Septic Tank Maintenance

Septic tank systems are on-site mini-sewage plants designed to safely dispose of biological sanitary waste within the boundaries of its owner’s property.

A septic tank system consists of two main parts — a “septic tank” where natural bacterial action decomposes human waste into environmentally acceptable components – and a dispersion field. Also called a drainfield or effluent field.

The septic tank contains water, sludge and scum. The sludge is the pooey, gooey stuff that sinks to the bottom of the tank and the scum is the fatty crust that floats on the top of the tank. The sludge is mostly decomposed faeces and food scraps and the scum is fat and soap residue.

Between the scum and the sludge is the water – the water you use to flush the toilets, empty out of your kitchen sink, bath and shower. Each time you flush the toilet or empty the sink Archimedes’ Principal comes into effect – an equal weight of liquid overflows from the other end of the septic tank out into the drainfield, buried beneath your garden.

The liquid leaving the tank should have had time to become separated from its accompanying faeces, toilet paper, food, soap residue and should be quite clear – if you are using your system with the care it needs to operate properly.

As time goes by the scum and the sludge builds and as the layers get thicker, the water layer gets narrower and periodically the whole thing needs to be stirred up into a slurry, pumped out and disposed of. This is important to realise — your poos, toilet paper, food gunk, soap residue aren’t disposed of in the septic tank, merely collected for disposal later, usually every three years or so.

Any gases created in the process are vented to the atmosphere via the plumbing vent system (the mushroom). It’s also very important to make sure the mushroom vent is clear and able to breath.

From the septic tank, the relatively clear liquid flows out to disperse over a large, sub-soil, drainfield consisting of a rock-filled trench, or network of trenches. The trench material defines the drainfield from the surrounding soil and acts as a filter to remove remaining small solids that may be suspended in the liquid.

This liquid seeps through the rock material and into the surrounding sub-soil where it becomes part of the moist sub-soil environment.

Some systems, especially those installed or renewed since the early 1980s, may consist of two separate tanks — one tank for the toilets, kitchen sink and dishwasher (brown water) and a smaller side-tank for the washing machine water (grey water). These may share the drainfield or have separate fields.

This is to stop the high volume of waste water from modern washing machines “flushing” through the septic tank disturbing the bacterial decomposing and separating process.

Conventional septic systems are not care free. They require regular maintenance. The undigested solids in the septic tank should be pumped out every two to four years. If the sludge is not removed periodically, it will eventually carry over into the drainfield and cause the field to block up with gunk, become saturated and generally fail to work— and then you have real problems.

It’s not the septic tank that fails, but the drainfield. It is at risk of becoming “gunked up” and unusable. Replacing a failed drainfield is a real pain. Say hello to major earthworks. Say goodbye to thousands of dollars.

A well-designed system can handle a minimum amount of normal household chemicals such as laundry detergent and bleach; but excessive usage can be detrimental. Look for cleaning products recommended for use with septic tanks. Do not pour away chemicals that are toxic to the bacteria, such as paint thinners, solvents, insecticides, etc. Absolutely do not pour products such as NapiSan down the sink. It is the system’s worst enemy.

Cooking fats and oils should also be avoided. Undersink garbage disposal units are a no-no.

Some of Wainui’s more recently built homes may have the latest multi-tank, aerated systems which claim to treat the water to a higher degree of purity before releasing it to the sub-soil drainfield.

People who have invested in such systems should have been provided with operating manuals and may have maintenance schedules that differ from the older systems. However the general concept is the same – care and respect at all times.

Ideally a tank should receive a small quantity of wastewater from shower and toilet use at breakfast, then be given eight hours rest during the day for the bacteria in the tank to Remember septic tanks systems were designed for households with rainwater tanks. The conservation practises used to save water are the practises needed to run a septic tank system. They are not designed for heavy water use.


Guides to septic tank care

There are many types of on-site wastewater treatment systems – the most common type of system is the septic tank and disposal field. On-site systems need to be designed and installed correctly and they require regular maintenance to work effectively. If your system fails it will cause a dirty, smelly and potential costly mess.

A failing system can put your family’s health at risk and it may also pollute the water in local streams, rivers or wells with pathogens and nutrients that are harmful to humans and the environment. To learn more about septic tanks and how to look after them see The Story of your Septic Tank System pamphlet (PDF 1.6MB) produced by the New Zealand Water and Waste Association and the Ministry for the Environment.

Waitakere City Council has a useful website.

The Smarter Homes website's sewerage system's overview is a good source of information on choosing and maintaining septic tanks.

For information specific to Wainui Beach contact GDC.