Cath Hassell from ech2o consultants explains what rainwater harvesting is, why it's useful and how to get started.
If you haven't heard our first podcast interview with Cath on the topic of saving water, check that one out first!
Interview with Cath Hassell
Rainwater harvesting is just one way of stopping rainwater from getting into our drains and sewers.
If the house is in a city and rainwater is going into a combined sewer, then there will be more waste water to treat and there may be times when water treatment plants can't cope and so the water spills out into local rivers or the sea.
Keeping rain out of the stormwater stream reduces the likelihood of this occurring.
What is Rainwater Harvesting?
Rainwater harvesting is collecting rainwater that falls on the roof of a building. It's taken from the roof because this is usually the cleanest water. This water is then stored outside but it can be used back in the building (for flushing the toilets, cleaning purposes, in washing machines, etc.) or it can be used outside the building for gardening.
Water is Stored Underground Because It's Heavy
There are two main reasons why water is stored underground:
1. It's heavy and therefore it does not want to be kept high up on top of a roof
2. The water will be cool if stored underground reducing the risk of bacteria
Water tanks can take up quite a lot of space so this might be another reason why it's best to locate them underground.
Setting Up Rainwater Harvesting is Easier for New Build
In a new build house there is already going to be earth-moving equipment on site and so it becomes cheaper to dig a big hole.
While it can be done in a retrofit situation, the initial set-up may cost more. There may also be issues such as no access to the back of a house (in a terrace, for example).
Rainwater is Not Fit For Drinking or Bathing
Astonishingly, in the UK, rainwater is actually given the same classification as water that has been swept from abattoir floors or contaminated with nuclear waste. For this reason there needs to be different pipework which runs to domestic taps, than for that which runs to toilets, washing machines etc.
An Air Gap is Required to Prevent Water From Syphoning Back Into The Pipework
The air gap is basically the space in between where the water can’t touch the taps. If a sink was overfilled the water would go down through the overflow or spill onto the floor and there would be no risk of the potentially dirty bathing water from re-entering the mains system through the taps.
A High Carbon Load Damages The Reputation of Rainwater Harvesting
The air gap does result in a loss of mains water pressure which can be overcome by pumping it to where it is needed, though using electrical pumping obviously does have a financial and carbon cost associated. The performance of the pumps can however be optimised by using particular valves.
A high carbon load is one of the arguments that gets used against rainwater harvesting, but Cath would welcome research to be carried out looking at how taking rainwater out of the stormwater stream impacts on the reduced carbon load of the sewers and sewage treatment plants by reducing pumping there.
Decisions About Rainwater Systems Can Influence The Designs of a New Building
Cath suggests that if you’re planning a green roof she would not recommend rainwater harvesting for use back in the building. This is because the water run-off from the green roof will be discoloured and residents of the building may start using bleaches and cleaning products to remove the staining to their toilet pans.
There is no reason why rainwater couldn’t be harvested by other areas of the building which do not have a green roof, and simply designed in with the gutters and downpipes.
A Calculation Determines The Size of The Required Rainwater Storage Tank
The storage tank holds approximately 5% of the anticipated rainwater that will be harvested in a year, which equates to around a 3 week supply. This is usually enough as the rain falls year round in the UK but there will still be some times where the tank will be too full or too empty. When empty the supply can be switched temporarily back to the mains, and when too full the overfill can be directed back towards the stormwater drains or into areas of garden that can cope with being inundated by water.
Cath Has a Kickstarter Project to Produce a Book Designed to Help Educate Children
A lot of Cath’s work is designing systems for schools. She has a Kickstarter project which will enable her to produce a book explaining about how the toilet systems work which she will be able to use when talking with the children.
The Storage Tanks Are Generally Either Made of GRP (Glass Reinforced Polyester) or Polyethylene
In the early days of rainwater harvesting the storage tanks tended to be made of concrete. Nowadays the two options are GRP (mostly for larger tanks) or poly (mostly for smaller tanks).
A life cycle analysis was carried out on the two by Judith Thornton (CAT, AECB) who found that poly tanks were far better in terms of overall carbon footprint.
Use Rainwater Harvesting Purely For The Garden if The Demand is Great Enough
Cath recommends that for a domestic situation where you have a large requirement for garden watering, that you just use the rainwater for that and do not try and bring it into the house. Her reasons for this are because of the extra pipework and pump (for the mains backup) that will be needed. An additional outdoor mains tap will provide extra if required.
If you’re harvesting rainwater purely for garden use then a different calculation for the storage tank is required. This reflects that as much water as possible will be collected during the non-growing months, in order to provide the best supply for the growing months.
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