Changing Prefilters

[rrasco]

I was doing some reading on prefilters (ran across a few of your posts across the interwebz Russ) and had a few questions.

I purchased my 3-stage kit from BFS in Dec 2011. I am still using the original sediment, carbon, and RO membrane that came with the kit. My DI has been changed a hand full of times. I have a dual meter reading after RO and after DI. After RO, my readings get down to 8-10. After DI, they are 0.

I am trying to determine when I need to replace the prefilters. I was under the impression the sediment filter should be changed when it gets dark, mine has yet to change colors. The carbon block is rated at 20,000 gallons, which is way more than I have made. I estimate at most I make about 2000 gallons a year, so being generous on that estimate I have made maybe 5000 gallons over the last 2 years. I have not noticed any drop in rejection from my membrane and the pressure is still kicking around 70PSI.

Additionally, I was thinking of adding a 4th stage for the Chloroguard Chloramine block. First off, does this cartridge fit in a 10" housing? Secondly, if added, where should this be added to the system? After sediment or after the regular carbon block? If I add the chloramine stage I'll probably just replace the sediment as they are cheap, but just curious what I should realistically be doing.

Thanks in advance.

[BuckeyeHydro]

Yes - chloragaurd fits our standard 10" housing. In your system, add it after the standard carbon block. Re when to change the filters, a good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the usable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. A good 0.5 micron carbon block for example will remove much of the chlorine from 20,000 gallons of tap water presented at 1 gpm. Some original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons. Remember that all the water you process, both waste water and purified water, go through the carbon block.

Regarding your RO membrane and DI resin, use your total dissolved solids (TDS) meter to measure, record, and track the TDS (expressed in parts per million [ppm]) in three places: 1) tap water, 2) after the RO but before the DI, and 3) after the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1,000 ppm. Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block (collectively called “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 it is still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in the feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce purified water (a.k.a. “permeate”) 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 through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the TDS in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce purified water more slowly as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the TDS in the RO water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker than would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal.

Additionally, don’t forget to sanitize the entire system at least once per year, and wash and lube your housing o-rings with food-grade silicone grease every filter change.

[rrasco]

Thanks for the reply Russ. This was one of the posts I had read on another forum earlier. So, if after 2 years I have had no decrease in pressure before the membrane (I have a pressure gauge), does that mean the sediment and carbon blocks don't require replacing? It's roughly $20 to do so, so not a big hit at all, but it wouldn't make much sense to replace them if there is no need to.

[BuckeyeHydro]

I would replace the carbon block at least. Remember that everything past the outside surface of the carbon block is dechlorinated, and microbes love dechlorinated water. So I would sanitize your entire system as well (before you put the new block in).

Russ

[rrasco]

Do you have a recommended procedure for sanitizing the system? I would be terrified to put any type of cleaner in it.

[SABOB]

He poste a Sanatizing procedure awhile ago, I'm sure he'll repost it


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[BuckeyeHydro]

http://i224.photobucket.com/albums/dd293/BuckeyeFS/Instructions/Sanitize.jpg (http://s224.photobucket.com/user/BuckeyeFS/media/Instructions/Sanitize.jpg.html)

Russ

[rrasco]

You're the man Russ. Thanks again.