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Smart guide

On-site sewage systems

Who needs an on-site sewage system, how do they work, and why is it important to look after them?

An on-site sewage system can be a good option if connection to the main system is not possible. They must be designed and sized correctly to work well. An undersized, poorly designed or under-maintained system may not adequately cope with your household's wastewater. It is the owner’s responsibility to ensure on-site sewage systems are maintained and operating correctly to avoid endangering human health and the environment.

Any system needs to be properly installed and maintained, as well as big enough and efficient enough to treat all of the wastewater from your household – both now and in the future.

Choosing an on-site sewage 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 rule, 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 which is 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.

Maintenance, running costs and other considerations

The maintenance and running costs of the system are an important factor in making a decision about what type of system to get. All on-site wastewater systems require regular maintenance and cleaning – you will usually need a specialist to do this. Many councils now require proof of maintenance as part of consent conditions for on-site wastewater systems.

Some types of systems require electricity to work, and that may be a consideration for households in remote areas.

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 install a system that's big enough for at least five 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.

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.

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Note that this document is published by the Ministry of Business, Innovation and Employment Chief Executive as Guidance under Section 175 of the Building Act 2004. This is a guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case.