Using a pump on permeate to increase throughput

[wobbletop]

A guy on reddit posted his RO setup and it included an additional pump on the permeate line. It's basically providing vacuum on the permeate line.

I asked if he had done any experiments to determine how his throughput improved but he had not had time to do any experiments during the busy sugaring season. It should improve becasue you are creating a higher pressure difference across the membrane.

Has anyone else done this and can provide feedback? I did search for permeate pumps, but they are mainly used in fresh water systems where you are trying to overcome the increasing pressure in the fresh water storage tank.

Interesting idea.

[carls47807]

I have thought about this, but theoretically speaking you would only increase the change in pressure by a maximum of 14.7psi, and that is if you pull a perfect vacuum. It would be much easier and cheaper to simply increase the system pressure by 14.7psi by adding an additional feed pump, or adjusting the return valve in the pump head to allow for a higher relief pressure.

[wmick]

I agree with Carls. It would be far more efficient to add more pressure before the membrane, that adding vacuum after it.... Pumps push a whole lot easier than they pull. Increasing your psi and flow on the pressure side, might also gain you some flow across the concentrate side of the membrane..... aiding in a healthy "recovery" rate.

[bmbmkr]

What Stephen Childs replied when I asked him the same thing a few years ago.

"Vacuum actually is very limited. The most pull pressure you can get with vacuum is a little less than 15 psi while the RO pumps you are using are likely pushing about 300 psi. The idea of effective RO is that you think of the membrane as a very fine screen with water and sugar moving down the screen. If the flow is too slow water still goes through the screen but the sugar gets stuck to the screen a lot as it bounces along, in the process blocking some of the holes. If you increase the flow over the screen from the pump the sugar just keeps flowing along without getting stuck much and more of the holes are open for the water to go through. So the best way to improve an RO is increase flow speed over the screen. Lots of pressure and not much flow means lots of plugging while lots of flow along with lots of pressure is the best for water removal results. Vacuum on the permeate line would likely stick more sugar to the screen without doing anything to increase the flow that would keep the screen clean. That’s why most big RO have recirculation systems, that makes for great flow and great pressure at the same time. Have you seen the NY Vacuum and Tubing Notebook? If not check it out at https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/7/5773/files/2016/06/Tubing-Notebook-6th-edition-2fpl9ze.pdf "

Stephen Childs

NYS Maple Specialist

G19 Fernow Hall

Ithaca NY 14853

Slc18@cornell.edu

Cornellmaple.com