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Discussion Starter · #1 ·
Hey, quick question:

My RODI unit (BRS 4-stage Value Unit) is reading 002 TDS for output. I'm assuming this means it's time to replace the filters.

I haven't done that before.

I assume this kit: http://www.bulkreefsupply.com/brs-4-stage-value-replacement-filter-kit.html is the one to get. Is it weird that it's a four-stage filter and there's only three replacements that come in the kit? Am I missing something?

Also, how do I know which one is the one that I ought to be changing, or does everyone just change all their filters when they start to read TDS?

Thanks folks.
 

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I change resin when the TDS varies 2-3ppm.

I change filters when the pre-membrane numbers climb from their normal 4-7, to 10, 12ppm and up.
 

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Sediment and carbon block filters have very little to absolutely noting to do with TDS, thye are there to trap suspended solids, particulates, silt and collidal materials (big floaty stuff not dissolved stuff) and chlorine. Sediment and carbon filters are usually rated for 5, 1 or maybe 0.5 microns, dissolved solids are just that dissolved not suspended and 0.0001 microns in size, thousands of times smaller than what the sediment and carbon can trap.
You use a pressure gauge to monitor headloss meaning your filters are plugging or fouling and less water is getting through, and a low range chlorine test kit to monitor for chlorine breakthru from exhausted carbon or you take the easy way out and replace them every 6 months like clockwork and disinfect the system while you have it apart.

You replace the DI resin when you first start seeing signs of anything other than 0 TDS in the finished water.

For the RO membrane which is the workhorse of the system you should be monitoring your tap water TDS and RO only TDS on a regular basis. When the rejection rate starts dropping off to where replacing the DI starts getting expensive it is time to change the membrane. Something to keep in mind is for every 2% the rejection rate drops it cuts your DI life in half so it does not take long to pay for a new more efficient RO membrane. The RO rejection rate should be around 98% and is calculated like this, tap TDS - RO only TDS / tap TDS x 100 and easy example using a tap TDS of 100 and a RO only TDS of 2 would be 100-2=98, 98/100=0.98, 0.98x100=98% rejection rate. If that RO only TDS was only 96 you would be money ahead to replace the RO membrane as it would double your DI life.

Again TDS is NOT and indicator of when to change the sediment and carbon filters as TDS passes right through since it is thousands of times smaller than they are capable of removing. My softened tap TDS is 560, my TDS after the 0.2 micron sediment filter is 560. my TDS after the 0.5 micron carbon block is 558-560 as carbon does remove the 1+/-ppm or so of residual chlorine so minimal TDS change at best. The RO only TDS is between 2 and 3 meaning my membrane is very very efficient at 99.4% rejection rate and the TDS after each DI stage is 0.

Personally I would buy better filters than BRS offers such as this kit:
http://spectrapure.com/CSPDI-System-Cartridge-Replacement-Kit-SuperDI-High-Capacity

You will get much longer filter life, longer membrane life (mine is already over 8.5 years old) and many times longer DI life.
 

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Discussion Starter · #5 ·
Thanks, that's really interesting stuff.

So basically if I want to go the simple route I change by charcoal/sediment/DI ever 6 months but my RO membrane can last for years? Am I reading that right?

I don't want to be wasteful, but at the same time it's a work tank that I'm not paying out of pocket for, and the stress/time management of testing is probably more expensive than just setting an interval and making it automatic.
 

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The simple route is use good sediment and carbon block filters and replace them every 6 months on a schedule, but at the same time paying attention to your pressure gauge in case for some reason you get a slug of silt that plugs the filters before they are in need of the 6 month replacement. Disinfect the system at least annually at filter change time.
Replace the DI when it is no longer 0 TDS. This could be a month or a year, everyone's mileage varies but after a couple swaps you will have a better idea.
 

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Discussion Starter · #8 ·
Well, I should have waited for an answer and saved my company $30 or so for a RO membrane. Maybe I'll return it.
 
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