I have tried it both ways and found the series works best for me. I run mine while I am sleeping or at work and found that a single pass to a holding tank gives me a better quality syrup in the end. Everytime it passes through the pump it heats up the sap - so by just going through once it limits the heating. Also, I have found that it takes roughly the same amount either in series or parallel to process. One drawback to running in series is you need to clean often. With this said I did build a new system this year with 3 - 400gpd membranes in series and although fixing some leaking issues I was taking 1.2% to 8% in a single pass at a rate of 12 gallons per hour. I am also using an RO soap instead of just peroxide to keep the membranes clean. I will be posting all my results at the end of the season to my website.