Daisy Chain RODI units

[prickles]

Hello,

I was hoping to reduce waste water, what would happen if I plug my 2nd RODI unit into the waste water of the first one? If they are both the same size, do I need to change the flow restrictor on either to compensate for them being one after the other?

thanks!

[Randy Holmes-Farley]

If you put back pressure on the waste line, the RO may not function well. I've not heard of anyone attempting this, but there are low waste RO/DI systems available. You can have a zero waste system if you eliminate the RO membrane and use just the particulate filters and the DI resins.

[bnumair]

https://www.youtube.com/watch?v=67e59Fr62c8

[disc1]

Originally Posted by Randy Holmes-Farley

You can have a zero waste system if you eliminate the RO membrane and use just the particulate filters and the DI resins.

But you'll burn through that DI resin like mad if you don't have super clean tap water to begin with.

[Randy Holmes-Farley]

That's certainly true.

[Randy Holmes-Farley]

FWIW, I think the video glosses over several important issues.

Doubling the membranes will not double the flow rate through them because the back pressure on the first reduces the flow through it. They didn't claim this, but folks also shouldn't expect it.

Reducing the pressure drop on the membrane also makes it less efficient, so you will eat up your DI resin faster due to more ions getting through the membrane per unit of water produced.

Finally, the water getting to the second membrane has a substantially higher TDS than the first, so it will be letting through higher TDS post RO water because of that. If the TDS is 33% higher in the waste, then the TDS of the effluent from that second membrane will be on the order of 33% higher than that same membrane would produce on raw tap, even ignoring the pressure drop issue.

How significant each of these is may depend on your tap pressure and the membranes selected, but it is not insignificant, IMO.

But if those things are less important to you than the wasted water, it clearly can function to some extent.

[phillrodrigo]

Would the waste even produce enough pressure for the second one to work. They usually need 60 psi I think you can get away with 40 but I doubt theres even that coming through the waste side.

[Randy Holmes-Farley]

So I realize I didn't describe the pressure issue properly and I'm not sure where the restrictor is in the setup in the video. The restrictor is normally keeping pressure on the RO membrane instead of letting all the water go to waste.

If you keep the restrictor in its normal place after the first membrane, then the second membrane with a second restrictor sees much less pressure. In that scenario, the second one will produce less of a lower quality water due to lower pressure and the higher TDS on its inlet side. The first RO membrane may need an adjustment to its restrictor or else the combination of its restrictor in its normal setting and the back pressure from the second RO membrane (and the second restrictor) may allow less flow through the "waste" end of the first membrane and the first membrane might actually see higher than normal pressure and lower than normal flow to waste. That combination produces less waste (as would just dialing the restrictor in tighter on a normal single membrane system), but keeps higher TDS water against the first RO membrane, reducing the quality fromt eh first membrane as well as the second.

If the only restrictor in the system is placed after the second membrane, and it is set properly to account for two membranes upstream of it, then the pressure effect on the first membrane is minimal. But the second one is seeing higher than normal TDS water and is necessarily seeing a lower pressure than is the first membrane since a significant portion of the flow (and hence the potential for pressure) has left the system through the first membrane. Consequently, in this scenario, if the membranes are identical, the second one will produce somewhat less water and of a higher TDS than the first.