How much does backwashing a pool filter cost in water usage in Ottawa?

A single backwash cycle on a standard residential pool sand filter in Ottawa uses approximately 750 to 1,500 litres of water, which costs roughly $2.50 to $5.00 based on the City of Ottawa's combined water and sewer rate of approximately $3.34 per cubic metre (1,000 litres). Over a full swimming season with backwashing every 7 to 14 days, your total backwash water cost typically ranges from $25 to $75 — a modest but often overlooked component of pool ownership expenses.

Understanding how much water backwashing actually uses requires knowing what happens during the process. When you set your multiport valve to "backwash" and run the pump, water flows backward through the filter media — reversing the normal direction — and flushes trapped dirt, debris, and contaminants out through the waste line to your yard or storm drain. The recommended backwash duration for a residential sand filter is 2 to 3 minutes, or until the sight glass on the multiport valve runs clear. Your pump flow rate, which for most Ottawa residential pools runs between 250 and 500 litres per minute depending on pump size and plumbing diameter, determines the actual volume discharged.

The math is straightforward for Ottawa homeowners who want to calculate their specific cost. If your pump moves 375 litres per minute (a common rate for a 1.5-horsepower pump on a typical 2-inch plumbing system) and you backwash for 2.5 minutes, that is approximately 940 litres per backwash. At Ottawa's 2026 combined water and wastewater rate of roughly $3.34 per cubic metre, that single backwash costs about $3.14. If you also run the "rinse" cycle for 30 seconds after backwashing — which you should, to resettle the sand bed and flush any remaining dirty water to waste rather than back into the pool — add another 190 litres and $0.63, bringing the total to approximately $3.77 per backwash-and-rinse cycle.

Seasonal backwash frequency in Ottawa varies more than most pool owners realize. Early in the season — late May through mid-June — your filter works harder because of heavy pollen loads, cottonwood fluff, spring algae bloom risk, and the general debris that accumulates during and after the opening process. You may need to backwash every 5 to 7 days during this period. By July and August, with the pool fully stabilized and less airborne debris, backwashing every 10 to 14 days is typical. September brings falling leaves and another increase in filter load, returning to weekly backwashing until closing. A reasonable estimate for a full Ottawa season is 12 to 20 backwash cycles, putting your total seasonal backwash water cost at $30 to $75.

Where your backwash water goes matters under Ottawa's municipal bylaws. The City of Ottawa's sewer use bylaw permits residential pool backwash water to be discharged to the sanitary sewer system, which is the default connection for most pool waste lines. Discharging chlorinated backwash water directly onto your lawn, into a garden, or into a storm drain is not recommended — storm drains in Ottawa flow untreated into the Ottawa River and Rideau River, and chlorinated water is harmful to aquatic ecosystems. If your waste line currently discharges to the surface, consider having it redirected to a sewer cleanout, which a plumber can typically do for $200 to $500.

Cartridge filters and DE filters have different water cost profiles than sand filters. Cartridge filters do not require backwashing at all — you remove the cartridge element and hose it off, using perhaps 50 to 100 litres of water per cleaning. This makes cartridge filters the most water-efficient option, with seasonal cleaning water costs of only $2 to $5. DE (diatomaceous earth) filters require backwashing similar to sand filters but use slightly less water per cycle because the backwash runs for a shorter duration — typically 60 to 90 seconds versus 2 to 3 minutes for sand. However, DE filters often need more frequent backwashing, so the seasonal water cost ends up similar at $20 to $60.

The hidden water cost that most Ottawa pool owners overlook is the water needed to refill the pool after backwashing. Every litre you send down the waste line during backwashing needs to be replaced by filling from your garden hose. This refill water carries the same $3.34 per cubic metre cost and needs to be chemically treated — each 1,000 litres of fresh Ottawa tap water added to your pool requires approximately $1 to $3 worth of chlorine, pH adjuster, and stabilizer to bring it into balance. Over a full season, the chemical cost of treating backwash replacement water adds another $15 to $40 on top of the water cost itself.

Some Ottawa pool owners try to reduce backwash water waste by cutting backwash duration short. This is a false economy. If you backwash for only 30 to 60 seconds instead of the recommended 2 to 3 minutes, you save perhaps 500 litres of water (about $1.67) but leave dirty, contaminated material in the filter bed. This material reduces filter efficiency, increases the pressure differential across the filter, makes the pump work harder (increasing electricity costs by more than the water savings), and forces you to backwash more frequently — ultimately using more water, not less. Always backwash until the sight glass runs clear, even if it takes a full 3 minutes.

Comparing Total Filter Water Costs

When choosing a filter type for a new Ottawa pool installation, water usage over the lifetime of the filter is worth considering alongside purchase price and maintenance effort. A sand filter uses approximately 3,000 to 15,000 litres per season in backwash water. A DE filter uses 2,000 to 10,000 litres. A cartridge filter uses 500 to 1,500 litres. Over a 10-year period, the difference between a sand filter and a cartridge filter amounts to roughly $250 to $500 in water costs — meaningful but not the dominant factor compared to equipment purchase prices, replacement media costs, and labour time for maintenance.

Want to optimize your pool's filtration efficiency and minimize water waste? Ottawa Pool Installation connects homeowners with knowledgeable local pool professionals who can recommend the right filter setup for your specific pool and budget.