How To Use Rainwater for the Garden, Toilet and Washing Machine
Standing at your downpipe during a decent summer downpour and seeing hundreds of litres of perfectly good water pouring down the drain gets evermore ridiculous when you look at your water bill each month. For us, that point of irritation eventually reached breaking point about two years ago. The difference is, we didn’t do it because we’d suddenly all decided to go Full Eco Warrior. The expenses associated with watering a garden that demands watering every few days when it doesn’t rain just isn’t viable any more. Add to that the toilets and washing machine and suddenly harvesting rainwater went from virtue signalling green tech to something akin to common sense.
What finally convinced us was the numbers. Toilet flushing makes up around 30 percent of a household’s water usage (Waterwise) and washing machines account for another 15 percent of domestic usage (Energy Saving Trust). That’s nearly half your water bill going on activities that you don’t actually need potable quality water for. At the same time, using rainwater for toilet flushing can save up to 20,000 litres per household per year (Environment Agency). That’s serious money off your bills and a significant reduction in demand form mains water supplies.
Ok, so we still use rainwater in the garden. But let’s look what else you can do with it…
Rainwater for Toilets and Washing Machines
First things first, what do you actually want the rainwater harvesting system to do? If you’re only ever going to use it for watering the garden you wouldn’t need to read much beyond my previous guide. But using it indoors for toilet flushing and/or the washing machine changes the goals quite substantially. So let’s talk about what you actually need to consider when setting up a rainwater harvesting system that you can use for more than just keeping the weeds at bay.
Different Applications, Different Requirements
Rainwater is essentially free of everything. It has ZERO hardness so acts as a naturally soft water supply for your washing machine meaning you use up to 50% less detergent (Green Building Store) when washing clothes. That in itself extends the life of your washing machine (Anglian Water) because you’ve got less lime-scale build up so everything works better for longer. Your toilets will work better for longer too.
So what do you need to consider?
Water Quality Requirements
Ok, so rainwater is pretty soft and clean. When it falls straight from the sky it actually IS clean. Fresh rainwater is cleaner than most people realise. The biggest concerns are dust, leaves, birds dropping and atmospheric pollution etc. But most of that can be easily managed with good first flush diversion.
Supply Demand
Household toilet flushing can take up a LOT of water. One study from Anglian Water suggests that the average person living in a house or apartment flushes the toilet almost 5 times per day (Which). That sounds high, but let’s crunch some numbers…
Four people x 5 Flushes = 20 Daily Flushes
If your toilets are low flow toilets (they should be these days), that’s roughly between 4-7 litres of water per flush (Which). Take the low number of 4 litres for each flush and our theoretical family of four are using 80 litres of water EVERY SINGLE DAY on flushing toilets. Now add in washing machine loads…
Assuming maybe 4 loads of washing per week that’s using between 40-60 litres of water per wash (£780 average across UK makes up):
4 Loads x 52 weeks = 208 Washes per Year
208 Washes @ 50 litres per wash = 10,400 litres of water
Which means our example family have a baseline NEED for 15,080 litres of water EVERY YEAR just for toilets and washing machines. That doesn’t factor in any irrigation requirements for the garden.
Catchment
Now this is going to vary massively based on where you live and how much rain you get. As a ball-park figure, every 100sqm of roof catching rainwater can collect approximately 62,000 litres every year. That’s with the UK having around 830mm of rainfall every year. Increase that roof catching area and you can collect more water. Decrease that roof space and you collect less water. Catch my drift?
Installation Requirements – Things To Think About
Typical System Components
I would absolutely argue that your requirements change depending on how you intend to use harvested rainwater. Something that’s purely supplying your irrigation system can be a lot simpler than what you need for supplying toilets and washing machines. So let’s talk about what you actually need…
To start with we’ve got the COLLECTION of rainwater, that won’t change. But you’re going to have to think about first flush diversion carefully. Supplying appliances and toilets demands better water quality than if you were just watering the garden.
Then you’ve got your STORAGE requirements. Nothing changes here really. Although… you might want a bigger tank. Again this comes back to your required usage.
DISTRIBUTION is probably where you’ll notice the biggest change. You’ll need a pump. You’ll definitely want a pump. You’ll also need a pressure vessel (tank) and a distribution manifold to connect your appliances/toilets into. There’s probably enough to say about distributing harvested rainwater that we’ll cover it separately.
Plumbing Principles – Don’t Mix Them Up
Normal household water supply plumbing mixes HOT and COLD water together to create a mixed supply. Often used for showers and baths where you don’t want scalding hot water blasting you (although you SHOULD get a thermostatic mixer on your showers).
You SHOULD NOT be mixing mains water with harvested rainwater. Ever.
I can’t stress that point enough.
Backflow prevention is REQUIRED when installing rainwater harvesting. Why? Because you don’t want contaminated water getting into your mains supply. So any time you install a rainwater harvesting system you need to think about cross connection control. For most households that means INSTALLING AND ATTENDING ISOLATION VALVES on each connection point. And ideally colour coding your rainwater pipes purple (check out what colours pipe should be here).
Rainwater Harvesting for Toilets & Washing Machines – Setup Matrix
To summarise, lets look at what you want your rainwater harvesting system to do…
| What Do You Want To Use Rainwater For? | Daily Demand | Pressure Required | Water Quality Required | Installation Complexity |
|---|---|---|---|---|
| Toilet Flushing | 80-120 litres | Gravity feed | Filtration probably sufficient | Moderate – depends if your replacing toilets or adding new pipework |
| Washing Machine | 40-80 litres | Pumped supply (see below) | Filtration probably sufficient | Low – easy to connect single appliance |
| Garden Watering | Dependent on household demands | Depends on how you set system up | Close enough to drink for most people! | See my other guide! |
The big question is always going to be water quality when it comes to using rainwater indoors. But even that’s fairly straightforward. Provided you ensure good first-flush diversion rainwater is more than adequate for toilet and washing machine use.
We’ve already got rainwater toilet systems plumbed in at home so that wasn’t too much of an issue. Cutting over to rainwater for the washing machine was pretty simple too. Most machines these days need a feed rate of between 10-15 litres per minute at around 1-2 bar pressure during the water fill cycle. So you’ll need a pump capable of delivering that demand, a decent sized pressure vessel (tank) and a distribution manifold to connect your appliances/toilets into.
Automatic water switching isn’t essential. However…. once you’ve gone through the effort of installing rainwater harvesting for toilets and/or your washing machine it kinda’ is silly not to cut over automatically. By automatically I don’t mean smart phones and apps. I mean a simple float switch in your storage tank that automatically tops up from mains water supply below a pre-set level. See below for more info on automated water switching.
Installation Errors to Avoid
Error #1 – Expecting harvested rainwater to meet drinking water standards
I can’t stress this point enough. If you’re using harvested rainwater for flushing toilets and/or laundry you don’t need Rainwater to be drinking water standards. It’ll never be that clean. And it never needs to be.
However… What you DO need is some crude filtration. Leaf matter, mud, dirt etc. That’s easily accomplished with good first flush diversion at source. Everything else can safely be harvested and used.
Error #2 – Treating a rainwater harvesting system like a normal gravity fed irrigation system.
Rainwater harvesting systems that don’t use rainwater for toilets and washing machines don’t require pumps. Or pressure vessels. All that’s required is a header tank in the roof to create enough gravitational pressure to push water around the system. As soon as you want to start using harvested rainwater indoors you’re gonna need a pump (storage tank) AND a pressure vessel (tank).
Error #3 – Forgeting to include automatic water switching.
Including automatic switching as part of your rainwater harvesting installation isn’t essential. But… do you really want to manually change over from mains water to rainwater every time you move house? Nope. Didn’t think so!
Rainwater Harvesting for Washing Machines & Toilets – Research Sources
WRAP – Quoting that Non potable indoor uses account for 45-50% of total domestic usage.
RICS – Research by RICS shows that over 60 percent of NEW eco homes specify rainwater reuse systems.
Carbon Trust – Recent Carbon Trust research which found rainwater harvesting systems can pay for themselves in 10 – 15 years
Effectively every water company … Yup, water companies. Despite investments in water efficient appliances and devices, USAGE of water continues to increase year on year.
Rainwater Harvesting Setups for Flats/Apartments/Terraced Houses
Flats/Apartments – The chances of you being able to install your own rainwater harvesting system are pretty slim. Outside roof areas are shared for most apartment style buildings. You MIGHT be able to install something community based if ALL your neighbours agree. But even then you’ll probably need your Building Management Company to take it on.
Terraced Houses – Really not much different to traditional house setups just… squashed together. The benefit of being a terraced house is that you can actually run pipes across your garage/storage room into your neighbours garden. Giving you a much larger catchment area than your roof size would suggest. Downside? You can’t bury your storage tank where everyone can see it!
Oversized houses – You know who you are. I could happily spend a week installing a system in your house and still not come close to using all that roof catching area. In cases like this? Focus on what will give you the best return. Toilet flushing and laundry before irrigation every time.
Benefits of Rainwater Harvesting For Toilets & Washing Machines
Financial – Did you read the big section above? Rainwater for toilets and laundry alone can save you between £150-400 per year on your water bill. Plus an additional £50-100 saving on detergent. And who knows how much longer your appliances will last…
Security Of Supply – Another biggy for me. Particularly living in an older property with old pipes and water supply interruptions every Summer during watering restrictions. Rainwater harvesting give you a buffer.
Water Quality – Soft water is better water. ALWAYS. Washing machines feel cleaner and your toilets will work better for longer when using soft water.
Flexibility – Rainwater Harvesting systems should last 20+ years before needing replacement. Your needs won’t. If you’ve got a big enough storage tank supplying toilets and laundry during the winter your tank will automatically become your irrigation store over summer when these appliances aren’t in use.
Installing a Rainwater Harvesting System For Toilets & Washing Machines – Step By Step
Phase 1 – Planning And Design (1-2 Months)
- Pump & Pressure requirements – Does your washing machine need 15 litres per minute at 2 bar pressure during fill cycles? Switching over? You’ll need that sort of flow.
Phase 2 – Storage and Collection (1 Month)
(1-2 Months)
Price Breakdown…
| What | Cost | Comments |
|---|---|---|
| 3,500L Underground Tank | £1200-£1800 | Including excavation and installation |
| Pump & Pressure vessle | £800 – £1200 | Includes submersible pump, pressure vessel and controls |
| Distribution pipework | £600 – £1000 | Purple pipe, Manifold & isolation valves |
| Collection Modifications | £300-£600 | First flush diversion, leaf guards, downpipe mods etc |
| Toilet & washing machine connections | £400 – £800 | Supply Modifications |
| Controls/Automatic Water Switching | £500-£800 | Necessary to automatically switch between mains and rainwater in my opinion |
Grand Total… Drumroll please..
£3,800 – £6,200
…I can hear your crying now. Seriously though. Toilet and Laundry alone knocks about 30% – 40% of your annual bill.
Don’t feel you have to install the full works in one go. Start with collection and storage. Run the irrigation system and see how much you save. Then invest in the pump and accessories when you’ve saved enough to cover the installation costs….
Want To See What Rainwater Harvesting For Toilets & Washing Machines Looks Like Installed?
Check out this project I did recently. Cost us less than £3k installed for the entire system…
Rainwater Harvesting for Toilets and Washing Machines
If your still on the fence about installing a rainwater harvesting system that DOES more than just water the garden…. STOP. Look at the comments I made earlier…
- Saves you up to 50% on annual water bills
- Automatically extends the life of your Appliances
- Harvesting rainwater for toilets substantially reduces lime scale build up around the flush mechanism AND inside your toilet cistern
- Gives you better washing results
- Reduces your demand on water supply networks
- Done in a day.. Seriously!
Installing a rainwater harvesting system that you can use for TOILETS and your washing machine IS achievable in the majority of houses. Takes a bit of planning and a bit of DIY ability but honestly… If we can do it… Anyone can!
Rainwater Harvesting MythBuster
MYTH – You can’t use harvested rainwater indoors
REALITY – Of course you can! Toilets and washing machines should always be run on rainwater if you have the ability.
Don’t want to use rainwater for drinking? No problem, just install a hydration system to connect to your water butt!
