Septic Tanks

The most common domestic wastewater treatment system used in rural areas is the septic tank-soil absorption system. The septic tank removes settleable and floatable solids from the waste water. The soil absorption field then filters and treats the clarified septic tank effluent and distributes it through the soil. Removing the solids from the wastewater protects the soil absorption system from clogging and failure. In addition to removing solids, the septic tank also promotes biological digestion of a portion of the solids and stores the remaining undigested portion.

Why Septic Tanks fail

The first stage of the treatment system, the septic tank, removes solids by holding wastewater in the tank. This allows the heavier solids to settle as sludge and the lighter particles to form scum at the top. Up to 50 percent of the solids retained in the tank decomposes; the remainder accumulate in the tank.

As the system is used, sludge continues to accumulate in the bottom of the septic tank. Properly designed tanks have enough space for up to three years safe accumulation of sludge. When the sludge level increases beyond this point, sewage has less time to settle before leaving the tank and more solids escape into the absorption area. If too much sludge accumulates, no settling occurs before the sewage flows to the soil absorption field. Infiltration of sludge into the soil absorption field can cause system failure, which will lead to the complete replacement of the absorption field, costing thousands of dollars. To prevent this, the tank must be pumped periodically.

The frequency of pumping depends on several factors:

  1. Capacity of Septic Tank
  2. Volume of wastewater (related to size of household)
  3. Amount of solids in wastewater (e.g. garbage disposals produce more solids).

Estimated Septic Tank Pumping Frequencies in Years*

 House Size(number of people)     

Tank Size (litres) *


* Montana State University Extension Service

Don't wait until you have a problem. Routine pumping can prevent failures, such as clogging of the absorption drains and sewerage backup into the home.

If you have just moved into a home, you may not know the size of the tank. In this case, you should have the tank pumped and inspected. The company pumping the tank will tell you its size, age and condition.

Septic tanks will not fail immediately if they are not pumped. However, a non maintained septic tank is no longer protecting the soil absorption field from solids. Continued neglect may result in system failure and even replacement of the soil absorption field, costing thousands of dollars. In some cases, site limitations may make replacement of the absorption field impossible.

Cleaning Septic Tanks

Septic tank pump contractors can clean your tank. The septic tank should be pumped through the large central manhole, not the baffle inspection ports. Pumping a tank through the baffle inspection ports can damage the baffles, resulting in a destroyed absorption field.

The use of additives in septic tanks to reduce the sludge volume or substitute for pumping is not recommended. In fact, relying on additives rather than conventional tank pumping may result in failure of the septic system.

Before closing the tank, check the condition of the baffles. If they are missing or deteriorated, replace them with appropriate sanitary tee baffles. It should never be necessary to enter a septic tank. Any work to replace baffles or repair the tank should be done from the outside. Decomposing wastes in the septic tank produce toxic gases which can kill a human in a matter of minutes!

To facilitate future cleaning and inspection, you could install risers from the central manhole and inspection ports to the surface. Also mark the location of the tank, so it can be easily identified.

Septic Tank Maintenance Record


Nature of Work:(pumped out, general inspection, remedial work-specify)

Name of company and person carrying out the work or inspection:

Preventative Care and Maintenance

Water Use - Minimise the amount of water used in the household. Excessive use of water could flush solids from the septic tank to the absorption drains which would lead to clogging or plugging of the absorption drains.

Each person uses between 200-300 litres of water a day, the majority of which is used in bathrooms by toilets, sinks, showers and baths. Approximately 45% is used for toilet flushing, 30% for bathing, 20% for dish washing and laundry and 5% for cooking.

Water usage in the home should be kept to a minimum to avoid exceeding the capacity of the septic tank system. If automatic washers and dishwashers are used, make sure full loads are washed each time and stagger their use to reduce peak flows. Retrofitting existing toilets to use less water will also ease the stress on the septic tank and the water supply systems.

Direct all surface waters, pumps and roof drains away from the absorption drain. This includes the lawn sprinkler.

Chemical Contamination - Moderate use of household drain solvents, cleaners, disinfectants, etc should not interfere with the operation of the septic tank system, however indiscriminate use may cause problems.

Do not dispose of fuels, grease, paints, thinners, weed or insect killers, cigarette butts, condoms, paper towels, nappies, sanitary napkins etc into the septic tank system. A septic tank system can only handle bio-degradable items.

There should be no need to use commercial "starters", "bacterial feeds", or "cleaners". Bacteria in a septic tank system occurs naturally.

Absorption Drains - The area of an absorption drain should have a good grass cover, but should not be planted with trees or shrubs.

Protect your absorption drains from damage by keeping cars, livestock and heavy equipment off the area.

You may make enquiries with Council's Environmental Health Officer concerning any issue with the maintenance of your septic tank system (03) 6261 8530.