A non-destructive method for screening foulants and particulates in full-scale reverse osmosis membrane processes

An alternative to membrane autopsies was employed to identify foulants and assess particle sizes in a 13.7 million gallon per day reverse osmosis (RO) process. In this non-destructive method, 0.45-micron rated silver pads were placed in a silt density index apparatus to filter a known volume (20-liters) from feed, interstage, and concentrate streams. The chemical composition of the foulants were analyzed via scanning electron microscopy and energy dispersive x-ray spectroscopy, commonly used in RO autopsies, as well as a novel technique, entitled optical photothermal infrared spectroscopy, that allowed for submicron imaging to quantify particle size. Feedwater samples primarily contained calcium sulfate, calcium hydroxide, organic matter, and silicates. Interstage and concentrate samples identified similar foulants, in addition to aluminosilicates. These findings aligned with historical RO autopsies, particularly in identifying silicates and organics. However, the filter pads uniquely detected residual scale inhibitor and cleaning chemicals not observed in the autopsies. Particles (20-30 µm) exceeding the nominal pore size (5-micron) of the pretreatment cartridge filter were observed in the interstage and concentrate streams that suggested agglomeration. Overall, this research demonstrated that the non-destructive method provided comparable foulant identification results to the autopsies without requiring operation downtime or damaging the elements. • A non-destructive foulant identification technique using filter pads was employed. • The predominant foulants in the RO process were calcium- and sulfate-based. • The interstage and concentrate particulate sizes were predominately 20-30 microns.