Caring for your Septic Tank

Your house like many others in the Otago District probably has a septic tank system to get rid of household sewage and wastewater. Properly installed and maintained this is a hygienic and economical way of disposing of household water. If neglected it can pollute the environment, put you and your neighbour’s health at risk and be expensive to fix.
Your septic tank can be kept in a healthy state so long as you understand how it works and carry out a few simple maintenance tasks. By following the instructions in this brochure, you have a better chance of operating an effective and environmentally friendly system which will safely dispose of all wastewater from your bathroom, kitchen, and toilet. If you do have problems or you need further advice on the operation of your system, officers of the Council’s Environmental Health Division are there to help.

How septic tanks work

There are a number of different types of disposal systems which use septic tanks. However, they all work according to the same basic principles. All the wastewater produced by your household flows down the house drain to the septic tank. Your septic tank should be large enough to hold the households daily wastewater flow for 24 hours. This is where the solid wastes settle and where some biological treatment takes place. From the septic tank, the partially treated effluent is piped out to a disposal field where it is cleansed by natural processes.

What does the septic tank do?

The septic tank pre-treats the wastewater before it goes to the disposal field.

Three main things happen in septic tanks:

  1. Solid wastes settle to the bottom of the tank and form a layer of sludge.
  2. Lighter wastes such as fat and grease float to the surface and form a scum layer.
  3. Bacteria which live in the septic tank help break down the solid wastes and reduce the volume of sludge collecting in the bottom of the tank.
An important job of the septic tank is to store sludge and scum so that they do not flow out and clog up the disposal field and stop it operating properly.

Where does the effluent go?

When the effluent leaves the septic tank, it is only partially treated. Final cleansing is carried out by natural processes occurring in the disposal field. The type of disposal field used depends on the soil conditions, slope of the section, vegetation, and underlying rock.
  • Soakage Trenches These are the most common type of disposal field and are commonly used on sections which are reasonably flat where water soaks into the soil readily in all weather conditions. Properly designed soakage trenches can also be used on sloping sections where soakage is good. Perforated pipes are laid in trenches filled with gravel or rock. The effluent flows out through the holes in the pipe into the trenches and then soaks into the surrounding soil. The soil and bacteria in the soil clean the effluent.
  • Evapotranspiration systemsWhere these systems are used, the effluent is piped through special beds of plants which absorb a proportion of the wastewater through their root systems and release it to the atmosphere through their leaves during that natural process of transpiration. Some of the effluent is also absorbed by the ground.
  • Lower pressure pipes These are sometimes used on bush covered sections. Small diameter plastic pipes are laid in shallow trenches or on the ground surface amongst the bush. The effluent wets the topsoil and is absorbed by the root systems of the plants. When this system is used the disposal area must be fenced to prevent access.
Combinations of all these methods of disposal may also be used. Some systems will have a distribution box located between the septic tank and the disposal field. This is designed to direct the flow of effluent to different parts of the disposal field in order to “rest” the balance of the field. This prevents trenches and pipes from becoming clogged through overuse.

How to find your septic tank

You can usually locate your septic tank by looking for the fresh air inlet pipe (the “mushroom”) in your lawn. Sometimes the level of the lawn is higher or lower over the tank or the grass does not grow as well over it. If you cannot find the mushroom and the location of the tank is not clear, you can locate it by following the line of the house sewer by probing the ground with a metal rod. This method can also be used to trace the drainage field down from the septic tank.
It is a good idea for future reference to draw a simple sketch plan of your property which shows the location of your house and the septic tank system. Measure the distance from the house to the vent and from the vent to the edge of the disposal field and show it on your plan.

Signs of a problem with your septic tank

Problems can occur with systems which have not been maintained and where drainage fields have become blocked or clogged.

 The warning signs are obvious:

  • Wastewater ponding on the surface of the ground.
  • The smell of sewage near the septic tank or drainage field.
  • Slow running drains and toilets.

A failed septic tank system is a serious health and environmental hazard and can lead to:

  • Infection for you and your family.
  • Pollution of beaches, stream and shellfish beds.
  • Contamination of bores and wells.
  • Breeding of flies, mosquitoes, and rodents.
You have a legal responsibility to keep your system in good working order. If you notice any of these warning signs contact the Environmental Health Officer at your Council Service Centre without delay.

Looking after your septic tank

Provided it is properly cared for, your septic tank is a simple and effective means of effluent disposal and will give you many years of reliable service.
Remember to:

Reduce sludge build up

  • DO scrape all your dishes to remove fats and grease.
  • DO keep all possible solids out of your system.
  • DON’T use insinkerators.
  • DON’T pass sanitary napkins etc into the system.

Conserve water

  • DO take showers instead of baths.
  • DO install water saving devices.
  • DON’T wash clothes until you have a full load.

Keep the bacteria working

  • DO use biodegradable soaps.
  • DON’T use powerful bleaches and disinfectant or put chemicals or paint down the drain.

Protect the disposal field

  • DO try to keep children off trenches or beds.
  • DO fence evapotranspiration area.
  • DON’T allow vehicles or stock on trenches or beds.
  • DON’T cover the vent or lid of the septic tank.
  • DON’T grow deep rooting trees or shrubs over soakage trenches or pipes.

Regular maintenance of your septic tank

By following a simple maintenance programme you can ensure that you will continue to have an efficient and healthy disposal system for many years.
  • Clean the septic tank regularly - This should be done about every four to five years to remove the build of up solids in the tank. If you need advice contact an office of the Council Environmental Health Division or your local waste disposal contractor.
  • Maintain evapotranspiration - Grass and plants should be maintained to ensure that these areas operate at maximum efficiency. Grass should not be cut too close nor should it be permitted to grow long. An adequate leaf area is required. Grass cut to a height of around 75mm is preferable.
  • Alternate disposal areasIf you have a dual drainage system you must alternate the flow to the trenches every two to three months. This is done by adjusting the baffle or valves in the distribution box.
Some houses do not have septic tanks and have other systems for effluent treatment such as “package” treatment plants. Such systems often have special operating requirements and must be maintained in accordance with the manufacturer’s instructions.
The Council has a set of guidelines explaining the requirements for design and construction of on-site effluent disposal systems. Copies can be obtained from your Council Service Centre.
12 Waverley Street, Dunedin 9012 Phone (03) 489 1802
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