there is a test BRS guys did calculating the benefit of clean vs waist water in this configuration. this is what I am after. and I have two membranes so why not ?!
First off some background, I'm an engineer for a company that designs and manufactures water treatment systems for municipal, industrial and commercial applications. That
isn't to say that I know better than anyone else, I'm an electrical engineer and I design the controls and other electrical components for said systems and I still learn things all the time. My point is that is I'm coming from a completely different background.
These guys are working with what they have and trying to optimize it, and cheers to them for that and sharing the knowledge in a way that definitely seems to be focused on helping people rather than just getting people to give them more money.
The primary reason I'd say why not is membrane fowling, but in my world we make RO systems that produce 30 - 100 GPM. the membranes are huge and cost 100X more. For the cost of these membranes, that is probably not a high concern for you. There are other ways to achieve the same results without putting your second membrane at such a disadvantage, but we're getting away from off the shelf components and into an area that is way beyond what is needed for your goals.
So go for it.
As for your permeate pump, the permeate lines should come together in a tee fitting and the single line would pass through that allowing your pump to create the lowest pressure possible on the permeate side of the membranes.
However looking at the diagram you have above, the brine is passing through the pump too. This makes me think it's a mechanical gear pump that is supposed to also proportion the permeate to brine ratio. That could be problematic for the configuration you are talking about. Dose the permeate pump use power?
