RO unit problem

[percula99]

I have a Kent Marine RO24TFC (25GPD) unit for producing RO water for my 180 reef tank. I have used it for several years with no problems. I wanted to change the membrane and filters and upgrade the unit to 50 GPD. I just bought a new MTFC50 membrane, a 1 micron sediment filter, a matrix carbon filter and a 50 GPD flow restrictor from an online service. When I installed the new components my inline TDS meter read over 400 but it quickly started to decrease as I flushed the unit. I got it down to a reading of 4 TDS and left it running normally overnight hoping to get it down to 0 TDS so I could start using my RO water. This morning the reading has gone from 4 up to 8. Instead of going down, the TDS is rising. I double checked everything and it all seems to be installed properly. What could be the problem?

Thanks in advance for any assistance you can provide.

 

[somethingphishy]

not really sure but.......i believe you should discard the first approx.20gallons of water that is made when you change the membrane.....that will allow it to fully saturate......might be the cause for a high tds reading.....

 

[AZDesertRat]

Nowhere do you mention having a DI filter. If you have RO only you will never see a TDS of 0. RO is 96 to 98% efficient and the DI acts as a final polishing filter.
What is your tap water TDS? With and RO only TDS of 4-8 I suspect it is in the 200 to 300 range and your RO is doing the best it can.

 

[percula99]

somethingphishy...The membrane packaging said to let it run for about two hours before using the product water. I have passed that much through it now and the reading has gone from 8 to 11, and now it is down to 9. I agree water has to pass though it before using the product water. Hopefully now that the TDS has gone from 11 down to 9 I am going in the right direction and in time the TDS will go down to 0.

AZDesertRat...I don't have a DI on this unit. It is a three stage RO unit bought from my LFS a few years back. I can buy a DI stage to add onto it, and I am thinking of doing that. In the past my TDS meter has always read 0. It is only recently that I started getting flukey readings of 80 when I tested. If I flushed the unit I would get it down to 0 eventually. I do flush the unit regularly. This happened twice which is why I decided to change all the filters and membrane. But I should be able to get a reading of 0 again.

Thanks guys. Any other replies are appreciated.

 

[AZDesertRat]

I would check the calibration on your TDS meter and clean it well. No RO unit will ever produce 0 TDS water on its best day. RO is a 96 to 98% device at best. TDS meters have a +/- 2% of full scale accuracy when they are brand new and kept clean. If you have not been cleaning your TDS meter with DI or distilled water after each and every use I suspect it is further off than you might imagine. They need to be kept clean or they can be significantly off. You might borrow a meter and compare readings to see what I am talking about.

 

[steelerguy]

<a href=showthread.php?s=&postid=10361932#post10361932 target=_blank>Originally posted</a> by AZDesertRat
I would check the calibration on your TDS meter and clean it well. No RO unit will ever produce 0 TDS water on its best day.

Is this really accurate? I have what seems to be very low TDS tap water at about 55-60. I have tested just the RO water from my unit and it usually gives a reading of 0 although I have seen 1 before. The RODI water is always 0. It just seems to me, that with the filter blocks and then a high rejection rate membrane, 0 TDS is possible with just RO. (Of course it is not truly 0, but 0 within the error of standard TDS meters used in the hobby)

Who knows, maybe my TDS meter is off, it is the only one I have had!

 

[AZDesertRat]

No its not possible. Prefilters and carbon blocks have very little to no effect on TDS. They are there to trap TSS or Total Suspended Solids (particles or particulates) which are much much larger than TDS or Total Dissolved Solids and to adsorb chlorine and volatiles. The membrane will filter 96 to 98% of the incoming TDS at best and some things like nitrates, silicates and phosphates will only be in the 85 to 90% removal range.
TDS meters have a +/- 2% of full scale accuracy and are usually good up to 1000 TDS in low range so can be up to 20 TDS off out of the box. If they are not kept squeaky clean and stored with the cap on at all times they are probably not very accurate. ALWAYS clean them with distilled or DI water after each use and always put them away clean and capped. Never put a dirty meter away.

 

[percula99]

AZDesertRat...I agree with you 100% that I should clean and calibrate my TDS meter. Mine is an inline meter however, not hand held. I also plan on looking around locally for someone who has a hand held meter so I can double check the readings.

An interesting development!! I just replaced my new 50 GPD membrane and 50 GPD flow restrictor with my old 25 GPD membrane and flow restrictor and immediately went from a TDS reading of 8 back down to 0. Even if my TDS meter is dirty and uncalibrated these readings cannot be ignored. This is telling me there is a problem with the new membrane I bought.

 

[AZDesertRat]

Inline meters are less accurate than handheld meters due to a lack of temperature compensation. They claim they are but if you look at the fat part of the probe you will see a small rectangular hole in the plastic. Inside that hole exposed to air not water is the temperature probe! Unless your water and air temperatures are exactly the same it will never be accurate. If you look at the temperature correction scales it can be quite significant at even 10 degrees difference. My air in my garage is 104 degrees and my water can be 58 degrees in the summer so you can see there is quite an error. I have tpw of the inlines but do not rely on them except as a guide. I always check things with a COM-100 handheld which is the only hobbyist meter on the market accurate at low levels like post RO and RO/DI due to the way it can be calibrated.

 

[percula99]

I wanted to install a DI cartridge today so I went to my LFS and bought the disposable one they had in stock. I installed it and my TDS went immediately to 0. I am very happy about that but it still leaves me wondering why my 3-4 year old membrane read 0 and my brand new one read 8. The manufacturer contacted me today and agrees that after all I have reported to them that my new membrane must be defective and they have agreed to send me a new one. Once it arrives I will test it without the DI to see if the one I have is truly defective.

 

[AZDesertRat]

Thats great. Its nice to see vendors back their products.