About Greywater

What counts as greywater?

Greywater is household wastewater that has not come into direct contact with toilet waste. In practical terms, it commonly includes water from showers, baths, hand basins, laundries, washing machines, kitchen sinks, and dishwashers.

Toilet wastewater is different. That is blackwater, and it belongs in a different category and needs to be managed accordingly.

One of the most common points of confusion is kitchen wastewater. MBIE’s household-water guidance includes kitchen sinks and dishwashers in its definition of greywater. However, kitchen water is usually one of the heavier and more demanding forms of greywater because it can carry higher grease, oil, food solids, and organic loading than lighter bathroom sources.

That does not change its category, but it does change how carefully it needs to be managed. Kitchen water needs suitable grease and solids management and an appropriate treatment arrangement; it should not be treated like shower water for ordinary direct garden diversion.

Greywater vs blackwater

The distinction matters because the reuse, discharge, and treatment options are not the same. Blackwater contains toilet waste and carries a different level of contamination risk and infrastructure requirement. Greywater, by contrast, can sometimes be reused or redirected more simply, but only where the source, site, and system support that outcome.

This is one reason composting toilet projects often lead people to think more carefully about greywater. A waterless composting toilet changes or removes the toilet-waste stream, but the household still produces wastewater from showers, basins, laundries, kitchens, and other fixtures. That greywater still needs to be properly managed.

Why greywater matters

Greywater often makes up a large share of household wastewater volume. In many homes, especially where water efficiency and onsite resource use matter, greywater becomes one of the key design questions.

Managed well, it can support more efficient onsite living and more purposeful use of wastewater. Managed poorly, it can create odour, loading, maintenance, and site-discharge issues.

For homes using composting toilets or other source-separated toilet arrangements, greywater is often the main wastewater stream left to solve. That is why it should never be treated as an afterthought.

Reusing greywater

One of the main reasons people become interested in greywater is reuse. Reuse can support better water efficiency, lower demand on water supplies, and more resource-conscious site planning. In the right situation, greywater can become part of an irrigation or controlled discharge strategy rather than simply being treated as waste with no further value.

That said, reuse is not automatic. It depends on the source of the water, the quality of the water, the site conditions, and the system chosen. Lighter greywater sources may lend themselves to simpler diversion-based outcomes in some projects, while heavier or kitchen-inclusive greywater may push the project toward more controlled treatment before reuse or discharge.

The most common reuse goal people think of is irrigation, and that can absolutely be part of a well-designed greywater project. But reuse only works well when the system and the site have been matched properly.

Can greywater be stored?

Untreated greywater should not be casually stored for extended periods. Once it sits too long, it can deteriorate, become more difficult to handle, and create odour or quality issues. This is one of the biggest misunderstandings around greywater design.

Short-term holding or surge management is a different matter. In many systems, greywater first moves into a surge or collection stage before being discharged onward to irrigation, diversion, or treatment. That is a controlled part of system function. It is not the same thing as long-term storage.

Treated greywater may be stored for a planned reuse when the system includes suitable treatment and storage controls. It is a different arrangement from simply holding untreated water in a tank.

How a greywater system works

At a broad level, a greywater system collects wastewater from selected household sources and moves it into the next stage of handling. In many cases, that begins with water from showers, basins, baths, laundries, or kitchens entering a surge or collection stage.

You may see two terms: Greywater Diversion Device (GDD), which directs water to a selected destination, and Greywater Treatment System (GTS), which improves water quality before reuse or discharge. A project can use both.

From there, suitable water may move by gravity or pump to a designed irrigation area, or it may pass through treatment before reuse or discharge.

Some systems are quite simple. Others combine several parts, such as surge management, filtration, pumping, irrigation, and treatment. Supporting components like filters and tanks may also need maintenance attention over time, because greywater is not a static or maintenance-free medium.

Not all greywater systems are the same

There is no single greywater answer that suits every property. The right system depends on the wastewater source, the level of use, the intended reuse or discharge outcome, and the site itself.

A lightly used tiny home can produce different amounts of greywater from a full-time family home. Actual occupancy and water use matter more than floor area alone. A standard residential site is not the same as a commercial or shared-use project. Kitchen-inclusive systems are not the same as bathroom-only systems.

Some projects suit simpler diversion-based arrangements. Some require treatment. Some benefit from pressurised irrigation or more controlled discharge layouts. Some require filters, pumps, or transfer components as part of the wider design. And some sites need a more developed commercial response rather than a domestic one.

That is why WCTNZ® treats greywater as a project-by-project system question rather than a one-size-fits-all product choice.

Explore Greywater in More Detail

These pages break the subject down further, from quick beginner answers through to system options, technology types, and choosing the right solution for a real project.

Need Help Choosing a Greywater System?

Every greywater project is a little different. Some sites are simple. Some are not. Some people already know they need a diversion-based setup, while others need help working out whether treatment, irrigation, pumping, or a broader system package is more appropriate.

That is why WCTNZ® scopes greywater projects case by case. If you already know what kind of project you have, use the enquiry form that best matches your project. If you are not sure, that is fine too — the right next step is still to enquire.