Greywater harvesting and price

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Greywater harvesting and price

What’s way to harvest greywater for a positive house?

How to collect greywater in a positive house?

What’s the price of collecting greywater in a positive house?

May you don’t know us yet, let’s have a quick introduction: we’re ERA architects.
Actually, I’m Esther Rovira, architect and passive house designer in Barcelona, Catalonia and Andorra for either passive houses, efficient houses and positive houses.
In this post ‘Greywater harvesting and price’, You will find our professional recommendation.
I hope it helps you!
Let’s go!


Greywater harvesting

Greywater is the water that despite being sensitively used from showers and bathroom sinks, it could again serve some other use.
It’s not recommended to re-use water from the washing machine, kitchen sink or dishwasher since they could contain massive amounts of oil, rest of food, cleaning products…
If you’re thinking of recovering greywater, you may also consider not to have foam party in your bathtub too often.

The most common is to harvest greywater to re-use it for: toilet tanks, garden irrigation and cleaning (for example cleaning the floors or washing your electric car).
However you need to know three important things.

This system consists of two tanks (usually located in the installation’s room)(note that there’s no underground system for this that we know yet).
Greywater is filtered to avoid rest of solids (like hair for example).
Then the bio-oxidation process brakes down the organic material.
Still in the first tank, the ultrafiltration membranes avoid damaging bacteria from going into the system.
Before leaving the first tank a system of membranes suctions and separates liquid from the solids, virus and bacteria.
Then only treated water goes into the second tank where it is chlorated and storaged to be used.

The first filter needs to be checked at least once a month to remove the hair rests and other solids.
This filter is very similar to the swimming pool basket filters.
So it’s not rocket science, but you need to remember to do this every month.
You also need to clean the membranes every six months by using sodium hypochlorite into the tanks.

Mind to calculate if your showers are coherent with your toilet use.
Meaning if you use the toilet more often than you shower, then you’ll need to feed your tank with paid water.
Let’s say showers harvest 30 litres and toilet flush take 6 litres.
Then we take a house with 3 people. Only one takes a shower once a week since the other two shower at the gym (1 people x 30 litres x 7 days= 210 litres).
But all 3 use the toilet three times a day (3 people x 6 litres x 3 times x 7 days= 378 litres).
Then you’ll need to feed the toilet tanks with paid water.
Of course this is not very common, and usually the greywater harvesting is higher than its use.
However, mind to install an automatic weather to feed the tank with paid water, just in case all family becomes gym maniacs!

So, let’s do a quick pros cons of greywater harvest:

– collect greywater from showers and bathroom sinks
– re-use greywater for toilet tanks, gardening and cleaning
– save water
– save money

– high maintenance
– if not used for a long time, then you’ll have problems
– occupies space system (not an underground system)
– occupies lots of space on the installation’s room
– no guarantees from manufacturers if not doing the maintenance

The greywater systems are very extended for hotel use or collective housing.
So manufacturers have usually big tanks, but your family house will do with a 500l/day regeneration tank. Meaning two tanks of 500l.
The price for the greywater two tank system for a family house is around 9.800€+21%iva.
Mind you’ll also need to cover the installation price.
Also mind that this will not be enough to cover all your garden irrigation needs in Spain all year long, so you’ll also need to install another deposit for either rainwater harvesting or blackwater treatment.

Our professional recommendation:
We recommend to harvest greywater from showers and bathroom sinks to feed toilet tanks, garden irrigation and cleaning (floors, car, bike…).
Mind to keep up with the maintenance once a month.
Mind to shower accordingly to your toilet use needs.
Mind not to do foam parties too often (greasy soaps exposure could damage the system).

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Hope this post ‘Greywater harvesting and price’ is helpful!

Best of luck on this adventure.
See you on the next post,


Founder of ERA architects 

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By |2023-03-20T18:51:42+01:00February 2nd, 2023|Passive House Architect Barcelona|0 Comments

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