Flexible fiber filters are recently developed modular filtration units which have been applied to wastewater and water treatments, satisfactorily removing solids even when operated at high application rates. In this paper, polypropylene fibers, in lieu of the commonly used polyamide fibers, were tested for constructing filtration modules containing parallel fibers. The studied fibers were analyzed by means of scanning electronic microscopy and through solubility assays in hydrochloric acid and sodium hydroxide, aiming to evaluate the risks of using them as filtering media. Three polypropylene filters with different lengths (25, 60, and 100 cm) were constructed and fed with the same raw synthetic water. In-line coagulation was applied by addition of aluminum sulfate (22.5 mg·L−1) and filtration rates from 20 to 80 m·h−1 were evaluated. Filtrates with less than 0.5 NTU could be produced by both 60 and 100 cm filters, operating at 80 m·h−1. High filtration rates, as well as significant backwashing water and air flows, could be applied to flexible fiber filters made of polypropylene, which shows their promising applications.