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Now reasons why you want good filters.
The prefilter and carbon block are there to protect the RO membrane and allow it to do its job effectively. The prefilter traps sediment and solids so they don't get to and damage the membrane. The smaller the micron rating the better for a couple of reasons. One is whatever passes through the prefilter gets trapped by the carbon block if it is sized properly but it also shortens the life of the carbon block as it is now pulling double duty. It was not designed to be a sediment filter.
Large micron ratings do a poor job of trapping solids so they pass to the carbon as mentioned and some then get passed to the membrane fouling its surface.
Carbon blocks as mentioned previously are there to remove chlorine but also to remove organic chemicals too. The better the filter the better job they do. A 0.5 micron block has massive amounts of surface area in its micro pores. If properly protected by a high quality prefilter they can go the full 20,000 gallons they are rated at with ease. Even if you change them every 6 months as recommended you still get much better filtration than with GAC or a cheap block.
As mentioned before some membranes are not even really Reverse Osmosis filters at all, they are micro or nano filters which are not nearly as effective. Thye are also not approved for use in drinking water systems in the US according to the National Sanitation Foundation (NSF) which certifies anything that comes into contact with water. Be very careful when evaluating filters and membranes, there are several different ANSI/NSF certifications and even though it may say NSF on th label look closely ate the number next to it. Case in point is the Dow Filmtec 100 GPD membrane, it is NSF certified for "POOL AND SPA USE ONLY" according to the NSF. That means it is less efficient than a RO membrane or 90% instead of 98% rejection. 90% rejection is a much larger micron size and cannot guarantee virus and bacteria removal like RO can. Not to metion like I said before "For every 2% you increase the RO efficiency you double the life of the DI resin."
A more efficient membrane makes DI resin last longer and work better. RO by itself is not real efficient at phosphates, silicates and nitrates, they are some of the things that are tougher for it to remove. The combination of RO and a good efficient DI cartridge can tackle things like this though if they are properly designed and maintained. Never ever rely on color changing DI resin. Its a poor indicator at best. Chances are by the time it turns colors its too late and you are passing phosphates and may not know it.
A good tool to tell you the condition of your equipment is a handheld TDS meter. Inline meters are the craze right now but they will never stand up to a good handheld. First off they are not temperature compensated and if you water and air temperature are not EXACTLY the same you will be off, sometimes significantly. Inlines are measuring air temperature not water temperature. They also are not portable and cannot bne used as such, they require flow past the probe to register and cannot just be immersed in a glass of water.
Another good tool is the pressure gauge. Pressure drop makes a huge difference in RO production and quality. Install at least one gauge aftre the carbon block and before the membrane and watch the readings often. I prefer two gauges myself, I also have one on the incoming tap water line so I can see the difference between the two at a glance.
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