RO Reject Membrane Recirculation

[Bubbles Galore]

I am plumbing my new spot free membranes and MEP had mentioned that plumbing the reject to get recirculated back through the membranes would help to extend membrane life along with reducing the amount of reject.

Any ideas how to plumb this? My system is going to have a 1.5 hp booster pump pumping water through (1) 2" membrane and (2) 4" membranes.

Thanks!

 

[MEP001]

I don't know if it's a good idea or not to try and use the smaller membrane with the two larger ones. What you can do is go straight from the pump outlet to one 4" membrane's inlet, then loop that membrane's outlet to the other 4" membrane's inlet. From there you need to split the flow between the reject and the recirculate. An easy way to control everything is to use a bypass regulator just like your high-pressure pump is set up, to return flow that would exceed 200 PSI back to the inlet of the pump. You can use a ball valve, but it's difficult to "steer" the flow and pressure to get it right, plus your system pressure will slowly rise as the membranes foul over time. You will need a ball or large needle valve to control flow to the drain.

If you really want to keep the 2 1/2" membrane in use, you could plumb that inline after you have tee'd off recirculation to the inlet of the pump. The full flow would be too much for such a small membrane. You'd still get some extra product from it, and when it eventually goes bad you can just remove it.

 

[Sudsy]

Reject Water

I have been following this discussion a-bit and wondering is there a min. pressure you need going thru the 4" filter?

gt

 

[MEP001]

It's more about flow and reject than pressure - I've had to run a system with city pressure (about 40 PSI) and made enough product to keep running.

 

[Greg Pack]

IIRC One of the posters here (Robtl)had three XLE membranes, plumbed in series, running on city pressure. He captured and used all reject so he wasn't worried about efficiency. Seems simple and cheap.

I never understood how the CW industry got by charging so much for RO systems and why so many operators are terrified of them.

 

[mjwalsh]

Is this wishful thinking or actually feasible???

IIRC One of the posters here (Robtl)had three XLE membranes, plumbed in series, running on city pressure. He captured and used all reject so he wasn't worried about efficiency. Seems simple and cheap.

I never understood how the CW industry got by charging so much for RO systems and why so many operators are terrified of them.

cfcw,

I wonder if there is any drawback to doing that. It seems like the TDS before the membrane would keep getting higher & higher which would eventually cause a problem. Also it seems that flushing is needed which in our case means more reject is passing to the drain leaving lower TDS water to flush the membrane.

Can someone share the long term implications of just constantly recycling the softened city water without any reject to the membrane. I see this approach as saving a lot of electricity besides never having to worry about the RO makeup water pump going out. The makeup water solenoid would obviously be open longer. Now that the business is less these days for some of us our storage tank may keep up fine even though the rate of make up volume is definitely less because of less pressure.

Our city water pressure after the softener etc is about 55 #'s of pressure --- I wonder if that pressure would be painfully slow ---- it seems like with the right kind of check valve the pressure could be maintained though. The car wash distributer had set us up with a 4" x 40" CA membrane ----- so I wonder if the type of membrane would make any difference.

We are getting kind of maxed out on different experimentation so I hope some one can explain their experience with a similar attempt to what CFCW & ROBTl appear to be implying.

MJ

 

[MEP001]

cfcw didn't say all the reject was recirculated through the membrane; he's saying that the reject from the membranes is used for other purposes.

If you try to use a reverse osmosis membrane with no reject flow, it's no longer a semi-permeable membrane and becomes a filter which will clog very quickly with no water running across it to keep it flushed.