Irreversible RO Fouling

Hi everyone, I just came on board with a team of engineers who are currently designing and will be constructing a Water Treatment Plant, and one of the treatment process that is currently been considered is the use of RO in place of some other conventional process, however, during the 1st pilot study, they found out that the fouling on RO became irreversible. So a 2nd pilot study is on the way, and our goal is to determine what is causing this irreversible fouling and how we can effectively prevent it. If I may add we currently pulling from the Red river and Red Lake river. I was wondering if anyone one of you gentlemen and ladies can share with me the best way to approach to design this study and what parameters I should be measuring during this study. Our current plan is to setup 4 parallel RO skids and try different pretreatment processes on the feed water before it goes through the RO. Less I forget, a UF will be placed upstream. Thank you very much for your time.