A septic tank is the main component in a small-scale sewage system that works with no connection to the main sewage pipes provided in most cities and small towns and is common in New Zealand’s more remote areas. The term ‘septic’ refers to the anaerobic bacterial environment that develops in the tank as the waste discharged into it from the toilet and household sinks and basins decompose. The tank is connected to a septic drain field, ponds, or other filtering systems to dispose of the liquid effluent it produces.

The system consists of one or more 4000-8000 litre chambers connected to an inlet wastewater pipe at one end and a septic drain field at the other. The tank may be just have a single chamber or may have two chambers separated by a wall with openings about halfway between the floor and the roof. Wastes enter the first chamber, where the solids settle and the liquids float, underneath a scum or “crust” layer which has the useful function of limiting the smell. The settled solids are food for the anaerobic bacteria, which reduce the volume of the solids. The liquid part then flows through to the second chamber if the tank has one, where further settlement and digestion take place, and the relatively clear liquid remaining then percolates out to the septic drain field in perforated pipes, buried about 1 to 2 metres down, that allows the water to be absorbed and filtered by the ground. The size of the field depends on the volume of waste you are dealing with and the porosity of the soil, so soil percolation tests are needed to determine how large a drainage field should be.

Routine maintenance is needed to get rid of the sludge that gradually accumulates at the bottom of the tank, reducing its efficiency. If you don’t do this, you will eventually pay the price as very nasty solids start to escape the tank and possibly contaminate the drainage field, leading to costly repairs. A well-maintained system can last for decades and give excellent service. Most rural towns have septic tank maintenance businesses that provide a mobile pumping and sludge removal service. On average, you may need the tank cleaned out every 2 to 4 years. In the event of a major disaster, it usually pays to call for expert assistance. You should definitely seek help if you notice sewage smells, surfacing sewage, or slow-draining sinks or toilets.

A septic system is a delicate balance, and while it normally works very well, you can make its life unnecessarily difficult, at your own peril! There are a few major cautions and don’ts to observe:

  • Cooking oils and grease can block the inlet pipes, so don’t tip your leftover cooking oils down the sink.
  • Flushing non-biodegradable items such as cigarette butts, cotton buds, condoms, sanitary pads and tampons down the toilet is a no-no! They rapidly fill and/or clog a septic tank. Provide a covered bucket beside the toilet for separate disposal of these items.
  • Don’t have a garbage disposal unit in your kitchen sink. The high organic load will overwhelm the system.
  • Be careful with cleaning chemicals. If they kill the bugs in your sink, they are likely to kill the useful bugs in the septic tank too. Avoid bleach, caustic soda, fabric softeners, solvents, dyes, nappy washes, antibiotics, and of course pesticides and herbicides.
  • Roots from plants growing near the tank or the leach field may clog the pipes or break them, so plan your planting carefully. You need shallow-rooted grasses, or flowering perennials and annuals, and can probably safely grow fruiting annual crops such as cucumbers which can be trained up off the ground. The level of health risk with edible crops probably depends on how deep your field drains are buried, and how good your filtering systems are. Best to avoid root crops like potatoes though!
  • Don’t put buildings or impermeable surfaces (such as driveways) over the drainage field as they will reduce its efficiency and possibly damage the tank.
  • Excess water getting into the system will overwhelm it, so check for plumbing leaks, and try to position the field and the tank in an area not prone to flooding.

With common sense and some regular attention, your septic tank should treat you well – and the carefully selected plants near or on the drainage field will enjoy the water and the nutrients you are providing for them.