Upgrading an RO system

[Buckeye Hydro]

For softened municipal water, and an RO system with 2 to 4 4040 membranes plumbed in series, a good rule of thumb is to set the concentrate to 3+ gpm and set the recycle for half that amount.

 

[OurTown]

For softened municipal water, and an RO system with 2 to 4 4040 membranes plumbed in series, a good rule of thumb is to set the concentrate to 3+ gpm and set the recycle for half that amount.

Wouldn't the downstream membranes get really high TDS water, especially the last in a series of four? Is the recycle the amount of water going back to the pump inlet? I don't think our system does that because it just has reject and product water flow meters.

 

[Buckeye Hydro]

Yes they would - that's one reason it is so important to pretreat for scale forming minerals like calcium and magnesium (aka hardness).

Yes - "recycle" refers to taking some of the concentrate and routing it back to the intake side of the pump.
Some lower-end RO's include a recycle function without a recycle flow gauge... but you are correct - you would want a flow gauge on the recycle.

 

[Buckeye Hydro]

If your demand for purified water is low enough, also consider using multiple 2.5" diameter membranes - where the minimum recommended concentrate flow is 1 GPM rather than 3 GPM for the 4" membranes.