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View Full Version : Does low PSI really increase TDS and if so why?


JasonTaylor
04/14/2010, 06:28 PM
I asked this question here (http://reefcentral.com/forums/showthread.php?p=16952597). To summarize that thread, nobody actually gave me an estimate of the likely decrease in TDS I would get if I increased my PSI from 25 PSI to 70 PSI by getting a booster pump. The only theory we came up with in the other thread that we have going so far is that low psi does not really lead to lower TDS but instead just faster junk per gallon RODI accumulating on the membrane. That problem can be solved just by flushing more often, and is the cheapest most logical option, which leads me to wonder why everyone suggests getting a booster pump.


Iamwrasseman suggested (http://reefcentral.com/forums/showpost.php?p=16949823) I ask you guys here the same question, since you are the best experts. Thanks in advance.

Thefilterguys
04/14/2010, 11:37 PM
Your low pressure is extreme and you will have increased tds and waste. Your pressure is at the point of near membrane failure to produce water. Filmtec membranes are made to operate at 50 psi or higher and all other brands require 60-65 psi. Is it possible your water connection is the problem, are you connected with a self piercing saddle valve?

Jim

JasonTaylor
04/15/2010, 07:42 AM
Is it possible your water connection is the problem, are you connected with a self piercing saddle valve?

Jim

Yes and I am not sure, but PSI does not change when using a different faucet at the same time, so I do not thing there is a plumbing aperature issue. I was unable to find a pressure regulator between the cold line in and the faucet. I will contact my utility company WSSC even so.

Your low pressure is extreme and you will have increased tds and waste.

My pre-membrane TDS is 200 and post-membrane is 30. What ball park range would TDS become if PSI is increased from 25 to 65 PSI? Would I get 20, 10, 5?

Thefilterguys
04/15/2010, 08:55 AM
I already see a performance issue in your tds readings at 85% rejection rate. Do you have any idea what size (gpd) membrane you have?

With your tap reading of 200 a Filmtec membrane 36, 50, or 75 GPD, with good pressure, should have a tds reading of about 4-6 after RO.

Jim

JasonTaylor
04/15/2010, 01:08 PM
I already see a performance issue in your tds readings at 85% rejection rate. Do you have any idea what size (gpd) membrane you have?


75 GPD.

With your tap reading of 200 a Filmtec membrane 36, 50, or 75 GPD, with good pressure, should have a tds reading of about 4-6 after RO.


Thanks Jim, this is the critical info I needed to make a decision.
So from what I gather, increasing pressure to something like 65 PSI should lower my TDS by about a factor of 30/5=6=also factor by which I am shortening my DI unit. So if my DI unit normally would last 6 months now it would only last about 1 month. So in about 6 months the investment in a cheapest possible $65 booster pump should pay for itself. Is that logic correct?

Strange thing is that I called up WSSC to ask them about the 25 PSI. A guy was kind enough to come out. He checked the pressure at a hydrant. It was at the top of a nearby hill and was 50 PSI. So something is decreasing pressure between that hydrant and the house, but there is no regulator to be seen inside the house. My guess is that all three meters used to test things are wrong. At any rate, I need to invest in a booster pump and just deal with the low pressure, which affects not only the RO unit but dish washing and showers as well.

Thefilterguys
04/16/2010, 07:57 AM
Dont buy a cheap booster pump as they are a bad investment. Cheap foreign booster pumps dont last but a year or two and I consider them poor quality. Buy the Aquatec 8800 booster it is American made and a excellent quality heavy duty pump. The few extra dollars you spend will be well worth it. We keep a stock of 18-24 Aquatec pumps on hand at all times and other companies are always out of stock with 4-6 week waits. It is possible you may have run your RO membrane at to low a pressure already and it is damaged and will not recover.

Jim

JasonTaylor
04/19/2010, 06:09 PM
Jim I just wanted to thank you for your kind advice. After reading a paper on the subject, I ended up ordering a unit booster pump and may post my new TDS readings here eventually, once I get it installed.

Jason