Higher nitrate after RO

[MNFish]

My rodi unit appears to be having issues.
I replaced all the pre filters today and tested nitrates.

The tap comes out at 2.4 than after ro membrane it measures 7.2 I am glad saltwateraquarium.com had hanna reagents in stock because I tested multiple times.

Could some sort of nitrate be leaching from a pre filter or the membrane itself?

I was not sure if this was a chemistry or equipment question but I cannot figure out how my nitrates would more than double even when tds is being reduced still?

I had been noticing tds had been creeping up so I ordered a new membrane, but I am really wondering where the added nitrates could be coming from.

Anyone experience this before?

 

[MNFish]

If it matters my current membrane is a DOW 100gpd, so nothing crazy, I have 3 stages of pre-filter which I change every 6 months and noticed increased nitrates prior to changing filters today just in case.

 

[homer1475]

And were using what to measure this?

A saltwater nitrate kit will not measure nitrates in freshwater with any accuracy.

 

[Octane13]

I have seen something similar. Getting somewhere between ~5 and 18 ppm with fresh mixed salt water. Using BRS 7 stage FWIW.

 

[homer1475]

I have seen something similar. Getting somewhere between ~5 and 18 ppm with fresh mixed salt water. Using BRS 7 stage FWIW.

Your salt mix, even though it says nitrate free, still has a small a mount of it in it. Everyone does.

 

[Octane13]

Your salt mix, even though it says nitrate free, still has a small a mount of it in it. Everyone does.

Agreed. I have done some exploratory tests the OP should try. Mix fresh SW using store purchased gallon ~2ppm using HW and less using TM and Hanna LR checker. Swapping all filters, membranes and resin. ~5 ppm using both salts. I have also had a source water test done by a lab with 0 nitrates present. So unsure where they come from. Can only assume the test kit is picking up something else as nitrate.

 

[MNFish]

Salifert nitrate which does do fresh and salt water, and Hanna HR nitrate which is matching the results from the salifert test, so I am assuming it works for fresh water as well. I might have some fresh water tests still sitting around I could try just in case......

 

[MNFish]

I will mix some salt into the RO water and ensure I recieve ~ the same reading from the fresh to salt water with both the salifert and Hanna tester just to double check.

 

[Randy Holmes-Farley]

Assuming you use drinkable water to start, there's no likelihood 2 ppm of nitrate is exiting a functioning RO/DI (0 ppm TDS).

Maybe the DI is depleted, or maybe it is test error.

What is the TDS of the effluent?

What type of RO membrane?

 

[MNFish]

Assuming you use drinkable water to start, there's no likelihood 2 ppm of nitrate is exiting a functioning RO/DI (0 ppm TDS).

Maybe the DI is depleted, or maybe it is test error.

What is the TDS of the effluent?

What type of RO membrane?

It is a DOW 100GPD membrane, the TDS at the tap is 700 and when the nitrates were 0 I was getting about 20 TDS after Membrane. Now the TDS is at about 160 so I have ordered a new membrane. But what I really cannot figure out is how is the nitrate increasing when sampling right after RO membrane. I would think if anything since the membrane is still filtering some the Nitrate would at worst be the same if not a little lower but the higher is what has me stumped.
I started with a salifert test and figured something was wrong, so I bought a Hanna checker which is showing as far as I can tell the same as a salifert. I did mix salt into the RO water and the test was ~ the same reading as just the straight RO water.

I guess what I am seeing does not seem possible, but hopefully when I swap out the membrane it will be back to zero.

 

[Randy Holmes-Farley]

Right, the value after the RO membrane must be lower than before it, but maybe only by a factor of 2 if it is a cellulose triacetate membrane. Thin film composite membranes reportedly do far better in removing nitrate. I'm not sure which type yours is.

Membrane Rejection Levels

Nominal Rejection Characteristics of Thin Film Composite Reverse Osmosis Membranes

www.watertreatmentguide.com

But the value after a depleting DI can sometimes be higher than before it, when measuring ions that are easily displaced by incoming competing ions. For example, the ammonia after an RO/DI when th4 DI is depleting an be much higher than before the RO/DI.

 

[MNFish]

Installed new ro membrane today and nitrates are back to 0.

Thank you all for the help.