ro unit?

[sharkboy]

Back in May I replaced the membrane and all of the filters including the di...the tds meter would read anywhere from zero ppm to 5ppm...it is now reading in the 100s...is this normal? If so, which filter would be the most likely to be replaced first?

[JimD]

Its probably the DI resin, check the TDS of the output of the RO membrane, it should be in the 12-20ppm range. If its there then the DI is spent, I always replace all the filters at the same time, excluding the RO membrane.

[sharkboy]

generally speaking, how long should the filters last? I'm sure it has something to do with the amount of water output, but any guesses as to how many gallons?

[caferacermike]

I'd say it depends upon the membranes and prefilters you ran. The Dow Filmtech is considered the best of the RO membranes and is said to last years with the proper care.

[sharkboy]

I changed out all filters except the membrane and the tds only went down to about 90ppm...i guess i need a new membrane unless the tds meter is out of wack...any local stores carry these membranes?

[caferacermike]

If it quickly went from 0 to 100 I'd agree with it being the membrane. You can check with your TDS meter, as was mentioned before, where the RO water enters the DI resin. If that reads over 100 you know your membrane is bad and your DI is working the best it can to lower it. Since your resins are letting that much go by, they are probably spent as well. My guess is that your membrane got wasted and your resin was absorbing the high TDS, as designed, and you noticed the jump when they were used up. Is your TDS a dual inline? I keep mine on the RO out and DI out lines. This way I can tell if my membrane is creeping up (004) or if my DI is spent (000). If my DI creeps up without my membrane going up I can safely say I need to replace my resins.

[BuckeyeHydro]

This may help (from our FAQ):

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable 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. The Matrikx+1 (“Chlorine Guzzler”) for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the tds (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (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 filter (collectively called prefilters) 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 its 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 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 less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant 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 total dissolved solids (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 less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, 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 then would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ

[sharkboy]

Thanks Russ, I guess presently my prefilters are doing all the work and are probably being exhausted at a greater than normal rate since my ro membrane needs replacing...it has been ordered but i guess i have to use the water being produced even though it is at about 65ppm..right now i only have one inline tds meter but will get another to be able to pinpoint which filters need replacing more accurately

[BuckeyeHydro]

If your membrane is bad, or your resin is bad - that won't accelerate how fast the prefilters are expended.