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 emptied periodically and have a system where treated effluent is disposed of via onsite drainage.

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

Products That Can Be Used With A Septic System

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 bio cycle systems.

All mainstream detergents eg. OMO, Dynamo, Biozet, can be used with a septic system without issue.

Stain removers, boosters and soakers eg. Vanish/Napisan, Sard, can be used with a septic system without issue.

Chlorine bleach can be used sparingly, as it breaks down into saltwater and oxygen in the system. Alternatively, it can be disposed of on paving. Chlorine bleach should not be used undiluted.

If 7L (1.85 gallons) of liquid bleach, at 1.85% sodium hypochlorite (% listed on the chlorine bleach label), is added to a 3780L (1000 gallon) septic tank. After addition of chlorine bleach, and within approximately 30 hours of normal septic system usage, the bacterial population will recover to its original concentration. 

If 19L (5 gallons) of liquid bleach, at 5% sodium hypochlorite, is added to a 3780L (1000 gallon) septic tank, the bacteria population will recover to its original concentration within approximately 60 hours (2.5 days).

Products That Cannot Be Used With A Septic System

Disinfectants with the active ingredient benzalkonium chloride eg. Canesten, Dettol, generic disinfectants, should not be used as they do not readily break down, they are detrimental because they will kill the bacteria in the system. The same applies for 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.

Another very 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. Wikipedia, Septic tank <>.
  3. Australian Government, Department of Health, 9 The septic tank <>.
  4. Gross. M, Assessment of the Effects of Household Chemicals Upon Individual Septic Tank Performances.