aqua-safe ro+di system
- Thread starter jospears
- Start date
AZDesertRat
In Memoriam
OK the real story.
You change a membrane when you see one of two things happening.
1. An increase in the rejection rate. Most good membranes are going to be in the 96-98% rejection rate or efficiency range when new. If you use high quality prefilters and carbon blocks a membrane should last 3+ years easily in any water conditions. Its not unheard of for membranes to go 7+ years provided you replace the filters as recommended and disinfect the housings while you have it apart.
Depending on your tap water quality to begin with, you might be able to wait until it drops to 90% rejection or even less but usually they should be changed at 92-94% or so. As an example, if your tap water TDS is 200, at 98% rejection you will be seeing a RO only TDS of 4. Once it begins to lose efficiency it will climb and the result is shortened DI resin life. It becomes a trade off, keep the membrane ad change DI more often or change the membrane and extend the life of the DI. Replacements add up pretty quickly so it is usually money in the bank to replace it.
In my case with a tap water TDS of over 800 I cannot afford to keep a membrane in service when it drops below 96% and try to keep them over 98%.
2. A drop in gallons per day produced. This will happen when you do not keep up with filter replacements and disinfection or you use inferior cheap replacements. One of the membranes worst enemies is colloidal material build up. Cheap filters do not trap the smaller stuff and it ends up on the surface of the membrane. Keeping your waste ratio at 4:1 helps and a flush valve may help if it is used EVERY time you shut the unit off but once it has been allowed to reach the membrane it hardens and no amount of flushing will ever remove it. If you have never seen a plugged membrane it can look like salt crystals or extremely small sand particles that flake off once it is dry.
The other thing I mentioned is disinfecting the unit. You really need to do this at least annually if not every 6 months. Slime bacteria can plug a membrane and cause health problems in both you and your tank inhabitants.
I mentioned high quality prefilters and carbons earlier. Lots of vendors, not those on RC for the most part, use cheap granular activated carbon instead of solid carbon blocks. GAC is right next to worthless as it can be totally exhausted in as little as 300 total gallons, that's 60 gallons on RO/DI and 240 gallons of waste! Think about it, all water goes through the prefilter and carbon even the waste. That is one very big reason I try to steer people away from e-bay quality units, there is a difference.
Same goes for prefilters, some are about as effective as a screen door. Buy the best prefilter and carbon you can afford. They are there to protect the most expensive part of the unit, the RO membrane which in turn protects the DI resin.
Prefilters and carbons need to be changed either at 6 month intervals which is by far the easiest method, when you experience a pressure drop at the membrane or whn you see chlorine break thru at the membrane. The last two methods require work on your part since you must monitor incoming tap water pressure and pressuer after the prefilter and carbon block or use a low range chlorine test kit.
You can see why the 6 month interval is the easiest method. Write the date down and change them 6 months later disinfecting the housings at the same time.
You change a membrane when you see one of two things happening.
1. An increase in the rejection rate. Most good membranes are going to be in the 96-98% rejection rate or efficiency range when new. If you use high quality prefilters and carbon blocks a membrane should last 3+ years easily in any water conditions. Its not unheard of for membranes to go 7+ years provided you replace the filters as recommended and disinfect the housings while you have it apart.
Depending on your tap water quality to begin with, you might be able to wait until it drops to 90% rejection or even less but usually they should be changed at 92-94% or so. As an example, if your tap water TDS is 200, at 98% rejection you will be seeing a RO only TDS of 4. Once it begins to lose efficiency it will climb and the result is shortened DI resin life. It becomes a trade off, keep the membrane ad change DI more often or change the membrane and extend the life of the DI. Replacements add up pretty quickly so it is usually money in the bank to replace it.
In my case with a tap water TDS of over 800 I cannot afford to keep a membrane in service when it drops below 96% and try to keep them over 98%.
2. A drop in gallons per day produced. This will happen when you do not keep up with filter replacements and disinfection or you use inferior cheap replacements. One of the membranes worst enemies is colloidal material build up. Cheap filters do not trap the smaller stuff and it ends up on the surface of the membrane. Keeping your waste ratio at 4:1 helps and a flush valve may help if it is used EVERY time you shut the unit off but once it has been allowed to reach the membrane it hardens and no amount of flushing will ever remove it. If you have never seen a plugged membrane it can look like salt crystals or extremely small sand particles that flake off once it is dry.
The other thing I mentioned is disinfecting the unit. You really need to do this at least annually if not every 6 months. Slime bacteria can plug a membrane and cause health problems in both you and your tank inhabitants.
I mentioned high quality prefilters and carbons earlier. Lots of vendors, not those on RC for the most part, use cheap granular activated carbon instead of solid carbon blocks. GAC is right next to worthless as it can be totally exhausted in as little as 300 total gallons, that's 60 gallons on RO/DI and 240 gallons of waste! Think about it, all water goes through the prefilter and carbon even the waste. That is one very big reason I try to steer people away from e-bay quality units, there is a difference.
Same goes for prefilters, some are about as effective as a screen door. Buy the best prefilter and carbon you can afford. They are there to protect the most expensive part of the unit, the RO membrane which in turn protects the DI resin.
Prefilters and carbons need to be changed either at 6 month intervals which is by far the easiest method, when you experience a pressure drop at the membrane or whn you see chlorine break thru at the membrane. The last two methods require work on your part since you must monitor incoming tap water pressure and pressuer after the prefilter and carbon block or use a low range chlorine test kit.
You can see why the 6 month interval is the easiest method. Write the date down and change them 6 months later disinfecting the housings at the same time.
