RO vs DI Only

[Nick potts]

Hi all.

I have used RO systems a lot in the past for marine setups, and I am looking at getting another one for my shrimp tanks.

There are 2 things I don't like about ro units, 1 is that they are very slow and a little more effort than tap water. I know can buy a larger GPD unit but still slow, this has the effect (on me at least) of meaning I will sometimes put off a WC because I forgot to make water etc. Second is the amount of wastewater, apart from watering plants there isn't really much you can do with it and I hate to see it going down the drain.

Anyway, this got me thinking of using just a De-Ionisation filter with a mixed bed DI resin. My water is not very high in TDS out of the tap at between 70-80 usually.

This would solve both issues as water can be passed through very fast, and there will be no wastewater. The main issue would be how quickly the DI resin would be used up, but I don't need a large amount, around 10 litres a week.

Anyone else tried this? Am I missing something, 0TDS is 0TDs whether from an RO or DI right? I remember reading something about this on a reef forum a few years ago but can't seem to find it.

Cheers

 

[Wookii]

My RO unit puts out water with a TDS at around 5-6ppm, they never get it to zero, you always need to add a DI resin post filter to get it to zero.

That said you have to remineralise for aquatic creatures anyway, so I don’t see any point in trying to get it to zero.

I would also say the same to you with your tap water. If I had tap water as low as 70-80ppm TDS I would be over the moon, and wouldn’t bother with an RO filter at all. You are going to need to remineralise that upwards anyway if you are keeping shrimp, so just find out what minerals are in it (water company report) and add in those that are missing. Job done, and no tedious filter unit to maintain.

If you are really dead set on RO, and you only need 10 litres a week, you can often get a large container or two filled at your LFS, or buy it online.

 

[alto]

Unless there is some issue with your tap water, your livestock should be fine without further treatment

You could go with either RO or DI or RODI, as to longevity, that would depend upon resin characteristics, so check with system manufacturers
(if you have significant peat etc in your tap water, prefilter cartridges to remove particles is strongly recommended)

 

[Nick potts]

My RO unit puts out water with a TDS at around 5-6ppm, they never get it to zero, you always need to add a DI resin post filter to get it to zero.

That said you have to remineralise for aquatic creatures anyway, so I don’t see any point in trying to get it to zero.

I would also say the same to you with your tap water. If I had tap water as low as 70-80ppm TDS I would be over the moon, and wouldn’t bother with an RO filter at all. You are going to need to remineralise that upwards anyway if you are keeping shrimp, so just find out what minerals are in it (water company report) and add in those that are missing. Job done, and no tedious filter unit to maintain.

If you are really dead set on RO, and you only need 10 litres a week, you can often get a large container or two filled at your LFS, or buy it online

Thanks Wookii.

I have had a few RO units over the years, both with and without DI stages and always got 0TDS water.

I see your point on the low initial TDS and the fact that I would need to add buffers. ( I add them to my tap water now as it is low gh/kh) One of the main reasons I started looking at a RO unit was my high PH out of the tap at 8.4 -8.6

LFS water is an option, I just like to have the option in an emergency or if I forget etc.

Unless there is some issue with your tap water, your livestock should be fine without further treatment

You could go with either RO or DI or RODI, as to longevity, that would depend upon resin characteristics, so check with system manufacturers
(if you have significant peat etc in your tap water, prefilter cartridges to remove particles is strongly recommended)

Thanks alto.

The main issue that got me think RO is high ph 8.4-8.6, though I am not sure if DI only would help with that, and just the fact I have total control of water parameters etc.

 

[Wookii]

Thanks Wookii.

I have had a few RO units over the years, both with and without DI stages and always got 0TDS water.

RO membranes are typically around 95-99% efficient - they are never 100% - so your resulting TDS obviously depends on your starting TDS. I come out with 5-6ppm but I’m starting starting at over 270ppm, so I’m running at about 98% efficiency which is pretty good.

As you are starting with 70-80ppm TDS, that is why you are measuring zero TDS as you’d be less than 2ppm (plus meter error, temp compensation etc).

I see your point on the low initial TDS and the fact that I would need to add buffers. ( I add them to my tap water now as it is low gh/kh) One of the main reasons I started looking at a RO unit was my high PH out of the tap at 8.4 -8.6

LFS water is an option, I just like to have the option in an emergency or if I forget etc.

Thanks alto.

The main issue that got me think RO is high ph 8.4-8.6, though I am not sure if DI only would help with that, and just the fact I have total control of water parameters etc.

You must be in one of those areas where they add sodium hydroxide (NaOH) to low KH water to boost the pH. I’m not sure if an RO unit can successfully remove this, or if there is another technique - @dw1305 might know (sorry to keep tagging you in all these threads Darrel, but you are the fount of all knowledge on these points).

 

[Nick potts]

RO membranes are typically around 95-99% efficient - they are never 100% - so your resulting TDS obviously depends on your starting TDS. I come out with 5-6ppm but I’m starting starting at over 270ppm, so I’m running at about 98% efficiency which is pretty good.

As you are starting with 70-80ppm TDS, that is why you are measuring zero TDS as you’d be less than 2ppm (plus meter error, temp compensation etc).



You must be in one of those areas where they add sodium hydroxide (NaOH) to low KH water to boost the pH. I’m not sure if an RO unit can successfully remove this, or if there is another technique - @dw1305 might know (sorry to keep tagging you in all these threads Darrel, but you are the fount of all knowledge on these points).

Thanks again mate.

TBH i am not sure either, but I would think that an RO would remove sodium hydroxide, though I DI only which I want is another matter. I have never needed to test PH of my RO water as it has always been for marine so high ph was needed.

Your right about Darrel

 

[dw1305]

Hi all,

I’m not sure if an RO unit can successfully remove this....

It should be able to, but it may not have much effect on pH however. I don't particularly like pH as a measurement, it isn't that it isn't fundamentally important, it is, but it is also quite a difficult measurement and often requires some interpretation. I tend to use conductivity as a proxy for pH, theoretically it doesn't matter whether the ions are acids or bases, but for us in the UK high conductivity nearly always means hard water.

The problem is that you would only need an incredibly low level of Na+ and OH- ions to raise the pH, potentially to ~pH10. You would have a very small amount of weak acid buffering (from dissolved atmospheric CO2), but that would be <"easily "overwhelmed"> by any residue of Na+ and OH- ions (these are strong bases).

Because pH is both a ratio and a log^10 value it is less and less meaningful as you approach pure H2O. In DI water the only ions are OH- and H+ (H3O+)), from the <"self-ionization of water"> and the electrical conductivity is 0 microS. As we add salts (what we call water isn't really H2O it is a dilute solution with H2O as a solvent) the conductivity rises in a <"linear relationship between ions and conductivity">.

cheers Darrel

 

[Nick potts]

Great info as always Darrel, even if I don't really understand it lol.

Would a DI only filter help with PH? I know stability is key with ph, but i would like mine lower as raising it is pretty simple, lower, on the other hand, is less easy, especially as most thing that lower PH also lover KH/GH which is already on the low side for me anyway.

 

[dw1305]

Hi all,

I know stability is key with ph

If you have really hard, carbonate buffered, water like the ocean or Lake Tanganyika, then the <"pH will be stable"> and always ~pH8. If you have black-water, with a large amount of weak acids from the tannic and humic compounds, the pH will be ~pH5. If you want to lower the pH of the very hard water you need to change the water chemistry by adding a lot of acid. When the pH starts to change it will change very rapidly, (the vertical drop in the graphs above). I don't think any-one is arguing with the fact that large changes in water chemistry, which cause large changes in pH, are very bad for fish.

But away from these highly buffered situations <"pH values are never stable">. That is one of the problems with pH. I look at it another way, if you are an aquascaper, and add CO2, you drop the pH by one unit before the photoperiod starts and it raises again by one pH unit (possibly very rapidly) when you turn the CO2 off. You have only altered the water chemistry by changing the equilibrium point of the CO2 ~ dKH ~ pH relationship.

I'm not a CO2 user, but that tells me that <"it isn't the change in pH that is important"> it is the change in water chemistry.

cheers Darrel

 

[Nick potts]

Thanks Darrel.

So, what would you suggest to use to reduce PH without affecting the KH/GH to much? If I wanted a PH of 6 for instance from my current of 8.4 and KH of 2.

I am looking at getting some bee shrimp and would like to get the params correct.

 

[dw1305]

Hi all,

So, what would you suggest to use to reduce PH without affecting the KH/GH to much? If I wanted a PH of 6 for instance from my current of 8.4 and KH of 2

Only really with pH buffers, or by adding CO2. You could use a compound like magnesium sulphate (MgSO4.7H2O) and/or calcium chloride (CaCl2.6H2O) which would only raise dGH, but you would still have a high pH because of the 2 dKH. If you add an acid it will reduce dKH and then the pH will fall, but you won't have any dKH left (the carbon (C), that was incorporated in bicarbonate (HCO3-), will be out-gassed as CO2).

I am looking at getting some bee shrimp and would like to get the params correct.

Just aim for a conductivity value of about 150 microS and feed them a good diet? I'll cc. in @Iain Sutherland, he looks to have got Bee shrimp keeping <"pretty much nailed down">.

cheers Darrel

 

[Nick potts]

Hi all,

Only really with pH buffers, or by adding CO2. You could use a compound like magnesium sulphate (MgSO4.7H2O) and/or calcium chloride (CaCl2.6H2O) which would only raise dGH, but you would still have a high pH because of the 2 dKH. If you add an acid it will reduce dKH and then the pH will fall, but you won't have any dKH left (the carbon (C), that was incorporated in bicarbonate (HCO3-), will be out-gassed as CO2).

Just aim for a conductivity value of about 150 microS and feed them a good diet? I'll cc. in @Iain Sutherland, he looks to have got Bee shrimp keeping <"pretty much nailed down">.

cheers Darrel

Thanks again Darrel.