Water
Designing homes to use water efficiently, reduce costs and cope with more frequent droughts or storms.
Designing a wastewater treatment system
Find out about:
- site considerations
- system designs
- system capacity
- design checklist.
Site considerations
Site features that must be considered when designing a wastewater disposal system, include:
- Is the water table is high or low?
- Does the land become saturated during periods of high rainfall?
- What area of land available for the system?
- What is the soil’s ability to absorb moisture, e.g. is the subsoil clay, sand, loam etc?
Note: The amount of land needed for the disposal field is dependent on the subsoil).
System designs
- Schematic of on-site domestic wastewater treatment
Where a sewerage connection is not available, the on-site wastewater treatment system must meet the performance requirements of the NZBC.
Local authorities or regional councils may have their own bylaws controlling an installation and resource consent may be required. Contact the local authority before you begin to design a system.
- Schematic of separate on-site domestic greywater and blackwater treatment systems
Greywater and blackwater may be treated separately. If separate treatment systems are selected, you should include a means of diverting the greywater to the blackwater treatment system.
- Schematic of an on-site domestic greywater treatment system used in combination with a waterless composting toilet
A composting toilet eliminates the need for a blackwater system but must incorporate a means of dealing with urine. The greywater can be dispersed to a land application or used for irrigation.
System capacity
Allow for a daily output of 200 litres per person when designing a system. This will enable it to cope with peak discharge rates or temporary overloads. It must also be able to retain the total flow for at least of 24 hours.
The system should also have enough capacity for 3–5 years of sludge at the following rates:
- for blackwater and greywater – 80 litres per person per year
- for blackwater only (where there is separate greywater system) – 50 litres per person per year
- for greywater only – 40 litres per person per year.
The septic tank system capacity must allow for:
- variations in the quantity of effluent to be disposed of
- the possibility that householders will not manage and maintain the system effectively.