Rodi ?

[432Reefer]

I purchased a RODI system that has four stages RO and 1 stage DI. MY question is the prssure feeding the system is only 20 PSI the membrane on my system is rated for 50 PSI. Will this cause my membrane to not function properly.

 

[Randy Holmes-Farley]

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Seems too low to me. You may need a booster pump. I discuss it a bit here:

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm

from it:

Various factors, such as temperature and pressure, impact not only the flow rate through the membrane but also the purity of the resulting water. Lower temperatures make the water more viscous and less likely to flow through the small pores, reducing the production of purified water. The effect of temperature on purity is much smaller, with purity decreasing slightly at higher temperatures. Higher line pressure across the RO membrane results in higher rates of production and quality, although a pressure that is too high can damage the membrane. Any backpressure on the effluent will degrade performance. Very high TDS (total dissolved solids) in the source water also leads to higher osmotic backpressure, reducing the membrane's effectiveness. As a rough guide, every 100 ppm of TDS produces 1 psi of osmotic backpressure.

 

[Octoshark]

20 is pretty low, and good post Randy.

 

[jonnybravo22]

Re: Rodi ?

Good article, thanks.

So does low pressure mean that the water coming out the other end isn't good or just that you should be getting more in that time period so you're running inefficiently?

If you have 40 (or 20) psi and it reads 0tds are you ok to use that water while you're getting the pressure worked out with anooster or new carbon block?


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[Octoshark]

Think of it like a cheese grater: if you lightly run the block of cheese against the grater then the light pressure will result in an inefficient job. If you increase the pressure and push harder then the cheese will start grating nicely and efficiently. Its kind of like that with the RO membrane, you need sufficient pressure in order for the osmosis to work efficiently and allow the unit to do its job.

EDIT: when you said "the water on the other end isn't good" I wasn't sure if you meant the water coming out of the unit or the water entering the unit. After re-reading it sounds like you mean the water coming out of the unit, and if that is what you meant then Randy would be able to provide a better answer.

 

[Buckeye Hydro]

20 psi is substantially below what you you need for the membrane to function. 40 psi would be the bare minimum, and more is better up to 90 psi max (the membrane can deal with more pressure easily, but there are likely other components in your system with a max working pressure of 100 psi).

At 20 psi the gpd from the membrane will be very very low, and the rejection rate (how well the membrane cleans the water) will be low as well (meaning you'll burn through DI resin quickly).

Russ

 

[KarlBob]

Octoshark, you made my day.

Let's finish the analogy:
If you press with all your might, you'll squash the cheese, and you could warp the cheese grater. You might also end up grating your fingers along with the cheese.

Too much water pressure can ruin an RO membrane in the same way that pressing too hard can ruin a cheese grater, or tear up your fingers.

 

[Octoshark]

Octoshark, you made my day.

Let's finish the analogy:
If you press with all your might, you'll squash the cheese, and you could warp the cheese grater. You might also end up grating your fingers along with the cheese.

Too much water pressure can ruin an RO membrane in the same way that pressing too hard can ruin a cheese grater, or tear up your fingers.

Well said

And that is what happens with too much pressure.

 

[Buckeye Hydro]

Funny analogy!

The max pressure for a Filmtec 75 gpd membrane (for example) is....

300 psi.


The max working pressure on certain other components in your rodi system is likely (depending on the brand, etc)....

100 psi.


So excessive pressure on a residential scale system will likely result in a failure of some other part long before the membrane.

Russ

 

[lostintheocean]

my pressure is about 30psi, on a 75gpd 5 stage system. i have nto had to change any filters or di and i have been using for about 1.5 years. i make about 20 gallons a week give or take. tds pre DI are 4, TDS after DI are 0. I thgought about gettign a pressure pump but its not worth the money for me. If you are not going to be making a ton of water adn your tds is 0 i would not worry about it.

 

[stealle]

Here is a post from Spectrapure (forum sponsors) that, I thought, summed it up nicely...

Another perspective on Booster Pumps for RODI systems

We often are asked the questions, “If you operate your RODI system at less than optimum tap water pressures, what is the downside? Is there a negative?”

To help answer these questions, I have provided data (below) that shows the actual results of various pressures applied to a typical RO membrane.


Membrane Pressure GPD Measured % Rejection % Increase
35 39.3 97.5
45 55.2 98.2 0.7
55 67.9 98.3 0.8
65 81.8 98.3 0.8
75 97.7 98.5 1.0
85 111.5 98.7 1.2

Note: 80.8 F, .932 TCF, 950ppm TDS, 1150 uS,
OPCF 7 psi; corrected psi 67


Generally speaking, production rate is directly proportional to operating pressure. It can be seen in this example that raising the pressure by the use of a booster pump from 35 pounds (a commonly found pressure in many homes and offices, especially as demand fluctuates during busy periods) to 75 pounds increases production rate by a whopping 2.5 times! This is the primary and most common reason for adding a booster pump to your system (either an RO or an RODI system)
you want to make more water faster! In cold-water locations, a booster pump can also increase the production rate, as production rate is proportional to water temperature.

Another reason for booster pumps that is often over looked, is that in hard water locations (many places in the country) % rejection improves significantly with the increase in pressure provided by a booster pump. Our data above shows a rejection performance increase from 97.5% to 98.5%, or about 1%. How important is 1%? The rule of thumb for DI resin capacity is for every 1% RO membrane rejection improvement you get a 25% increase in DI cartridge life (due to the reduced ionic load). Thus, with this particular RO membrane as an example (they do vary somewhat), you can see that operating an RODI system at higher pressures (75 psi) will gain a 25% increase in cartridge life over lower pressures (35 psi). In other words, with a 25% gain, every fifth DI cartridge is free! As DI cartridge changes are a significant cost of producing DI water over the life of your system, a 25% increase in cartridge life can add up fairly quickly.

bh