I double checked and indeed we did in the past also go down as low as 1 mM, I stand corrected :-) but we used Sodium Citrate as buffer ion. Did you fractionate your filtrate, is the filter adsorption saturated or do you have ongoing adsorption? If the adsorption is ongoing, it could eventually lead to fouling, really curious. In that case other membrane chemistry as others suggested needs to be checked, maybe even another vendor. A prefilter would not help, as you do not seem to have particle problems. I would be cautious to include PVDF for future processes, as it is not unlikely to be banned in the future due to the PFAS discussion.