Those filters on your RO/DI unit may look only a little dirty, or clean even, but when you take them out and compare them with a clean filter, the differences could be astounding. Over a long period of use, those pre-filters in your RO unit get very dirty. This process is slow enough that we don’t really see it until it’s too late, but when you realize that the TDS is starting to creep up and pressure through the unit has decreased, all of the sudden those filters look exactly how they are. Replacing the filters every six months to a year (depending on how much water you run through it) is crucial, no matter how they look. This keeps the system running at an ideal pressure level and it keeps the water entering into the membrane clean, which prolongs the effective life of the membrane.