RO/DI Not Getting all NH3

R

rrkss

Biology is Fun
Dec 2, 2005
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Hi,
My RO unit lowers the TDS of my tap water from 254 ppm to 1-2 ppm. Then the water goes through the DI to strip the rest. My tapwater has 0.75 ppm of ammonia from the chloramines. Once it goes through the RO membrane, the leftover NH3 is about 0.25 ppm. After the Deionizer I am left with approximately 0.15 ppm of ammonia. These tests are being done with a freshwater test kit. Shouldn't the ammonia be 0 after going through my RO/DI system?
 
How old is the membrane? Pulling out that much, it's probably going to need frequent replacement. But, RO.DI gets you to about 99% pure, and if you're using standard hobbyist test kits, they tend to have a variability within those parameters, as well.
 
Yeah, the membrane should last quite some time. Because it excludes, rather than filters, it should not be too badly affected by high TDS.

It's weird that the membrane is not rejecting more NH3. I'll have to check the stats on mine, but I would expect a 80-90% exclusion.
 
Do you let the Ro unit run about 2 gallons through it before you use it each time? Usually that helps keep the readings down.
 
When I first start the unit up I get a TDS reading of around 18-25 ppm from the purified water. This soon drops to 1 ppm as the membrane gets flushed. Once it reaches that level I start extracting the purified water for drinking (I bypass the DI unit for drinking water) and aquarium use (I don't want to use any stagnant water especially for drinking).
 
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Resin has a slight color change at the bottom but I would say 98% of it is still black. I think I found out the problem. Ammonia is a gas and at the pH of my tapwater, lots of it will be in the NH3 form. Gasses pass right through the RO membrane and the DI unit. Both of these can only trap the molecules in the ionic form hence why I still get NH3 in my readings after running it through the unit. I guess I could buy special carbon designed for chloramines to take out the ammonia like the AquaFX chloramine blaster. Any experience with this product?
 
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I'm certainly not a chemist but ammonia (NH4) in your water supply is not in gaseous form. Rather, in water NH4 is a dissolved ion - a postively charged one or "cation".

The larger the amount of dissolved ions, the higher the conductivity of the water. I believe this is how TDS meters work.

The charged nature of various ions is also how and why DI ("deionization") works: the resins literally strip out the charged ions from the water. As a result, I'd check closer to see if your DI resin is exhausted because that's the part of your filtration system (not the carbon or the RO of the sediment filters) that is most suited to removing NH4.
 
Test Kit

I notice that you're using a freshwater testkit, could this be testing for total ammonia both ionized and unionized???

See these websites: http://www.novalek.com/kpd62.htm
http://www.novalek.com/kpd66.htm

If it is testing for total ammonia you might get some left over at the end, but if you use the chart in the second link, you will be able to get the total of un-ionized ammonia (the bad stuff). I had a test kit that was like this at the start and when I took it into the lfs it would always show up as 0, couldn't understand it until I read up on the web, needless to say I've since ditched the testkit in favour of a salifert.

I might be wrong, but worth a try.
 
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