Identifying active concentrations of biopolymers for enhancing membrane nanofiltration performance: From bench-scale tests to real production considerations

https://doi.org/10.1016/j.scitotenv.2021.151808Get rights and content

Highlights

  • Biopolymers in surface waters may alleviate NF membrane fouling.

  • Polysaccharide-like substances could mitigate NF flux decline.

  • An “active” range of biopolymers' concentration is proposed.

  • MF filtrate is suggested to be the feed of a NF unit.

  • A novel design of membrane-based NF plant was established.

Abstract

In the last decades, membrane-based nanofiltration (NF) technique has been widely applied for safe and high-quality drinking water production worldwide. NF membrane fouling has become one of the main obstacles in its application due to high operation cost, and thus numerous efforts have been made. However, there is still a large disconnect between academic findings and their applications. Hence, novel approaches for further exploitation and application are required based on feasibility of implementation. In this work, an optimized design of membrane-based NF plants was proposed, inspired by natural biopolymers present in feed water of NF unit. Specifically, we found beneficial functions of biopolymers, including NF membrane fouling alleviation and effluent quality improvement; these advantages could only be “activated” under a certain concentration range of biopolymers (0–1 mg C/L here), and less or more is not acceptable. This indicated that a NF unit is better to follow a microfiltration (MF) (instead of ultrafiltration (UF) which removes biopolymers) process during which natural biopolymers could be remained; also, this approach is suggested to be valid across different seasons when biopolymers' concentrations could be controlled within an “activated” range by mixing MF and UF permeates. Furthermore, three representative reference biopolymers with different, confirmed spatial structures and molecular weight (MW) were used to elucidate the micro-level functions of natural biopolymers on NF membranes, suggesting that cake layer structures shaped by various biopolymers determine the resulting NF performance. Overall, this innovative proposal is expected to be considered and adopted towards more energy-efficient NF technology for drinking water supply.

Introduction

Over 80% of wastewater is discharged to the environment without adequate treatment, which has become a challenge for providing safe drinking water worldwide (World Water Assessment Programme (United Nations), 2017). Source waters are suffering from external pollution such as eutrophication, and therefore purification processes for drinking water production towards greater efficiency and economic feasibility are required. Particularly, due to undesired deterioration of raw water quality in recent decades, membrane-based techniques are increasingly important for supplying clean and safe drinking water for urban and rural communities globally (Elimelech, 2006; Xia et al., 2005). Pressure-driven membrane separation systems, including MF, UF, NF and reverse osmosis (RO), have been designed and widely applied for rejecting suspended particles, pathogens, and parts of organic matter. Recently, things have become challenging because of stricter standard and more demands for improving water quality (arising from health-related concerns) (Naujokas et al., 2013). Additionally, an improved operational efficiency of water treatment plants for reducing carbon dioxide emissions is needed for global ecosystems (Molinos-Senante and Guzmán, 2018; U.S.EPA, 2014).

Among the aforementioned systems, MF emerged as a solution for purifying surface water in the 1980s and was also implemented in large-scale during the mid-90s in the U.S. and Canada in response to Cryptosporidiosis outbreaks, resulting in stricter laws regarding water-borne pathogen removal (Warsinger et al., 2018). The MF market for membranes used in drinking water treatment is estimated at $185 million in 2005 and is increasing at an average annual growth rate (AAGR) of 9.5% (Alspach et al., 2008). Compared to MF, UF membrane, with smaller nominal pore size (MW cut-off, MWCO: 1 kDa–100 kDa), is more capable of improving effluent quality and biosecurity. Normally, the pore size of MF membrane ranging from 0.1 μm to 0.45 μm, and viruses are not well retained whereas UF membrane performs effectively. In contrast, NF and RO membranes, which are energy-consuming, are more efficient in terms of removing metal ions (Gherasim et al., 2015), inorganic contaminants at the nanoscale (Fujioka et al., 2013) and dissolved organic matter which was found as a contributor of cancerogenic disinfection by-products (DBPs) formation (Bond et al., 2010; Blau et al., 1992). Therefore, NF and RO membrane-based systems are selected for obtaining high-quality drinking water. Recently, Sedlak reported a concern that drinking RO-treated water consistently could lead to nutritional deficiencies because it is too clean (Sedlak, 2019). In contrast, we note that NF could be of great value for producing potable water with relatively less energy consumption and better supply of healthy nutrients simultaneously.

For this reason, achievements have been made to improve membrane NF process (Lively and Sholl, 2017; Yang et al., 2019). In this regard, pretreatment of feed and optimization of conventional process are suggested as practical options to control membrane fouling. For example, conventional coagulation was found to be contributable in controlling NF membrane fouling in numerous researches (Wang et al., 2020; Listiarini et al., 2009a). J. Kohler et al. concluded this positive function of aluminum coagulation on NF membrane system operated at full scale using surface waters in Sweden; the removal efficiency of humic substances increased from 65% to 83% via switching coagulation to coagulation followed by NF, and more than 90% of the dissolved organic carbon (DOC) from incoming lake water was removed during this combined process (Köhler et al., 2016). Furthermore, an extended strategy by applying additional membrane-based pretreatments aimed at lowering the load of contaminants in NF unit was well developed, and was successfully applied in drinking water treatment plants (Li et al., 2009; Chellam et al., 1998). However, whether or not to reduce the contaminants to an extremely low level prior to NF unit, what kind of contaminants should be particularly eliminated, or even need to be reserved during pretreatment, requires a further discussion regarding their complicated transportation processes and the relevant membrane fouling mechanisms.

In previous studies, the contaminants in surface water are commonly categorized as autochthonous natural organic matter (NOM), including macromolecules produced by microorganisms (i.e., biopolymers; such as cellulose, etc.), and allochthonous low-medium MW terrestrial organics mainly constituted of humic-like substances, and these different pollutants dominate distinct membrane filtration behaviors (Guo et al., 2020; Pivokonsky et al., 2015). In particular, deterioration of porous membrane permeability caused by membrane pore blockage, channel shrinkage and cake layer formation can be specifically attributed to various constituents of foulants (Duclos-Orsello et al., 2006). In addition, as for organic pollutants, the degree of flux decline is not only determined by their concentration, but also by their characteristics, e.g., spatial structures, chemical groups, as well as adhesive force (Trinh et al., 2020). Previous studies have well evaluated the influences of contaminants (usually considered as a whole) sourced from various surface water on NF membrane fouling (cake layer and pore blockage) (Contreras et al., 2009; Listiarini et al., 2009b). In comparison, compositional contributions of aquatic pollutants (e.g., with different structural characteristics) to NF membrane fouling have not been teased out to date. Practically, as reported, a widely recommended strategy to achieve better NF performance is to lower the NOM concentration in NF feed as much as possible; in this way, e.g., using UF membrane (rather than MF) as the pretreatment of a NF unit is adopted (Li et al., 2019). However, this consensus has never been questioned by considering some possible effects (maybe different) arising from various components of pollutants.

In this work, we investigated NF performance and the relevant membrane fouling mechanisms influenced by matrices of natural organic contaminants (macromolecules-biopolymers, and medium and low humics) in surface water. In addition, three representative standard macromolecules (bull serum albumin (BSA), sodium alginate (SA), and xanthan gum (XG)) with various molecular weights and confirmed spatial structures were employed to systematically elucidate the roles of natural biopolymers in NF performance when co-existing with humic-like substances in typical surface water. More importantly, results are not only expected to obtain an improved understanding of NF membrane fouling mechanisms, but also to provoke a rethinking of the design of membrane-based NF drinking water production towards better water quality and cost efficiency, via considering more possibilities led by different aquatic pollutants.

Section snippets

Raw water preparation

Surface water, the river Jingmi (JM) in Beijing, China, as one of the representative source waters for drinking water production in northern cities in China was collected in summer and winter. The characteristics of the collected samples were analyzed immediately after sampling and the typical properties of water quality are presented in the Supporting Information (SI, Table S1). For the following filtration tests, water samples were first pre-filtered using a mixed cellulose ester (MCE) MF

Natural biopolymers in surface water

Compared to the biopolymers (also denoted as extracellular polymeric substance, EPS) in wastewater (Ni and Yu, 2012; Sheng et al., 2010) and sludge systems (Seviour et al., 2010; Bura et al., 1998), previous studies paid little attention to biopolymers in drinking water production, due to their relatively low concentration in natural waters. We herein uncovered the composition and physic-chemical properties of biopolymers collected from the river JM (Tier 1) across two seasons including summer

Conclusion

In this work, natural biopolymers in surface water (the river JM) were collected and employed to investigate their possible influence on membrane-based NF process. Three representative reference biopolymers were selected to further elucidate the effects caused by natural biopolymers, in terms of effluent quality and the underlying membrane fouling mechanisms. The main concluding remarks are as follows:

  • BSA, a protein-like substance, was found to cause severe NF membrane fouling continuously with

CRediT authorship contribution statement

Zhaoyang Su: Conceptualization, Investigation, Writing – original draft. Ting Liu: Methodology, Investigation. Thomas Seviour: Writing – review & editing. Shuo Li: Investigation. Long Tian: Investigation. Guotao Zhang: Investigation. Wenzheng Yu: Writing – review & editing, Supervision.

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

We thank China Postdoctoral Science Foundation (No. 2018M641497), Beijing Natural Science Foundation (No. 8192042), National Natural Science Foundation of China (No. 51108444), and Key Research and Development Plan of the Ministry of Science and Technology (No. 2019YFD1100104 and No. 2019YFC1906501) for the financial support of the work. The authors would also like to acknowledge and thank the research support offered by Singapore Centre for Environmental Life Sciences Engineering, Nanyang

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      From the above observations, the weaker effectiveness of Al-based salts with/without particles at low temperature was confirmed, and an increased Al dosage of 0.2 mM (Fig. 1b) was not acceptable. Other factor(s), e.g., biopolymers' absence potentially, significantly hindering coagulation process in winter need to be further clarified, because biopolymers’ concentration in surface water in summer is usually higher than that in winter (Su et al., 2022). For further understanding the influence of biopolymers on the coagulated surface water at low temperature, their roles in particles coagulation process at normal temperature (25 °C) were first investigated for clarifying the mechanisms; two-sourced biopolymers, collected from the river JM and the lake OP, were applied comparatively.

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