Close this search box.

Dual Rainwater & Greywater Harvesting Systems to Save Water (2024)

Water is essential for life as we know it. Every living organism requires water in some form, and life initially emerged in the ocean.

It is no surprise then, that water is so important to our everyday lives, as we use it not just for survival, but also to clean ourselves, our clothes, and our dishes.

We use it to dispose of waste, and to ensure the survival of the animals and plants that we choose to keep around us, be it for food or companionship (plants can also be our companions, and I refuse to hear otherwise).

If more than 70% of the Earth’s surface is covered in this all-important liquid, why then is water scarcity often such an issue?

Well, first of all, 95% of the water found on Earth is located in the ocean, which doesn’t help any of us land-based life forms, it’s high salt content.

The other 5% is all around us, contained in places such as rivers, lakes, clouds, and underground aquifers. This is the water that provides for all land-based life, and it seems to be getting scarcer by the year.

Due to this water-based crisis many governments, people and organizations are continuously looking to reduce their water usage, especially in regards to water sourced from municipal water schemes.

So outside of not showering or washing your clothes (for all our sakes, please don’t do this one), and having a barren home with no garden or lawn; what can you do at home to reduce your mains water usage, or cease your reliance on external water supply all together?

Let’s find out!

What is Greywater?

Most people who have heard the term greywater before think of it as any wastewater used by the household that would otherwise run into a septic or sewer system. Studies put the water consumption of the average American at 80-100 gallons of water a day.

This definition is a little too broad, however, and as such slightly inaccurate, as not all water used by the household is classified as greywater, and as such not all can be used in a greywater recycling system.

rainwater and greywater harvesting - image of hand washing in soapy water

Typically, any water used in the bathroom or laundry is classified as greywater, along with some of the water used in the kitchen (the water used to rinse fruits and vegetables, pasta water, and similar food-related uses).

Water from toilets and dishwashers or kitchen sinks are often filled with a higher concentration of chemicals, bacteria, fats, and other solids that are not suitable for use in your garden, making it best to try and cut back on water use in the toilet and kitchen, rather than recycle it.

The water you can collect from the bathroom and laundry comes in at about 25 gallons per person, per day – or a whopping 9000 gallons a year. That’s enough to fill a hot tub a fortnight (disclaimer: not a recommended use for greywater).

Where Can I Use Greywater?

Can greywater be used on your entire garden? Well, that’s a complicated question, and the answer depends on the garden you have.

Untreated greywater is great for watering lawns and trees and some garden plants, but due to the impurities in the water, is not recommended for use on vegetables or herbs and other plants that are sensitive to alkalinity or impurities in the water.

If you are using untreated greywater, it is important to keep an eye on the pH levels of your soil and to ensure that any potted plants are not being watered with this system, as the impurities of the water will have more of an effect in a closed environment.

There are several fertilizers on the market that can be added to untreated greywater to reduce its alkalinity before watering lawns and garden beds with it.

rainwater and greywater harvesting - sprinkler watering flowers

Treated greywater, however, is much more versatile, as not only can it be used in the garden with much less stress relating to alkaline levels.

It can also be reused within the home for the toilet and washing machine – two areas in the home where we typically use significant amounts of water.

Greywater systems range from very simple to more high tech, with simple diversion systems available – with or without a filtration system – in a more affordable price bracket, and diversion and treatment systems coming in at a higher price tag but general more robust.

Before we look at combining rainwater and greywater harvesting systems, let’s take a look at what rainwater systems are:

What is Rainwater?

Rainwater is any water harvested from the roof of your home, shed, or even water tank. Really any surface that you can attach a gutter or water harvesting system to divert precipitation into a water tank.

The process of rainwater harvesting can be traced thousands of years as humans were developing farming techniques for drier areas: Before the advent of the aqueduct, The Romans were even using rainwater from rooftop water storage systems to have water available for homes and farms.

Rainwater is also safe to drink, provided you don’t live in an area with too much pollution. If you want to make extra sure that your water is fit for consumption, simply affix a filter to your kitchen taps to remove any impurities.

How Much Rainwater Can I Harvest?

Several factors affect how much your rainwater harvesting system can increase your water supply, and it’s not just limited to the size of your rainwater tank.

The two major factors are the annual rainfall that falls on your home, and the size of your catchment area.

Your catchment area is the combined space of the roof of your dwelling, and of any sheds or other structures you are catching water from.

Converting your annual rainfall, and square footage of catchment area into the metric system (if you’re not already using it) allows you to easily calculate how much water you will be able to harvest in an average year:

rainwater and greywater harvesting - Image text says "Use the following formula to calculate how much water you can harvest in an average year: Annual Rainfall (mm) x Catchment Area (m2) = Total Rainwater Harvesting Capacity (L)"

So using the equation above, if you were in an area like San Marco, Texas, which receives 890mm (35 inches) of rainfall a year, and someone hooked up a rainwater tank to a home and shed with a combined 185m2 (2000ft2) catchment area, they would be able to harvest 164,650L (43,496gal) of rainwater each year.

However, this doesn’t mean it would make sense to install a 165,000L/45,000gal rainwater tank, as this water is going to be steadily used throughout the year, meaning for this hypothetical property; a 110,000L/30,000gal tank would probably be the best way to go.

The good news is, thanks to modern manufacturing technologies, the prices of steel water tanks are often not as high as you might expect, and there is a model for every budget and project size.

Making Rainwater and Greywater Harvesting Systems Work Together

If you’re aiming for full sustainability, combining rainwater and greywater harvesting systems is arguably one of the most effective ways to do it, but if you’re just looking to reduce your footprint, even utilizing part measures could help.

For full sustainability, the rainwater tank would account for all household water use for the kitchen, shower, and drinking water, while the greywater supported the laundry, toilet, and garden.

Water would gradually downcycle from the bathroom to the toilet and laundry, and onto parts of the garden where non-edible plants are watered.

Take a look at this video by Josh’s House to see how a full rainwater and greywater harvesting systems can work:


For smaller systems where you are just looking to reduce your water footprint, you could install a smaller water tank and not worry about the treatment of greywater.

You can still greatly reduce your water usage in the garden and toilet without spending as much money or taking up as much space. This is especially helpful for smaller blocks, without the space for a large water tank or greywater system.

Next Steps

The best thing about trying to become more sustainable is that you don’t have to do it all at once – and we all love planning home improvements, no matter how small.

Figure out what your budget is, and what you want to work on first, make some google searches and start making phone calls.

For a rudimentary greywater system, you can probably set this up yourself with the help of some instructional videos, but for more technical rainwater and greywater harvesting systems you’d want to get a professional in to install it.

If you have the budget to get it all done at once, call around to water tank and water storage companies, and greywater system experts, figure out what suits your property and available space most, and get some plans in place.

Good luck, and happy harvesting!

About the Author

Photo of the Author - Jack Turner Jack Turner is a digital marketer and content creator for the Australian water storage company Pioneer Water Tanks. While he now lives in the city for work, he grew up in rural Western Australia and remains a country boy at heart.

Frequently Asked Questions

Is harvested rainwater safe to drink and cook with?

Provided that you live in an area without high levels of air pollution, the rainwater that falls on your property will typically be safe as potable water. If you want to make extra sure, you can filter or boil the water first – or both! However, ensuring the cleanliness of your harvesting system is important.
The first step is to make sure your tank is made from food-grade materials, either fitting it with a food-safe tank liner or a UV stabilized polyurethane water tank. Ensure that the roof being used for rainwater harvesting isn’t made from treated wood or bituminous products, and hasn’t come into contact with lead paint or a chimney from a wood burner. Read the full guide for more info on harvested rainwater.

Can I use greywater on fruit and vegetables?

So long as the water isn’t coming into contact with the edible parts of the plant, this is fine. This means for most vegetables and herbs, grey water irrigation is going to be a no-go (unless you are using subsurface irrigation or drip irrigation systems), whereas, for fruit and nut trees greywater reuse is ideal.
Always make sure to wash your fruit and vegetables before consumption (and as a bonus, you can add the water used to wash them to your greywater recycling system). Check the guide for information on what water is safe for use as greywater.

How much water will a rainwater and greywater system save me?

This will depend on your roof size, annual rainfall, household water usage, and the sophistication of your greywater harvesting and treatment system.
The more rainfall you have, the larger your rainwater tank, and the more advanced your greywater system is, the more water you will be able to save in the long term. For more information check the respective sections of the guide.

References and Useful Resources

Australian Broadcasting Corporation: Guide to Greywater

Southern Cross Water: Is it safe to drink water from a rainwater tank?

Renew Magazine: Greywater system buyers guide

Josh’s House: 10-star energy-efficient family home

HarvestH20: Comparing Rainwater Storage Options

Get cutting-edge Climate Solutions Delivered to Your Inbox

The climate tech essentials. Bite-sized monthly updates for busy changemakers.

Pin It on Pinterest

Share This
Scroll to Top