# RO water remineralisation through CaCl2 NaHCO3 MgSO4



## Roberto WS (Sep 13, 2013)

Hello everybody,

I'm brand new and I have no aquariums at the moment, but I'm here because many things liase with my past and present works. I will talk about them soon in my presentation post.

By now please let me explain what disturbs my nights: CaCl2 NaHCO3 MgSO4.

Basically I have an RO water flowrate that I have to remineralize through an high concentration solution:
- Ca++ has to increase from 10 to 30 ppm;
- Mg++ from 2 to 7 ppm;
- Na+ from 1 to 7 ppm; 
- HCO3- from 2 to 85 ppm (more on this later);
- SO4-- from 0 to 7 ppm;
- Cl- from 15 to 65 ppm;
- pH passes from 5,2 to 7,5.

So basically it's needed to add something like:
- 20 ppm of Calcium;
- 5 ppm of Magnesium;
- 80 ppm of Bicarbonates;
- 7 ppm of Sulphates;
- 6 ppm of Sodium;
- 50 ppm of Chlorures.

So bicarbonates increase because from a pH of 5,2 to 7,5 we go from 10% to 95% HCO3-:









Now, the molecular weight of CaCl2 is 110, MgSO4 is 120 and NaHCO3 is 84, so:
- to add 20 ppm of Ca++ there's need of 20*(110/40)= 55 ppm of CaCl2 (and we add also 35 ppm of Cl-);
- to add 5 ppm of Mg++ there's need of 5*(120/24)= 25 ppm of MgSO4 (and we add also 20 ppm of SO4--);
- to add 6 ppm of Na+ there's need of 6*(84/23)= 20 ppm of NaHCO3 (and we add also 14 ppm of HCO3-).

So, if you think that's too easy, here come the troubles.
I dose the concentrated solution with a flowrate that is 1/90th of the main waterflow, so of course the concentration of the "mother solution" must be 90 times higher, so:
- 55 ppm x 90 = 4.950 ppm = 4,95 g/l CaCl2;
- 25 ppm x 90 = 2.250 ppm = 2,25 g/l MgSO4;
- 20 ppm x 90 = 1.800 ppm = 1,80 g/l NaHCO3.

The water used to prepare the solution is RO water (Ca++ 10 ppm; Mg++ 2 ppm; Na+ 1 ppm; HCO3- 2 ppm; SO4-- none; Cl- 15 ppm; pH 5,2).

It means that I add:
- 4,95 / 110 = 45 mmol/l CaCl2 = 45 mmol/l Ca++ and 90 mmol/l Cl-;
- 2,25 / 120 = 19 mmol/l MgSO4 = 19 mmol/l Mg++ and 19 mmol/l SO4-- ;
- 1,80 / 84 = 22 mmol/l NaHCO3 = 22 mmol/l Na+ and 22 mmol/l HCO3-.

And I have to take care of:
Kps Mg(OH)2 = 1,8 x 10-11
Kps CaSO4 = 2,4 x 10-5
Kps MgCO3 = 2,6 x 10-5
Kps CaCO3 = 1,7 x 10-8

Qps CaSO4 = [0,25+0,045][0,019] = 0,056 = 5,6 x 10-2
Qps CaSO4 > Kps CaSO4 so there's precipitation

Do you see any other salt precipitating?
How can I avoid all the precipitations using these three salts?

Is a separate preparation into two batches the only way to avoid this problem?
So for example MgSO4 in one solution and NaHCO3 plus CaCl2 on the other solution?

Thank you in advance for your help.

Roberto


----------



## Zapins (Jul 28, 2004)

Ok so to highlight what I think you are saying, you have or want to have a constant dosing system that re-mineralizes your water to the predetermined levels in your post and you are having issues with cloudiness (precipitation)?

The problem I see is that at some point your solution cannot become more concentrated, this is based on the Ksp value of each chemical and is particularly poor for CaCO3 which is not very soluble in water as fertilizers go. Only 47 mg/L of CaCO3 is soluble in water at normal pressure/temps, so you won't be able to get to the concentrations you need with this chemical. CaCl2 is far more soluble (745g/L) so perhaps you should switch to that. The bulk of the precipitation in your concentrated mix is CaCO3 (1000x less soluble than MgCO3 [0.39 grams/L]).

Also, as you suggested the common ion effect will decrease the solubility of the Mg and Ca, so switching out one or both to another salt is a good idea, preferably CaCO3 to CaCl2. In fact, carbonate compounds in general are not very soluble so you should avoid using them when trying to make highly concentrated solutions. Using MgSO4 you can get the concentration up to 337g/L, so this is a better salt to use, and probably the highest concentration you can make unless you want to switch to MgNO3 which is about 2x more soluble but will add Nitrates to your water as well.

To answer your question if you should mix it separately, it will help, but not very much, as mentioned carbonate salts are sparingly soluble so the increase won't get you where you want to be. Have a look at this page on wikipedia it will help you get an idea of which salt is better to use for solubility. http://en.wikipedia.org/wiki/Solubility_table

Alternatively you could just have a larger reservoir for your concentrated mix and simply add more of the liquid to your water than you are now.

Why do you want to add Na, and CO3? Is this water for growing plants or for another purpose? Na is not desirable in any significant quantity and will cause issues with your plants. Carbonates are also generally not desirable for aquatic plants, a lower KH and pH are generally better for aquatic plants.

Have a look at the fertilator (top left of APC's screen in the banner) it will help you calculate the ppm of a given chemical without doing it all out by hand:
http://www.aquaticplantcentral.com/forumapc/fertilator.php


----------



## niko (Jan 28, 2004)

All this chemistry that disturbs your sleep is fine and dandy. A minor but not bad thing to keep in the back of your mind is common sense:

Discus breeders. These are weird folk. Humongous water changes every day, super clean tanks and systems. VERY touch about the quality of their water. Probably touchier than the discus themselves. So what do these weird guys do? Why don't al of them mix their own RO concoction? Most of them use a lot of RO anyway.

If you figure out the answer to that let me know.


----------

