Sensitivity of physical membrane damage detection on low pressure membranes of commercialized specification

Abstract

In this study, a pressure decay test (PDT) was applied to microfiltration (MF) and ultrafiltration (UF) membranes that were in use for more than 7 years at a wastewater treatment plant. Laboratory-scale hollow-fiber membrane modules were made from treatment plant used membranes having areas of 0.18 and 0.027 m2 for MF and UF membranes, respectively. Filtration was performed on a feed solution containing organic, inorganic and turbidity elements as foulants for 1 h followed by clean-in-place (CIP) with 1 N HCl and 3000 mg L−1 NaOCl solutions for 3 h and continued for 6 CIP cycles. Membrane characteristics were analyzed by using Fourier-transform infrared analysis, scanning electron microscopy and atomic force microscopy. Changes in mechanical strength were estimated with a universal testing machine and integrity was evaluated by comparing the upper control limit (UCL) of the pressure decay rate (PDR) with the experimental PDR of these membranes. Results showed that UCLs of both membranes were less than the actual decay rate, confirming the compromising of 4 log removal value (LRV) removal in both membranes in a Ptest of 100–160 kPa, with a maximum of 3.78 LRV at the end of the experiments at a 100 kPa Ptest. Membrane characterization revealed the loss of hydrophilicity and strength due to the oxidation and dehydrofluorination of the membranes, particularly in MF membranes. Scratches on membrane structures due to complex foulants and exposure to CIP chemicals are the most likely causes of integrity loss associated with membrane operation in a treatment plant.