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Membrane electro-oxidizer: A new hybrid membrane system with electrochemical oxidation for enhanced organics and fouling control
Accepted Manuscript Membrane electro-oxidizer: A new hybrid membrane system with electrochemical oxidation for enhanced organics and fouling control Naresh Mameda, Hyung-June Park, Kwang-Ho Choo PII:
S0043-1354(17)30742-X
DOI:
10.1016/j.watres.2017.09.009
Reference:
WR 13203
To appear in:
Water Research
Received Date: 12 May 2017 Revised Date:
10 July 2017
Accepted Date: 3 September 2017
Please cite this article as: Mameda, N., Park, H.-J., Choo, K.-H., Membrane electro-oxidizer: A new hybrid membrane system with electrochemical oxidation for enhanced organics and fouling control, Water Research (2017), doi: 10.1016/j.watres.2017.09.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Membrane electro-oxidizer: A new hybrid
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membrane system with electrochemical oxidation
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for enhanced organics and fouling control
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Naresh Mameda a, Hyung-June Park b, Kwang-Ho Choo a,b,*
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Advanced Institute of Water Industry, Kyungpook National University, 80 Daehak-ro, Buk-
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gu, Daegu, 41566, Republic of Korea b
Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea
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*
Corresponding author
Tel: +82-53-950-7585
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Fax: +82-53-950-6579
E-mail: [email protected]
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The first two authors contributed equally
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Abstract
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The synergistic combination of membrane filtration with advanced oxidation is of particular
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interest for next-generation wastewater treatment technologies. A membrane electro-oxidizer
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(MEO) hybridizing a submerged microfilter and an electrochemical cell was developed and
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investigated for tertiary treatment of secondary industrial (textile) wastewater effluent.
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Laboratory- and pilot-scale MEO systems were designed and evaluated for treatment
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efficiency and membrane fouling control. The MEO achieved substantial removal of color
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(50–90%), turbidity (>90%), and bacteria (>4 log) as well as chemical oxygen demand (13–
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31%) and 1,4-dioxane (~25–53%). Fluorescence-based parallel factor analysis disclosed the
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degradation of humic-like organics with fluorophores. Size exclusion chromatograms with
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organic carbon detection confirmed the removal of specific organic molecules with ~100 Da.
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While investigating the effects of oxidant quenching agents, reactive chlorine species and
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hydrogen peroxide were found to be most responsible for the anodic oxidation of secondary
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effluent organics. The efficacy of membrane fouling mitigation by the MEO was greater
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when higher electric current densities were applied, but was not dependent on the number of
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electrochemical cells installed. The MEO is a promising technology for enhanced organics
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removal with simultaneous fouling control due to its multifunctional active oxidants.
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Keywords
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Membrane electro-oxidizer; Electrochemical cell; Hybrid process; Fouling mitigation;
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Oxidant formation
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Introduction
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The application of membrane filtration for advanced wastewater treatment has become
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popular in recent decades due to its advantages over other treatment processes (Brunetti et al.,
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2015). These advantages include high product water quality (Ioannou-Ttofa et al., 2017), a
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relatively small footprint (Sutzkover-Gutman et al., 2010), and affordable installation costs
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(Gao et al., 2011). However, the major factor limiting the extensive use of membranes is
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fouling (Meng et al., 2017), which leads to reduced productivity, increased maintenance costs,
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and high energy consumption (Fan et al., 2001). In addition, porous membranes are unable to
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handle micropollutants, which are frequently encountered in industrial wastewater. Advanced
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oxidation processes (AOPs) are thus often considered the most attractive alternative for
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removing refractory/hazardous materials present in wastewater. The AOPs generate and
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utilize reactive oxygen species (e.g., ·OH, O2-•, 1O2, H2O2) to break down large organic
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compounds into smaller molecules and eventually mineralize them (Lee, 2015; Liu et al.,
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2012). AOP technologies can be linked to membrane processes to control high fouling
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potential and hence reduce the severity of membrane fouling (Tang et al., 2017; Wei et al.,
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2016; Zhang et al., 2015). Integrating the AOP treatment with membrane filtration is of great
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interest because this combination can improve the quality of the product water (Puspita et al.,
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2011) and alleviate membrane fouling (Prado et al., 2017; Zhang et al., 2015).
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Recently, electrochemically aided AOPs have received significant attention as promising
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tools for advanced water treatment (Chaplin, 2014; Moreira et al., 2017; Zheng et al., 2017).
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Compared to conventional AOPs (e.g., UV/H2O2 and O3), the electrochemical processes have
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several advantages: 1) chemical-free operation, 2) enhanced organic degradation via direct
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electron transfer reaction, and 3) low-cost operation. Some efforts have been made to
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incorporate electrochemical techniques into membrane filtration processes and thereby to 3
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sulfanilic acid) in wastewater (Zaky and Chaplin, 2014; Zheng et al., 2017). Besides, a
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number of studies focused on combining electrochemical methods with membrane
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bioreactors to reduce biofouling (Ensano et al., 2016; Li et al., 2016; Liu et al., 2013; Ma et
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al., 2015; Wang et al., 2013). Membrane fouling was mitigated by the enhanced electrostatic
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repulsive force between sludge flocs and the membrane as well as the oxidation of foulants.
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In our previous study, electrochemical oxidation before microfiltration was able to mitigate
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membrane fouling during the membrane treatment of municipal wastewater by the
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decomposition of dissolved and particulate organic matter (Park et al., 2013). A combination
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of electrocoagulation and membrane filtration is another technique to remove heavy metals,
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microorganisms, and colloids with simultaneous membrane fouling reduction (Bani-Melhem
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and Elektorowicz, 2010; Mavrov et al., 2006; Zhu et al., 2005). However, sacrificing anodes
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and producing chemical sludge are the challenges to be overcome in this electrochemical
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membrane process.
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The purpose of the present study was to evaluate the membrane electro-oxidizer (MEO),
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which is a new hybrid membrane process for advanced treatment of industrial wastewater
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effluent, coupled with submerged ceramic microfilters with stably oxidizing anodes in a
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single reactor. The effects of electric current density, air sparging, and membrane relaxation
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on the treatment performances and membrane fouling control were investigated using batch
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and continuous process operations. In-depth examinations on the formation of
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electrochemically activated oxidants responsible for the decolorization of textile dyes and
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disintegration of organic colloids were performed. Pilot-scale field tests were conducted
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under different electric current densities to demonstrate the feasibility of the newly developed
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MEO process.
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Materials and methods
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Feed wastewater
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Secondary effluent samples were collected right before the sand filtration process in the
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Dalseocheon Industrial (Textile) Wastewater Treatment Plant, Daegu, Korea (Fig. S1). The
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samples were shipped to the laboratory for hybrid membrane treatment experiments and
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water quality analyses. The characteristics of the wastewater samples are presented in Table 1.
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Membrane and electrode
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Flat sheet ceramic membranes (Ceraflo, Singapore), made of alumina and with a nominal
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pore size of 0.5 µm, were used for laboratory- and pilot-scale membrane systems. For
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laboratory experiments, one flat sheet with an effective surface area of 0.1071 m2 was used,
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whereas a CP42 module with 42 flat sheets corresponding to a membrane area of 4.5 m2 was
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used for field tests.
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An IrO2/Ti anode (Elchemtech, Korea) and a 316-L stainless steel cathode were used. For
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the laboratory MEO, an electrochemical cell consisted of two IrO2/Ti anodes with dimensions
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of 50 cm × 11 cm each and three stainless steel cathodes with the same dimensions and an
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overall active surface area of 0.22 m2. The pilot-scale electrochemical cell was composed of
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electrodes with dimensions of 100 cm × 11 cm. Each cell had twelve IrO2/Ti anodes and
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thirteen 316-L stainless steel cathodes with an active surface area of 2.64 m2.
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Membrane electro-oxidizer operation
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Figure 1 shows the laboratory-scale MEO system composed of a 10-L working volume
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rectangular reactor, a single flat sheet ceramic membrane, a 0.22-m2 electrode cell, an air 5
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electrode cell, and air diffuser were aligned vertically to effectively convey the gases and
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oxidants generated from the diffuser and electrode to the membrane. The membrane was
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operated at a constant flux of 28 L/m2-h corresponding to a flow rate of 3.0 L/h. Continuous
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and cyclic membrane filtration were applied. For the cyclic operation, membrane relaxation
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was performed for 1 min every 20 min. The electric current density was changed from 0 to
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0.5 A/L (corresponding to 0-23 A/m2). The air was supplied at a flow rate of 1.0 L/min
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through a diffuser underneath the electric cell. A level sensor was installed to keep the reactor
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water level constant during continuous MEO operation. The secondary effluent was fed to the
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reactor in quantities equal to that of the permeate discharged. During the batch MEO
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operation, no feed was supplied, but continuous suction was performed and permeate was
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returned to the reactor. A pressure transducer (ZSE40F, SMC, Japan) was used to monitor the
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variation in the transmembrane pressure with time and the pressure data was recorded on a
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laptop computer using a data logger (Petit Logger GL100, Graphtec, USA). Permeate
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samples (final products) were collected every hour using a fraction collector (Universal
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Fraction Collector, Eldex, USA).
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The pilot-scale MEO had a similar reactor design with a working volume of 1.2 m3, a
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Ceraflo CP42 membrane module, two or three electrochemical cells with an active electrode
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area of 2.64 m2 each, several sensors (pH, temperature, conductivity, dissolved oxygen,
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oxidation-reduction potential in addition to a level sensor), on-site/remote real-time data
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acquisition units, and automated control systems (Fig. 1b). The secondary effluent was
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pumped to a feed tank and then supplied to the reactor using a feed pump controlled by the
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level sensor. The pilot-scale membrane flux was set at 15 L/m2-h corresponding to a flow rate
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of 1.125 L/min controlled by a volumetric flow controller. The air was supplied through
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every 10 min. The electric current density was changed from 0 to 150 A/m3 (corresponding to
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0-23 A/m2) while installing two or three electrochemical cells in the reactor. Detailed
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conditions are presented in Table 2 with the treatment performance results. Feed and
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permeate samples were collected daily.
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When each membrane experiment was completed in the laboratory, the membrane was
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cleaned chemically. The cleaning procedure included the following steps: 1) the membrane
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was washed with deionized water and then soaked in a 1.0% citric acid solution for 2 h; 2)
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the membrane was flushed using deionized water and then immersed in a 0.12% NaOCl
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solution for another 2 h. 3) Finally, the chemically cleaned membrane was rinsed using
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deionized water. The pilot-scale membrane was cleaned on-site after each experiment almost
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in the same manner, but with a longer cleaning duration. The membrane module was soaked
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in 1.0% citric acid for 24 h and then in 0.24% NaOCl for another 24 h.
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Oxidants quenching
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To determine the oxidants/radicals that are responsible for electrochemical reactions in the
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MEO, several different chemical quenching agents, such as t-butanol (99%, Duksan, Korea),
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vitamin C (99%, Duksan, Korea), cinnamic acid (99%, Sigma-Aldrich, USA), ammonium
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acetate (99%, Dae Jung, Korea), and sodium sulfite (97%, Yakuri, Japan) were used. It is well
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known that t-butanol can quench hydroxyl radicals (Fang et al., 2017); vitamin C, superoxide
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and hydroxyl radicals (Xu et al., 2017); cinnamic acid, ozone (Flyunt et al., 2003);
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ammonium acetate, chlorine/dichloride radicals (Kristiana et al., 2014); and sodium sulfite,
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reactive chlorine species and hydrogen peroxide (Keen et al., 2013). For each quenching test,
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the chemical agent (1.5 eq) was dosed to an 800-mL reactor at the beginning of the test
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considering the electric current density (0.5 A/L) during the 2-h reaction.
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Analytical methods
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The true color was determined with Hach Method 8000 using a Hach spectrophotometer
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(DR/4000U). Feed (secondary effluent) samples were filtered using a 0.45-µm filter before
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color measurements, but no pretreatment was performed on permeate samples. The solution
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pH, conductivity, and turbidity levels were measured using a calibrated pH meter (4 STAR,
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Orion, USA), conductivity meter (Cond 340i, WTW, Germany), and a turbidity meter (Model
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2100, Hatch, USA), respectively. The total bacterial count was determined by cultivation on
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Petrifilm plates (3M Petrifilm Aerobic Count Plates, USA) in an incubator (MIR-553,
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Sanyo, Japan) at 35 ± 2 °C for 48 h.
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The total organic carbon content was determined using a Sievers 900 TOC analyzer. The
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metal concentration was measured using an inductively coupled plasma spectrophotometer
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(ICP-OES Optima 2100DV, Perkin-Elmer, USA). The chemical oxygen demand was
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determined based on the dichromate closed reflux method. The size distribution of particles
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present in the secondary effluent (feed) was determined using a laser diffraction particle size
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analyzer (LS 13320, Beckman Coulter, USA).
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Fluorescence excitation-emission matrix (Ex/Em) spectra were obtained using a
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spectrofluorophotometer (RF-6000, Shimadzu, Japan) for the excitation and emission
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wavelength range of 200–550 nm at a 2-nm interval. To characterize the chemical
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composition of organic matter in the feed and the permeate samples, the parallel factor
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analyses of the fluorescence Ex/Em peak matrices were conducted according to the tutorial
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published by Murphy et al. (2013). The data sets were modeled using the drEEM toolbox in
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Matlab®.
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used for the characterization of organic matter present in the feed and the permeate samples.
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The analytical system was composed of a high-performance liquid chromatograph (Infinity
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1200, Agilent Technologies, USA) with size exclusion columns. The separated compounds
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were detected by UV absorbance at 254 nm and total organic carbon measurements.
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Results and discussion
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Effect of electrical current density on color and turbidity control
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The effects of electrical current on color and turbidity removal were examined using a
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laboratory membrane electro-oxidizer in a batch mode (Fig. 2). No color removal was
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achieved without electricity (0 A/L), whereas significant disappearance of color occurred
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with electricity (0.1–0.5 A/L). The larger the current density, the faster the color removal. For
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instance, >90% of the color was removed within an hour at an electric current density of 0.5
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A/L. Observed experimental data fit the first-order kinetics better at a low current density
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(e.g., 0.1 A/L), whereas they fit the zeroth-order kinetics at high current densities (0.3 and 0.5
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A/L) (Table S1). Although electrochemical reactions are very complicated, this can be
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interpreted as follows: at low current densities (with limited electrons/holes at the electrodes),
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the competition between the organic compounds (e.g., dyes) at the anode plays a more
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important role in the reaction than their transport to the electrode surface. Therefore, the rate
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of color removal can be expressed as the first-order kinetics. In previous studies, the first-
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order kinetics was applied to the anodic oxidation of organic pollutants (e.g., azo dye, phenol,
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chemical
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electrochemically generated mediators (i.e., oxidants such as active chlorine species) (Gupta
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et al., 2007; Panizza and Cerisola, 2009; Szpyrkowicz et al., 2005). At high current densities
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oxygen
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reaction if the contaminants are degraded as soon as they reach the anode surface. Thus, the
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electrochemical oxidation can fall into a zeroth-order rate expression in this case. Another
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previous study reported the zeroth-order kinetics for electrochemical oxidation of ammonia
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with NaCl addition at 15.4 mA/cm2 (Li and Liu, 2009).
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The removal of turbid materials was well achieved (>80%) primarily due to the rejection
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of particulate matter by the 0.5-µm membrane. Indeed, the majority of the particles present in
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the feed were bigger than the size of the membrane pores (Fig. 3). The slightly higher
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turbidity removal rate (~10–15%) with electricity could be attributed to the anodic
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electrochemical degradation of organic colloids that otherwise may pass through the
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membrane. Throughout the rest of the laboratory study, the MEO was run at a current density
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of 0.5 A/L (i.e., 23 A/m2), whose performance was compared with that of the control run
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(with no electric current).
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Continuous MEO treatment performance: color and turbidity removal
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Figure 4 shows the color removal efficiencies with time under different electrical and
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physical cleaning conditions during continuous membrane reactor operation at a hydraulic
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residence time of 3.3 h. As expected, the MEO with a current density of 0.5 A/L achieved
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substantial and sustained color removal (>60–80%), but a relatively small percentage (<30%)
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of the color was removed without electricity (submerged microfiltration alone). Membrane
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cleaning strategies, such as aeration and relaxation, had minor impacts on color removal.
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Continuous aeration (1 L/min) and/or periodic relaxation (1 min every 20-min cycle) for
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membrane cleaning had a slightly negative effect on the color removal. When both aeration
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and relaxation were employed, the color removal decreased to ~65% and 10% with and
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without electricity, respectively (Fig. 4). This is likely because of the disturbance or
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dislodgement of a secondary dynamic layer formed on the membrane surface; otherwise, it
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may play a role in rejecting the molecules. The removal of particulate and colloidal matter during continuous treatments was not
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significantly affected by the application of electricity, aeration, and relaxation (Fig. 4). It is
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likely that the microfiltration membrane itself (which has a nominal pore size of 0.5 µm)
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rejects them sufficiently with the particle size distribution in the feed.
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The MEO can not only decompose dissolved organic fraction but also reject particulate
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matter by the microfilter. This demonstrates the potential of the MEO, a hybrid of a
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membrane and an electrode, for advanced wastewater treatment of both organic and
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particulate contaminants.
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Spectroscopic and chromatographic investigation of organic substances degraded by
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MEO
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To investigate the degradation of unidentified organic matter in the feed wastewater,
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fluorescence excitation-emission (Ex/Em) matrix spectroscopy was employed. Figure 5
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shows the two component peaks extracted through parallel factor analyses from the original
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fluorescence peaks observed in the feed and the permeate samples (Figs. S2 and S3). The
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peak region located at the wavelength range of Ex/Em = 250–350/275–400 nm (component 1)
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had very strong intensities and appeared to be associated with protein-like compounds present
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in the feed (Murphy et al., 2014; Yang et al., 2014). The other peak region located at the
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wavelengths of Ex/Em = 225–400/400–500 nm (component 2) represents humic-like organic
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matter (Yu et al., 2015). Without electricity, there was no perceptible intensity change in the
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two components, but the intensity of component 2 (humic-like compound) was reduced
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greatly with electricity (left bottom of Fig. 5). This component should be susceptible to the
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attack of oxidants or radicals generated on the anode. This will be further discussed in next
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section. Further chromatographic analyses of the permeate samples obtained from microfiltration
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alone and MEO were conducted using a liquid chromatograph with ultraviolet and total
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organic carbon detectors (Fig. 6). The MEO permeate showed a relatively large reduction in
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UV254 absorbance at the elution time of 24–35 min (corresponding to the molecular weight
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range of <3,400 Da) compared to that of microfilter permeate. A substantial reduction of
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organic carbon with molecular weights of approximately 100 Da was observed with
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electrochemical oxidation (Fig. 6). This should be related to the removal of humic-like
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substances by MEO treatment.
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Mechanisms of electrochemical degradation of organic matter with chromophores and
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fluorophores
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We further investigated the types of oxidants generated by MEO that are responsible for the
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degradation of organic matter in the secondary effluent. The formation of several potential
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oxidants, such as hydroxyl radicals (OH), chlorine/dichloride radicals (⋅Cl/Cl2-•), superoxide
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radicals (O2-•), hydrogen peroxide (H2O2), and ozone (O3), were postulated and tested to
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determine the most responsible one for the MEO treatment. By quenching each oxidant as
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described in the experimental section, color removal was monitored (Fig. 7). The addition of
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a hydroxyl radical scavenger (t-butyl alcohol) had no impact on color removal, suggesting
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that hydroxyl radicals are not the main oxidizing agent. Vitamin C and cinnamic acid had no
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effect either, indicating that superoxide radicals and ozone are not generated by MEO. When
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ammonium acetate and sodium sulfite were dosed, however, the color removal decreased
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quench both reactive chlorine species and hydrogen peroxide. Therefore, anodic conversion
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of chloride ions to chlorine/dichloride radicals (Park et al., 2013; Park et al., 2009) as well as
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the oxidation of water to hydrogen peroxide (Viswanathan et al., 2015) contributes to the
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degradation of color (chromophores) and humic-like organic fluorophores via MEO.
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Membrane fouling control by MEO
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The effects of electric current density on membrane fouling were primarily tested in a batch
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membrane reactor (Fig. S4). The transmembrane pressure build-up with the electric current
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(0.5 A/L) was slower than that without electricity (control run), although the overall pressure
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build-up was not very high after 15 h of operation. Membrane fouling was delayed by
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electrochemical oxidation during MEO operations.
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The effects of electric current, air sparging, and membrane relaxation on fouling control
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were further evaluated during continuous MEO operations (Fig. 8). The transmembrane
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pressure build-up occurred relatively rapidly with the control reactor (with no electric current
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applied), even though the periodic membrane relaxation (i.e., filtration stop for 1 min every
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20 min) was provided. However, the MEO delayed membrane fouling by more than three
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times (i.e., the ratio of the time for the MEO to reach the transmembrane pressure to 30 kPa
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to that of the control was >3). In place of the periodic membrane relaxation, continuous
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aeration was applied to reduce the transmembrane pressure build-up more for both control
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and MEO systems, but a further delay was achieved with the MEO. When both aeration and
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relaxation were employed, it took more than 30 h for the MEO (with a current density of 0.5
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A/L) to reach 40 kPa in transmembrane pressure, slowing down the fouling rate markedly.
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The actions of the electrochemically generated oxidants destruct fouling layers leading to
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longer, sustained membrane permeation.
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Pilot-scale testing of MEO
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Pilot-scale MEO field tests were conducted with both aeration and relaxation because the lab-
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scale experiments demonstrated that both physical membrane cleaning strategies could
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mitigate membrane fouling effectively during continuous reactor operations. Figure 9 shows
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the transmembrane pressure build-up with time under different electric current densities. The
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transmembrane pressure increased sharply without electric input (control run) while reaching
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60 kPa within a day. However, the transmembrane pressure build-up was delayed drastically
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with electric input. For instance, the MEO at a current density of 150 A/m3 (i.e., 23 A/m2)
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was operated for 40 d until the transmembrane pressure reached 60 kPa. The transmembrane
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pressure increase with an electrode surface area of 7.92 m2 (three cells) was similar to that of
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5.28 m2 (two cells) as long as the same current density of 100 A/m3 was applied. The result
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demonstrated that the electrode surface area (or the number of electrochemical cells) had no
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significant impact on the MEO performance, but the current density did.
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Table 2 summarizes the treatment efficiency during the pilot-scale MEO tests. The
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steady-state color removal efficiency kept increasing with electricity inputs and reached 76%
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at 150 A/m3. The turbidity removal efficiency increased from 77% to 94% when the current
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density increased from 0 to 150 A/m3. The high turbidity removal efficiency without
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electricity occurred due to the rejection of particles by the membrane. The chemical oxygen
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demand removal efficiency increased up to 31%, whereas the removal of a specific chemical
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compound (1,4-dioxane) used in the textile industry was 52.9% at a current density of 150
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A/m3. The total bacteria disinfection efficiency reached ~4.5 log removals with 100 and 150
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A/m3. Without electricity, the total bacterial log removal was only <1.33, which was
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considerably lower than that achieved by the active MEO. In summary, the MEO enabled the
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removal of organics (color, chemical oxygen demand, and 1,4-dioxane) and particulate matter
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(turbidity and bacteria) by means of membrane rejection as well as anodic oxidation. Concerning the energy consumption, the pilot-scale MEO was found to require 3.11-12.0
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kWh/m3 depending on the electric current densities applied (Table S2). It seems that the
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MEO is relatively expensive compared to biological treatment processes. When compared to
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other electrochemical oxidation processes (Aquino et al., 2014; Rajkumar and Kim, 2006;
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Rajkumar and Palanivelu, 2004; Vlyssides et al., 1999), however, the MEO appears to be
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more efficient in energy demand than those (50-140 kWh/m3) while producing final effluents
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of high quality. Further investigations on economic analysis are needed with larger scale field
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tests.
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Conclusions
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A new membrane-electrode hybrid reactor (i.e., MEO) for advanced wastewater treatment
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was developed and investigated for organic and particulate matter removal as well as
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membrane fouling control. The main conclusions include:
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(1) The MEO achieved substantial color (>90%) and turbidity (>80%) removal efficiencies
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with 15-h batch reactor treatment of biologically treated secondary effluent. The
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disappearance of color followed first-order kinetics at low current densities, and zeroth-
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order kinetics at a high current density.
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(2) Physical membrane cleaning strategies, such as aeration and relaxation, had a small
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impact on color removal and a negligible effect on turbidity removal during the
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continuous operation of the laboratory MEO system.
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(3) The fluorescence spectroscopy and liquid chromatography-total organic carbon detection 15
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weight of ~100 Da during MEO treatment. Reactive chlorine species and hydrogen
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peroxide were found to be most responsible for the removal of chromophores and
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fluorophores in secondary effluent.
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(4) Membrane fouling retardation was dependent most significantly on electric current inputs, although membrane aeration and relaxation also had a certain contribution to fouling
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mitigation.
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(5) Pilot-scale tests confirmed the efficacy of MEO in the removal of organics (color,
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chemical oxygen demand, and 1,4-dioxane) and particulate matter (turbidity and total
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bacteria) as well as a substantial delay in membrane fouling (more than 40 times delayed
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Acknowledgements
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This research was supported by a convergence technology project (No. 2015001640004)
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from the Korea Environmental Industry & Technology Institute, funded by the Korea
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Ministry of Environment. The authors would like to thank Dr. Xiaolei Zhang and Huarong Yu
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for their assistance with the parallel factor analysis and discussion.
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Value 7.4 ± 0.2 3.4 ± 0.3
Turbidity (NTU)
2.5 ± 1.1
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Conductivity (mS/cm)
Color (Pt-Co Unit)
126 ± 14.5
Chemical oxygen demand (mg/L)
43.5 ± 1.5 9.5 ± 1.2
Suspended solids (mg/L)
2.5 ± 0.1
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Total organic carbon (mg/L)
Total nitrogen (mg/L)
6.94 ± 0.03
Cl- (mg/L)
306 ± 40.8
NO3- (mg/L)
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SO42- (mg/L)
1380 ± 200
Al (mg/L)
0.501 ± 0.187
71.2a ± 6.86
Ca (mg/L) Cr (mg/L)
0.062 ± 0.007 0.144 ±0.017
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Cu (mg/L) Fe (mg/L) Mg (mg/L) Mn (mg/L)
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Zn (mg/L) 178 mg/L as CaCO3
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52.0 mg/L as CaCO3
0.596 ± 0.179 12.5b ± 3.46 0.180 ± 0.040 0.066 ± 0.014 0.127 ± 0.014
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Table 2. Summary of the treatment performances of the pilot-scale MEO system operated under different current density and electrode area conditions 1,4-Dioxane removal (%)
Total bacteria removal (log removal value)
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23.9 ± 10.5
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14 ± 5.1
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91 ± 2.1
17.9 ± 4.6
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4.49 a ± 0.43
94 ± 2.5
31 ± 5.6
52.9 ± 11.6
4.51b ± 0.51
Color removal (%)
Turbidity removal (%)
0
5.28
4.8 ± 3.8
77 ± 4.3
50
5.28
49 ± 3.8
93 ± 1.1
67
5.28
52 ± 5.1
90 ± 3.3
100
5.28
65 ± 2.4
91 ± 1.9
100
7.92
71 ± 1.4
150
7.92
76 ± 5.4
Chemical oxygen demand removal (%) 6.1 ± 5.4
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13 ± 5.5
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Total bacterial log removal value before applying 150 A/m3: 0.43
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Fig. 1. Schematics of the membrane electro-oxidizer composed of submerged flat sheet ceramic microfiltration membranes and electrochemical cells used for tertiary treatment of secondary effluent: (a) laboratory-scale and (b) pilot-scale systems.
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Time (h) Fig. 2. Effects of electric current density on color and turbidity removal during batch MEO operations.
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10 0.375 µm to 100 µm
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100% 16.47 µm 15.78 µm 10.64 µm 2.088 µm 31.65 µm
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Fig. 4. Color and turbidity removal efficiencies with time as a function (f) of electric current (0 or 0.5 A/L), aeration (0 or 1 L/min), and relaxation (0 or 1 min every 20 min).
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Fig. 5. Variations in the intensity of fluorescence peak components extracted via PARAFAC model decomposition from the original fluorescence excitation-emission matrix spectrograms shown in Figs. S2 and S3.
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Fig. 6. Liquid chromatograms with ultraviolet (top) and total organic carbon (bottom) detectors for the feed and membrane permeate samples from control (left) and MEO (right)
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Sodium sulfite
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t-Butanol
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Fig. 9. Transmembrane pressure build-up over time during pilot-scale MEO testing under various electric current densities. The MEO tests at 100 A/m3 (#2) and 150 A/m3 were run with an electrode area of 7.92 m2, whereas the others were run with an electrode area of 5.28 m2.
ACCEPTED MANUSCRIPT ► A membrane electro-oxidizer integrates a membrane and an electrode for advanced treatment ► Substantive, simultaneous removal of organics and particles is achieved in a single reactor
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► Reactive chlorine species and hydrogen peroxide are dominant oxidants in the process ► Membrane fouling is alleviated due to electrochemically generated oxidants
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► Pilot-scale testing successfully demonstrates the potential of the new hybrid treatment
S0043-1354(17)30742-X
DOI:
10.1016/j.watres.2017.09.009
Reference:
WR 13203
To appear in:
Water Research
Received Date: 12 May 2017 Revised Date:
10 July 2017
Accepted Date: 3 September 2017
Please cite this article as: Mameda, N., Park, H.-J., Choo, K.-H., Membrane electro-oxidizer: A new hybrid membrane system with electrochemical oxidation for enhanced organics and fouling control, Water Research (2017), doi: 10.1016/j.watres.2017.09.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Membrane electro-oxidizer: A new hybrid
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membrane system with electrochemical oxidation
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for enhanced organics and fouling control
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Naresh Mameda a, Hyung-June Park b, Kwang-Ho Choo a,b,*
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Advanced Institute of Water Industry, Kyungpook National University, 80 Daehak-ro, Buk-
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gu, Daegu, 41566, Republic of Korea b
Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea
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*
Corresponding author
Tel: +82-53-950-7585
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Fax: +82-53-950-6579
E-mail: [email protected]
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The first two authors contributed equally
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Abstract
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The synergistic combination of membrane filtration with advanced oxidation is of particular
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interest for next-generation wastewater treatment technologies. A membrane electro-oxidizer
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(MEO) hybridizing a submerged microfilter and an electrochemical cell was developed and
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investigated for tertiary treatment of secondary industrial (textile) wastewater effluent.
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Laboratory- and pilot-scale MEO systems were designed and evaluated for treatment
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efficiency and membrane fouling control. The MEO achieved substantial removal of color
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(50–90%), turbidity (>90%), and bacteria (>4 log) as well as chemical oxygen demand (13–
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31%) and 1,4-dioxane (~25–53%). Fluorescence-based parallel factor analysis disclosed the
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degradation of humic-like organics with fluorophores. Size exclusion chromatograms with
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organic carbon detection confirmed the removal of specific organic molecules with ~100 Da.
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While investigating the effects of oxidant quenching agents, reactive chlorine species and
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hydrogen peroxide were found to be most responsible for the anodic oxidation of secondary
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effluent organics. The efficacy of membrane fouling mitigation by the MEO was greater
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when higher electric current densities were applied, but was not dependent on the number of
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electrochemical cells installed. The MEO is a promising technology for enhanced organics
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removal with simultaneous fouling control due to its multifunctional active oxidants.
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Keywords
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Membrane electro-oxidizer; Electrochemical cell; Hybrid process; Fouling mitigation;
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Oxidant formation
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Introduction
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The application of membrane filtration for advanced wastewater treatment has become
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popular in recent decades due to its advantages over other treatment processes (Brunetti et al.,
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2015). These advantages include high product water quality (Ioannou-Ttofa et al., 2017), a
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relatively small footprint (Sutzkover-Gutman et al., 2010), and affordable installation costs
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(Gao et al., 2011). However, the major factor limiting the extensive use of membranes is
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fouling (Meng et al., 2017), which leads to reduced productivity, increased maintenance costs,
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and high energy consumption (Fan et al., 2001). In addition, porous membranes are unable to
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handle micropollutants, which are frequently encountered in industrial wastewater. Advanced
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oxidation processes (AOPs) are thus often considered the most attractive alternative for
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removing refractory/hazardous materials present in wastewater. The AOPs generate and
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utilize reactive oxygen species (e.g., ·OH, O2-•, 1O2, H2O2) to break down large organic
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compounds into smaller molecules and eventually mineralize them (Lee, 2015; Liu et al.,
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2012). AOP technologies can be linked to membrane processes to control high fouling
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potential and hence reduce the severity of membrane fouling (Tang et al., 2017; Wei et al.,
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2016; Zhang et al., 2015). Integrating the AOP treatment with membrane filtration is of great
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interest because this combination can improve the quality of the product water (Puspita et al.,
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2011) and alleviate membrane fouling (Prado et al., 2017; Zhang et al., 2015).
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Recently, electrochemically aided AOPs have received significant attention as promising
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tools for advanced water treatment (Chaplin, 2014; Moreira et al., 2017; Zheng et al., 2017).
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Compared to conventional AOPs (e.g., UV/H2O2 and O3), the electrochemical processes have
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several advantages: 1) chemical-free operation, 2) enhanced organic degradation via direct
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electron transfer reaction, and 3) low-cost operation. Some efforts have been made to
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incorporate electrochemical techniques into membrane filtration processes and thereby to 3
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sulfanilic acid) in wastewater (Zaky and Chaplin, 2014; Zheng et al., 2017). Besides, a
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number of studies focused on combining electrochemical methods with membrane
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bioreactors to reduce biofouling (Ensano et al., 2016; Li et al., 2016; Liu et al., 2013; Ma et
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al., 2015; Wang et al., 2013). Membrane fouling was mitigated by the enhanced electrostatic
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repulsive force between sludge flocs and the membrane as well as the oxidation of foulants.
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In our previous study, electrochemical oxidation before microfiltration was able to mitigate
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membrane fouling during the membrane treatment of municipal wastewater by the
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decomposition of dissolved and particulate organic matter (Park et al., 2013). A combination
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of electrocoagulation and membrane filtration is another technique to remove heavy metals,
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microorganisms, and colloids with simultaneous membrane fouling reduction (Bani-Melhem
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and Elektorowicz, 2010; Mavrov et al., 2006; Zhu et al., 2005). However, sacrificing anodes
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and producing chemical sludge are the challenges to be overcome in this electrochemical
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membrane process.
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The purpose of the present study was to evaluate the membrane electro-oxidizer (MEO),
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which is a new hybrid membrane process for advanced treatment of industrial wastewater
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effluent, coupled with submerged ceramic microfilters with stably oxidizing anodes in a
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single reactor. The effects of electric current density, air sparging, and membrane relaxation
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on the treatment performances and membrane fouling control were investigated using batch
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and continuous process operations. In-depth examinations on the formation of
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electrochemically activated oxidants responsible for the decolorization of textile dyes and
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disintegration of organic colloids were performed. Pilot-scale field tests were conducted
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under different electric current densities to demonstrate the feasibility of the newly developed
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MEO process.
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Materials and methods
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Feed wastewater
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Secondary effluent samples were collected right before the sand filtration process in the
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Dalseocheon Industrial (Textile) Wastewater Treatment Plant, Daegu, Korea (Fig. S1). The
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samples were shipped to the laboratory for hybrid membrane treatment experiments and
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water quality analyses. The characteristics of the wastewater samples are presented in Table 1.
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Membrane and electrode
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Flat sheet ceramic membranes (Ceraflo, Singapore), made of alumina and with a nominal
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pore size of 0.5 µm, were used for laboratory- and pilot-scale membrane systems. For
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laboratory experiments, one flat sheet with an effective surface area of 0.1071 m2 was used,
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whereas a CP42 module with 42 flat sheets corresponding to a membrane area of 4.5 m2 was
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used for field tests.
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An IrO2/Ti anode (Elchemtech, Korea) and a 316-L stainless steel cathode were used. For
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the laboratory MEO, an electrochemical cell consisted of two IrO2/Ti anodes with dimensions
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of 50 cm × 11 cm each and three stainless steel cathodes with the same dimensions and an
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overall active surface area of 0.22 m2. The pilot-scale electrochemical cell was composed of
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electrodes with dimensions of 100 cm × 11 cm. Each cell had twelve IrO2/Ti anodes and
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thirteen 316-L stainless steel cathodes with an active surface area of 2.64 m2.
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Membrane electro-oxidizer operation
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Figure 1 shows the laboratory-scale MEO system composed of a 10-L working volume
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rectangular reactor, a single flat sheet ceramic membrane, a 0.22-m2 electrode cell, an air 5
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electrode cell, and air diffuser were aligned vertically to effectively convey the gases and
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oxidants generated from the diffuser and electrode to the membrane. The membrane was
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operated at a constant flux of 28 L/m2-h corresponding to a flow rate of 3.0 L/h. Continuous
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and cyclic membrane filtration were applied. For the cyclic operation, membrane relaxation
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was performed for 1 min every 20 min. The electric current density was changed from 0 to
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0.5 A/L (corresponding to 0-23 A/m2). The air was supplied at a flow rate of 1.0 L/min
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through a diffuser underneath the electric cell. A level sensor was installed to keep the reactor
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water level constant during continuous MEO operation. The secondary effluent was fed to the
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reactor in quantities equal to that of the permeate discharged. During the batch MEO
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operation, no feed was supplied, but continuous suction was performed and permeate was
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returned to the reactor. A pressure transducer (ZSE40F, SMC, Japan) was used to monitor the
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variation in the transmembrane pressure with time and the pressure data was recorded on a
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laptop computer using a data logger (Petit Logger GL100, Graphtec, USA). Permeate
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samples (final products) were collected every hour using a fraction collector (Universal
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Fraction Collector, Eldex, USA).
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The pilot-scale MEO had a similar reactor design with a working volume of 1.2 m3, a
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Ceraflo CP42 membrane module, two or three electrochemical cells with an active electrode
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area of 2.64 m2 each, several sensors (pH, temperature, conductivity, dissolved oxygen,
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oxidation-reduction potential in addition to a level sensor), on-site/remote real-time data
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acquisition units, and automated control systems (Fig. 1b). The secondary effluent was
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pumped to a feed tank and then supplied to the reactor using a feed pump controlled by the
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level sensor. The pilot-scale membrane flux was set at 15 L/m2-h corresponding to a flow rate
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of 1.125 L/min controlled by a volumetric flow controller. The air was supplied through
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every 10 min. The electric current density was changed from 0 to 150 A/m3 (corresponding to
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0-23 A/m2) while installing two or three electrochemical cells in the reactor. Detailed
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conditions are presented in Table 2 with the treatment performance results. Feed and
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permeate samples were collected daily.
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When each membrane experiment was completed in the laboratory, the membrane was
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cleaned chemically. The cleaning procedure included the following steps: 1) the membrane
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was washed with deionized water and then soaked in a 1.0% citric acid solution for 2 h; 2)
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the membrane was flushed using deionized water and then immersed in a 0.12% NaOCl
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solution for another 2 h. 3) Finally, the chemically cleaned membrane was rinsed using
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deionized water. The pilot-scale membrane was cleaned on-site after each experiment almost
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in the same manner, but with a longer cleaning duration. The membrane module was soaked
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in 1.0% citric acid for 24 h and then in 0.24% NaOCl for another 24 h.
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Oxidants quenching
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To determine the oxidants/radicals that are responsible for electrochemical reactions in the
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MEO, several different chemical quenching agents, such as t-butanol (99%, Duksan, Korea),
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vitamin C (99%, Duksan, Korea), cinnamic acid (99%, Sigma-Aldrich, USA), ammonium
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acetate (99%, Dae Jung, Korea), and sodium sulfite (97%, Yakuri, Japan) were used. It is well
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known that t-butanol can quench hydroxyl radicals (Fang et al., 2017); vitamin C, superoxide
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and hydroxyl radicals (Xu et al., 2017); cinnamic acid, ozone (Flyunt et al., 2003);
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ammonium acetate, chlorine/dichloride radicals (Kristiana et al., 2014); and sodium sulfite,
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reactive chlorine species and hydrogen peroxide (Keen et al., 2013). For each quenching test,
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the chemical agent (1.5 eq) was dosed to an 800-mL reactor at the beginning of the test
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considering the electric current density (0.5 A/L) during the 2-h reaction.
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Analytical methods
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The true color was determined with Hach Method 8000 using a Hach spectrophotometer
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(DR/4000U). Feed (secondary effluent) samples were filtered using a 0.45-µm filter before
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color measurements, but no pretreatment was performed on permeate samples. The solution
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pH, conductivity, and turbidity levels were measured using a calibrated pH meter (4 STAR,
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Orion, USA), conductivity meter (Cond 340i, WTW, Germany), and a turbidity meter (Model
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2100, Hatch, USA), respectively. The total bacterial count was determined by cultivation on
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Petrifilm plates (3M Petrifilm Aerobic Count Plates, USA) in an incubator (MIR-553,
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Sanyo, Japan) at 35 ± 2 °C for 48 h.
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The total organic carbon content was determined using a Sievers 900 TOC analyzer. The
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metal concentration was measured using an inductively coupled plasma spectrophotometer
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(ICP-OES Optima 2100DV, Perkin-Elmer, USA). The chemical oxygen demand was
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determined based on the dichromate closed reflux method. The size distribution of particles
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present in the secondary effluent (feed) was determined using a laser diffraction particle size
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analyzer (LS 13320, Beckman Coulter, USA).
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Fluorescence excitation-emission matrix (Ex/Em) spectra were obtained using a
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spectrofluorophotometer (RF-6000, Shimadzu, Japan) for the excitation and emission
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wavelength range of 200–550 nm at a 2-nm interval. To characterize the chemical
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composition of organic matter in the feed and the permeate samples, the parallel factor
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analyses of the fluorescence Ex/Em peak matrices were conducted according to the tutorial
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published by Murphy et al. (2013). The data sets were modeled using the drEEM toolbox in
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Matlab®.
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used for the characterization of organic matter present in the feed and the permeate samples.
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The analytical system was composed of a high-performance liquid chromatograph (Infinity
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1200, Agilent Technologies, USA) with size exclusion columns. The separated compounds
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were detected by UV absorbance at 254 nm and total organic carbon measurements.
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Results and discussion
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Effect of electrical current density on color and turbidity control
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The effects of electrical current on color and turbidity removal were examined using a
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laboratory membrane electro-oxidizer in a batch mode (Fig. 2). No color removal was
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achieved without electricity (0 A/L), whereas significant disappearance of color occurred
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with electricity (0.1–0.5 A/L). The larger the current density, the faster the color removal. For
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instance, >90% of the color was removed within an hour at an electric current density of 0.5
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A/L. Observed experimental data fit the first-order kinetics better at a low current density
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(e.g., 0.1 A/L), whereas they fit the zeroth-order kinetics at high current densities (0.3 and 0.5
200
A/L) (Table S1). Although electrochemical reactions are very complicated, this can be
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interpreted as follows: at low current densities (with limited electrons/holes at the electrodes),
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the competition between the organic compounds (e.g., dyes) at the anode plays a more
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important role in the reaction than their transport to the electrode surface. Therefore, the rate
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of color removal can be expressed as the first-order kinetics. In previous studies, the first-
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order kinetics was applied to the anodic oxidation of organic pollutants (e.g., azo dye, phenol,
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chemical
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electrochemically generated mediators (i.e., oxidants such as active chlorine species) (Gupta
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et al., 2007; Panizza and Cerisola, 2009; Szpyrkowicz et al., 2005). At high current densities
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oxygen
demand,
ammonia),
assuming
their
indirect
reactions
with
9
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reaction if the contaminants are degraded as soon as they reach the anode surface. Thus, the
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electrochemical oxidation can fall into a zeroth-order rate expression in this case. Another
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previous study reported the zeroth-order kinetics for electrochemical oxidation of ammonia
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with NaCl addition at 15.4 mA/cm2 (Li and Liu, 2009).
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The removal of turbid materials was well achieved (>80%) primarily due to the rejection
215
of particulate matter by the 0.5-µm membrane. Indeed, the majority of the particles present in
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the feed were bigger than the size of the membrane pores (Fig. 3). The slightly higher
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turbidity removal rate (~10–15%) with electricity could be attributed to the anodic
218
electrochemical degradation of organic colloids that otherwise may pass through the
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membrane. Throughout the rest of the laboratory study, the MEO was run at a current density
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of 0.5 A/L (i.e., 23 A/m2), whose performance was compared with that of the control run
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(with no electric current).
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Continuous MEO treatment performance: color and turbidity removal
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Figure 4 shows the color removal efficiencies with time under different electrical and
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physical cleaning conditions during continuous membrane reactor operation at a hydraulic
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residence time of 3.3 h. As expected, the MEO with a current density of 0.5 A/L achieved
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substantial and sustained color removal (>60–80%), but a relatively small percentage (<30%)
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of the color was removed without electricity (submerged microfiltration alone). Membrane
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cleaning strategies, such as aeration and relaxation, had minor impacts on color removal.
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Continuous aeration (1 L/min) and/or periodic relaxation (1 min every 20-min cycle) for
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membrane cleaning had a slightly negative effect on the color removal. When both aeration
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and relaxation were employed, the color removal decreased to ~65% and 10% with and
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without electricity, respectively (Fig. 4). This is likely because of the disturbance or
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dislodgement of a secondary dynamic layer formed on the membrane surface; otherwise, it
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may play a role in rejecting the molecules. The removal of particulate and colloidal matter during continuous treatments was not
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significantly affected by the application of electricity, aeration, and relaxation (Fig. 4). It is
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likely that the microfiltration membrane itself (which has a nominal pore size of 0.5 µm)
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rejects them sufficiently with the particle size distribution in the feed.
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The MEO can not only decompose dissolved organic fraction but also reject particulate
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matter by the microfilter. This demonstrates the potential of the MEO, a hybrid of a
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membrane and an electrode, for advanced wastewater treatment of both organic and
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particulate contaminants.
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Spectroscopic and chromatographic investigation of organic substances degraded by
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MEO
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To investigate the degradation of unidentified organic matter in the feed wastewater,
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fluorescence excitation-emission (Ex/Em) matrix spectroscopy was employed. Figure 5
249
shows the two component peaks extracted through parallel factor analyses from the original
250
fluorescence peaks observed in the feed and the permeate samples (Figs. S2 and S3). The
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peak region located at the wavelength range of Ex/Em = 250–350/275–400 nm (component 1)
252
had very strong intensities and appeared to be associated with protein-like compounds present
253
in the feed (Murphy et al., 2014; Yang et al., 2014). The other peak region located at the
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wavelengths of Ex/Em = 225–400/400–500 nm (component 2) represents humic-like organic
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matter (Yu et al., 2015). Without electricity, there was no perceptible intensity change in the
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two components, but the intensity of component 2 (humic-like compound) was reduced
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greatly with electricity (left bottom of Fig. 5). This component should be susceptible to the
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attack of oxidants or radicals generated on the anode. This will be further discussed in next
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section. Further chromatographic analyses of the permeate samples obtained from microfiltration
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alone and MEO were conducted using a liquid chromatograph with ultraviolet and total
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organic carbon detectors (Fig. 6). The MEO permeate showed a relatively large reduction in
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UV254 absorbance at the elution time of 24–35 min (corresponding to the molecular weight
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range of <3,400 Da) compared to that of microfilter permeate. A substantial reduction of
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organic carbon with molecular weights of approximately 100 Da was observed with
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electrochemical oxidation (Fig. 6). This should be related to the removal of humic-like
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substances by MEO treatment.
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Mechanisms of electrochemical degradation of organic matter with chromophores and
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fluorophores
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We further investigated the types of oxidants generated by MEO that are responsible for the
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degradation of organic matter in the secondary effluent. The formation of several potential
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oxidants, such as hydroxyl radicals (OH), chlorine/dichloride radicals (⋅Cl/Cl2-•), superoxide
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radicals (O2-•), hydrogen peroxide (H2O2), and ozone (O3), were postulated and tested to
275
determine the most responsible one for the MEO treatment. By quenching each oxidant as
276
described in the experimental section, color removal was monitored (Fig. 7). The addition of
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a hydroxyl radical scavenger (t-butyl alcohol) had no impact on color removal, suggesting
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that hydroxyl radicals are not the main oxidizing agent. Vitamin C and cinnamic acid had no
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effect either, indicating that superoxide radicals and ozone are not generated by MEO. When
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ammonium acetate and sodium sulfite were dosed, however, the color removal decreased
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quench both reactive chlorine species and hydrogen peroxide. Therefore, anodic conversion
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of chloride ions to chlorine/dichloride radicals (Park et al., 2013; Park et al., 2009) as well as
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the oxidation of water to hydrogen peroxide (Viswanathan et al., 2015) contributes to the
285
degradation of color (chromophores) and humic-like organic fluorophores via MEO.
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Membrane fouling control by MEO
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The effects of electric current density on membrane fouling were primarily tested in a batch
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membrane reactor (Fig. S4). The transmembrane pressure build-up with the electric current
290
(0.5 A/L) was slower than that without electricity (control run), although the overall pressure
291
build-up was not very high after 15 h of operation. Membrane fouling was delayed by
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electrochemical oxidation during MEO operations.
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The effects of electric current, air sparging, and membrane relaxation on fouling control
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were further evaluated during continuous MEO operations (Fig. 8). The transmembrane
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pressure build-up occurred relatively rapidly with the control reactor (with no electric current
296
applied), even though the periodic membrane relaxation (i.e., filtration stop for 1 min every
297
20 min) was provided. However, the MEO delayed membrane fouling by more than three
298
times (i.e., the ratio of the time for the MEO to reach the transmembrane pressure to 30 kPa
299
to that of the control was >3). In place of the periodic membrane relaxation, continuous
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aeration was applied to reduce the transmembrane pressure build-up more for both control
301
and MEO systems, but a further delay was achieved with the MEO. When both aeration and
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relaxation were employed, it took more than 30 h for the MEO (with a current density of 0.5
303
A/L) to reach 40 kPa in transmembrane pressure, slowing down the fouling rate markedly.
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The actions of the electrochemically generated oxidants destruct fouling layers leading to
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longer, sustained membrane permeation.
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Pilot-scale testing of MEO
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Pilot-scale MEO field tests were conducted with both aeration and relaxation because the lab-
309
scale experiments demonstrated that both physical membrane cleaning strategies could
310
mitigate membrane fouling effectively during continuous reactor operations. Figure 9 shows
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the transmembrane pressure build-up with time under different electric current densities. The
312
transmembrane pressure increased sharply without electric input (control run) while reaching
313
60 kPa within a day. However, the transmembrane pressure build-up was delayed drastically
314
with electric input. For instance, the MEO at a current density of 150 A/m3 (i.e., 23 A/m2)
315
was operated for 40 d until the transmembrane pressure reached 60 kPa. The transmembrane
316
pressure increase with an electrode surface area of 7.92 m2 (three cells) was similar to that of
317
5.28 m2 (two cells) as long as the same current density of 100 A/m3 was applied. The result
318
demonstrated that the electrode surface area (or the number of electrochemical cells) had no
319
significant impact on the MEO performance, but the current density did.
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Table 2 summarizes the treatment efficiency during the pilot-scale MEO tests. The
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steady-state color removal efficiency kept increasing with electricity inputs and reached 76%
322
at 150 A/m3. The turbidity removal efficiency increased from 77% to 94% when the current
323
density increased from 0 to 150 A/m3. The high turbidity removal efficiency without
324
electricity occurred due to the rejection of particles by the membrane. The chemical oxygen
325
demand removal efficiency increased up to 31%, whereas the removal of a specific chemical
326
compound (1,4-dioxane) used in the textile industry was 52.9% at a current density of 150
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A/m3. The total bacteria disinfection efficiency reached ~4.5 log removals with 100 and 150
328
A/m3. Without electricity, the total bacterial log removal was only <1.33, which was
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considerably lower than that achieved by the active MEO. In summary, the MEO enabled the
330
removal of organics (color, chemical oxygen demand, and 1,4-dioxane) and particulate matter
331
(turbidity and bacteria) by means of membrane rejection as well as anodic oxidation. Concerning the energy consumption, the pilot-scale MEO was found to require 3.11-12.0
333
kWh/m3 depending on the electric current densities applied (Table S2). It seems that the
334
MEO is relatively expensive compared to biological treatment processes. When compared to
335
other electrochemical oxidation processes (Aquino et al., 2014; Rajkumar and Kim, 2006;
336
Rajkumar and Palanivelu, 2004; Vlyssides et al., 1999), however, the MEO appears to be
337
more efficient in energy demand than those (50-140 kWh/m3) while producing final effluents
338
of high quality. Further investigations on economic analysis are needed with larger scale field
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tests.
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Conclusions
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A new membrane-electrode hybrid reactor (i.e., MEO) for advanced wastewater treatment
343
was developed and investigated for organic and particulate matter removal as well as
344
membrane fouling control. The main conclusions include:
345
(1) The MEO achieved substantial color (>90%) and turbidity (>80%) removal efficiencies
346
with 15-h batch reactor treatment of biologically treated secondary effluent. The
347
disappearance of color followed first-order kinetics at low current densities, and zeroth-
348
order kinetics at a high current density.
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(2) Physical membrane cleaning strategies, such as aeration and relaxation, had a small
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impact on color removal and a negligible effect on turbidity removal during the
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continuous operation of the laboratory MEO system.
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(3) The fluorescence spectroscopy and liquid chromatography-total organic carbon detection 15
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354
weight of ~100 Da during MEO treatment. Reactive chlorine species and hydrogen
355
peroxide were found to be most responsible for the removal of chromophores and
356
fluorophores in secondary effluent.
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(4) Membrane fouling retardation was dependent most significantly on electric current inputs, although membrane aeration and relaxation also had a certain contribution to fouling
359
mitigation.
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(5) Pilot-scale tests confirmed the efficacy of MEO in the removal of organics (color,
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chemical oxygen demand, and 1,4-dioxane) and particulate matter (turbidity and total
362
bacteria) as well as a substantial delay in membrane fouling (more than 40 times delayed
363
at a current density of 150 A/m3 compared to the control).
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Acknowledgements
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This research was supported by a convergence technology project (No. 2015001640004)
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from the Korea Environmental Industry & Technology Institute, funded by the Korea
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Ministry of Environment. The authors would like to thank Dr. Xiaolei Zhang and Huarong Yu
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for their assistance with the parallel factor analysis and discussion.
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Zheng, J., Ma, J., Wang, Z., Xu, S., Waite, T.D., Wu, Z., 2017. Contaminant removal from source waters using cathodic electrochemical membrane filtration: Mechanisms and implications. Environ. Sci. Technol.
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ACCEPTED MANUSCRIPT Table 1. The characteristics of the secondary effluent (feed) used Parameter pH
Value 7.4 ± 0.2 3.4 ± 0.3
Turbidity (NTU)
2.5 ± 1.1
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Conductivity (mS/cm)
Color (Pt-Co Unit)
126 ± 14.5
Chemical oxygen demand (mg/L)
43.5 ± 1.5 9.5 ± 1.2
Suspended solids (mg/L)
2.5 ± 0.1
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Total organic carbon (mg/L)
Total nitrogen (mg/L)
6.94 ± 0.03
Cl- (mg/L)
306 ± 40.8
NO3- (mg/L)
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1.56 ± 0.34
SO42- (mg/L)
1380 ± 200
Al (mg/L)
0.501 ± 0.187
71.2a ± 6.86
Ca (mg/L) Cr (mg/L)
0.062 ± 0.007 0.144 ±0.017
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Cu (mg/L) Fe (mg/L) Mg (mg/L) Mn (mg/L)
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Ni (mg/L)
b
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Zn (mg/L) 178 mg/L as CaCO3
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52.0 mg/L as CaCO3
0.596 ± 0.179 12.5b ± 3.46 0.180 ± 0.040 0.066 ± 0.014 0.127 ± 0.014
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Table 2. Summary of the treatment performances of the pilot-scale MEO system operated under different current density and electrode area conditions 1,4-Dioxane removal (%)
Total bacteria removal (log removal value)
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23.9 ± 10.5
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14 ± 5.1
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91 ± 2.1
17.9 ± 4.6
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4.49 a ± 0.43
94 ± 2.5
31 ± 5.6
52.9 ± 11.6
4.51b ± 0.51
Color removal (%)
Turbidity removal (%)
0
5.28
4.8 ± 3.8
77 ± 4.3
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5.28
49 ± 3.8
93 ± 1.1
67
5.28
52 ± 5.1
90 ± 3.3
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5.28
65 ± 2.4
91 ± 1.9
100
7.92
71 ± 1.4
150
7.92
76 ± 5.4
Chemical oxygen demand removal (%) 6.1 ± 5.4
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Electrode surface area (m2)
13 ± 5.5
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Current density (A/m3)
Total bacterial log removal value before applying 100 A/m3: 1.33
b
Total bacterial log removal value before applying 150 A/m3: 0.43
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Fig. 1. Schematics of the membrane electro-oxidizer composed of submerged flat sheet ceramic microfiltration membranes and electrochemical cells used for tertiary treatment of secondary effluent: (a) laboratory-scale and (b) pilot-scale systems.
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1.0
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Color (C/C0)
No electricity
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Time (h) Fig. 2. Effects of electric current density on color and turbidity removal during batch MEO operations.
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10 0.375 µm to 100 µm
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Volume Frequency (%)
100% 16.47 µm 15.78 µm 10.64 µm 2.088 µm 31.65 µm
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Volume : Mean : Median : S.D : d10 : d90 :
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100
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Fig. 4. Color and turbidity removal efficiencies with time as a function (f) of electric current (0 or 0.5 A/L), aeration (0 or 1 L/min), and relaxation (0 or 1 min every 20 min).
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Fig. 5. Variations in the intensity of fluorescence peak components extracted via PARAFAC model decomposition from the original fluorescence excitation-emission matrix spectrograms shown in Figs. S2 and S3.
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Fig. 6. Liquid chromatograms with ultraviolet (top) and total organic carbon (bottom) detectors for the feed and membrane permeate samples from control (left) and MEO (right)
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Cinnamic acid
Ammonium acetate
Sodium sulfite
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t-Butanol
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Color (Pt-Co Unit)
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Fig. 8. Transmembrane pressure build-up with time as a function (f) of electric current (0 or 0.5 A/L), aeration (0 or 1 L/min), and relaxation
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Fig. 9. Transmembrane pressure build-up over time during pilot-scale MEO testing under various electric current densities. The MEO tests at 100 A/m3 (#2) and 150 A/m3 were run with an electrode area of 7.92 m2, whereas the others were run with an electrode area of 5.28 m2.
ACCEPTED MANUSCRIPT ► A membrane electro-oxidizer integrates a membrane and an electrode for advanced treatment ► Substantive, simultaneous removal of organics and particles is achieved in a single reactor
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► Reactive chlorine species and hydrogen peroxide are dominant oxidants in the process ► Membrane fouling is alleviated due to electrochemically generated oxidants
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► Pilot-scale testing successfully demonstrates the potential of the new hybrid treatment