Elsevier

Desalination

Volume 527, 1 April 2022, 115568
Desalination

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

https://doi.org/10.1016/j.desal.2022.115568Get rights and content

Highlights

  • Hydrophobicity and roughness increase confirmed the loss of permeability.

  • High pressure for PDT detects precise pores for membranes integrity.

  • Increase in Al percentage on membrane verifies scaling by oxidation with CIP chemical.

  • Pressure decay rate is higher in MF membrane as compared to UF membrane.

  • Integrity compromised due to scratches after long term operation.

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.

Introduction

Membranes have been used to separate pollutants from surface water and wastewater for several decades [1], [2]. Different types of polymeric membranes including polyethersulphone (PES) [3], polyvinylidene fluoride (PVDF), polysulfone (PS) etc. [4] were used for providing the portable water to society in various combination of treatment, including pre-coagulation [5], sedimentation [6] and pre-ozonation [7] depending on the treatment requirements [8]. Constant flux and constant pressure are used as a mode of operation depending on feed strength [9], [10]. Propagation of membrane fouling due to deposition of contaminants on and/or in the membrane surface is considered a serious obstacle to successful operation [11]. In addition to that, different cleaning methods, such as backwashing, air scrubbing, and chemical cleaning, are used to restore membrane performance [11], [12], [13]. However, these methods can also damage membrane structures and increase the operational cost particularly chemical cleaning is problematic in this respect [14], [15].

Various combination of oxidants with acids and bases have been reported in studies of chemical cleaning of membranes along with their impacts on membrane structure to long-term analysis [3], [15], [16]. For example, Ahmed et al. described the impact of HCl and NaOH on the zeta potential of membranes in a study of membrane integrity loss [17], [18]. Other studies of PVDF membrane aging [19] report that dehydrofluorination and oxidation under the impact of sodium hypochlorite (NaOCl) cleaning in low-pressure membrane operations is associated with a loss of membrane hydrophilicity [20], [21], [22]. Both PES and PSF [23] membranes suffer morphological losses and decreases in performance after long-term exposure to NaOCl [24], [25], [26]. These studies confirm that exposure to chemical cleaning can change the morphological structure of membranes, but less attention has been paid to the loss of membrane integrity due to long-term interactions with feed solutions and chemicals [27], [28].

Several methods have been used to confirm membrane integrity loss, including direct integrity (pressure decay test [PDT] [29] and bubble-point testing [30]) and indirect integrity (turbidity monitoring [31], particle counting [9], [32] and particle monitoring [10], [33]) measurement as described in guidance manuals for membrane operation provided by the United States Environmental Protection Agency (EPA) [34], [35], [36]. Successful operation of membrane processes requires that membranes achieve a 4log (99.99%) removal value (LRV) for Giardia and Cryptosporidium [35], [37], [38], [39], [40]. A variety of surrogate materials can be used to confirm membrane sensitivity [41], [42]. Following a challenge test of a PVDF membrane using silver nanoparticles as surrogate materials to evaluate membrane integrity, Antony et al. reported an LRV of 2.9 for nanoparticles without effecting membrane performance [43]. Moreover, PDT offers a convenient and precise method of measuring membrane integrity [29]. In addition to that, Ultrasonic spectroscopy has also been used to determine filter integrity [44], [45]. Alvarez et al. identified a correlation between the magnitude of ultrasonic waves and the bubble point and pore size, suggesting this relationship can be used to evaluate membrane integrity [45]. These studies were conducted in the lab and pilot-scale membranes; fewer studies are available on membrane integrity at real wastewater treatment plants that have been operating for more than 5 years. Evaluations of the impact of chemical cleaning on sensitivity loss are also not well explained, and limited studies are available that analyze the type of damage responsible for membrane sensitivity loss and the compromising of removal efficacy.

This study was conducted to use a pressure decay test (PDT) for the integrity of microfiltration (MF) and ultrafiltration (UF) membranes that have operated for almost 7 years in a treatment facility. Standard factors were set according to a guidance manual for a laboratory-scale PDT apparatus. Integrity was evaluated according to the type of damage suffered by the membrane using pre-specified membrane specifications and exposure conditions. The effect of the clean-in-place (CIP) method on pressure decay was examined in a lab-scale module The characteristics variation of membranes was illustrated by Fourier transform infrared (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Atomic force microscopy (AFM), universal testing machine (UTM) and contact angle measurement before (pristine and received after the 7 years of facility operation) and after exposure to lab CIP conditions.

Section snippets

Membranes and materials

Chemicals and reagents used in this study are of analytical grade, unless otherwise stated. UNI Chemicals and Co. provided 12% NaOCl while Duksan Pure Chemicals (Korea) provided 1 N HCl and 1 N NaOH. Ferric chloride anhydrous (FeCl3), anhydrous calcium chloride (CaCl2), and alum (KAl[SO₄]₂·12H₂O) were purchased from Daejung Chemical Reagents and Metals (Korea). Humic acid (HA) and kaolin were purchased from Sigma-Aldrich (St, Louis, MO. USA). Mn(II) sulfate was obtained from Junsei Chemical

Membrane characterization

The variation in membrane characterization after 7 years of operational exposure and 6 cycles of CIP with the lab-scale module in comparison with pristine membranes is illustrated in Fig. 4(a)–(f). Fig. 4(a1)–(b3) shows the change in the roughness of the membrane and membrane surface variation as revealed by AFM and SEM. respectively. The roughness of membrane A (Ra) increased from 24.92 nm to 51.02 nm and 54.74 nm after 7 years of operation and 6 CIP cycles, respectively, compared with

Conclusions

Direct PDTs were applied to MF and UF membranes in use for 7 years under different test pressures to analyze the filtration potential and impact of CIP on membrane integrity. Lab-scale hollow-fiber membrane modules were assembled and tested with a PDT test device. Membrane characterization confirmed the variation in membrane structure. Loss in hydrophilicity of membranes resulted to increase in membrane roughness and contact angle due to dehydrofluorination and oxidation of PVDF membrane under

CRediT authorship contribution statement

Yong-Soo LeeConceptualization, Writing–Original draft
Khan Imtiaz AfzalConceptualization, Data validation, Revising draft
Kang Hoon LeeValidation, Formal analysis
Jong-Oh KimProject administration, Funding acquisition

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by Korea Ministry of Environment as a “Global Top Project (2016002100004)”. The authors would like to thank the Higher Education Commission of Pakistan scholarship grant under project “HRD INITIATIVE of FACULITY DEVELOPMENT FOR UESTPs/UETs.”

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