RO rejection

[bob115]

So about a year ago I bought a refurbished spectrapure rodi system. It has a standard 90gpd membrane and I've noticed my rejection rate is only 96%, even though the system is just over a year old and the fairly small quantities of water I produce. My pressure is only about 50psi, however, and I think this may be the source of the poor performance of the membrane.

 

Tap tds: 399

RO only tds: 16

Waste to output is about 7:1

[AZDesertRat]

If you adjust your flow restrictor back to 4:1 you will increase the pressure available to the RO membrane and improve the rejection rate. To give you an idea, temporarily crimp the end of the waste line or install a ball valve on it and restrict it back to 4:1 +/- and see how much the pressure goes up once you are not losing it down the drain.

 

Questions I have are, what are you testing with, is it calibrated, are you drawing water in a clean clear glass drinking water glass and rinsing it with RO/DI or distilled water between each TDS test and putting both the glass and meter away clean each time? What is your water temperature and do you have softened water? How often do you replace the sediment and carbon block filters and have you done the annual disinfection on the system yet? Do you make water in 5 gallon or larger batches so you don't suffer from TDS creep and do you use the system at least every 10-14 days and make sure the housings always have water in them?

[bob115]

I figured my 7:1 was pretty high. This was the first time I had actually measured it, I had thought it came as 4:1 from spectrapure. I just replaced the filters for the first time now, but I only make 5 or so gallons a week. I added a ball valve to the waste line and dialed it back to about 5:1 and my RO only readings are 16ppm still. Pressure read ever so slightly above 50, but I didn't see terribly large pressure gains.

 

Testing is done with a TDS3 pen meter. Water samples were collected in a (foodsafe) plastic cup rinsed at least 3 times with the test water. Water temperature is 20.2C (as measured by the TDS3). Water is not softened. I always make a minimum of 5 gallons. System is used around once a week.

 

I've not heard of disinfecting the system each year, so that is a negative.

 

 

I've got some extra pex stuff lying around and was thinking of putting in a flush valve, would this be worthwhile?

[jamescstein]

 

 

I've got some extra pex stuff lying around and was thinking of putting in a flush valve, would this be worthwhile?

 

Yes, especially with that low production. I always open my flush valve for the last 5-10 minutes of any run.

[AZDesertRat]

No. Flush valves have no documented proven value other than to lighten your wallet and give you a warm fuzzy feeling.

 

Adjust the waste ratio to 4:1, it would be better to do so by trimming the capillary tube flow restrictor following the instructions starting on page 8 of your owners manual. The change is rejection rate is not going to be instantaneous, it will gradually go down with usage.

When sampling use a squeaky clean clear glass drinking water glass washed by hand in extremely hot water so it does not have any dishwasher soap residue, hard water spots or antispotting agents on it to read as TDS. Start with the tap water, draw a sample, insert the meter and allow the reading to stabilize. Write it down the triple rinse the glass and meter in good RO/DI or distilled water if you suspect your RO/DI is not 0 TDS. Next draw a sample of RO only water directly from the RO membrane, if you don't have a DI bypass valve or sample point then disconnect the quick fitting between the RO and the DI at the DI end and draw the sample there. I prefer to install a tee and ball valve at this point so I can draw samples, use RO only for other purposes and to flush out TDS creep before it gets to the DI. (This is very different from a flush kit and actually has value). Stick the meter in the RO sample, let it stabilize and write it down. Again triiple rinse the glass and the meter in good RO/DI or distilled water and then draw your final RO/DI sample and insert the meter. Let the reading stabilize, write it down, triple rinse the glass and meter, place the glass somewhere out of the way upside down so it stays clean and cap the meter.

 

What are the tree readings?

[bob115]

Huh. My manual make no mention of trimming the flow restrictor to dial in the waste:product ratio, just says that its 3:1. Looking at a pdf version of the manual it says that if the ratio is above 6:1 that the restrictor needs to be replaced, not cut. Here's mine, and like I said I've never taken it out or trimmed it before, but aren't these supposed to be longer than 4" out of the box?

[bob115]

These numbers are with the ball valve at the end of the waste line bringing us into about 4:1.

 

Tap: 410

RO: 19

RO/DI: 0

 

Pressure gauge is reading 50psi on the nose or 3.4ish bar.

Temp: 17.3C

[AZDesertRat]

Starting on page 8 it walks you through how to measure the waste ratio and how to adjust it by trimming the flow restrictor. In some cases the restrictor is pre trimmed and too short for your conditions. In this case you would buy a new untrimmed restrictor and following the directions again starting on page 8 you measure the ratio then trim the length accordingly. Restrictors are $5-$6 at Buckeye or Spectrapure.

http://spectrapure.com/manuals/MPDI.pdf