When to replace RO membranes?

[sterlingh20]

I have a 4 year old 3 membrane ginsan RO system and in the past six months the TDS has gone from 19 to 35, is it getting time to replace the membranes? Do they usually only last 4 years? Can I just replace 1 of the 3 membranes and see if it lowers the TDS?

 

[lag]

Since the unit has been slowly going up,replace all 3. I replace when the TDS gets above 20. I have seen 25 spot.

I would not be to concerned,about them lasting only 4 years, but before you change them make sure your carbon filter is working. You will want to run a chlorine test ,compare the incoming city water to water after the carbon filter. You may not always have chlorine coming in to the building ,so I test incoming water daily,and when I see chlorine ,I then test it after the carbon filter.

Bottom line; Make sure your carbon filter is OK ,change all 3 membranes if you can.

 

[sterlingh20]

I checked the chlorine and there is 1.5ppm in city water and none in the spot free tank, how long does a carbon filter last? Do you just refill it?

 

[lag]

I checked the chlorine and there is 1.5ppm in city water and none in the spot free tank, how long does a carbon filter last? Do you just refill it?

You can recharge(refill) it,when the time comes. Sounds like it is ok. Agian how long it lasts will depend on the amount of usage. Checking the carbon filter water is one of the things I have found out over the years that people do not do for some reason. I have a T installed after the carbon filter to test it daily.

 

[lag]

I might add we recharge our units,every 5 years or so,but that is our preference,not any kind of standard.

 

[tobaccofarmer]

I have to change membranes every 8 months, and change centaur the same. Centaur btw is a replacement for carbon in locations where the incoming water contains chloramine, which regular carbon cannot treat. I would recommend checking to see if your water provider uses chloramine and if they do replace carbon with centaur. Also I have to use 2 centaur tanks in series, not required just helps. I will replace my membranes if tds is 5 or higher!

 

[mac]

The chloramine issue is a new one to us, and like most anything new, there is a lot of misunderstanding about it. I found this out by talking to my charcoal supplier, whom I've always regarded as an expert. Chloramine contains chlorine and ammonia. A lot of suppliers are pushing the Centaur charcoal, which costs 3 to 4 times more than regular granulated charcoal. While Centaur does remove the ammonia, so does the regular stuff. You have to increase the contact time with both charcoals to do this. You increase the contact time by slowing the flow through the filter, or adding another one in series. You are right that most operators have never checked chlorine levels. To do it accurately, you need something more than a pool test kit. You can find digital testers online in the $200 range. Your water supply determines how long the RO membranes last. Here in FL I see some places where 6 months is good, and others where 6 years is the norm. Of all the aspects of the business, water treatment is the least understood.

 

[MEP001]

Can I just replace 1 of the 3 membranes and see if it lowers the TDS?

You could disconnect each of the product lines from the three membranes and test the TDS of the membrane outouts individually. If only one is bad and two are still very good it might not hurt, but if one is bad and two are borderline you should just change them all. Obviously if all three are bad, you'll have to change them all.

You may not always have chlorine coming in to the building ,so I test incoming water daily,and when I see chlorine ,I then test it after the carbon filter.

I always test for chlorine in the spot-free reject, since it will be more concentrated and more easily detected. It's also easier since many systems don't have a tap for getting a sample, but they all have a line to the drain.

I checked the chlorine and there is 1.5ppm in city water and none in the spot free tank, how long does a carbon filter last? Do you just refill it?

Did you check it after the charcoal filter? There will never be chlorine in the spot free tank because the membranes filter it out.

How long the charcoal lasts depends on water flow and the amount of chlorine in the water. There is no standard length of time.

The tank is disconnected, emptied and refilled.

I might add we recharge our units,every 5 years or so,but that is our preference,not any kind of standard.

My standard of practice is to replace the charcoal whenever the membrane(s) is(are) changed. Charcoal is cheap.

 

[sterlingh20]

I was reading through the maintenance part of the manual and it said that increasing TDS can be caused by a dirty tank and that every 6 months or so that you should clean the tank out, I drained it and cleaned it out and it went down to a TDS of 16. The water coming out of the feed line was 14, i didn't check that the first time around, I just stuck the meter in the tank. So membranes can last over 4 years, you just need to clean the tank out. I makes sense.

 

[Buckeye Hydro]

I was reading through the maintenance part of the manual and it said that increasing TDS can be caused by a dirty tank and that every 6 months or so that you should clean the tank out, I drained it and cleaned it out and it went down to a TDS of 16. The water coming out of the feed line was 14, i didn't check that the first time around, I just stuck the meter in the tank.

Yes - test the TDS of the RO water BEFORE it reaches any storage tank. Install a sampling port, or easier yet, install an inline TDS meter.

When should you replace an RO membrane? Use a TDS meter to measure, record, and track the TDS in at least two places: 1) RO feedwater, and 2) after the RO but before any tank.

The TDS in your RO system feedwater will likely range from about 50 ppm to upwards of 1000 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 that won't spot?

If you do some experimenting with your TDS meter, you'll see that your prefilter equipment (e.g., softener, carbon filter, sediment filter) 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 membrane 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. Check the specs on any membrane for its stated "rejection rate." You'll see a number like 96% or 98% or 99% (i.e., the membrane should reject 96+% of the dissolved solids in the feed water). So the purified water coming from your membrane would be about 16 ppm (a 96% reduction). Or 400 * 0.04 = 16. If the TDS of the feedwater is higher, then the TDS of the RO water will also be higher.

Membranes can function well for up to a year, two years, or more. The lifespan is dramatically affected by the quality of the pretreatment. Commonly the pretreatment consists of a backwashing carbon tank to treat chlorine or chloramine, and a softener to remove hardness. If you allow either (chlorine or hardness) to reach your membranes, their lifespan will be reduced. We worked with an industrial customer recently who was changing membranes out every two weeks. He had very hard water, no softener, and had the concentrate flow set way too low.

Russ