Greywater Systems for Residential Builds in Australia

What it is

Greywater is the wastewater from showers, baths, basins, washing machines and laundry tubs in a dwelling. It does not include water from toilets, kidney dialysis machines or kitchen sinks. Kitchen wastewater carries fats, food solids and detergents that push it into the blackwater category under most state health rules.

For a builder, greywater matters at design stage because reusing it on site changes the plumbing layout, the wastewater discharge point and the approval pathway. Get it wrong and the property can fail final plumbing inspection or be flagged by the local council during occupancy certification.

Greywater vs blackwater

State environmental health departments draw a hard line between the two. Greywater is lower strength wastewater suitable for sub-surface irrigation after the right level of treatment. Blackwater includes toilet waste and kitchen waste and must go to sewer or an on-site sewerage system. Mixing the two streams in a tank or pipework converts the whole flow into blackwater for regulatory purposes.

Plumbing on a new home must keep the two drainage stacks separable if any greywater reuse is planned. Retrofitting separation later means cutting slabs and rerouting waste lines so it pays to decide at the floor plan stage.

Diverter systems

A greywater diverter takes untreated greywater straight from the waste pipe and pushes it to a sub-surface irrigation area. There is no storage and no treatment. Most states allow diverters for laundry and bathroom waste with a manual switch back to sewer for wet weather or when the garden does not need water.

Diverters suit single dwellings on reticulated sewer with a back yard large enough to absorb the daily flow. Council approval is usually a simple permit rather than a full system approval. The plumber installs the diverter under AS/NZS 3500.2 drainage rules and the discharge area follows the state greywater guideline for trench depth and setback.

Treatment systems

A greywater treatment system filters, disinfects and stores the water before reuse. Treated greywater can flow to toilet cisterns, cold water washing machine inlets and surface or sub-surface irrigation. The product must hold a state health approval and the installation must follow the manufacturer manual plus AS/NZS 1547 for any land application area.

Treatment units use a combination of filtration, biological digestion and ultraviolet or chlorine disinfection. Tank size is matched to the dwelling occupant load. The system needs a power supply, a service contract and an annual report to council in most jurisdictions.

AS/NZS 1547 obligations

AS/NZS 1547 covers on-site domestic wastewater management for flows up to 14,000 litres per week from up to 10 people. It sets out the design rules for land application areas which is the part of a greywater install that does the long-term work. Soil category, hydraulic loading rate, area calculation and setback distances all come from this standard.

The standard does not directly cover greywater diverters or surface reuse. Those gaps are filled by state guidelines such as the NSW Department of Health greywater guidance or the Victorian EPA code of practice for on-site wastewater management. A builder needs both documents on hand for any greywater work outside the sewer network.

Council approval

Approval pathways vary by state. In NSW a greywater diverter for a single dwelling on sewer goes through council under the Local Government Act. A treatment system needs both a NSW Health accreditation and council approval. Victoria splits the work between the EPA, which approves products, and the local council, which approves installations. Queensland uses the Plumbing and Drainage Act and councils issue compliance permits.

Builders should check the council website for the specific application form and the lodgement timing. Many councils require the greywater paperwork before the plumbing permit can be issued.

Common problems

The most frequent failure point is undersized irrigation area. A four person household generates around 200 to 300 litres of greywater per day. The land application area has to absorb that flow without ponding or run-off. Sandy soils take it fine. Clay soils need a much larger area and sometimes a raised mound.

The second issue is cross connection between potable and treated greywater pipework. Lilac coloured pipe and clear labelling on every outlet protects against this and is mandatory under AS/NZS 3500.1.