How do Septic Tanks work?

Septic tanks work as an on-site sewage system. Wastewater is fed to the tanks (usually via gravity but also by pumps) which is why septic tanks are usually placed underground. The wastewater travels through a filtering system and is partially processed by bacteria. On-site sewage systems can either break waste down using oxygen (aerobically) or without oxygen (anaerobically).

A healthy septic tank ecosystem has three layers:

  • A layer of fats called scum, which floats on the surface of the liquid waste.
  • A layer of liquid waste called effluent.
  • A layer of solids called sludge, which sinks to the bottom.
The scum (or layer of fats) helps prevent odours and air entering the ecosystem. Bacteria living in the tank work on breaking down organic solids (or sludge). At this stage, the effluent still contains large amounts of pollutants, salts and high levels of nutrients such as nitrogen and phosphorus (which can cause toxic algae blooms in waterways). It can also contain diseases, such as viruses, harmful bacteria, and parasites. Viral and bacterial infections are the most common causes of sickness in New Zealand. Campylobacter and Salmonella are two common forms of bacteria that are carried in human waste that can cause serious illness. That’s why it’s so important to have a wastewater system that is safe and of the correct size for your household.

From here the treated water is distributed out to an absorption field or soakage treatment area. This is where natural soil processes kill off more pathogens and break down the harmful toxins. It’s important that this is a slow, safe process as too much effluent can overwhelm the soil and flow into subsoil pathways and drainage channels. In New Zealand, we commonly plant native grasses and other plants that are very effective in a soakage area (link to plant list).  Having a site visit and expert come to look at potential areas to have your soak area is key in the process of installing a septic tank as there are several things to consider before introducing a soakage treatment area.

The soakage treatment area should be:

  • large enough to cope with the amount of wastewater your household produces
  • as dry as possible – pathogens survive better in waterlogged soil
  • shallow – this allows plants to absorb nitrates and organisms in the soil and the heat of the sun to act on pathogens to remove them
  • away from waterways, flood-prone areas and areas of stormwater runoff
  • while surface water should be diverted around any effluent disposal field

Grey Water and Black Water – What is it? How can you utilise your water?

Greywater is wastewater that can safely be reused, so household wastewater from laundry tubs, washing machines, showers, baths, and basins. This water, with building consent, can be reused to water the garden or flush the toilet. A grey water system can be put in for example, where grey water is collected and filtered (for things such as lint and fats are filtered out). There are some important lifestyle guidelines and several options that go along with installing and collecting any grey water, so it’s best to discuss this with one of our experienced team before deciding to go ahead.
Blackwater is wastewater containing human waste. You can’t reuse it, it must go into the mains sewage system or (if rural) into an on-site sewage system – your septic tank.

Septic Tank Maintenance

Septic tanks also have to be properly maintained. They should be cleaned on a regular basis to help with regular drainage and increase the life-expectancy of your tank. We recommend a regular clean and maintenance check every six months, to make sure everything is running smoothly. On average, a septic tank should be pumped clear of sludge every three to five years, however, some require pumping every year.  If you are having issues with your septic tank, contact us today to arrange an onsite visit. Often, it’s something as simple as extending the disposal area, or a quick maintenance check as opposed to needing a new tank installed.

Living with a Septic Tank

The Dunedin City Council have this great Do’s and Don’ts guide to life with a septic tank, and how to get the best out of your Septic tank and keep it healthy.


  • Do scrape your dishes clean of food and fats before washing
  • Do remove sand and soil from your clothes before washing them
  • Do install water saving devices
  • Do use showers instead of baths
  • Do use biodegradable soaps and washing powder
  • Do use detergents and cleaners compatible with your system
  • Do protect your septic tanks and disposal fields from vehicle access to avoid cracking the pipes and tank.
  • Do fix leaky taps
  • Do try and avoid doing more than one full load of laundry a day.


  • Don’t use a waste disposal unit
  • Don’t flush sanitary products, disposable nappies etc
  • Don’t flush expired medicines
  • Don’t use washing machines or dishwashers unless you have full loads
  • Don’t overuse strong bleaches and disinfectants
  • Don’t put paints, weed killers and other chemicals down the drain
  • Don’t put oils, fats, or greases down your drain
  • Don’t use septic tank cleaning chemicals unless specified by the manufacturer
  • Don’t allow storm-water to enter the disposal field
  • Don’t put large volumes of water through your system (e.g. from spa pools)
  • Don’t plant deep rooting trees or shrubs over the disposal field and pipes.

How to spot if your septic tank isn’t healthy and what it could be:

If you have any of these things happening on your property, with your current Septic Tank please don’t hesitate to give us a call. It could be something as simple as getting the sludge pumped, or extending your soakage area.

What Septic Tank Failure usually looks like:

  • Waste-water ponding at the surface
  • Smell of sewage near the septic tank or disposal field
  • Slow draining sinks and toilets
  • Sinks backing up when the toilet is flushed
  • Black and slimy areas around your tank or disposal field
  • Evidence of discharges around vents or gully traps.

Septic Tank failure can result in contamination of local rivers and streams, or have negative effects on people and the surrounding environment, please don’t hesitate to contact us today if you spot any of these things with your current waste water system.

12 Waverley Street, Dunedin 9012 Phone (03) 489 1802
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