# RO filter question



## MichaelJ (16 Aug 2021)

Hello, I know my RO cartridges and membranes _theoretically_ are due for a replacement, but I am wondering what compounds passes the filter stages when the membranes gets "old" and the efficiency drops?  When brand new I usually measure a TDS around 3-4 ppm on the outlet, which is supposedly very good. I now measure around 12-15 ppm. What makes up that additional ppm?



Cheers,
Michael


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## dw1305 (16 Aug 2021)

Hi all,


MichaelJ said:


> .... When brand new I usually measure a TDS around 3-4 ppm on the outlet, which is supposedly very good. I now measure around 12-15 ppm. What makes up that additional ppm,?


Usually it is just more calcium (Ca++) and bicarbonate (HCO3-) ions, assuming they are the majority of ions in the water flowing in.

Cheers Darrel


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## MichaelJ (16 Aug 2021)

dw1305 said:


> Hi all,
> 
> Usually it is just more calcium (Ca++) and bicarbonate (HCO3-) ions, assuming they are the majority of ions in the water flowing in.
> 
> Cheers Darrel


@dw1305 thanks for the answer. I only do the RO to lower my KH (conductivity really) and KCl levels (we use KCl in our household water softener) which otherwise would probably be in the +150 ppm range. Now, since the tap water going into the RO unit is essentially "0" GH some additional HCO3 wouldn't be a problem at all. I suppose I could get more milage out of the cartridges and membranes and wait until the water out of the RO shows a bit higher ppm before switching them out. Nice.
Cheers,
Michael


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## Zeus. (16 Aug 2021)

Once the membrane goes completely the ppm will shoot up quite high, had one at work and only changed the RO membrane when the DI filter (which I fitted as a machine at work need to have a very low ppm) didn't last very long, surprizing how much mileage you can get out of them.


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## MichaelJ (16 Aug 2021)

Zeus. said:


> Once the membrane goes completely the ppm will shoot up quite high, had one at work and only changed the RO membrane when the DI filter (which I fitted as a machine at work need to have a very low ppm) didn't last very long, surprizing how much mileage you can get out of them.


@Zeus.  Thanks, that is good to know. Yes, I think I am just going to keep using the membranes until I get up around 20 ppm.  I was just wondering what particular compounds slip through first when the membrane starts to decay - I suppose the biggest molecules will be the last ones to make it through when the membrane goes completely. Of course, there is also the issue of water waste as efficiency goes down, as it presumably takes longer to produce the same amount of RO water ? - or is that just a trick by the manufacturer to make me change the membrane more often... I can't really tell.
Cheers,
Michael


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