Choosing an on-site sewage system
Choose a system that can deal with all of your household’s wastewater safely.
You’ll need to consider how big the tank needs to be, what type of system to use, and where the treated effluent will discharge to.
You’ll also have to comply with legal requirements aimed at protecting your own and neighbouring properties and waterways from contamination.
Do you need an on-site sewage system?
Under the Building Code, if a connection to the mains sewer is provided, you must use it. However, if there is no connection to the mains sewer then an on-site system is allowed.
Most people in New Zealand have access to a mains sewer, so an on-site system is not an option.
How do on-site sewage systems work?
An on-site sewage system is one or more underground tanks or chambers where wastewater from your home is held while it decomposes. The chambers can be made of concrete, fibreglass, or plastic.
After a period, liquid waste from the chambers is piped to a soakage treatment area where decomposition continues. Solid waste remains in the chambers and has to be pumped out every few years.
The liquid waste that leaves your on-site sewage treatment system ultimately ends up in the groundwater. The better your sewage treatment system, the safer it will be for human health and the environment.
Choosing the right system
What type of system?
On-site sewage systems can either break waste down using oxygen (aerobically) or without oxygen (anaerobically).
Traditional septic tanks generally have only one chamber and process wastewater anaerobically (this is sometimes referred to as ‘primary treatment’).
Other, more advanced systems have two or more chambers and use both anaerobic and aerobic process to break down the waste. (The aerobic part of this process is sometimes referred to as ‘secondary treatment’.)
By providing secondary treatment, the more advanced systems produce effluent that is safer for human health and the environment.
Choosing the right system for your property can be complex. There are many systems available, with different numbers of chambers and different treatment processes.
But, as general rules, you’re better off with a system that:
- has at least two chambers, and
- provides both primary and secondary treatment.
Buying a cheaper system that provides only primary, anaerobic treatment could turn out to be a false economy - you could end up having more problems with it and spending more on maintenance, as well as facing potentially greater health risks.
Many councils require new systems to have at least two chambers. More chambers isn’t necessarily better (five may not be better than four, for example) - but two is definitely better than one.
A multi-chamber system can be added to an existing septic tank to improve its effectiveness. You can also add an outlet filter to improve the efficiency of a septic tank.
How big should your system be?
Your on-site sewage system needs to be big enough to deal with all of your house’s wastewater - an average New Zealander uses 160-250 litres of water a day. For a three-bedroom house, you’ll need a tank with at least 3000 litres capacity. Some councils require at least this size.
Note that your system needs to be big enough to meet future needs as well as current ones. If you have a four-bedroom home but only three people live there, you’ll need to buy a system that’s big enough for at least four people.
If your system isn’t big enough, the consequences can be serious. You may be responsible for ground or surface water containing harmful bacteria (such as campylobacter and salmonella), protozoa (such as giardia and cryptosporidium), and other contaminants.
Many New Zealand on-site sewage systems are too small for the households they’re serving.
The soakage treatment area
Treated effluent from your septic tank is discharged to a soakage treatment area, where any remaining pathogens are removed before the effluent reaches nearby groundwater or waterways.
There are several types of soakage treatment area. Most use pipes with small holes to distribute effluent slowly either on or immediately under the surface of the soil.
Effluent gets to the soakage area either using gravity or a pump. Gravity won’t always disperse the effluent evenly. This can cause clogging. With a pump, clogging shouldn’t be a problem - but an unreliable power supply might be.
Native grasses, sedges, rushes and other moisture-loving plants will grow in your soakage area and will enhance the soakage effect. Check with your regional council for more information on what’s suitable to plant in your area. Don't grow deep-rooting trees over the 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.
Soil type and depth will influence the size of the soakage treatment area. Some soil types are not suitable as drainage fields ‑ clay, for example, can cause wastewater to pool on the surface.
Legal requirements
To install an on-site sewage system, you’ll need a building consent.
You may also need a resource consent - but even if you don’t, you’ll need to meet regional council requirements regarding the discharge of effluent. Some councils will require additional treatment of wastewater with ozone, UV, filtration or chlorine to make the soakage treatment area safe.
An environmental engineer’s report and a soil analysis may be required, to determine where tanks should be sited and where treated waste may be released to, as well as how large the soakage area has to be.
The system will have to be installed by a qualified installer. Look under Water Treatment or Water & Wastewater Services in the Yellow Pages (www.yellowpages.co.nz).
If you have access to a mains sewage system, you can’t have an on-site sewage system.
Costs
On-site sewage systems can cost many thousands of dollars. As well as the system, you may have to pay:
- design fees
- an engineer’s fee
- building and resource consent charges
- an annual inspection fee (this may be included in the system cost.
If you skimp on system size or type, your system may not work effectively. This could endanger health and harm the environment.
More information
From Smarter Homes
- Maintaining your on-site sewage system
- Waterless toilets
- Easy ways to save water
- Reducing water flow
- Reusing greywater
- Collecting and using rainwater
- Managing stormwater
- Outdoor water use
- Onsite sewage systems
From ConsumerBuild
Your local or regional council will have information about on-site sewage systems in your area. Consumerbuild’s Council finder web page has contact details for New Zealand local authorities.
From other sites
The Waitakere City Council’s website has a page about on-site sewage systems.
The Greater Wellington Regional Council’s website has guidelines for onsite sewage treatment.
You can download The Story of Your Septic Tank (PDF, 1.2MB), a joint Ministry for the Environment/NZ Water and Wastes Association booklet, from the association's website.
You can buy copies New Zealand Standards relating to on-site sewage systems and waterless toilets from the Standards New Zealand website.
