Disentangling membrane wetting, compaction, and fouling during ultra-low-pressure liquid filtration using a phase-inverted PVDF membrane

This study disentangles membrane wetting, compaction, and fouling during ultra-low-pressure filtration using phase-inverted polyvinylidene fluoride (PVDF) membranes. Pressure-stepping tests (0.04-0.11 bar) and repeated one-hour filtration cycles at 0.07 bar, each followed by 10 min of relaxation, were performed with clean water and river water to separate the hydraulic contributions of the three phenomena. With clean water, permeability increased most strongly in the 4-7 kPa range and then approached a plateau, indicating progressive pore wetting. During repeated clean-water filtration, permeability declined because of compaction but recovered from 337.72 to 372.10 L m^-2 h^-1 bar^-1 after the first relaxation, confirming that part of the compaction was reversible. River water showed consistently lower permeability and only partial recovery (217.40 to 299.30 L m^-2 h^-1 bar^-1 after the first relaxation), demonstrating the superimposed effect of fouling. The results show that wetting dominates the initial pressure response, compaction governs time-dependent permeability loss under clean water, and fouling becomes decisive in natural water filtration. This mechanistic separation provides a practical basis for pressure optimization, relaxation scheduling, and fouling control in ultra-low-pressure membrane processes.