Acceptable Phosphate Reading for RODI Water?

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Gary Ellis

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Isn't it a good idea to do that to make sure your filters are good? What kind of testing should you do with RODI water? None?
 

92Miata

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Isn't it a good idea to do that to make sure your filters are good? What kind of testing should you do with RODI water? None?
The problem is that you're probably chasing ghosts.

ULR Phosphate checker:
Resolution: 0.01 ppm
Accuracy: ±.02 ppm ±5% of reading

Its just as likely that your phosphate is 0 as it is .02.
 

esther

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Do you have a PO4 problem in your tank? Or are you just testing for the sake of testing? There's nothing wrong with testing for peace of mind, but if you're trying to figure out why you have a PO4 issue in your tank, it's a good idea to test to be able to exclude things from the list.
 

Doctorgori

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Isn't it a good idea to do that to make sure your filters are good? What kind of testing should you do with RODI water? None?
TBH testing RO never occurred to me, I’ve always just looked at TDS: whether the output is 0 or thereabouts. If not I look at the inline TDS meter:both sides of the membrane.
 
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Gary Ellis

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I have no problem with my tank. Just testing. Just curious what my rodi water is.
 

chipmunkofdoom2

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Isn't it a good idea to do that to make sure your filters are good? What kind of testing should you do with RODI water? None?
Phosphate in the parts per billion does not say anything about the state of your RO filters. It might tell you something about your DI resin since that's where the phosphate is remediated, but even then it's not an accurate indicator of filter health. The only final test that's really worth anything is the TDS. It's difficult or impossible to test anything else in deionized water.

You have a basically two options on how to replace filters. The first is you can replace them on a schedule. Replace the prefilters every 6 months, replace the RO membrane every 2 years or so, and replace the DI resin when the TDS goes above zero. This will basically ensure you always have clean water, but it can be pretty wasteful.

The other option is to test. Every filter in your RO unit does a very specific task, and you can tell when to replace them by testing.

The sediment filter's main job is to protect your carbon blocks from plugging before they lose the ability to process chlorine. To know when your sediment filter needs replacing, put a pressure gauge before the prefilters and put one after. When the pressure starts dropping more than 5 - 10 PSI, replace the sediment filter. If the pressure doesn't go back up after replacing the sediment filter, the carbon blocks may be plugged and may need to be replaced as well.

The carbon blocks are designed to remove chlorine and other VOCs from the water. This is important because chlorine is really damaging to your membrane. To know when carbon blocks need replacing, test the brine (the waste water) of your RO unit with a chlorine test strip. If there is any chlorine, the blocks need to be replaced.

The RO membrane does most of the filtering in your RO unit. It will also probably last the longest, especially if you keep up with the prefilter changes (my last one lived for over 10 years). To know when your RO membrane needs to be replaced, test the TDS before the membrane, but after the prefilters). Then, test your TDS after the membrane, but before the DI resin. Divide the "before membrane" by the "after membrane" number. This gives you a rough percentage of the solids that the membrane is letting through. I would replace the membrane when the number is between 0.05 and 0.10, but you may want to replace it sooner if you have high TDS tap water.

The DI resin is the easiest to know when to replace. Simply test your TDS after the DI resin. If it's greater than 0, replace the resin.

I very much prefer testing to blindly replacing the filters every 6 months. As an example, my prefilters were last replaced in may of 2017. I still have great pressure and no chlorine breakthrough. My water's pretty clean, so this won't be the case for everyone. But it's a good reason to consider replacing the filters only when the tests say to do so.

Here's a source for the above info from one of our water filter vendors.
 

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