When to replace RO prefilters

sabbath

Premium Member
How do you know when to replace the RO/DI "prefilters". I have the dual digital TDS meter, reading RO output and DI output.

Tap=163 TDS (hand held, Water flushing in a glass)
RO = 1-2 TDS
DI = 0 TDS

Should I get a second and put one on Sediment output and one on the Carbon? Would that work at all?

Thanks in advance.
 

Billybeau1

Premium Member
A pressure drop is a good indication of spent filters.

I just change them every 6 months or so. The sediment and carbon filters are pretty cheap. :)
 

sabbath

Premium Member
I don't have a presser gauge on it.

It was just a thought, I will just change them every 6 months or so.

One more filter change question. What TDS reading am I looking for to see the RO is spent? 5% tap water TDS?

Thanks.
 

HowardW

New member
<<< One more filter change question. What TDS reading am I looking for to see the RO is spent? 5% tap water TDS? >>>


Assuming your tap water TDS remains steady, the RO membrane should be changed when you notice the post membrane TDS begin to climb from what you normally get.
 

Randy Holmes-Farley

Reef Chemist
Premium Member
I normally get on the order of 5% through my RO membrane even with a perfect membrane, so I might back that off a bit for replacement considerations. My first one lasted more than a decade, so it doesn't require frequent monitoring.

It was just a thought, I will just change them every 6 months or so.

FWIW, mine get clogged before that. If you notice the effluent is collecting a lot more slowly, replace at least the sediment filter.
 

Billybeau1

Premium Member
<a href=showthread.php?s=&postid=12209761#post12209761 target=_blank>Originally posted</a> by Randy Holmes-Farley
FWIW, mine get clogged before that. If you notice the effluent is collecting a lot more slowly, replace at least the sediment filter.

Your tap water must be a little worse than mine. :lol:

My tap runs around 125 ppm. :)
 

Buckeye Hydro

.Registered Member
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 â€"œ remember that all resins are not created equal!

Russ
 
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