RO filter water pressure.

[gonewild]

Poor results from RO filter.
Does low water pressure reduce the amount of salts removed or only the volume of filtered water per hour?

 

[merc]

what do you mean by poor results? the filters should catch a good chunk of the salts. we have a 4 stage rodi from bulk reef supply and it cranks out a slow trickle. i'm thinking low pressure just reduces volume.

 

[gonewild]

Poor result is high TDS.
Well water tests 721ppm and the RO filtered water comes out 150 ppm.

 

[merc]

50-300 TDS is considered excellent

 

[gonewild]

50-300 TDS is considered excellent

After filtering through RO filter? I was expecting closer to 30ppm.

 

[merc]

80% reduction in TDS is pretty awesome, but maybe i'm too easily impressed by any positive improvement. heh! hoping someone else with an rodi unit can chime in.

 

[orchid527]

I used softened water to supply my RO filter at 60 psi and collect the output in a 30-gallon drum, not a pressure tank, so I get the full 60 psi drop across the membrane. My RO water is less than 5 ppm dissolved solids. Mike

 

[SelectOrchids]

Over 50ppm TDS is very bad for a RO System. We usually target a max of 20-30 microsiemens.

Check 3 things:

- The input water pressure must be at least 40psi, preferably higher. Below that, it does not work.
- If the membrane is well secured inside its cartridge. Sometimes there are leaks.
- How much water is rejected. Depending on the membrane models, the ratio RO water/rejected water will vary. High performance membranes that are expensive can be set up to 1 part of rejected water to 20 of RO pure water, but most cheap household ones will work on a 1:3 ratio. If you block too much water from being rejected, then the EC of the RO water will sharply increase.

RO Membranes do not filter physically all the ions and molecules. A part of the larger molecules cannot pass through the semipermeable membrane, but the ions are repelled from the membrane. If the water pressure is too low, the repelling effect will not happen, and the ions go through. Basically it removes many molecules, and it repels most ions, if the pressure is right. At low pressure, the electrostatic and flush effects won't happen, and they can pass through the membrane.

That's why too boron is a huge problem. The molecules of boric acid are small enough to pass through the membrane, and as boric acid does not dissociate readily, most RO systems will not filter boron. In high boron areas, it can be deadly to the plants.

One last note, if the EC of the imput water is 600 microsiemens as an example, and the RO water is by accident at 200 microsiemens, which would be very bad. it does not mean that there is 1/3 of the calcium or minerals of the input water. Some ions are repelled more easily than others, and a high EC output RO water might well be loaded with most of the input sodium, the calcium having been rejected as a priority in a poorly working setup, as an example.

 

[Ray]

For the 25 years I was building and selling RO systems, I used standard residential membranes, not the more expensive industrial ones. With the residential membranes, within a reasonably broad range - typically 35-100 psi - pressure affects the ratio of permeate to brine volume, far more than the degree of solids reduction.

The pure-water production standard membranes are rated at concentrate water at 65 psi (0.48 MPA) and 77°F (25°C) at the membrane, and they typically passed no more than 2-3% of the dissolved solids. When folks had very low water pressure, I used membranes designed for that application, specified at 25 psi (0.17MPA) and the same temperature. Those passed no more than 7%-8% of the dissolved solids, but had no minimum pressure spec.

 

[gonewild]

Looks like low water pressure is the problem. I ordered a booster pump.

 

[gonewild]

Over 50ppm TDS is very bad for a RO System. We usually target a max of 20-30 microsiemens.

Check 3 things:

- The input water pressure must be at least 40psi, preferably higher. Below that, it does not work.
- If the membrane is well secured inside its cartridge. Sometimes there are leaks.
- How much water is rejected. Depending on the membrane models, the ratio RO water/rejected water will vary. High performance membranes that are expensive can be set up to 1 part of rejected water to 20 of RO pure water, but most cheap household ones will work on a 1:3 ratio. If you block too much water from being rejected, then the EC of the RO water will sharply increase.

RO Membranes do not filter physically all the ions and molecules. A part of the larger molecules cannot pass through the semipermeable membrane, but the ions are repelled from the membrane. If the water pressure is too low, the repelling effect will not happen, and the ions go through. Basically it removes many molecules, and it repels most ions, if the pressure is right. At low pressure, the electrostatic and flush effects won't happen, and they can pass through the membrane.

That's why too boron is a huge problem. The molecules of boric acid are small enough to pass through the membrane, and as boric acid does not dissociate readily, most RO systems will not filter boron. In high boron areas, it can be deadly to the plants.

One last note, if the EC of the imput water is 600 microsiemens as an example, and the RO water is by accident at 200 microsiemens, which would be very bad. it does not mean that there is 1/3 of the calcium or minerals of the input water. Some ions are repelled more easily than others, and a high EC output RO water might well be loaded with most of the input sodium, the calcium having been rejected as a priority in a poorly working setup, as an example.

I wondered if some minerals might pass easier than others. My sodium content is high and I was hoping it might pass with a preference but no luck.
I had tests done on both well and filtered water. Here are the results that show how the ro filter functioned under low pressure..

Well water.

RO filtered water

 

[SelectOrchids]

What I was unfortunately expecting... Calcium and Magnesium are removed with the highest priority, leaving the sodium. You definitely need a booster pump and re-analyze the water if the EC is not below 30-40 microsiemens...

Another trick is to 'open' the pipe that releases the waste/drain water. Usually it is blocked in a way or another, a clamp or a tap, and if you release more waste/drain water you get automatically a much more pure RO water to start with.

 

[Ray]

Another trick is to 'open' the pipe that releases the waste/drain water. Usually it is blocked in a way or another, a clamp or a tap, and if you release more waste/drain water you get automatically a much more pure RO water to start with.

In residental RO systems, there must be a flow restrictor in the flush water line, or there will be insufficient pressure to push the water through the membrane.



The yellow boxes are the typical pairings of the various membranes and restrictors, bright green are the pairings that improve the productivity without resulting in excessively fast fouling, and the lighter green boxes represent "doable, but not advisable".

Lance - is this a new system? New membrane? Installed properly? Something is obviously not right, but I really doubt it's pressure. I had a customer once whose water supply was an elevated tank, so there was almost no pressure whatsoever coming into the system. A system rated at 50 gpd @ 25 psi only put out about 5 gpd, but it was still very pure.

 

[gonewild]

Ray it's not a new install. However the poor results are from startup and remain constant until now.
I have two ro units on the same water supply. One installed a year ago in our kitchen purchased from Costco. The other is a Hydrologic installed 6 months ago in my plant room. Both produce water with similar tds.
Our water pressure stands at 46 psi. What I was unaware of is that when a tap is opened elsewhere the pressure drops to 20 psi at the filter units. Apparently there is a water volume restriction somewhere in the underground lines.
I hope to have the booster pump installed today and hopefully see better results. I just need to reduce the sodium content now.

 

[Ray]

Were the membranes left open or the systems allowed to dry before use? If so, they can degrade fairly rapidly.

I have never seen nor heard of that poor of a reduction level in any system. It's not your water chemistry, and I still doubt it's pressure-related...

 

[gonewild]

Both units were purchased by new. I installed the same day I unpacked them. I'm my possession they have never dried out.
Since the results are the same from two different models of filters and it's no the water chemistry or pressure what else could it be?
I was so surprised when I first tested the results using my cheap tds meter I had the water tests done at a trustable lab.
We'll see if the booster pump solves the problem.

 

[Ray]

Well...Something ain't right. 721 only being reduced to 150 ppm suggests a defective system to me. 15 seems more reasonable, and even the LP membranes I used would be expected to reduce it to no more than about 50-60. It really seems that you've got "raw" water mixing with the permeate.

 

[gonewild]

Some progress.
I installed the booster pump. Pressure raised to just below 80 psi. Filtered water drop to 50 ppm.
I went to the local grow shop and bought new filter cartridges hoping it will drop more. RO cartridge says to run it for 24 hours to flush the system. So tomorrow we'll see.

 

[Ray]

Filter cartridges take no dissolved solids out of the water, only suspended solids (sediment), and the carbon cartridges are for chlorine and organic compounds.

Was the 50 after installing the new stuff?

 

[gonewild]

The 50 was with the old cartridges with the pressure boosted.

The instructions on the new RO cartridge said it takes 24 hours run time to remove a temporary protective coating before peak efficiency is reached.