Do I need DI before RO membrane if the water has high GH?

[materia]

Hi, I was thinking of setting up an RO system, but I'm currently living in an area with very hard water. I did some measurements and I'm getting around 18-19 German Degrees of general hardness (320-340 ppm) and around 8 °D of alkaline hardness (140 ppm).

Admittedly the online resources on the topic are not what I would call reliable, but a lot of places seems to suggest that hard water like this will wreck a membrane pretty fast. I don't need a lot of water - maybe 20-40L a week - and so I was thinking of just getting a can with regenerable cation resin as an additional stage just before RO.

I know this will introduce some additional work of keeping a second batch in a saline bath and swapping them between uses of the system. I was wondering if there are any alternatives or if the membrane could potentially maintain a lifetime that is shorter, but to an acceptable degree?

 

[MichaelJ]

Hi @materia , welcome to UKAPS A DI stage will only remove charged particles (through ion exchange) that may have slipped through the RO stage (usually smaller charged particles that are too small for the RO to reject). A decent RO only system will offer a rejection rate of about 95-99% (varies by minerals). If the water you feed in is 300 ppm you will typically see a TDS around 3-5 ppm on the outlet. With the addition of a DI stage you can reduce that to 0-1 ppm. If your only concern is to remove the Calcium and Magnesium (what makes up the dGH) you will probably be fine without the DI stage. Regardless of configuration, the higher the mineral contents of the water you feed into your RO(DI) system the lower the life expectancy of your membrane, carbon and sediment filters generally will be. If you only need 20-40 L per week I would get a small affordable system.

Cheers,
Michael

 

[dw1305]

Hi all,
Welcome to UKAPS.
What @MichaelJ says.

but I'm currently living in an area with very hard water.

I know it isn't possible for everyone, but I have hard tap water and <"I use rainwater in the tanks">.

cheers Darrel

 

[hypnogogia]

I know this will introduce some additional work of keeping a second batch in a saline bath and swapping them between uses of the system.

I wouldn’t advise that. If you regenerate it with salt, the resin will start exchanging. Ions for sodium ions, and you do t want sodium ions in your fish water.

 

[bazz]

and so I was thinking of just getting a can with regenerable cation resin as an additional stage just before RO

Hi,
@MichaelJ has described the chemistry but DI resin is generally used after the RO unit to polish the water. I only have limited scientific knowledge (but years of experience using RO Systems) but I cannot see where exchanging ions before entering the RO unit is of any benefit, plus the DI resin will become depleted (or whatever the word is) in next to no time.

but a lot of places seems to suggest that hard water like this will wreck a membrane pretty fast

Regardless of configuration, the higher the mineral contents of the water you feed into your RO(DI) system the lower the life expectancy of your membrane, carbon and sediment filters generally will be.

My tap water is marginally less hard than yours and I produce 130l/week and I'm still using the same membrane I've had for the last 2 years. Changing out your prefilters and flushing before and after will use greatly increase its life expectancy, so if you are only going to produce 20-40l/week I would imagine your membrane should last quite a long time.
Cheers!

 

[MichaelJ]

Hi,
@MichaelJ has described the chemistry but DI resin is generally used after the RO unit to polish the water.

Absolutely correct there @bazz … DI is really icing on the cake… hence after the RO stage,,, putting it before would be like raking before tilling…

Cheers,
Michael

 

[_Maq_]

I don't need a lot of water - maybe 20-40L a week

Be aware that RO membrane must not dry up. It should be working no less than once per week to avoid this danger.

 

[Hanuman]

All has been said above I think. If you want your membrane to last as much as possible though you need to have pre-filters prior to the RO membrane. Have a good sediments prefilter followed by a carbon one, possibly 2 to remove odors, chlorine and chloramines. These prefilters will remove any large particles that will otherwise clog the RO membrane prematurely. Both these pre-filters are far cheaper than an RO membrane.