Sensitivity of physical membrane damage detection on low pressure membranes of commercialized specification
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 Lee Conceptualization, Writing–Original draft Khan Imtiaz Afzal Conceptualization, Data validation, Revising draft Kang Hoon Lee Validation, Formal analysis Jong-Oh Kim Project 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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