RE-Filtering RODI wastewater?

[gregbenner]

Hello all, I am new here, I have relatively new to me 150 gallon salt water mixed reef tank. I have read several threads on potential uses for wastewater from the RODI process. Is there any particular reason why I wouldn’t just run it back through the system again, since it’s already been partially cleaned it seems like it would be a good use rather than pushing it down the drain?

I have a 30 gallon container with RODI, which recently I noticed it was slightly contaminated (TDI of 8). My original thought was to dispose of the water but instead I bought a $20 Amazon pump and pump the water back through my aqua life system until the TDI was back to zero. Now I’m wondering why I couldn’t just save my SOME Waste water and then use that instead of tapwater to run through the system?.

I’m new to saltwater aquariums so I apologize if this is it a stupid question

Greg

 

[The_Paradox]

You could but the electricity will cost magnitudes more than the water in most locations.

 

[aquaman67]

Waste water is highly concentrated with TDS.

Think of it like this. You have taken a 1/2 of the H2O out with the RO filter (the good water) and what is left is the other 1/2 of the water with twice the contaminants.

(It’s not really 1/2. More like 4 to 1 but you get the idea)

You technically could use it if you really had to but it’s not either the effort and extra work on your RO membrane.

At 95 percent efficiency of your RO membrane at tap water TDS of 100 your TDS coming out of your RO would be 5.

If the TDS of your waste water is 200, the TDS of your “good” water would be 10.

 

[The_Paradox]

Waste water is highly concentrated with TDS.

Think of it like this. You have taken a 1/2 of the H2O out with the RO filter (the good water) and what is left is the other 1/2 of the water with twice the contaminants.

(It’s not really 1/2. More like 4 to 1 but you get the idea)

You technically could use it if you really had to but it’s not either the effort and extra work on your RO membrane.

At 95 percent efficiency of your RO membrane at tap water TDS of 100 your TDS coming out of your RO would be 5.

If the TDS of your waste water is 200, the TDS of your “good” water would be 10.

Should come out about the same or cleaner when you factor in sediment filters and carbon blocks. Either way at less than a few dollars per thousand gallons it’s easier to just water plants or something with it.

 

[gregbenner]

Thanks for the replies. The TDS for my tapwater is actually about 400+ (Oceanside, California).

I didn’t realize that TDS for wastewater would be substantially higher, I was thinking it would be lower.

 

[gregbenner]

If I understand it, then, the RODI does not remove TDS from the water, but rather separates the TDS into the wastewater, which is then put down the drain.?

 

[The_Paradox]

If I understand it, then, the RODI does not remove TDS from the water, but rather separates the TDS into the wastewater, which is then put down the drain.?

It removes the waste which is then back flushed off the membrane.

 

[City_Boy_Jay]

Thanks for the replies. The TDS for my tapwater is actually about 400+ (Oceanside, California).

I didn’t realize that TDS for wastewater would be substantially higher, I was thinking it would be lower.

Check the tds of the waste water

 

[EeyoreIsMySpiritAnimal]

Search this site for the word "concentrate" posted by Buckeye hydro. There are several threads where buckeye explains exactly how RO membranes work... There's also one thread about water over 700 tds and what type of system to use... might have some tips for you.

*When people run double membranes, they actually DO re-filter the concentrate ("wastewater"), but iirc, this isn't recommended when tds from the tap is already high.

 

[EeyoreIsMySpiritAnimal]

Should come out about the same or cleaner when you factor in sediment filters and carbon blocks. Either way at less than a few dollars per thousand gallons it’s easier to just water plants or something with it.

Not really since the sediment and carbon shouldn't be removing anything else from water that's already been through them.

 

[EeyoreIsMySpiritAnimal]

If I understand it, then, the RODI does not remove TDS from the water, but rather separates the TDS into the wastewater, which is then put down the drain.?

Each section of your RODI removes some things from the water. You use a sediment filter to remove larger particles, and carbon to remove chlorine (it can also trap some particles), before the water gets to the reverse osmosis membrane. You don't want to clog the membrane and chlorine can damage it. Once the water has passed through the membrane, it goes through one or more filters containing deionizing resin, which removes charged particles that have made it past the other stages. Each stage removes something that helps the subsequent stage operate more effectively.

*Others, please correct any errors in my explanation

 

[gregbenner]

EMSA, Your explanation makes perfect sense, and is what I sort of thought I guess. But. If that is the case, then it doesn’t seem like TDS should be higher in the wastewater?

 

[EeyoreIsMySpiritAnimal]

EMSA, Your explanation makes perfect sense, and is what I sort of thought I guess. But. If that is the case, then it doesn’t seem like TDS should be higher in the wastewater?

Why not?? The "wastewater" is the portion of water that does not get filtered by the reverse osmosis membrane. It rinses out the stuff the membrane has removed from the rest of the water, adding that amount of solids (TDS=total dissolved solids) to what's already in the water... That's why it's more appropriately called "concentrate" rather than wastewater.

And, generally, with normal water pressure, it takes 2-3 times the unfiltered water to rinse out the membrane.
*This should not be confused with the routine use of the system's flush valve...

 

[EeyoreIsMySpiritAnimal]

This helped me understand it better

Which chlorine/chloramine RO/DI should I buy?

Well, I often use it for freshwater and do near-total water changes for QT/hospital tanks. I'm not sure what the minimum detection level of chlorine using the color wheel is but I wouldn't be confident it's below 0.05 mg/L. And I wouldn't bet my life that it's below 0.1 mg/L either. OK, I...

www.reef2reef.com

 

[EeyoreIsMySpiritAnimal]

Here's a pretty straightforward description of how the membrane works.

How does an RO membrane work? | D-D The Aquarium Solution

www.theaquariumsolution.com

 

[EeyoreIsMySpiritAnimal]

One more with a diagram

*When I first started, I seriously could not understand how this process worked until I looked into it... Hope this helps

 

[Randy Holmes-Farley]

If incoming water is 400 ppm TDs, and the waste water to processed water ratio is the typical 4:1, then the waste water will rise in TDS by about 20% to about 480 ppm (since most of the TDS of the effluent water is diverted into the waste water).

It's not impossible to use it, certainly, but as folks point out, you need to repressurize the water with a pump.

Alternatively, if waste is a big issue in your local area, then you can use an RO/DI with a smaller waste to product ratio (which itself is also not always a win win, unless water is especially precious).

 

[SpyC]

Hello all, I am new here, I have relatively new to me 150 gallon salt water mixed reef tank. I have read several threads on potential uses for wastewater from the RODI process. Is there any particular reason why I wouldn’t just run it back through the system again, since it’s already been partially cleaned it seems like it would be a good use rather than pushing it down the drain?

I have a 30 gallon container with RODI, which recently I noticed it was slightly contaminated (TDI of 8). My original thought was to dispose of the water but instead I bought a $20 Amazon pump and pump the water back through my aqua life system until the TDI was back to zero. Now I’m wondering why I couldn’t just save my SOME Waste water and then use that instead of tapwater to run through the system?.

I’m new to saltwater aquariums so I apologize if this is it a stupid question

Greg

You can get a second ro membrane and have the waste from the first go through the 2nd membrane. Both clean outputs would then go into the DI canister. You will always have waste water, but you can prodice less waste yhis way. You do need at least 65psi to make this work. A booster pump helps with pressure.

 

[Malcontent]

I remember years ago Spectrapure had some kind of system that used a pump to re-pressurize and recirculate the "waste" water back to the membrane.

 

[Randy Holmes-Farley]

You can get a second ro membrane and have the waste from the first go through the 2nd membrane. Both clean outputs would then go into the DI canister. You will always have waste water, but you can prodice less waste yhis way. You do need at least 65psi to make this work. A booster pump helps with pressure.

IMO, that is not nearly as effective as using a pump to repressurize since the method you suggest reduces the pressure across each membrane to a very large degree. It's like putting two light bulbs in series and hoping to get light out of them; each is much dimmer than one alone.