A good rule ofthumb is to replace your sediment filter and carbon block after six months. Amore precise way to maximize the usable life of these two filters is to use apressure gauge to identify when pressure reaching the membrane starts todecline. This is your indication one or both of the filters is beginning toclog.
Also be cognizant of the chlorinecapacity of the carbon block. A good 0.5 micron carbon block for example willremove 99% of the chlorine from 20,000 gallons of tap water presented at 1 gpm.Some original equipment suppliers commonly provide carbon cartridges rated at2,000 to 6,000 gallons. Remember that all the water you process, both wastewater and purified water, go through the carbon block.
Regarding your RO membrane and DIresin, use your total dissolved solids (TDS) meter to measure, record, andtrack the TDS (expressed in parts per million [ppm]) in three places: 1) tapwater, 2) after the RO but before the DI, and 3) after the DI.
The TDS in your tap water will likelyrange from about 50 ppm to upwards of 1000 ppm. Common readings are 100 to 400ppm. So for sake of discussion, let's say your tap water reads 400 ppm. Thatmeans that for every million parts of water, you have 400 parts of dissolvedsolids. How do we go about getting that TDS reading down to somewhere nearzero?
If you do some experimenting withyour TDS meter, you'll note that your sediment filter and carbon block (collectivelycalled “prefilters†because the treat the water before it reaches the membrane)do very little to remove dissolved solids. So with your tap water at 400 ppm,you can measure the water at the “in†port on your RO housing and you'll see itis still approximately 400 ppm.
The RO membrane is really theworkhorse of the system. It removes most of the TDS, some membranes to agreater extent than others. For instance, 100 gpd Filmtec membranes have arejection rate of 90% (i.e., they reject 90% of the dissolved solids in the feedwater). So the purified water coming from your 100 gpd membrane would be about40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce purifiedwater (a.k.a. “permeateâ€) more slowly, but have a higher rejection rate (96 to 98%).The lifespan of a RO membrane is dependent upon how much water you run throughit, and how dirty the water is. Membranes can function well for a year, twoyears, or more. To test the membrane, measure the TDS in the water coming in tothe membrane, and in the purified water (permeate) produced by the membrane.Compare that to the membrane’s advertised rejection rate, and to the samereading you recorded when the membrane was new. Membranes also commonly producepurified water more slowly as their function declines.
After the RO membrane, water willflow to your DI housing. DI resin in good condition will reduce the TDS in theRO water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1ppm, you know that your resin needs to be replaced. Sometimes people complainthat their DI resin didn't last very long. Often the culprit is amalfunctioning RO membrane sending the DI resin “dirty†water. This willexhaust the resin quicker than would otherwise have been the case. Sometimesthe problem is poor quality resin – remember that all resins are not createdequal.
Additionally, don’t forget tosanitize the entire system at least once per year, and wash and lube yourhousing o-rings with food-grade silicone grease every filter change.
Russ
