Checking Your Septic System

Your septic tank is a living ecosystem where bacteria digest waste. Like any living system, it can become sick if it is flooded, poisoned with chemicals, or not looked after.

CHECKLIST: IS YOUR SEPTIC HEALTHY?                                                      1536-septic2.jpg

Your septic may need attention if any of these conditions occur -

  • The air around it smells - usually like rotten egg gas.
  • The ground is damp or soggy or pools form downhill of the system.
  • There's lots of dark green grass growing around the absorption area.
  • The toilet or drains are slow to clear, or keep backing up and overflowing.
  • There are lots of weeds growing on the shore if the system is near a waterway.
  • The tank has not been checked in the past 12 months.
  • The tank has not been de-sludged in the past 5 years.

If any of these factors apply to you, you should act quickly to ensure that the damage, and the cost, does not get worse.

Here's what to do

  1. Call Council's Environmental Health Officer for advice. Often a phone call to council will either solve the problem or put your mind at rest


2. Call a plumber or septic expert (find them under Septic Tank Cleaning Services in the Yellow Pages).

To catch septic problems before they get out of hand, do this simple 20 minute check-up each year.


1. Check the level of the tank at the outlet. It should be no higher than the outlet pipe.

2. If you have a filter, check it is clean and working

Action: If it's clogged - rinse it clean with a hose.

Warning - the filter may have disease-carrying organisms. Wear gloves and wash it so the drainage goes back into the septic.

3. If you have absorption trenches, check the absorption area is not soggy, doesn't smell and doesn't have prolific grass growth.

Action: If it's soggy, smells or is overgrown with dense grass, there may be too much water flowing into your septic, or the trench may be exhausted. Call a plumber.

4. Check all drains and toilets in the house are working properly.

Action: If drains and toilets are slow to empty, you may have blocked pipes or your septic system may be full. Call a plumber.

If you are unsure, it's best to consult an expert. Look in the Yellow Pages for a list of plumbers or septic specialists.

Tank too full

If you have a pump-out system, the tank should be no more than 2/3 full.

Solution: See next section, Pumping out.

Too much sludge and scum in the tank

These two solid components of sewage don't move out of the tank. They just stay behind and build up. If you don't de-sludge the tank, it will eventually get to the point where the digestion process fails and untreated wastewater (with human waste in it) flows out of the tank, clogging pipes and absorption trenches. You should de-sludge your tank every 3 to 5 years.

Solution: See next section, De-sludging.

Too much water going into the system

This causes the effluent to flow too quickly through the tank before the bacteria have a chance to work. As a result, solids can be pushed through the system, polluting the holding tank or clogging the absorption trenches.

Solution: Use less water.

Homes on tank water are already used to conserving water, but in homes connected to reticulated water, there is much more temptation to overuse water. See How to maintain a healthy system for tips on reducing water use.

Chemicals going into the system

Chemicals like solvents, oils, paints, disinfectants, pesticides and bleaches can kill the essential bacteria in your septic system. This may "kill" the system and stop it digesting effluent.

Solution: Switch to natural cleaners and use smaller amounts.

You can protect your septic system by switching to traditional non-toxic cleaners, like vinegar and bicarbonate of soda, in the kitchen and bathroom.

SHOPPING TIP - use low-phosphorus detergents

Changing washing powders can make a difference to the amount of phosphorus entering rivers from on-site systems. Using phosphorus-free detergents can mean less phosphorus in the waterways and that means less risk of algal blooms.

Septic systems don't work well either if too much phosphorus is going into the system. Always look for low-phosphorus detergents.


What's that smell? If your visitors have said this recently, it might be a sign that the septic system needs some tender loving care.

If you have absorption trenches

De-sludging (every 3 to 5 years)

You will need to have the solid sludge and grease pumped out of your tank every 3-5 years.

Septic tanks need de-sludging because the level of the solids builds up in the tank, the wastewater has less time to settle properly and so solids flow into the absorption field and clog it up.

Newly pumped-out tanks should be filled with clean water and a handful of lime added to prevent odours and encourage bacteria.

Trench warfare

The other place to check if your septic system isn't working properly is around the absorption trench area.

The absorption trench is where the effluent flows after it leaves the tank. An archway or perforated pipe is laid in a gravel trench and covered with soil. Effluent seeps through the archway or pipe and is absorbed by the soil. Soil bacteria digest the pollutants and pathogens.

Don't wait until your trench system fails to have your tank pumped. With septic systems, an ounce of prevention is worth a ton of cure!

The failure of the trench is one of the most common causes of problems with septic systems. They usually fail the trenches get blocked up and the effluent is unable to evaporate or drain away.

You can tell if the trench has failed because the area will be soggy, smelly and covered with dense grass.

Disposal trenches should last for 15-25 years, but if they are not built and maintained properly the trench life can be reduced to as little as two years.


What can you do to fix a failed trench? It's best to contact your council or consult a septic expert (find them in the Yellow Pages).

In the meantime, here are some DOs and DON'Ts to keep your trench working well.

Trench DO's

  • Ensure that the proper soil tests are done to determine how long your trench should be. An alternative disposal area could also be identified.
  • Plant small trees down-slope and away from your trench to absorb effluent, provided they are water-loving and shallow rooted, such as tropical palms, banana palms, paperbark trees and wetland plants. (See the plant list at the back of the booklet.)
  • Consider installing an alternative disposal area. This allows the soil to rest. Disposal areas should be swapped over every 12 months.
  • Build a small earth berm (a small ridge about 15 cm high and longer than the length of the trench) uphill from the trench to divert runoff water around the actual disposal area. This will help to reduce the load on your trench in wet weather.

Trench DON'Ts

  • Do not level diversion contour mounds.
  • Do not build structures or plant trees that will shade the disposal area it should be in full sun to help aid breakdown and evaporation. Small trees should be planted at least 5m away, large trees such as figs should be 20m away, otherwise the roots will get into the trench and pipework.
  • Do not flood the disposal area with sprinklers or hoses.
  • Do not drive cars over disposal area or graze animals there. Any movement of vehicles over the trench may break the pipework or the dome cover and compress the soil. A small fence will let visitors know which areas to avoid.
  • Do not cover trenches with concrete, pavers etc.
  • Do not store loads of soil or other materials on your disposal area.
  • Do not place extra topsoil on top of your trench to "soak up" overflowing effluent if the trench is failing. If there is water pooling over the trench, it's best to call a plumber and have it checked.
  • Do not allow children to play in the disposal area

Irrigation systems: Do's

Irrigation systems are susceptible to blockage.

Experts should be employed to install an irrigation system. If you move into a house which has an irrigation system, get advice from an expert on how to maintain it (look under irrigation systems' in the Yellow Pages).

The plants which are being watered by your irrigation system must be able to tolerate high amounts of water and nutrients. Seek advice from a horticulturalist or landscape gardener when choosing plants for your irrigation area.

Stay safe!

Don't attempt to repair a failing system yourself - get an experienced contractor.

If you are inspecting your system, remember-

  • sewage contains germs that can cause disease;
  • never enter a septic tank;
  • toxic and explosive gases in the tank are a hazard;
  • electrical controls are a shock and spark hazard;
  • secure the septic tank lid so that children cannot open it.

There are many septic system additives such as enzymes and cleansers available on the market. The truth is, these are only suitable for problems which are minor and temporary (e.g. antibiotics in the system, or occasional water overuse). A well maintained system which has the correct amount of wastewater entering the system should not need these additives.

No amount of additives will help a tank if it really needs to be pumped out or has a failed trench system.