Poor rejection rate on RO system

Boxofpurplerocks

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Hey guys, I was hoping you could help me out with some issues I'm having with my RO system. Basically, I'm getting poor rejection and cant figure out why. TDS coming in is ~313, out is ~35.

For some background, I have an olympia water systems 5 stage 50gpd unit, that I've been using to make my RO. I made some modifications when I originally got it such as removing the pressure tank, removing the post RO carbon filter, and adding a spectrapure dual DI filter.

From day one with this unit (about 2 years ago), I had gotten poor rejection, pretty similar to what I posted above. I always blamed this on the unit coming with a bad membrane, and figured when it was time to buy a new membrane my rejection rate would go way up.

Fast forward to now, and I just bought and received a DOW Filmtec 50gpd membrane. Excited, I hooked it up, let it run for a few hours, and boom... still the same crappy rejection rate.

A few more background details on my system...I dont have a pressure guage, but I'm fairly confident water is above 60psi, out of the last few places I've lived this place is by far the highest. I have an adapter that hooks up right to the faucet, so I turn on both hot and cold water to feed the unit. RO membrane seems to be seated perfectly fine in the housing. Flow restrictor is around 300mlm. Due to the high water pressure, my unit can actually produce ~61 gallons/day.

Any suggestions on how to up this crappy Rejection rate??? My DI usage is ridiculous with 30ppm TDS going into it.
 

Deezill

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How much pressure you got going into your RODI? You might need a pressure booster
mine was low until i got the booster last week. My rejection rate is much better now 0 tds out


P.S
I just saw your rodi does not have a pressure gauge.
 

KStatefan

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Knowing what your pressure would be helpful.

What ratio of waste to good water is it running?
 
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Boxofpurplerocks

Boxofpurplerocks

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Unfortunately ill have to wait until I buy a guage to get the actual pressure, however I believe the pressure is definitely 60psi+, my 50gpd unit produces closer to 60gpd, and pressure is definitely the highest of all places I've lived. I'll check the clean/waste water ratio when I get home.
 

adrianr514

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First off only feed cold water, hot water or in this case “warmer” since you’re running both hot and cold, leads to worse rejection.

Secondly don’t just assume your working pressure. The reason behind this is that the pressure coming out of your tap is not the same pressure going into your membrane. Your pre filters will lower your pressure and even more so as they become dirty.

Third what is your current rejection rate? You may need a lower rejection. You can achieve this by getting a larger flow restrictor or use a ball valve instead of the flow restrictor to fine tune your rejection.
 
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Jay Z

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Like said above. Never use hot water for rodi. It’s filled with sediment and leeching from the hot water tank.

I have a whole house pressure pump. Keeps my psi at 80psi at the faucets closed, 62psi when wide open. 55 on my rodi unit. Added a rodi booster pump. Now I’m at 85psi on the rodi when running. Made a world of difference.
 

BZOFIQ

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Agree with other posters, what made you think running hot into the filter would be a good thing?

Also agree with Adrian above - stick a pressure gauge right before membrane and see your actual pressure going into the membrane housing. Pre-filters will lower your pressure significantly especially after few weeks of use.
 
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Boxofpurplerocks

Boxofpurplerocks

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Agree with other posters, what made you think running hot into the filter would be a good thing?

Also agree with Adrian above - stick a pressure gauge right before membrane and see your actual pressure going into the membrane housing. Pre-filters will lower your pressure significantly especially after few weeks of use.
Will do on the pressure guage, I'll grab one to see what the pressure is after the prefilters, hopefully this is my issue (low pressure entering RO membrane).

Also to clarify, I do not run hot water into my membrane, I run it with the cold and hot water taps both wide open, resulting in an influent water temp of ~80 degrees. Is this still too warm?
 
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Boxofpurplerocks

Boxofpurplerocks

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First off only feed cold water, hot water or in this case “warmer” since you’re running both hot and cold, leads to worse rejection.

Secondly don’t just assume your working pressure. The reason behind this is that the pressure coming out of your tap is not the same pressure going into your membrane. Your pre filters will lower your pressure and even more so as they become dirty.

Third what is your current rejection rate? You may need a lower rejection. You can achieve this by getting a larger flow restrictor or use a ball valve instead of the flow restrictor to fine tune your rejection.
Not sure on the current rejection in terms of clean to waste, I'll be testing that later today. What would you consider an optimal ratio? 4:1?
 

madweazl

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So you arent running a carbon block at all now? This removes quite a bit before the water hits the RO membrane.
 

KStatefan

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Will do on the pressure guage, I'll grab one to see what the pressure is after the prefilters, hopefully this is my issue (low pressure entering RO membrane).

Also to clarify, I do not run hot water into my membrane, I run it with the cold and hot water taps both wide open, resulting in an influent water temp of ~80 degrees. Is this still too warm?
For a Dow Filmtech TW30-1812-50HR
Permeate flow and salt rejection based on the following test conditions: 500 ppm NaHCO3, 77°F (25°C), pH 8.0, 15% recovery and the specified applied pressure
Maximum Operating Temperaturea113°F (45°C
 
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adrianr514

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Not sure on the current rejection in terms of clean to waste, I'll be testing that later today. What would you consider an optimal ratio? 4:1?
So rejection ratios are permeate:brine. You generally want more brine than permeate as that means your membrane is letting more dirty water out and only keeping cleaner water. The rejection rate that’s best for you would be different from everyone else’s so there’s no “optimal” rate. I saw that I misspoke and said you needed a lower rejection rate when I meant a higher rejection rate. So something more like 1:4 instead of 4:1.

Warm water even if it’s only 80 degrees still affects your membrane. Membranes are tested at 76 degrees for nsf certification, generally the colder the better so completely close off the hot water. I realize this will affect your production amount but it will make the prefilters and membrane more efficient.
 
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Boxofpurplerocks

Boxofpurplerocks

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So you arent running a carbon block at all now? This removes quite a bit before the water hits the RO membrane.
There is a sediment block and two carbon blocks before the ro (changed a month or so back). Essentially I removed stage 5.
 
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Boxofpurplerocks

Boxofpurplerocks

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For those curious, when I got home today I checked and the current flow restrictor was only 250mlm, leading to a 1:2 ratio of clean:reject. As luck would have it I had a 425mlm restrictor lying around, that when hooked up gave me about a 1:3.75 ratio. Between that, and running cold water only, I'm down to 12 TDS, which is like ~97% rejection! Much better than I had it before.
 

adrianr514

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For those curious, when I got home today I checked and the current flow restrictor was only 250mlm, leading to a 1:2 ratio of clean:reject. As luck would have it I had a 425mlm restrictor lying around, that when hooked up gave me about a 1:3.75 ratio. Between that, and running cold water only, I'm down to 12 TDS, which is like ~97% rejection! Much better than I had it before.
Glad you listened to the advice. Get yourself a 550mlm restrictor if you can find a cheap one. That should bring you to 1:4 which could get you in the single digits with proper pressure.
 
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Boxofpurplerocks

Boxofpurplerocks

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Glad you listened to the advice. Get yourself a 550mlm restrictor if you can find a cheap one. That should bring you to 1:4 which could get you in the single digits with proper pressure.
I'm glad just glad you guys could help out . Excited not to change my DI every month. I found a 550mlm restrictor on amazon for $7 so definitely going to pull the trigger on that.
 
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