RO Crazy Question

SumpinFishy · Sep 25, 2024

Ok, so I might get some backlash for this but I have to ask about RO waste water.

I am cycling my new 100gallon w/ 30 gallon sump. My nitrates are spiked maxing out Hannah at over 75ppm. 50%water change and still topped it out. I did theb1ml sample and 9ml RO it's 11.6 so 110.6 Nitrates. So need to do same major water changes.
This is when my crazy kicked in.
What if I rig the RO to pump aquarium through it?
Yep it would probably pull the salt, and then waste is like 4 to 1 so 100 gallons would turn into 25...
SO HERE IS THE CRAZY QUESTION.S.

1. HOW BAD is the waste?
2. Has anyone had a deep test done on Waste?
3. If you were going to test, (in a lab) what would you want to check?

My RO is 0 nitrite and nitrate. My waste is 1.4ppm nitrate and nitrite is 6ppb.
Ph, ammonia, Cal and Mag are all the same as well. These are all better than tap when I test so the RO is filtering
So part of me is thinking I am going to have to do either a full 130 water change or several 50-75% water changes. I get about 70 gallons a day of RO. So in my head I can use the RO good and waste at this point. It's better than the Tanksat this time.. do a couple 50% with both, then maybe go just to pure RO.
Is there concern that filters use or produce some of the metals could still be in there? Lead, iron...
I am thinking about getting the test from department or water. The full range for metals and inorganic would be $200.
It has me curious, is the waste not as bad as I thought it was. It's still going through filters and that's taking a lot out.

And fyi... Ro is set up in basement, into Rubbermaid which i forgot to turn off last night and this am overflow/flood... so if I need to produce 200+ gallons that's 4-5 days of running this thing and remembering to shut it down at bed. So 17 hours a day...

Randy Holmes-Farley · Sep 25, 2024

If I understand correctly, it will not be useful in any way.

You are talking about pushing salt water up against the RO membrane to try to make something?

The TDS of the salt water is something on the order of 50,000 ppm TDS.

After the RO it will no longer match seawater (because some ions are rejected more than others) and will still have pretty high TDS but won't be at all close to full strength seawater, so you will have to add a bunch of salt to get to seawater strength, but it will be too salty for evaporation replacement. It also won't really be "purified" in any meaningful way. .

SumpinFishy · Sep 25, 2024

Randy Holmes-Farley said:
If I understand correctly, it will not be useful in any way.

You are talking about pushing salt water up against the RO membrane to try to make something?

The TDS of the salt water is something on the order of 50,000 ppm TDS.

After the RO it will no longer match seawater (because some ions are rejected more than others) and will still have pretty high TDS but won't be at all close to full strength seawater, so you will have to add a bunch of salt to get to seawater strength, but it will be too salty for evaporation replacement. It also won't really be "purified" in any meaningful way. .

Thanks. Yeah I'm nit going to try and run my tank water through it. But if I need to burn through 200+ gallons of water. I'm tempted to use the RO waste in an initial change better than tap and better than current water conditions. Then use the RO full for subsequent changes. But again I'm tempted to have the RO waste fully tested. The waste has gone through a level of filtration seems to be not terrible.

Randy Holmes-Farley · Sep 25, 2024

sham_mi said:
Thanks. Yeah I'm nit going to try and run my tank water through it. But if I need to burn through 200+ gallons of water. I'm tempted to use the RO waste in an initial change better than tap and better than current water conditions. Then use the RO full for subsequent changes. But again I'm tempted to have the RO waste fully tested. The waste has gone through a level of filtration seems to be not terrible.

RO waste is not better than tap. In most ways, it is worse. Most contaminants in it are about 25% higher than in tap water.

KStatefan · Sep 25, 2024

What are you trying to reduce/improve by using waste water?

SumpinFishy · Sep 25, 2024

KStatefan said:
What are you trying to reduce/improve by using waste water?

Time and water. RO 100 gallons is going to use 400 gallons. I could use the waste and get a 50% change done in a few hours.

Randy Holmes-Farley · Sep 25, 2024

sham_mi said:
Time and water. RO 100 gallons is going to use 400 gallons. I could use the waste and get a 50% change done in a few hours.

Tap water with dechlorinator is likely better than ro waste, unless there is chloramine in the source water, in which cause neither method is desirable.

WvAquatics · Sep 25, 2024

IF you want to speed up production you will have yo piggy back your ro. So for me i run a 75gpd membrane first. Waste water goes into a 150gpd membrane then into di resin. This being said im at 50tds going into ro system so low tds to start.

SumpinFishy · Sep 25, 2024

Randy Holmes-Farley said:
RO waste is not better than tap. In most ways, it is worse. Most contaminants in it are about 25% higher than in tap water.

That is what I thought but then I ran all the tests. And the Nitrates were 1.44
I would have assumed the waste would be a concentration of what is pulled out of the good stuff. I was really kind of shocked think the waste should be horrible. I almost want to take tap, RO good and RO waste and send it in and get all 3 lab just to see what's really there. More of a scientific curiosity at this point.

Tavero · Sep 25, 2024

sham_mi said:
Thanks. Yeah I'm nit going to try and run my tank water through it. But if I need to burn through 200+ gallons of water. I'm tempted to use the RO waste in an initial change better than tap and better than current water conditions. Then use the RO full for subsequent changes. But again I'm tempted to have the RO waste fully tested. The waste has gone through a level of filtration seems to be not terrible.

If you are talking about tying to put saltwater through your RO membrane, I implore you to reconsider. Don't. Just don't.
In the best case you will just clog the RO membrane. In the worst case you will completely destroy it due to the high Cl- concentration.
As Randy has mentioned, saltwater has a ion concentration of 50.000 ppm. You can calculate how much pressure you need to push this concentration through a semipermeable membrane and it's around 200 bar (I was able to calculate it after University, not anymore though). That's around 192 bar more than your RODI system is meant to resist. Not a single piece of your equipment is able to survive these pressures.
But the the highest probably is that you will just destroy the membrane due to excessive salt concentration.

If you're talking about using RO wastewater again, that's possible. I am doing this sometimes to reduce wastewater but the quality will drop and resin consumption will probably increase. Depending on your setup it may be too much work though.
My setup doesn't take water from the tap but from a canister that's constantly filled with tap water. Sometimes I put the RODI wastewater outlet into that canister so I can use part of the waste again.

Randy Holmes-Farley · Sep 25, 2024

sham_mi said:
That is what I thought but then I ran all the tests. And the Nitrates were 1.44
I would have assumed the waste would be a concentration of what is pulled out of the good stuff. I was really kind of shocked think the waste should be horrible. I almost want to take tap, RO good and RO waste and send it in and get all 3 lab just to see what's really there. More of a scientific curiosity at this point.

I do not know if what you measured was nitrate or a much smaller amount of nitrite, but neither is a big concern. Concerns in tap water and RO waste include copper, ammonia, chlorine/chloramine (in tap), and phosphate. Alkalinity and calcium can also be significant.

Drinking water is limited to 10 ppm nitrate in most places, and is often very low. Mine is about 0.02 ppm, but silicate is quite high (several ppm)..