Mitigation of organic fouling on ceramic membranes by selective removal of microbial-oriented organic matters in wastewater effluents

https://doi.org/10.1016/j.seppur.2019.03.032Get rights and content

Highlights

  • Microbial-oriented organic matters (MOOMs) were the major organic foulant in WWTP effluent.

  • Pretreatment steps for the selective removal of MOOMs can extend the ceramic membrane fouling.

  • Graphene-structured carbon materials can selectively adsorb MOOMs.

Abstract

The fouling propensity of organic matter categorized by their origins has been investigated during the reuse of tertiary wastewater effluent by a ceramic ultrafiltration membrane with molecular weight cut-off (MWCO) around 1 kDa. Selective removal of organic matter was carried out with three carbon-based adsorbents including granular activated carbon (GAC), expanded graphite (EG), and multi-walled carbon nanotubes (MWNTs). Analysis on the residual organic matter after the adsorption revealed that EG and MWNTs mainly adsorbed microbial-oriented organic matters (MOOMs) such as the soluble microbial products (SMP)-like and aromatic protein (AP)-like organic compounds, while GAC showed non-selective organic removals. At the identical organic loading, feed solution treated by EG and MWNTs exhibited significantly lower fouling propensity than that of the GAC-treated sample. Consequently, the selective removal of MOOMs from tertiary effluents can provide an effective strategy to reduce organic fouling on ceramic membranes.

Introduction

Wastewater reclamation is one of the essential strategies to manage water resources for sustaining urban developments and addressing water shortages [1]. Conventional biological wastewater treatment plants (WWTP) cannot provide the required water quality for various reuse applications, therefore, additional advanced processes such as activated carbon adsorption, chemical coagulation / precipitation, and membrane filtration, are required to provide the required water quality for domestic and industrial reuse [2].

Recently, the membrane technology is widely used to guarantee acceptable water quality with a small footprint for wastewater treatment [3], [4], [5]. Generally, membranes can be divided into ceramic and polymeric membranes on the basis of the raw materials. Previous studies have reported that ceramic membranes have gained popularity for the treatment of tertiary treated wastewater due to their various advantages over polymeric membranes [6], [7], [8]. In more detail, the advantages of ceramic membranes included superior chemical and thermal resistivity, enabling chemical or thermal regeneration and sterilization by strong chemical or high-temperature processes [9]. Their high mechanical stability enables high-pressure backwashing [10]. Moreover, ceramic membranes are generally more hydrophilic than polymeric membranes resulting in lower fouling and high fluxes at low trans-membrane pressure (TMP) [7]. Thus, despite their higher cost, ceramic membranes are more economically feasible for use in wastewater treatments that requires strong and frequency chemical and physical cleaning [11], [12].

However, a major issue for applications of ceramic membrane technology is the phenomenon of membrane fouling. Several types of research have shown that the performance of ceramic membrane can be significantly degraded by membrane foulants such as dissolved organic matter (DOM), soluble inorganic compounds, colloidal or particulate matters, and microorganisms in the feed solutions [13], [14]. In previous studies, DOM has been identified as a major foulnats of membrane fouling during the filtration of the WWTP tertiary effluent [14], [15], [16], [17].

The DOM in the WWTP tertiary effluent is composed of various organic compounds, such as humic acid (HA), fulvic acid (FA), soluble microbial by-products (SMP), proteins, amino sugars, organic acids, and cell components with a wide range of molecular weight [18], [19], [20]. More details, these organic compounds can be divided into naturally-occurring organic matters (NOOMs) and microbial-oriented organic matters (MOOMs). The representative NOOMs are HA and FA-like substances mostly originated during the decomposition of plant and animal decay [21]. On the other hands, MOOMs can be found in the products of substrate metabolism, biomass growth, and biomass decay [22]. SMP-like and aromatic protein (AP)-like substances are considered as representative organic compounds in MOOMs [23]. Especially, SMP-like substances are known as the majority of DOM in the effluent from biological wastewater treatment processes [22].

For organic fouling, the characteristics of organic compounds such as the molecular weight distribution, hydrophobicity, and surface charge from the functional group content are important to understanding the interaction with the membranes [24]. The previous studies revealed that the NOOMs and MOOMs in the WWTP tertiary effluent represent a complex of hydrophobic/hydrophilic fraction and a wide range of molecular weight distribution [25], [26].

To mitigate the organic fouling of polymeric membranes, several studies have investigated the effects of organic foulants in the WWTP tertiary effluent [20], [27], [28], [29]. However, few studies have been proven to organic fouling on the ceramic membrane, especially applied for treatment of the WWTP tertiary effluent. For a fundamental understanding of the organic fouling propensity, a commonly used approach is the use of a single organic matter to investigate the individual effects under well-defined conditions [30]. However, it is not available to investigate the individual effects of organic matter in the WWTP tertiary effluent consist of a complex mixture of organic and inorganic substances. Furthermore, Law et al. [31] and Zazouli et al. [32] reported that individual fouling behavior with the representative organic matter has exhibited distinct fouling from real feed water due to the interactions between various foulants in the WWTP tertiary effluent. Therefore, for a better understanding the fouling propensity of ceramic membranes, using a complex organic matter divided into NOOMs and MOOMs are required.

In order to better understand the effect of NOOMs and MOOMs on organic fouling of ceramic membranes, a preferential process for selective removal of NOOMs and MOOMs in the WWTP tertiary effluent is required to prepare feed solutions of ceramic membranes. In particular, recent studies have demonstrated that carbon-based adsorbents (CBAs) exhibit selectivity toward aromatic contaminants by the π-π interaction between hydrophobic adsorbents and organic foulants [33], [34]. Therefore, the adsorption using CBAs can be applied as a pretreatment step for the preparation of feed solutions with the various organic compositions at the identical organic loading.

The purpose of this study is to reveal the most influencing organic components in WWTP tertiary effluent during the organic fouling of ceramic membranes, thus, to provide effective pretreatment strategies of WWTP effluent. For the selective removal of organic components, three CBAs as a pretreatment step prior to the ceramic membrane filtration.

Section snippets

Preparation of feed and adsorbents

The WWTP tertiary effluent collected from Daejeon WWTP in South Korea was filtered through a GF/C glass-fiber filter (Whatman, Germany) to remove particulate matter. To investigate the role of organic components in WWTP tertiary effluent, the selective removal of organic matter was conducted using three CBAs: granular activated carbon (GAC), expanded graphite (EG), and multi-walled carbon nanotubes (MWNTs). For the preparation of adsorbents, GAC (Norit, Nederland) was washed using deionized

Properties of ceramic membranes

Ceramic membranes were prepared by the filtration-coating method with Al2O3 and ZrO2 nanoparticles as reported in our previous work [42]. Fig. 1 shows the SEM images of cross-section and surface of the ceramic membranes. The active and support layer of the ceramic membrane was clearly shown in the cross-section image and the average thickness of the active layer was approximately 12 μm inside of membrane surface. The surface of the ceramic membrane was completely deposited with aluminum and

Conclusions

In this study, we clearly showed that the composition of organic matters can significantly influence the fouling propensity of ceramic membranes even at the similar organic concentration. The presence of MOOMs including AP-like and SMP-like substances led to more severe flux decline and frequent chemical cleaning of ceramic membranes. These observations clearly suggest that the proper pretreatment process for selective removal of MOOMs can be an effective strategy for mitigation of organic

Acknowledgments

This study was supported by the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT and Future Planning (MSIP) (2017R1A2B3006354) through the National Research Foundation of Korea.

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