Septic systems

A septic system is a watertight tank through which domestic wastewater is collected for primary treatment onsite, instead of being transported via mains sewage plumbing to a wastewater treatment plant. The tanks are pumped out periodically.

Septic systems cannot handle excessive amounts of oils or grease, food or chemicals. ‘Flushable’ wipes or liners should not go into the system, they should be disposed of in the general waste.

Products that can be used with a septic system

Many members ask if they can use their regular laundry products with a septic system. The answer is yes! Most mainstream detergents, laundry boosters and chlorine bleach can be used without issue.

All detergents sold in Australian supermarkets are now either phosphorus-free (P) or contain very little phosphorus (NP). Detergents with the symbols P or NP are all suitable for use with septic systems. If they comply with the biodegradability standard for detergents they are also suitable for greywater or biocycle systems.

Chlorine bleach can be used sparingly, as it breaks down into saltwater and oxygen in the system. Our chlorine bleach sanitise instructions provide dilutions for sanitising and stain removal, these quantities are safe for use in septic systems.

A study added 7 litres of chlorine bleach to a 3780L septic tank and measured how long it took the system to recover. This is far more bleach than a household would under any normal circumstances. The bacterial population of the septic system recovered within 1.5 days (30 hours).

According to the experimental data, a 1000 gallon septic tank required 7 lbs of bleach, or approximately 2-6 gallons of liquid bleach, for all the bacteria to be killed. This corresponded to about 600 gms of HTH powder which contained 65 percent chlorine.

Mark A. Gross, Assessment of the Effects of Household Chemicals Upon Individual Septic Tank Performances

Products that cannot be used with a septic system

Do not use disinfectants with the active ingredient benzalkonium chloride (for example Canesten, Dettol, and generic disinfectants). These products do not break down readily and will kill the bacteria in the system. The same applies to anti-bacterial products and pesticides.

Caustic products such as oven and drain cleaners, and ammonia-based cleaners should also not be used as the concentration is too strong for the system.

The likelihood of an individual homeowner using 1.3 gallons of liquid bleach or 2.5 gallons of Lysol liquid in one day is remote. How ever, 0.65 ounces of Drano crystal could possibly be used in a short time period during the course of unclogging a drain. The use of large amounts of Drano crystals is not recommended for septic systems.

Mark A. Gross, Assessment of the Effects of Household Chemicals Upon Individual Septic Tank Performances

Safety considerations

An important safety factor regarding septic tanks is that they should be completely covered with strong material. The covering should be checked periodically.

There should also be signage around and on the tank as a warning to prevent people from walking on it. People have been known to fall through coverings that have degraded or have been damaged over time and drown in the tank. The effluent inside acts like quicksand, and becoming submerged is fatal.

References and further reading

  1. Unilver, Septic tanks (February 16, 2018) <>.
  2. U.S. Environmental Protection Agency, How Septic Systems Work (September 29, 2023) <>.
  3. Australian Government, Department of Health, The septic tank (February 16, 2018) <>.
  4. Gross. M, Assessment of the Effects of Household Chemicals Upon Individual Septic Tank Performances (September 29, 2023) <>.
  5. Environmental Protection Authority of South Australia, Disposal and reuse of septic tank sludge (September 29, 2023) <>.