Professor Stuart Khan, an expert in water quality and treatment at the University of Sydney, says thousands of different chemicals flow down showers, sinks and toilets into the sewerage system. Wastewater carries low concentrations of herbicides, pesticides, detergents, chlorine bleaches, antibacterial agents and fragrances, he says.
“Most people probably don’t think much at all about what goes down the drain and where it goes,” Khan says. Anything used or disposed of outside, such as paints, oils or petroleum-based solvents, risks ending up in the storm water system. Storm water can run off into freshwater environments, local creeks and rivers, where high concentrations of some chemicals can have ecological consequences.
What is in our dust and soil?
A recent study of indoor dust and soil samples from across Victoria detected trace metals, PFAS compounds, pesticides, products of fossil fuel combustion and pharmaceuticals – including antibiotics, antidepressants and anti-inflammatories – although most were below toxic thresholds.
Jess Walker, head of the Western Treatment Plant and sewage transfer at Melbourne Water, says every drain in the house – kitchen sink, shower, washing machine and toilet – feeds into the same sewerage system. When chemicals such as fats, oils, paint, solvents or harsh cleaners go down drains or are flushed, they can block pipes and interfere with the natural biological processes used in wastewater treatment.
The three Ps rule
Ben Armstrong, principal manager environment at Sydney Water, says the only things that should be flushed down the toilet are the “three Ps”: pee, poo and toilet paper. In the sink or shower, it should be just soap suds and water.
Khan says pharmaceuticals and antimicrobial agents such as triclosan are not well removed by treatment systems and can end up in the environment. Measurable concentrations of paracetamol, aspirin and ibuprofen and many other pharmaceutical chemicals can be found.
Cooking oils, out-of-date milk or residues from a roast dinner add to the buildup of fats, oils and grease – known as FOG – in wastewater pipes, and the growth of fatbergs.
Products containing PFAS, such as stain-resistant carpets and upholstery, water-resistant clothing, food packaging and cosmetics, can also end up in waterways when used and washed. “The main problem with Pfas is its environmental longevity,” Khan says. “It’s very, very slow to naturally degrade – potentially hundreds of years for some of these chemicals to degrade in the environment.”
Treatment varies by location
The level of wastewater treatment differs depending on where you live. In Sydney, sewage goes straight to three big ocean outfalls – Malabar, Bondi and North Head – with limited treatment, so many chemicals are barely removed at all.
Consumer choices can have an impact at scale. Khan points to the shift towards phosphorus-free detergents, which has reduced the amount being discharged into the environment.
Safer alternatives and disposal
For cleaning, the Victorian government’s Detox Your Home program recommends choosing low-toxic, phosphate-free materials, such as baking soda or fibre cloths for heavy-duty scrubbing, or a mix of vinegar and water for surfaces and floors. Khan says washing in hot water – even though it requires more energy – can be an effective way of disinfecting rather than relying on chlorine-based bleach or ammonia products.
Melbourne Water’s advice: if it’s not rainwater, it doesn’t belong in a storm water drain. “That one rule covers most of what people need to know,” Walker says. Chemicals washed into gutters or drains find their way into rivers, creeks and oceans, usually without treatment. Some affect water quality, while others such as phosphates and nitrogen can contribute to algal blooms or harm wildlife.
In the garden, Detox Your Home suggests using compost and mulch instead of synthetic fertilisers, and vinegar, hot water or manual weeding instead of herbicides. If washing a car, do it on the grass or take it to a commercial wash.
“Never pour paint, chemicals, herbicides and pesticides or pool water down a stormwater drain or sewer. Take them to an approved collection or disposal point. It protects both your pipes and the waterway at the end of them,” Walker says.
Many local councils offer chemical collections or drop-off points for paints, petroleum products and common chemicals. Unused medicines can be returned to your local pharmacy.
