Journal of Membrane Science 563 (2018) 843–856 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Characteristics and formation mechanism of membrane fouling in a full- scale RO wastewater reclamation process: Membrane autopsy and fouling T characterization Libing Zhenga,b,c, Dawei Yua,c, Gang Wangd, Zenggang Yued, Chun Zhangc,e, Yawei Wanga,c, ⁎ Junya Zhanga,c, Jun Wanga,b, Guoliang Liangf, Yuansong Weia,b,c, a State Key Joint Laboratory of Environmental Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China b University of Chinese Academy of Sciences, Beijing 100049, China c Department of Water Pollution Control Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China d Huaneng Jiaxiang Power Generation Co.Ltd, Shangdong 272400, China e College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China f Huaneng Jining Power Generation Co.Ltd, Shangdong 272000, China ARTICLE INFO ABSTRACT Keywords: Membrane fouling is the bottleneck of stable operations for reverse osmosis (RO), which is the key technology Autopsy for reclaimed water reuse in thermal power plants. The foulant composition, formation mechanism, and key Membrane fouling contributors were analyzed in this study. The primary scaling substances are Ca, Mg, Al, Fe, and Si, and Ca RO accounts for 1.49 wt%. Humic substances, proteins, and polysaccharides are the primary organic constitutes, and Bio-fouling among them bio-derivatives were main composition revealed by FTIR, EEM, and SEC. Thus, bio-fouling was Vertical distribution believed to be the key contributor together with the results of the SEM and the microbial community analysis. The three leading bacteria were α-, β-, γ-proteobacteria at class levels, 38.11%, 14.19%, and 34.31%, respec- tively, while starkeya, acidovorax, luteimonas, and pseudoxanthomonas were the leading bacteria at the genus level. Among these bacteria, the one with metabolic processes related to nitrogen fixation and proteolysis showed higher abundance for the high concentrations of nitrogen and protein. Foulants on the endcap and membrane entrance also indicated severe bio-fouling, where Acidovorax presented significant abundance. A Vertical distribution of the microbial community was found in the cross-section of the foulant, in particular, a significant decrease of starkeya (from 48.4% to 4.65%) and acidovorax (from 17.64% to 5.61%) and an increase of pseudoxanthomonas (from 1.04% to 12.89%) from the top layer to the bottom layer were observed. The γ- proteobacteria was recognized as the pioneering bacteria for its significantly higher abundance in the deeper layer. This study helps to elucidate the RO membrane fouling composition and its key contributors, and improves our understanding of the membrane fouling mechanism and a controlling strategy. 1. Introduction remain [4–6], although numerous wastewater treatment plants have updated to tertiary treatment to meet the needs of reclaimed water Thermal power plants (TPPs) are heavy water consumers in which reuse. reclaimed water reuse had found a great opportunity for application Advanced treatment is therefore needed for reclaimed water appli- [1–3]. Approximately 90% of the TPP water consumption was used as cations in TPPs, and reverse osmosis (RO) has become the first option recirculated cooling water and boiler feedwater; the application of re- with an increasing demand of high quality reclaimed water [7–9]. claimed water is feasible in TPPs [1]. Thus, reclaimed water was However, membrane fouling is the most notable obstruction, the key strongly recommended in TPPs all over the world, and it was manda- concerns of the pretreatment processes were the removal of suspended tory to use reclaimed water for new TPPs in certain parts of China, such particulates, colloids, and dissolved matters, preventing bacterial as Shangdong Province. However, scaling and corrosion were the two growth, and inhibiting scaling formation [5,10]. Coagulation, adsorp- biggest problems for reclaimed water reuse, as certain contaminants tion, and microfiltration (MF)/ultrafiltration (UF) were widely used to ⁎ Correspondence to: 18 Shuangqing Road, Haidian District, Beijing 100085, PR China. E-mail address: [email protected] (Y. Wei). https://doi.org/10.1016/j.memsci.2018.06.043 Received 9 April 2018; Received in revised form 20 June 2018; Accepted 21 June 2018 Available online 26 June 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved. L. Zheng et al. Journal of Membrane Science 563 (2018) 843–856 alleviate the membrane fouling, as well as the dosage of antiscalants [28,33–35]. Tan et al. [29] reported that the use of non-oxidizing and bactericides and the application of online and offline chemical biocides can significantly increasing the abundance of pseudox- cleaning [5,11]. However, RO membrane fouling is inevitable because anthomonas, which belonged to γ-proteobacteria and was believed to be the pretreatment cannot completely remove all components, especially more resistant to non-oxidizing biocides. bacteria. Colloidal precipitation, inorganic scaling, organic fouling, and However, membrane fouling always appeared in a combined pat- bio-fouling were four typical fouling types in municipal wastewater tern, where the four types of membrane fouling cannot be clearly de- reclamation (MWR) by RO [12]. marcated [14]. Therefore, understanding their composition, structure Surface and bulk crystallization were two main processes for in- and contribution is important for fouling control. Membrane autopsy organic scaling because of the salt concentration effect, and especially technology, including investigations on morphology, structure, com- in the tail membrane elements, the concentration polarization effect position, and the microbial community structure of fouled membranes deteriorates the scaling, as the concentration was determined to be and foulants, as well as the interaction between membrane surface and approximately 4–10 times in the polarization layer [5,12]. Natural or- foulants was the key technique for the membrane fouling study in terms ganic matter (NOM) and bacteria were demonstrate to accelerate of the fouling mechanism analysis, selection of pretreatment technol- scaling even when the concentration was lower than the critical sa- ogies, and generating effective but targeted scale inhibiting, steriliza- turation value [13–15]. Organic fouling was another important con- tion, and membrane cleaning strategies [36,37]. The RO membrane tributor, and effluent organic matter (EfOM) and extracellular poly- autopsies were widely reported; however most of them focused on the meric substance (EPS)/soluble microbial polymers (SMP) were the lab-scale or pilot-scale RO process [6,33,38–40]. For the full-scale RO main constituents [16]. The EPS and SMP can significantly promote process, seawater and brackish water desalination processes have trans-membrane pressure (TMP) and increase up to 1000-fold of the gained more attention in the recent five years [20,40], the study of the resistance to bactericides [17]. That finding means that the organic full-scale RO membrane for the MWR is still lacking. Hu and his col- fouling and bio-fouling were mutually related and reinforced [18]. leagues have conducted a series of researches, and they investigated the Meanwhile, the EfOM acts as a nutrient for microbial growth—espe- membrane fouling composition [41], the effect of chemical cleaning cially for the pioneering bacteria— and it can easily adhere to the [42], and the fouling characteristics at different positions in a full-scale membrane surface due to the phenolic and carboxylic functional groups RO reclamation system [43]. However, to the best of our knowledge, of NOM like humic substances (HA), fulvic acids, and humin. As a re- severe membrane fouling in the summer has never been reported. sult, they act as the bridge for inorganic matter and bacteria and de- Meanwhile, most membrane autopsy studies only focus on the com- teriorate the membrane fouling [13,14,19–21]. Colloidal fouling also position of the foulants, key contributor to the membrane fouling and plays a key role in RO membrane fouling, and biopolymers, such as their synergistic effects have not been thoroughly elucidated to date. polysaccharides and protein, and inorganic colloids such as silica, iron The objective of this work is thus to understand the foulant com- hydroxide, and aluminum hydroxide, have been the main contributors position and the key factor of membrane fouling in a full-scale RO [22,23]. process. The performance of the wastewater reclamation process, fou- Bio-fouling was thought to be the most stubborn fouling, as it lant morphology, inorganic and organic matters (OMs), as well as the cannot be effectively removed by pretreatment processes and is less microbial community were studied to analyze the membrane fouling thoroughly understood [12,16,24]. The bacteria irreversibly adhere on formation mechanism. The foulants on the endcap of the membrane the membrane surface, and fast multiplication later occurs in the pre- module and foulants in the entrance of the leading element were also sence of feed water nutrients, afterwards, the stubborn biofilm forms studied, as they are also important contributors to the increase of the together with the EPS and SMP [16,25,26]. However, the microbial TMP. This work offers a systematic