When it comes to managing household wastewater, understanding the difference between greywater and blackwater is essential—especially if you’re considering sustainable water solutions or dealing with drainage systems on your property.
While both types of water come from everyday household use, they differ significantly in their content, potential uses, and how they should be treated or disposed of. Whether you’re planning a new build, upgrading your drainage system, or simply curious about water recycling, knowing what separates greywater from blackwater can help you make smarter, safer decisions.
In this blog, we’ll break down what greywater and blackwater are, how they’re generated, the pros and cons of reusing each, and how they’re handled in New Zealand residential and commercial properties.
What Is Greywater?
Greywater is the relatively clean wastewater that comes from:
- Bathroom basins
- Showers
- Baths
- Washing machines
- Some kitchen sinks (depending on local regulations)
This water has been used once but does not contain human waste, making it suitable for reuse in certain applications after minimal treatment. Greywater typically contains soap, hair, food particles, detergent, grease, or dirt—but not anything directly harmful like faecal matter or urine.
Greywater makes up a large percentage of a home’s total wastewater. In fact, it’s estimated that greywater accounts for around 50–80% of residential wastewater, making it a valuable resource in water conservation efforts.
What Is Blackwater?
Blackwater is the wastewater that comes from:
- Toilets
- Kitchen sinks (in many jurisdictions)
- Dishwashers (in some cases)
This type of water contains organic waste, pathogens, and harmful bacteria, which makes it more hazardous to human health. Blackwater must be properly treated before disposal and is not suitable for domestic reuse without specialised filtration and sanitation systems.
Because blackwater is high in contaminants, it’s often directed straight to a septic tank or the public sewer system, where it undergoes strict treatment before being reintroduced into the environment.
Key Differences Between Greywater and Blackwater
While both greywater and blackwater are considered forms of wastewater, there are important distinctions between the two:
- Level of contamination: Greywater contains fewer pathogens and contaminants, while blackwater contains faeces, urine, and disease-causing organisms.
- Potential for reuse: Greywater can be reused after light treatment. Blackwater requires intensive treatment and is generally not reused at the household level.
- Regulations: Greywater reuse is permitted (with restrictions) in many parts of New Zealand. Blackwater reuse is tightly regulated or prohibited due to health risks.
- Treatment requirements: Greywater systems may include simple filters and storage tanks. Blackwater treatment involves septic tanks, advanced filtration, or connection to the main sewer.
Can Greywater Be Reused?
Yes! In fact, greywater reuse is becoming increasingly popular in New Zealand as households look to reduce their water usage and environmental impact. A properly installed greywater system can capture and redirect water from showers and washing machines to irrigate gardens, flush toilets, or wash external surfaces.
Common uses for greywater reuse include:
- Subsurface garden irrigation
- Toilet flushing (in some systems)
- Car and window washing (if filtered properly)
Benefits of greywater reuse:
- Reduces demand on mains water supply
- Decreases household water bills
- Environmentally friendly and sustainable
- Reduces load on septic systems or sewer infrastructure
Important considerations:
- Greywater should not be stored for more than 24 hours due to bacterial growth
- Only use biodegradable, low-sodium detergents to avoid damaging plants or soil
- Filters or treatment systems are required to remove lint, soap scum, and other particles
- Some councils have restrictions on how and where greywater can be reused
If you’re interested in installing a greywater system, it’s best to consult a licensed plumber to ensure it complies with local building codes and health standards.
Why Blackwater Must Be Handled Carefully
Blackwater poses significant health risks due to its high bacterial content and organic matter. For this reason, it’s not reused at the household level in New Zealand and must go through rigorous treatment in a septic or municipal system.
Trying to reuse or treat blackwater without proper infrastructure can result in contamination of soil and groundwater, creating serious health and environmental hazards. That’s why any system dealing with toilet waste must be sealed, watertight, and professionally maintained.
If your property is on a septic system, regular servicing is essential to prevent failure, backups, or pollution. Citywide Plumbing offers inspections and maintenance for both greywater and blackwater systems to keep everything running safely and efficiently.
How Are These Systems Managed in NZ Homes?
Most New Zealand homes use a single plumbing network to carry both greywater and blackwater to a central treatment system—either the public sewer or a septic tank. However, more homeowners and developers are now designing homes with separate greywater collection systems to allow for safe reuse.
If your property is:
- Connected to mains water and sewer: Both greywater and blackwater go to the treatment plant.
- On a septic tank system: Greywater and blackwater enter the same tank and are processed together.
- Off-grid or rural: A custom greywater recycling system can help reduce tank overload and stretch water resources.
Systems like diversion devices, filtration tanks, and irrigation pipes can be added to collect greywater separately from blackwater. This is particularly helpful in dry regions or on lifestyle blocks where water conservation is a priority.
Common Questions
Can kitchen water be considered greywater?
In many regions, water from kitchen sinks is classified as blackwater because of the high concentration of food waste, grease, and potential bacteria. However, some systems may still collect this water separately if treated appropriately—check local council guidelines.
Is greywater safe for vegetable gardens?
Greywater should not be used on edible plants unless it has been thoroughly treated. It’s safer to use it on ornamental gardens, trees, or lawns.
Can I install a greywater system myself?
Basic systems (like a laundry-to-landscape hose) can be DIY-friendly, but permanent installations or plumbing alterations should always be handled by a licensed plumber to ensure they meet building codes and don’t interfere with your main drainage system.
Understanding the Flow
Knowing the difference between greywater and blackwater isn’t just plumbing trivia—it’s an important part of making your home more efficient, eco-friendly, and compliant with local water regulations.
By reusing greywater where appropriate and managing blackwater responsibly, you can reduce your household’s water footprint, lower your utility bills, and contribute to a more sustainable future.
Whether you’re building new, upgrading your plumbing, or just curious about water systems, Citywide Plumbing is here to help. We provide expert advice and installation for greywater-friendly systems, septic maintenance, drainage solutions, and more.
Get in touch with the team today to explore smarter, cleaner ways to manage water on your property.
