system. A Class 4 septic system, which is what most Ontario homes have, is designed to receive all of it. Our septic classes guide covers the full set.
This is also why your tank is sized by bedroom count rather than by toilet count. The design flow assumes every fixture in the house.
Is Kitchen Sink Water Grey Water?
Yes, and this one is worth settling because there is a genuine argument about it online.
A kitchen sink is not a water closet, urinal, bidet, or bedpan washer, so under the Ontario definition its waste is grey water. Some US jurisdictions treat kitchen waste as “dark grey” and exclude it from grey water systems. Ontario does not make that distinction.
What is true is that kitchen water is the hardest on any system it enters. Fats, oils, grease, and food solids thicken the scum layer in a septic tank and, if they escape, coat and seal the soil in the bed. The legal answer and the practical answer point in different directions here — it is legally grey water, and it is also the thing most likely to shorten the life of whatever receives it. Worth reading what kitchen grease actually does if you cook a lot.
Can I Divert Grey Water Out of a Failing Septic System?
This is the question underneath most searches on the topic, and it usually arrives when a leaching bed is showing signs of failure. The reasoning is sound on the face of it: if the bed is overloaded, send it less water.
You cannot just do it. Three reasons.
It is a permitted change. Rerouting fixtures away from a sewage system is a modification to that system, and modifications require a permit. Doing it quietly creates unpermitted work, which surfaces at the next sale, refinance, or inspection — and in the City of Kawartha Lakes, bringing unpermitted work into compliance costs the applicable permit fee plus an additional 100%. Our guide to permits for repairs and modifications covers what triggers one.
The water still has to go somewhere legal. Diverting grey water means building a Class 2 grey water system to receive it. A pipe to a hole in the yard, a French drain, a dry well of your own construction, or a line to a ditch are not options. Discharging sewage to the ground surface or to a watercourse is a separate and more serious problem, and the City is explicit that you must not connect a grey water system to an agricultural field drain either.
The math usually does not work anyway. A Class 2 system cannot be built where the design flow exceeds 1,000 litres per day, and with pressurized water each fixture unit counts as 200 litres. A kitchen sink, bathroom sink, tub with shower, and clothes washer come to 5.5 fixture units — 1,100 litres a day, over the cap before you have counted a second bathroom. For a normal house on pressurized water, diverting the grey water to a Class 2 is not permittable. The grey water pit guide has the full sizing method.
So for a typical failing bed, grey water diversion is not the fix. Bed rehabilitation or replacement is, and the honest version of that conversation starts with finding out why the bed is failing.
What About Laundry Specifically?
Laundry water is grey water under the Code — a clothes washer and a laundry tub each count as 1.5 fixture units in the Code’s own table, which is how they get included in a Class 2 design flow calculation in the first place.
That said, laundry is a real load on a septic system, and it is one of the few loads you can genuinely manage without touching the plumbing. Volume arrives in bursts, and several loads back to back can push enough water through the tank to carry solids out to the bed before they settle. Spreading laundry across the week is the single easiest thing most households can do for their system — more detail in our laundry and septic guide.
If a designer excludes laundry from a proposed Class 2 system, that is usually a capacity decision against the 1,000 litre cap rather than a rule about laundry itself.
What Should Not Go Into the Septic System
Grey water belongs in the septic system. Some other things people connect to it do not, and these are the ones that actually cause damage:
- Sump pump discharge and foundation drains. Clean groundwater has no business in a septic tank. It floods the tank, pushes solids into the bed, and can hydraulically overwhelm a system that was working fine. This is a common and genuinely destructive mistake — see sump pumps connected to septic.
- Water softener backwash. Contested, and worth understanding rather than guessing about. Our water softener guide covers where it lands.
- Roof and surface drainage. Same problem as the sump pump, in larger volumes.
- Hot tub or pool water. Volume and chlorine, both bad for the tank’s biology.
The pattern: grey water from household fixtures is what the system is for. Clean water from outside the house is what breaks it.
What If My Cottage Has a Separate Grey Water Pit?
Some older cottages, and some legitimately permitted small systems, do split it — a composting toilet or privy handling the blackwater, and a Class 2 grey water pit taking the sink and shower water. That is a real configuration, and where it is permitted and sized correctly it works.
Two things to know about it.
First, since 1 January 2025 this route has narrowed sharply. The 2024 Building Code requires a water closet in any new dwelling unit with a water distribution system, and no longer accepts a drainless composting toilet as a substitute. Existing setups are unaffected and stay legal, but for new construction the combination is largely off the table. The cottage grey water guide covers what that means in practice.
Second, a lot of what looks like a grey water pit is not a permitted system at all. A stone pit behind the cottage taking the kitchen sink, a dry well of unknown vintage on the laundry line, an outdoor shower draining onto the grass — these were common decades ago and most were never permitted. The City of Kawartha Lakes holds sewage system records back to 1974, so an absent record generally means an absent permit rather than a lost file.
Frequently Asked Questions
Does grey water go into the septic tank in Ontario?
Yes. Grey water is sanitary sewage under the Ontario Building Code, and on a conventional Class 4 septic system all household fixtures — sinks, tub, shower, dishwasher, and washing machine — drain to the same tank as the toilet. The system is sized on the assumption that they do.
Can I separate grey water from my septic system?
Only by building a permitted Class 2 grey water system to receive it, which requires a permit and a design by a licensed sewage system designer. For most houses it is not permittable regardless, because a Class 2 cannot exceed 1,000 litres per day of design flow and an ordinary set of fixtures on pressurized water already exceeds that.
Will diverting grey water save a failing leaching bed?
Almost never, and it is not a legal shortcut. Rerouting fixtures is a modification requiring a permit, the diverted water still needs a permitted system to go to, and the flow calculation usually rules out a Class 2 for a full house. If a bed is failing, the real options are rehabilitation or replacement.
Does laundry water go into the septic tank?
Yes. Laundry is grey water and drains to the septic system like every other fixture. It is worth spreading loads across the week rather than running several back to back, because the volume can carry solids out of the tank and into the bed.
Is it legal to drain a sink onto the ground in Ontario?
No. A permanent fixture discharging to the ground surface is not a permitted sewage system, and discharging sewage where it can reach a watercourse creates additional exposure beyond the Building Code. An occasional bucket of water on the lawn is a different situation from permanent disposal infrastructure.
Does grey water still need a septic tank if I have a composting toilet?
Yes. A composting toilet deals with blackwater only. The sink, shower, and laundry water still has to go to a permitted system — historically a Class 2 grey water system. The belief that a composting toilet removes the need for any sewage system was never correct.
Not Sure What Your System Actually Does?
On a lot of older Kawartha properties, nobody can say with confidence where every drain goes. There is a tank somewhere, a bed somewhere, and possibly a pit out back that predates anyone’s memory of it.
That is a solvable problem, and the answer is usually cheaper than people expect. We pump, inspect, and assess septic systems and grey water arrangements across Kawartha Lakes — Lindsay, Bobcaygeon, Fenelon Falls, Coboconk, and the rural and waterfront properties between them — with no travel surcharge anywhere in the city.
If the tank is simply due, our septic tank pumping page has what that involves. If something is not draining the way it should, call (705) 702-5526 or book online.