TDS on waste output?

[FishyMike]

Hey guys just trying to get a better understanding of How the RO units work. As of right now out of my Tap (going thru a sediment filter and 2 carbon filters) i have a respectable 78ppm TDS at right around 7 ph. I purchased an RO unit a couple months ago and haven't installed it yet. I'm guessing my good output will be around 0 TDS so what should i expect for my WASTE water to look like? could i use the "waste " for some grow out tanks and other stuff. Really didn't want to waste water if i didn't have to. 

[Mmiller2001]

On 11/29/2022 at 1:49 PM, Mikeo said:

Hey guys just trying to get a better understanding of How the RO units work. As of right now out of my Tap (going thru a sediment filter and 2 carbon filters) i have a respectable 78ppm TDS at right around 7 ph. I purchased an RO unit a couple months ago and haven't installed it yet. I'm guessing my good output will be around 0 TDS so what should i expect for my WASTE water to look like? could i use the "waste " for some grow out tanks and other stuff. Really didn't want to waste water if i didn't have to. 

It will be significantly higher. Just use it on plants outside.

[tolstoy21]

The output will be close to zero, but might not be there totally unless you have DI resin chambers to polish up the last bit of TDS.

I would expect your waste water TDS to be pretty high (I'll have to check mine to see that the waste water TDS ratio is it compared to the input water).

Can you use it for something?  I think that all depends on what is getting rejected by the membrane.  Whatever you have in your water that the membrane rejects will be super concentrated in the water.  I would not put anything living in it, like fish.

Use it for plants? Again, I'd say it depends on what's in your tap water to start and how high the levels of that are in the waste water.

Plants don't love waste water. They love nitrates, potassium, phosphates, magnesium, etc. So if your waste water has that, then they will appreciate it. If you're just stripping out a lot of Kh, then using it on plants could shift the Ph of their soil in a way that they like or maybe hate.

Some plants have a hard time picking up and processing nutrients is the Ph is shifted in a way not amenable to them. I keep roses as another spring/summer hobby. When their soil Ph is out of whack, they begin to show other side-effects like nitrogen and phosphorous deficiencies that can't be fixed by feeding, etc.

Aquarium water change water is great for plants because it's full of organic wastes they can convert to food, and unused plant fertilizers. RO waste water is not the same as aquarium waste water, so your mileage may vary. 

 

Edited November 29, 2022 by tolstoy21

[Remi de Groot]

As Tolstoy is saying the TDS probably won't be exactly 0. Mine comes out at 6/7.

Just tested the outcome of the waste water for you (never did it before). And it is 280 where my tap water is 240.
So actually not that much higher, which suprises me a bit.

My RO unit is outside next to my pond. So i've always been using the waste water to refill my pond.
Kind of with the idea that the rain water is softening the water, so adding extra hard water would balance this out a little bit.
Never really tested the parameters of the waste water though. Wasn't to worried that a few gallons of waste water would make a big difference in a 46.000 gallon pond.

[FishyMike]

On 11/30/2022 at 6:04 AM, Remi de Groot said:

As Tolstoy is saying the TDS probably won't be exactly 0. Mine comes out at 6/7.

Just tested the outcome of the waste water for you (never did it before). And it is 280 where my tap water is 240.
So actually not that much higher, which suprises me a bit.

My RO unit is outside next to my pond. So i've always been using the waste water to refill my pond.
Kind of with the idea that the rain water is softening the water, so adding extra hard water would balance this out a little bit.
Never really tested the parameters of the waste water though. Wasn't to worried that a few gallons of waste water would make a big difference in a 46.000 gallon pond.

 Thank you for your data!! it is greatly appreciated! I happen to come accross a video on yourtube from Jack Watttley discus and Gabe said he would use the Waste water for his grow out tanks and he would use the RO for Breeding Tanks. i might have toi hook it up and see what kinda results I get.  

[tolstoy21]

On 11/30/2022 at 6:04 AM, Remi de Groot said:

And it is 280 where my tap water is 240.
So actually not that much higher, which suprises me a bit.

Same here, tested mine yesterday. The difference is approx. 300 (water in) and 370 (waste water out).  I had assumed it would be much higher.

@Mikeo If you're going to use it for grow outs, again I guess it just depends on how high the waste water TDS is and what's in it.  I'd test what the resulting GH/KH measures at and determine if that's amenable to the species you're growing out.

I guess think of it this way, a portion of the water you're pumping into the membrane housing goes into your RO bucket, so that portion is removed from the amount of water discharged and therefore the waste water will be a small ratio per TDS vs the water going in.  So if your water is hard, it will be harder. If your Ph is high, it will be higher.

Looking at my water for reference, the waste water is approx. 20% less by volume, but carries all the TDS of the original water. Since you TDS is low to begin with, maybe you'll be Ok.  The TDS of the waste water can vary depending on the rejection rate of your membrane the pressure of the over all system (lower pressure, worse rejection rate -- 60  to 80 PSI is the sweet spot). 

My water is hard, full of nitrates (40ppm) and has fun stuff like arsenic in it, so I'm not going to be subjecting any fish to an even more concentrated form of that.

Also, an RO membrane alone typically doesn't result in zero TDS.  I get about a 93% rejection rate from mine, so the 'RO' water is about 20 TDS. I use DI resin to remove that residual amount for a final TDS of zero. This is a typical setup for a lot of reefers and shrimp keepers, otherwise known as RO/DI.  

Anyway, just do some experiments etc. and you'll be fine and figure it out.  If you can use it on grow outs, that would be great. Mine goes down the drain, but I am on a private well so I feel less guilty than if i was on a public reservoir . 

Edited November 30, 2022 by tolstoy21