Rainwater tanks - in the greenhouse or outside

Hi folks.

I have to decide where to place my rainwater tanks: inside the greenhouse or outside.
Either way, they will be at least partially buried since there's not a lot of space available.
And since an off the shelf tank is too much $ for me, i'm thinking of doing them from ferocement.

Now, my preference would be to place them in the greenhouse to be sheltered from the zone 6 frosts and to do double duty as thermal mass.
However, during summer it will be hot in the greenhouse and the water will get warm.
Warmer water is more likely to favor bacteria proliferation.
And since i want to use this water for washing there's an infection risk, think legionella.

Even if burying the tanks 1/3rd in the greenhouse ground, i'm unsure how much thermal buffering the ground is going to offer.
I hope to build about 3 x 650ga (2500 l) tanks.

If putting them outside the greenhouse, insulation will be mandatory plus a higher burial depth.



Any recommendations / experience with this ?

i've seen only a few water reservoirs, and i'm not a professional.

usually in hot places, the problem is ALGAE,
and/or particles from the container (asbestos, lead): the process of disintegration is sped up by hot climate.

the best in my idea is natural hard stone containers, but that's nealy impossible to achieve. all the old natural pools here are from stone, and exposed to the elements, and besides leeches, which can be easily filtered, i don't remember special side effect to heat.

however, algae requires sunlight. if the container's ceiling is totally opaque -> no algae.

personally I wouldn't fear bacteria. We used to drink water from water pools in the desert after just a casual filtering through a cotton cloth, which doesn't really block bacteria, and it was fine.

another problem that might rise from warm water is amoeba or even leeches, but that's more common in 3rd-world countries;
Where in the world are you? that's kind of important.
if the container is closed or even sealed, it's usually ok.

i'm from israel, and sometimes still water containers and reservoirs get pretty warm during the .... most of the year, and it was never a concern by itself.

Since you are not opposed to multiple tanks, why not put some in the greenhouse and some in the ground? I would build an in-ground cistern for drinking water, of either parraged block or solid concrete so the lime creates an unfriendly environment for bacteria. It is how most people in Bermuda get their drinking water. If you use block, do some research as they frequently develop cracks and leak, causing owners to install a liner which defeats the purpose of the block. I read once of a technique where little or no mortar is used between block, and then porraged with a skim coat (like stucco) to waterproof, which is supposed to be more stable and less suseptable to cracking than using mortar​. Steel reinforced concrete would be best, but most expensive.

Never get into a hole deeper than 4 feet without something in place to hold back the dirt. Hole collapses kill a lot of people.

I read an article about building a solar hot water storage tank the guy built out of 2x4 lumber, 3/4 ply, and a pond liner for about $200. It was about 4x4x4 (450 gallons), if I remember. If I find the link I'll post it later.

Around here (east coast of USA)  you can buy plastic tanks about that size cheap, that were used for food grade soilbean oil or some such.

Btw, are you familiar with first flush rainwater collection? You could build a small tank of 50 gal (foodsafe 55 gallon drum for $20) to catch the rainwater, and when the tank fills it backs up the downspout and through a T into your drinking water tank. That way, all the bird poop, pollen, and dirt on your roof stays out of your drinking water. After the rain, you empty your first flush tank into your greenhouse water so it is empty and ready for the next rain. Smaller roofs would use smaller firstflush tanks.

Best of luck!

Hi Asaf.

The tanks will be closed but not hermetically sealed.
Material will be ferrocement.
After the first flush, water will be stored in these tanks under darkness.
I don't fear algae but i do legionella.

I am considering putting some outside, fully buried since we have the ground hard frozen for 2 months a year, even if this will complicate matters (digging, insulation, etc).

Brett, if i put tanks buried outside, the hole will have to be deeper than 4ft.
And insulation placed on top due to winter frost.
And i'd have to put a layer of sand between the tank and the hole to accommodate soil movement.

For legionella, you need a rather high temperature (higher than 20C where they are dormant, up to around 45C).

Personally, I would put them outside the greenhouse and grow something over the top of it to provide shade (so the tank serves multiple functions). Greenhouse space is limited, and the the thermal mass maths don't actually add up if you run the numbers.

It turns out that the efficiency of heat transfer from the greenhouse to the water mass is not good enough to facilitate a thermal inertia effect (e.g. a ton of water will absorb less than 100 watts of power in a typical day) when you account for the thermal loss of the greenhouse itself (i.e. the greenhouse loses 150 watts to the cold night air).

So the mass will keep your greenhouse a degree or so warmer for a few hours after the sun goes down, but by 4 am, it's run out of steam so to speak, which is exactly when you want it working.

You are better off aligning your greenhouse carefully (not directly south if you are in the Northern hemisphere for example), using good insulation (and double skin), coupling it to the thermal mass of the earth, and heating it via compost or some other waste heat stream.

Food grade hydrogen peroxide (32%) works well to preserve and purifie snow and rain water It is safe for human consumption and plants excluding algae love it.

See here for more on legionella
http://www.hse.gov.uk/healthservices/legionella.htm

Hydrogen peroxide will only work for a certain time depending on conditions as it degrades to water and oxygen

Power is in watts, energy storage is in watt hours. 1 ton of water (1 cubic metre) stores about 1 Kwhr/oC rise

http://www.what-is-permaculture.co.uk/wp-content/articles/water-temperature-moderation-permaculture-design-greenhouse-GGI-33.pdf

Best wishes to you.

Thanks Tony.

Tanks installed outside (buried) will have a temp around 10 celsius. Here main concern is frost and damage due to frost heaving.
Tanks in the greenhouse could get to the 20-25 celsius mark during summer.
It's these that i would like to get a little cooler. Partially burying them (3rd to half height) is something that should mitigate this somewhat.