RO System Pre-Filter Sequence

Thanks, AZ! So if I load the 3 stages before the DI as you mention, I should expect longer filter life? I ask b/c I seem to always have 0 TDS, and don't have to change my prefilters out more than once every 9 months.

 

Prefilters and carbons have nothing to do with TDS, they protect the membrane which is what deals with most of the TDS.
To monitor prefilter and carbon condition the best two tools are an inline pressure gauge (or two which I will explain further) and a low range chlorine test kit.
The purpose of the pressure gauge is to monitor for pressure drop across the filter array. If your incoming water pressuer is a constant, which usually isn't the case then a single gauge would be fine since your static pressure minus the flowing pressure would be your head loss or pressure drop acrosss the filters, more loss means fouling or plugging of the prefilter and or carbon block(s). Since pressure fluctuates according to demands in the distribution system, seasonal adjustments, your sprinklers kicking on etc., I prefer two gaiuges, one on the incoming line before the prefilter and the second one as most have it either on the membrane housing or between the carbon block and the RO membrane.
This way you can see at a glance what the difference is between the two readings and know without having to shut things off what the incoming pressure is.
The other tool is a low range chlorine test kit. If your carbon is exhausted or the pores are plugging you will get a chlorine reading at or after the RO membrane which should not be. It does not take much chlorine breakthru to damage a RO membrane permanently.

Since many of us do not have the gauges or chlorine test kits it is easiest to stick with the 6 month rule on prefilter and carbon block replacements. Remember its not just the treated water that passes through the prefilter and carbon, the waste at 4:1 waste ratio also passes through them too. So say you made 1000 gallons of RO/DI in a 6 month period, you actually passed somewhere over 5000 gallons through the filters if your waste ratio is set correctly. Many factory flow restrictors are poorly designed and sized incorrectly so it could be much more than that too. If you are going into a pressure tank, which I do not advise for an RO/DI, or even a closed floa valve or switch, the waste increases to as high as 20:1 or higher before the ASOV finally shuts the system down.

I recommend sticking with the 6 months unless you are a geek like myself and monitor everything under the sun.