Water

The satisfactory design and operation of a system depends on:

• the personal habits of the users – what they put in the system
• the amount of wastewater output
• the size of the site
• the soil conditions of the site
• the type of usage required (i.e. whether it is for irrigation only or for re-use within the home)
• good maintenance and monitoring.

As part of the design process, you’ll need to consider statutory requirements, plumbing/installation, filtering and treatment options, and the irrigation system.

Statutory requirements

The installation of greywater recycling systems is not covered by a NZ Building Code compliance document. Applications to install a system are an alternative solution under the Code. Whether a system is accepted or not will be at the discretion of the building consent authority (BCA) and few have guidelines on greywater recycling. It is therefore important to consult with the BCA at the outset.

All internal plumbing and drainage must meet the requirements of the NZBC Clause G13 (Foul Water).

NZBC G12/AS1 requires backflow prevention to be provided where it is possible for water or contaminants to backflow into a piped potable water supply. The responsibility of owners and building professionals to prevent contamination of potable water supplies cannot be overemphasised.

Installation

The type of ground floor construction has an effect on plumbing. Suspended timber floors allow for more fall in the wastepipes, flexibility in the pipe arrangements, and the ability to make changes as needed. With concrete slab floors it is difficult and expensive (if not impossible) to change or add to wastewater pipe layouts. Concrete slab floors also don’t allow as much fall in the wastepipes.

Plumbing in new houses and in major renovations can be installed to suit the intended greywater use. Installations designed in accordance with NZBC G13/AS1 Sanitary plumbing are suited to greywater installations because:

• greywater and blackwater are separated until they are outside the building
• greywater intended for recycling can be separated and directed to a single gully trap where it can be diverted to re-use
• wastewater not required (such as kitchen) can be directed to a separate gully and then to the outfall drainage.

Installations designed in accordance with AS/NZS 3500 Part 2 Plumbing and drainage may not be suitable where greywater installations are required because:

• greywater and blackwater are not necessarily separated (i.e. they may be both connected to the same drain)
• all wastewater may be connected directly to the outfall drainage (i.e. there may be no gully trap, however there must be an overflow relief gully).

Under AS/NZS 3500, you can specify a purpose-designed plumbing installation which will separate greywater for re-use.

All greywater systems must be designed so that if the irrigation system is overloaded by heavy rainfall or surface flooding all wastewater can be discharged to a sewer or on-site blackwater treatment system.

Calculating the quantity of greywater

For a typical household, greywater is produced at about 100 to 200 litres per person per day. However, because greywater generation is so variable the amount generated must be calculated for each situation.

For your clients

To help your clients understand onsite wastewater treatment options, safety issues, and the importance of maintenance, refer them to www.smarterhomes.org.nz/water/on-site-sewage-systems and www.smarterhomes.org.nz/water/re-using-greywater.