Ask A Question: TDS Creep
Question: Hi. I have well water. The TDS in the tap water is 700. I have a 3 stage RO system. Number 1 is the sediment filter. Number 2 is the TFC membrane. Number 3 is the carbon post filter. I have added a permeate pump. The water coming out of the RO is 200 ppm. Is this too high? Is there anything else that I can add to the system to reduce the ppm even more? Thanks in advance, Paul
Answer: I have a permeate pump RO in my house and you are seeing what is called "TDS Creep." Permeate pump RO systems have trouble with TDS Creep because they respond to the smallest pressure change in the distribution system. When a slight amount of water is used the permeate pump system will start processing water. If the amount of water being used is small; such as a cup of water; the system doesn't run very long to replenish the water used. Short system run times like this causes TDS creep because the system doesn't run long enough to compress the membrane and create quality water. The membrane needs full pressure to compress and give the best quality water. Full pressure will not be present when small amounts of water are used. It is best to use the tank contents daily to ensure the TDS creep will not be a problem. There's nothing worse than a large storage tank of water that is only used to fill a 32 oz. glass 1-2 a week. After a few weeks the water quality in the tank will be very poor. In regards to a permeate pump RO, it is better to size the RO tank to make sure it will be used within 1-3 days. A standard system with an automatic shutoff valve (ASO) is used water can be dispensed until the distribution system pressure falls below 65% of the incoming line pressure. The system will then start to run and will run for a long period to replenish the tank and the distribution system pressure. Since the run time is longer TDS creep is not likely. A permeate pump RO is wonderful for increased pressure, especially if you have a fridge with an icemaker and/or water dispenser. In my case, that is why I have it. However, I simply drain my tank once a week and all is well. In your case, drain the RO a couple of times and then test the TDS. It should be below 100 ppm!
14 Comments
Joe, flushing the permeate side of the membrane before every start is what will make the most difference in reducing TDS creep. That may actually give you better results than a permeate pump alone. The only downside is that auto flush timers also periodically flush during the production cycle. Theyre designed for the brine outlet of the membrane, but if you dont mind the extra wasted permeate a few minutes every hour, theyll do the trick.
Youll need to have 24 VAC power to run the auto flush timer. If using a booster pump, you can use the same transformer to feed power to the auto flush timer using a wiring harness. Im not aware of any autoflush timers on the market for RO systems that are set up to do only an initial start up flush.
Using a solenoid valve controlled by a bypass timer will do the same thing but without needless permeate flushing during the production cycle. This approach takes some DIY effort because youll need a bypass timer, solenoid valve, wiring harness, and some electrical wire terminals to connect to the bypass timer. It can be done for less than $50 in parts.
What are your thoughts on auto-flush valves or manually flushing the membrane?
Mark, how do you empty the tank weekly? Is it a turn on the sink valve for a certain amount of time or take the tank to the sink and empty it? I ordered a larger tank to have the capacity with the permeate pump because I have the fridge with Ive maker. I am guessing that I should empty the tank every once in a while. Thanks.
Will the use of a powered booster pump prevent TDS creep?
Mark, I have the same issue.
What do you think of setting up 2 tanks (I have an extra one that has not been used yet) and a few valves that would allow me to toggle those two tanks between being refilled and providing water to the tap, fridge, etc? Once a tank is empty I would flip the valve which would provide water from a full tank and completely refill the empty one.
Seems like less work than draining the tank each week.
Thanks!
Great article. My RO system is challenged with TDS creep in my system, a 5 stage RO system w/ the Aquatec ERP 500 permeate pump. My city water pressure is only 45 psi. What about adding a in-line booster pump to the RO water IN feed to increase overall pressure on membrane. I would think I would then not need the permeate pump either. Thoughts?
What about a solenoid powered by a timer and set to flush the membrane for 30 seconds every hour. Wouldnt that keep the water fresh in the membrane?
I guess you can try it I would not go to that trouble, but it might work.
Yes!
Absolutely, not a problem.
They are more style than substance, but they are OK.
30 or 60 seconds. Sure. That works.
Just turn on the RO faucet and let it run until it drips out.
Will check valve on permeate out line prevent TSD creep