How to Look After Your ATU

An ATU is also called an Aerated Wastewater Treatment Plant (AWTP) in Australia. These systems rely on different types of bacteria to digest your wastes and reduce the nutrient loading to such a degree that the treated effluent could then be discharged to irrigation. While they are a proven technology and generally work well, it is possible that problems arise – and these are most often caused by the particular cleaning products you use and what you put in the washing machine.

Every ATU or septic or wastewater treatment system gradually accumulates sludge and scum. This is because the scum that floats on the surface is mainly oils, grease and fats, and these substances are not readily broken down by the bacteria. Sludge builds up on the bottom as this is foodstuffs and materials that aren’t broken down, as well as a build-up of dead bacteria. Literally millions of bacteria die every day and this accumulates as sludge. Eventually, your system will require a pump out – typically anywhere from two to five years.

Cleaning Products

You must try to use environmentally-friendly cleaning products as any product used that kills bacteria is harmful to your system. If you wish to use some of the harsher cleaning products, such as bleach, nappy cleaning products and any disinfectants, it is suggested that you use a bucket and discard the contents in a hole in a disused area of the garden. Some other things that may cause problems are antibiotics or people who are on chemotherapy. Antibiotics are designed to kill bacteria in your body and they also wipe out these good bacteria working hard in the ATU. We do not suggest that you stop your medication, just expect that the treatment system may not function well for a short time.

Washing Machines

Try to evenly spread your washing over a period of a week. Avoid where possible to wash everything in one day. It puts too much water and alkaline substances in the system in a short time and your treatment plant will struggle to cope. Liquid soaps breakdown easier than powder types, and they generally contain less salt and are at a lower pH – both of which will affect soils and plant growth. Try not to use too much soap of any type.

Things to Avoid

Chlorine, disinfectant, ammonia, acids, bleaches, caustics and heavy chemical products, nappy pre-soaker products, antibacterial products, fat, oil, grease, milk, toilet deodorizers and cleaners etc are some of the types of products that will cause the bacteria to die off in your ATU. It will recover but may cause some odours in the short term.


Don't allow foreign objects, (eg. Nappy liners, disposable nappies, tampons, pads, condoms etc) to enter the system. They do not breakdown and can cause problems.

Avoid pouring large quantities (½ litre or more) of beer, wine, milk or fruit juice or oils into the system. Products that are acidic may affect bacteria production. (Milk, beer and fruit juices, for example, often contain large amounts of sugar which is digested by some bacteria that rob the water of oxygen, and this, in turn, stops the growth and working of those bacteria that rely of oxygen to survive).

Suitable Products

It is fair to suggest that all products should be used in moderation.


Surface cleaners - when using surface cleaners try to wipe excess cleaner with a disposable cloth and discard contents into the bin.

Toilet cleaners - cream cleaner or washing soda (Sodium carbonate).

General cleaners – bicarb soda (Sodium hydrogen carbonate), vinegar (not too much).

Floor cleaners - use hot water and detergent.

Laundry powders and liquids - use suitable products that contain low phosphorus and low salt (used as bulking/fill agents).

Do Not Allow These to Enter Your ATU (not down the drain)


  • oil, paint and chemicals
  • drain cleaning or clearing products
  • methylated spirits, kerosene, acetone or any other solvents
  • flea or tick wash
  • oven cleaners
  • plastics of any type
  • disposable nappies, sanitary napkins

Servicing of Wastewater Treatment Plants

All aerated treatment plants are required by law to be serviced. Most AWTS or ATU’s are required to be serviced every quarter, and service reports to sent to yourself and to your Local Council.


Treatment systems can only be serviced by registered authorised wastewater service personnel, and it is an offence for a homeowner to undertake this specialised work themselves.

How You Can Help

Service Personnel need clear access to your system so that they can carry the required testing and cleaning equipment from their vehicle to the tank/s, and so that they can easily remove hatches and undertake the range of tasks performed at each service.


  1. Do not cover tanks and hatches with soil, cement, paving or any material. (Mulch can be placed on the lids of systems that rely on earthworms as part of the treatment process).
  2. Do not prevent quick and easy access to any inspection openings.
  3. Do not allow roof or surface water to enter any part of the system.
  4. Allow free access to your property for the service person. Ensure that the gates are unlocked, your dogs are locked up and your children are supervised.
  5. Keep plants and grass monitored and maintained on land application areas (irrigation zone/s).

If you have any other questions please call our office during normal business hours (08) 9295 6263.

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