Ontario rentals: greywater diversion or treatment system?

For a rental, have the contractor provide a written maintenance and emergency-response plan with the design. It should cover:

- What tenants can and can’t put into showers, tubs, or laundry drains, and who they contact for an odour, alarm, backup, or loss of service.
- How filters, pumps, tanks, and inspection points can be accessed without disrupting the unit.
- Winterization for any outdoor diversion or irrigation components, including power outages and prolonged vacancies.
- The approved fallback if treatment fails, such as a properly designed bypass or a procedure for taking the reuse system offline without affecting normal drainage.
- Who is responsible for cleaning, inspections, records, and emergency callouts, with expected response times.

That exercise may show that a basic diversion setup is not as maintenance-free as it first appears. A treatment system may still be worthwhile if its alarms, fallback arrangement, and servicing responsibilities are practical for the owner to manage.

The fallback and service plan should also include commissioning documentation, especially if treated greywater will feed toilet-flushing lines. Before connection, require a documented cross-connection-control test and record showing the potable supply is protected, along with the system’s isolation, backflow device, and any air-gap arrangements.

That record should identify the tester, date, test results, and what happens if the protection fails. Keep it with the plumbing and maintenance records, then repeat testing at the interval required by the applicable authority or device standard. A treatment alarm alone doesn’t prevent contaminated water from entering the potable system.

I’d push back on calling a basic diversion setup simpler just because it has fewer components.

- In Ontario, approval can depend heavily on where the water is discharged, the soil and drainage conditions, required setbacks, and which greywater sources are allowed.
- A shower-only diversion to a properly designed subsurface system is a different proposition from sending laundry or kitchen-related water outdoors. The source water, detergents, solids, and expected flow all matter.
- A diversion system still needs protection against ponding, freezing, odours, backups, and accidental tenant contact. Access for inspection and cleaning has to be practical too.
- The contractor should document the proposed discharge location, soil assessment, setbacks, allowable sources, and the authority having jurisdiction’s requirements before comparing equipment and maintenance costs.

So the treatment system may be more mechanically involved, but diversion isn’t automatically the lower-complexity or lower-risk option.

Diversion can be the lower-risk option when it is limited to suitable sources, such as shower or bath water, and discharged through a properly designed, approved subsurface system.

The key distinction is how failures are managed. A diversion system can still pond, freeze, smell, or back up, so it needs accessible inspection points and an approved overflow or bypass arrangement. A treatment-and-reuse system adds stored water, pumps, filters, disinfection, alarms, and indoor non-potable piping. If treatment or backflow protection fails, it can interrupt toilet service or create a broader cross-connection concern.

That does not make diversion automatically suitable. Laundry water, kitchen water, poor soils, high groundwater, or inadequate setbacks may rule it out. For a suitable site with restricted source water, avoiding storage and indoor reuse plumbing may reduce the owner’s operational risk, even though the diversion system still needs careful design and maintenance.