Where a pool's water goes when it leaves

Two states, two opposite answers

Start with the sharpest disagreement on this site, because it decides how you read every other page here. In South Australia, SA Health's page says that in sewered areas the water used to backwash or rinse the filter must always be disposed to sewer, and for properties not on a sewer it points to the state EPA's fact sheet on disposing of swimming pool backwash water. In Western Australia, Water Corporation's page says the opposite in as many words: domestic pool and spa backwash is not trade waste there, and it is not to be put into the sewer at all. It sends the owner to the local council for disposal options instead.

Both are right, for their own pipe. Each sewer has its own utility's rules, and a pool owner follows the one where the pool is. Nothing in the rest of this guide is national either, which is why every rule below names the utility it came from.

What each utility says

From SA Health, Water Corporation, Sydney Water, Icon Water, Queensland Health.

What a utility will accept, at the point of discharge

Where a utility does take pool water, it takes it on conditions, and two of them publish the numbers. SA Water's guideline is written for commercial and public pools, spas, hydrotherapy pools, and ornamental fountains and ponds, not for a home pool, and this is what it asks of the water leaving one of those.

Behind the numbers sit the mechanics of getting the water in. SA Water's guideline sends small regular discharges, the filter backwash among them, to the sewer at no more than half a litre a second, and any discharge over 20,000 litres that no trade waste authorisation covers down the non-domestic hauled waste route, with its charges, and it may ask for pre-treatment or a flow meter at the outlet. Icon Water adds two of its own: solids that settle in a holding tank are removed for disposal off site rather than discharged, and water from cleaning pool surfaces with strong detergents or acids may not go into the sewer at all, with the cleaning compounds' details, including brand name and quantity and the data sheet where appropriate, given to Icon Water in advance.

Why the utilities care

SA Water's own page gives the reasons a pool's water is a problem for a sewer: discharges from commercial pools can carry contaminants that exceed the Restricted Wastewater Acceptance Standards, a significant flow rate can exceed the hydraulic capacity of the downstream network, and water that is essentially clean dilutes the general sewage stream, which reduces how efficiently the treatment plant removes what it should. Icon Water puts the consequence in legal terms: a discharge that breaches its guideline will probably breach section 35 of the territory's Utilities (Technical Regulation) Act 2014, and the person who discharged it, or let it be discharged, may be prosecuted.

From SA Water, Icon Water.

Topping up, and where the fill water comes from

A pool loses water all year and is topped up, and the first rule is about the source. SA Health's page says only treated mains water from a domestic water supply should be used to fill a pool, and where another water source is used, advice should come from an environmental health officer at the local council. It also sets the method: the pool should be topped up regularly with a hand-held hose or bucket, all top-up water should be added through the skimmer box with the filtration system running, and the disinfectant and pH levels should be checked afterwards.

Two other pages put conditions around the same job. Sydney Water says to check the Water Wise Guidelines before any water is removed, in case a permit is needed to refill the pool: it considers each application individually and will approve one only where draining and refilling is unavoidable. In the ACT, the Permanent Water Conservation Measures apply to swimming pool use, and Icon Water's page says not to overfill a pool, because an overfull pool stops the filter working effectively and overflows. It also records where the water goes: evaporation is a major cause of water loss from a pool, and it is greatest in the early evening, when the air cools and the water is still warm.

Not all water loss is evaporation, and a spa has its own schedule. Queensland Health's spa page says: "Some of the water in your spa pool will need to be replaced when the level of total dissolved solids (TDS) becomes elevated." It defines that measure as a measure of the inorganic salts and the small amounts of organic matter that are dissolved in the water, notes that it generally increases with increased use of the spa pool, and gives the alternative: "Alternatively, you can replace at least one quarter of the water in your spa pool monthly." Its rural-properties page has one rule for the smallest pools of all: "Inflatable and toddler pools should only be filled with water from a disinfected supply and should be emptied daily after use", and small collapsible wading pools should be emptied and cleaned daily.

From Icon Water, Queensland Health on rural properties.

What this has to do with cleaning a pool

Backwashing the filter is part of cleaning a pool. The rule that decides where it goes is never the pool's: it is the sewer utility's, and the two pages named at the top of this guide say opposite things about the same bucket of water. Reading the page for your own utility is the whole of the work. The next guide is the cleaning itself, and the filter whose rinse this is.