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Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater
Accepted Manuscript Title: Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater Authors: Kamyar Sardari, John Askegaard, Yu-Hsuan Chiao, Siavash Darvishmanesh, Mohanad Kamaz, S. Ranil Wickramasinghe PII: DOI: Reference:
S2213-3437(18)30401-9 https://doi.org/10.1016/j.jece.2018.07.022 JECE 2517
To appear in: Received date: Revised date: Accepted date:
2-4-2018 19-6-2018 8-7-2018
Please cite this article as: Sardari K, Askegaard J, Chiao Y-Hsuan, Darvishmanesh S, Kamaz M, Wickramasinghe SR, Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.07.022 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.
Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater
Kamyar Sardari1, John Askegaard2, Yu-Hsuan Chiao1, Siavash Darvishmanesh1, Mohanad
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Kamaz1, S. Ranil Wickramasinghe11
Ralph E. Martin Department of Chemical Engineering, University of Arkansas,
Fayetteville, AR, USA Tyson foods, Inc., Springdale, AR, USA
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Graphical abstract
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Corresponding author: Tel: +1 479 276 479; Fax: +1 479 575 8475; E-mail: [email protected]
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Highlights Electrocoagulation-ultrafiltration used for treating poultry processing wastewater.
Fouling was the main obstacle when operating ultrafiltration with poultry wastewater.
5 min electrocoagulation significantly reduced fouling on ultrafiltration membrane.
Integrated system achieved higher permeate flux and contaminate rejection.
Intermittent cleaning led to 50% increase in water recovery in a 7 days period.
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Abstract
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Ultrafiltration (UF) is an emerging technology of interest for the treatment of highly impaired
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industrial wastewaters. Here we focus on treating poultry processing wastewater (PPW) using UF.
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Although UF suffers from severe flux decline due to membrane fouling when treating PPW, we show that the flux decline could be significantly reduced using a pretreatment step.
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Electrocoagulation (EC) is studied as the pretreatment method. EC is shown to be effective in
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removing suspended solids and organic compounds which foul the membrane during UF. Higher EC reaction times result in higher contaminate removal. We show that for an EC reaction time of
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5 min, equivalent to energy consumption of 0.15 kWh m-3, over 85% reduction of fats, oil and
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grease and total suspended solids results. Compared to individual UF, the EC-UF process results in lower flux decline and enhanced contaminate rejection. Finally, we run long-term EC-UF
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experiments, with and without membrane cleaning, to study the feasibility of the combined process.
Key words: membrane, flux, fouling, pretreatment, membrane cleaning, water reuse.
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1. Introduction Water is extensively used in the food industry [1]. In 2016, the poultry industry within the United States (US) processed approximately 8.9 billion broilers, using an average of 26.5 L of
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water per bird and produced over 60 billion gal of poultry processing wastewater (PPW) requiring treatment [2,3]. PPW contains proteins, fats, carbohydrates, feather, blood and skin, resulting in much higher biochemical oxygen demand (BOD) and chemical oxygen demand (COD) compared to municipal wastewater [4,5]. Several unit operations have been reported in the literature for the treatment of PPW including biological treatment involving aerobic and anaerobic systems [6–10].
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Dissolved air floatation (DAF) is the most popular treatment method currently utilized in US
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poultry processors with approximately 80% of slaughter plants employing the technology [11].
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While in most cases, the DAF-treated PPW is sent to a biological wastewater treatment plant for
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be recycled and reused [2,12,13].
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further removal of contaminants and discharge into environment, at least a portion of PPW could
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In a poultry processing plant, live birds are slaughtered, scalded, de-feathered, eviscerated, cleaned and chilled [8,14]. PPW effluent (at the end of a poultry processing plant) is a combination
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of different wastewater streams, originating from the process steps listed above. The actual composition of PPW depends on the type of system used, the method of operation and the
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processing loads [15]. Here, we focus on the wastewater effluent coming from the chilling process. In the chilling process, the birds are chilled in water containing antimicrobial agents [16,17]. The
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main contaminants in chiller water are blood, oil, grease and fat particles, with the total suspended solids (TSS) in the range of 200 to 600 mg L-1 [14]. Membrane filtration is an established technology for treatment of highly impaired wastewaters [18,19]. Reverse osmosis (RO) and nanofiltration (NF) can remove the majority of pollutants
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including dissolved organics. However, their operational costs are high due to their high energy requirement. Ultrafiltration (UF) is a low-pressure and cost-effective option in terms of higher permeate flux compared to NF and RO [20–22]. UF has been widely used in the food processing
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industry for the past 20 years [23]. UF provides a variety of advantages over conventional treatment technologies (such as diatomaceous earth filtration and dissolved air flotation (DAF)) including no added chemicals, low energy consumption and small footprint. In addition, UF can provide an absolute barrier for pathogen removal which could be crucial for recycling PPW. In this work, we investigate the application of UF for PPW recycling and reuse.
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Similar to all membrane separation processes, UF suffers from membrane fouling, which
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causes a dramatic drop in the efficiency of the water recovery, due to accumulation of separated
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particles both on the membrane surface and inside the membrane pores [12,24–26]. Pretreatment
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as well as membrane cleaning are the main strategies for fouling minimization and membrane regeneration. A number of researchers have focused on membrane fouling control as well as single
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reagent and multiple-step cleaning of fouled membranes [23,27]. In a previous study, we have
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pretreatment [5].
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shown that the membrane cleaning process can only provide moderate flux recovery without using
Numerous pretreatment processes have also been considered prior to membrane filtration [28].
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Chemical coagulation, adsorption, pre-oxidation and pre-filtration are among the most popular pretreatment methods prior to membrane filtration [29]. Here, we focus on electrocoagulation
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(EC) as a pretreatment step prior to UF. Development of a viable UF process for treating PPW will require the inclusion of a viable pretreatment step. Here we consider the use of EC prior to UF.
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EC is an emerging technology in water and wastewater treatment, as it combines the benefits of coagulation, flotation and electrochemistry [30]. In EC, sacrificial electrodes are utilized to release coagulant counter ions into solution using electricity. The following electrode reactions
At the anode:
𝑛+ 𝑀(𝑠) → 𝑀(𝑎𝑞) + 𝑛𝑒 −
At the cathode:
2𝐻2 𝑂 + 2𝑒 − → 2𝑂𝐻 − + 𝐻2
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occur at the anode, cathode and consequently, in the solution [31]: (1) (2)
where M is the electrode metal material (usually Al or Fe). Analogous to chemical coagulation,
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the metal and hydroxide ions form various monomeric species such as M(OH)(n-1)+ , M(OH)2(n-2)+
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and also polymeric species such as M6(OH)15(6n-15)+ [32,33]. As the solution ‘ages’, polynuclear
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complexes develop and amorphous M(OH)n(s) forms in the solution, as given by the following
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general scheme, according to complex precipitation kinetics [34]: 𝑛+ 𝑀(𝑎𝑞) + 𝑛𝑂𝐻 − → 𝑀(𝑐𝑜𝑚𝑝𝑙𝑒𝑥) → 𝑀(𝑂𝐻)𝑛(𝑠)
(3)
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In solution:
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Metal complexes eventually transform to solid M(OH)n(s) with a large surface area that can adsorb organic compounds, trap suspended particles and form flocs. Finally, M(OH)n(s) flocs (with
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adsorbed organics and colloidal particles) will polymerize and deposit according to the following reaction [35]:
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𝑥 𝑀(𝑂𝐻)𝑛 → 𝑀𝑥 (𝑂𝐻)𝑥𝑛
(4)
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A number of researchers have looked at EC though not EC combined with UF for treatment
of PPW [36–39]. While the fundamental chemical basis for chemical coagulation (e.g. alum or ferric chloride coagulation) and EC are similar, EC has gained significant attention from many researchers [40–42]. EC also has the potential of treating oily wastewaters, where the presence of an electric current can contribute to the electro-coalescence of oil droplets [43]. 5
In this study, we investigate the application of PPW pretreatment via EC prior to UF for fouling mitigation and stable water recovery. We focus on chiller wastewater that is produced during chilling operations. The chiller step is one of the last unit steps prior to cooking operations.
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An antibacterial agent is added to the water to suppress bacterial growth. We design and develop an EC system as a pretreatment operation prior to UF. In addition, we investigate the impact of membrane cleaning on UF water recovery from raw and EC pretreated PPW. Finally, due to lack of studies on long-term operation of EC and UF for PPW treatment in the literature, we perform
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long-term EC-UF experiments with and without intermittent membrane cleaning.
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2. Materials and Methods 2.1. PPW samples
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PPW samples were obtained from Tyson Foods Inc. (Springdale, AR) facilities, processing
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200,000 broilers per day. All samples were stored in 4⁰ C prior to testing in order to minimize
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bacterial growth. All samples were analyzed at the Food, Safety and Research Laboratory, Tyson Foods Inc. Following wastewater parameters were measured: BOD, COD, TSS, total dissolved
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solids (TDS), fats, oils and grease (FOG), proteins, total Kjeldahl nitrogen (TKN) and pH. In
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addition, the particle size distribution of the raw and EC pretreated PPW samples were determined using a laser diffraction particle size analyzer (Beckman Coulter, LS 13 320, Brea, CA).
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2.2. EC setup
Figure 1 is a schematic diagram of the established EC setup. Five aluminum electrodes
(6061 aluminum alloy, Sapa, Rosemont, IL) with effective surface area of 180 cm2 per electrode were placed in a 1 L polycarbonate reactor. The electrodes were arranged to have 9 mm spacing between them. The spacing between each two electrodes was selected wide enough in order to 6
prevent short circuiting of the used (curved) electrodes. The first and last electrodes were connected to a DC power source (Hewlett Packard, Palp Alto, CA). All EC experiments were carried out in batch mode. For each experiment, 1 L raw PPW was placed in the EC reactor, the
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voltage was maintained at 9.0 V and current was recorded every 20 seconds. Different reaction times were investigated. After EC, pretreated samples were transferred to a separatory funnel for sludge sedimentation/flotation. After 6 h of sedimentation, the clear portion of the sample was recovered and the deposited sludge as well as floating skimmings were wasted. Used aluminum electrodes were soaked in a 1 mM sulfuric acid solution for 30 min and
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rinsed with DI water several times before storage.
Figure 1. Schematic diagram of the EC setup
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2.3. Membrane Regenerated cellulose (RC) membrane with embedded polypropylene support and 30 kiloDalton (kDa) nominal molecular weight cut-off was used. The membrane samples were
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provided by Pall Corporation (New York, NY) and received in rolled flat sheets. 2.4. UF Experiments 2.4.1. Filtration setup
Tangential flow filtration (TFF) experiments were conducted in order to evaluate the water recovery efficiency of the UF process. Figure 2 illustrates the experimental set up. For TFF
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experiments without EC, the feed was passed through a 300 µm stainless steel mesh in order to
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remove any large particulate matter. Then, the PPW feed was placed in a 1 L feed tank. Initially
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the permeate outlet was closed and feed was recirculated through the membrane module by means
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of a peristaltic pump (Masterflex I/P, Cole Parmer, Vernon Hills, IL). The membrane module consisted of a Minitan S tangential flow filtration cell (MilliporeSigma, Billerica, MA). The
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membrane surface area available for filtration was 33 cm2. The feed flow rate was 1.2 L min-1 and the feed pressure was 70 kPa. The permeate side pressure was essentially atmospheric pressure.
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After 5 min of operation, once steady state had been reached, the permeate outlet was opened and permeate was collected in the permeate tank which was placed on a computer-connected analytical
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balance (Mettler Toledo, Columbus, OH). Permeate flux was calculated based on the rate of
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permeate collection in permeate tank. 2.4.2. Membrane pre-compaction Prior to testing, each membrane was first pre-compacted. The feed tank was filled with de-ionized (DI) water and the feed pump was started while the permeate outlet was closed. The feed flow rate was 2.3 L min-1 and feed pressure was 200 kPa. DI water was recirculated through
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the membrane module for 5 min after which the permeate outlet was open. The permeate side was essentially atmospheric pressure. After a steady state flux was observed, DI water pressure was reduced to 70 kPa and the permeate weight was recorded in 1 min intervals over a period of 5 min
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for DI water flux calculation. Then the DI water in the feed tank was replaced with PPW and filtration commenced. UF experiments were conducted till 500 mL of permeate was collected. The permeate weight was recorded in 1 min intervals. The permeate flux was then calculated. 2.4.3. EC –UF
EC, see section 2.2, was used to pretreat the PPW prior to UF. The RC 30 kDa membrane
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was tested as described above using 1 L of PPW pretreated using EC. The variation of flux as a
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function of permeate volume was determined. Finally, a long-term filtration experiment was
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conducted using the RC 30 kDa membrane with raw and EC pretreated PPW. RC 30 kDa
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membrane was first compacted with DI water using the method described in section 2.4.2. Then,
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10 L feed PPW was placed in the feed tank and the experiment was continued over a week. Fresh
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the feed tank.
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feed PPW was intermittently added to the feed tank to maintain a volume of greater than 5 L in
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2.5. Membrane cleaning
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Figure 2. Schematic diagram of the filtration (TFF) setup
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Since the permeate flux decreases during filtration due to membrane fouling, membrane
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cleaning was performed in order to recover the initial permeate flux. The cleaning cycle consisted of four steps: cleaning with a detergent, rinsing with DI water, cleaning with acid and rinsing with
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DI water. (1) 0.2% Triton X-100 in DI water (pH adjusted to 9.0 with 0.1 M sodium hydroxide at room temperature) was placed in the feed tank and pumped through the membrane module at a
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flow rate of 1.2 L min-1 and 70 kPa pressure for 30 min. Next, the feed tank was filled with DI water which was pumped through the membrane module under the same conditions for 30 min. The feed tank was filled with DI water containing citric acid (pH adjusted to 4.0). Again, the feed was recirculated for 30 min through the membrane module under the same conditions. Finally, DI water was placed in the feed tank and recirculated through the membrane module for 30 min, again 10
under the same conditions. After completion of the cleaning procedure, the membranes were retested with feed PPW. 3. Results and discussion
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3.1. Wastewater characterization
Ten PPW samples were collected over a period of three months at Tyson Food Inc. facilities and were analyzed for the level of contaminations. Measured pH of the PPW samples were in the range of 6.2 to 7.5 with an average of 6.7. Figure 3 represents the other characterization results of
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the ten PPW samples. As can be seen, the value of all wastewater parameters is relatively higher
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than average domestic wastewater. BOD in the range of ~275 to 500 mg L-1, COD in the range of
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~800 to 1050 mg L-1, FOG in the range of ~100 to 190 mg L-1 and TSS in the range of ~160 to
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280 mg L-1 were observed. Asselin et al. [44], Del Nery et al. [45] and Basitere et al. [15] report much higher values (more than twice) for BOD, COD, TSS, TKN and FOG in their tested PPW
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compared to the values we report in Figure 3. However, PPW tested by them is obtained from the
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effluent wastewater of the poultry processing plant, where the PPW is a combination of different wastewater streams; whereas the PPW in this work is obtained from the chilling process. Figure
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3 shows proteins in the range of ~125 to 200 mg L-1, which is almost half of the value reported by
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Lo et al. [46].
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Figure 3. Characterization of collected PPW samples from Tyson Foods Inc. facilities processing
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200,000 birds per day.
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Results of particle size distribution analysis for raw PPW is shown in Figure 4.a. The size range of the particles in the raw PPW are within 0.05–100 µm with an average size of 0.084 µm
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(number-based) and 36.24 µm (volume-based). The volume-based size distribution diagram is skewed to right compared to the number-based diagram mainly due to presence of a small number
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of larger particles which in terms of the total volume of particulate matter are significant. Majority
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of the total particle volume comes from the particles around 36.24 µm, whereas individual particles in the range of 0.084 are more abundant.
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3.2. EC performance 3.2.1. Removal mechanism In this study, EC was investigated as the pretreatment step prior to membrane filtration. According to Eqs. (1) and (2) (see
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Aluminum electrodes were used in the EC reactor.
introduction), Al3+ and OH- ions were produced at the anode and cathode, respectively. Aluminum and hydroxide ions react and form a variety of monomeric (e.g. Al(OH)2+) and polymeric (e.g. Al6(OH)153+) species in the solution that eventually transform into insoluble Al(OH)3(s) solids. For pH in the range of 6 to 8, which is the case for tested PPW here, formation of amorphous Al(OH)3(s)
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prevails [47]. Due to their large surface area, Al(OH)3(s) particles adsorb organic compounds, trap
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suspended particles and form agglomerated flocs. Formed Al(OH)3(s) flocs could be easily
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separated from the solution by sedimentation or flotation. Here, EC pretreated PPW samples were
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transferred to a separatory funnel for sludge deposition.
Thus, flocs were separated by
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sedimentation following a 6 h sedimentation time. Figure 5 shows digital images of a PPW sample
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during different stages of EC pretreatment. While the overall reaction mechanism of EC may seem ordinary, the actual series of reactions that undergo to form aluminum hydroxide solids is much
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more complex and require in-depth investigation.
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Figure 5.a shows raw PPW and Figure 5.b shows EC pretreated PPW after 5 min reaction time. As can be seen, the original yellow color of the raw PPW was transformed to grey after 5
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min EC which indicates the formation of Al(OH)3(s) solids [48]. Figure 5.c illustrates the recovered EC pretreated PPW after 6 h sedimentation (deposited sludge and floating skimmings were wasted). As can be seen, the majority of agglomerated flocs are removed. However, the efficiency of physical floc separation can be enhanced by employing a floatation process (e.g. DAF) after the EC [49]. Although removal mainly takes place by Al(OH)3(s) agglomeration and sedimentation, 14
the remainder of positively charged aluminum hydroxide species such as Al(OH)2+ can also help with contaminate removal [32]. These species can contribute to destabilization of the organic macromolecules by charge neutralization and their consequent agglomeration as neutral colloidal
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entities and finally, physical separation by sedimentation [50,51].
Figure 5. Digital image of a) raw PPW, b) EC pretreated PPW after 5 min reaction time, and c)
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recovered EC pretreated PPW after 6 h sedimentation.
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3.2.2. Contaminate removal efficiency
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The efficiency of pollutant removal by EC process depends on variety of operating parameters including: the material of the electrodes used, current density, voltage, reaction time,
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etc. Here, we conduct all EC experiments at steady voltage of 9.0 V (autonomous current change) and vary the reaction time. The effects of EC on removal of different contamination factors are
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given in Figure 6. Removal efficiencies are calculated using Eq. (5): 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =
𝐶𝑝𝑝𝑤 −𝐶𝑟𝑤 𝐶𝑝𝑝𝑤
ꓫ 100
(5)
where, Cppw and Crw are the concentration in the raw PPW and recovered water, respectively. As it can be seen in Figure 6, EC has significantly decreased the amount of impurities in PPW and 15
could be an alternative pretreatment method. In Figure 6, it can be seen that increasing the reaction time above 5 min does not significantly increase the removal efficiency of impurities after 6 h sedimentation time. After 5 min reaction time, up to 94% of FOG, 87% of BOD, 59% of COD
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and 84% of TSS are removed. Kobya et al. [38] report the same level of COD and FOG removal from their tested PPW after 5 min EC reaction time, employing aluminum electrodes, 11 mm electrode spacing and constant current density of 150 A m-2. Here we maintained a steady voltage of 9.0 V throughout EC experiments, resulting in an average of ~30 A m-2 current density. TDS was not significantly removed by EC and is not shown in Figure 6 (TDS removal less than 5%).
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Due to the generation of ionic species during EC, the current was decreased to ensure a
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constant voltage of 9.0 A. The electrical energy consumption per volume during EC was
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𝑉∗𝐼∗𝑡 𝑉𝑟
(6)
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𝐸=
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calculated using Eq. (6) [34]:
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where V is applied voltage, I is average current, t is reaction time and Vr is volume of feed water. Voltage was fixed at 9.0 volts for all experiments and the current provided by DC power source
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was recorded through the experiments. Figure 6 represents the energy consumption of EC system as a function of reaction time in the secondary vertical axis. Higher removal was achieved for
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longer reaction times which resulted in higher energy consumption. Increasing the EC reaction time from 1 to 5 min resulted in over 24% increase in BOD and FOG removal as well as over 14%
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increase in TSS removal after 6 h sedimentation. However, increase in reaction time over 5 min did not significantly increase the removal of contaminants after sedimentation. Consequently, 5 min was chosen as the EC reaction time for EC-FO experiments (see section 3.4). For 5 min reaction time, 0.15 kWh m-3 is estimated as the EC energy consumption using Eq. (6).
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Figure 6. Removal efficiency of wastewater parameters as well as EC energy consumption as
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functions of reaction time. Voltage was fixed at 9.0 V and the current from the DC power source
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was recorded every 20 seconds in order to determine energy consumption. TDS removal is not
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shown in the figure due to minimal removal in EC process. 3.2.3. Particle size distribution
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Results of particle size distribution analysis for EC pretreated PPW after 6 h sedimentation
are shown in Figure 4.b for the recovered water. The size range of the particles in the EC pretreated
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PPW are within 0.05–100 µm, similar to raw PPW, with majority of the total particle volume coming from the larger particles (~25.70 µm). However, the average particle size of EC pretreated PPW is different from the raw PPW. As can be seen, the number-based size distribution for pretreated PPW is skewed to right compared to raw PPW, denoting agglomeration of particles
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after EC. Aside from Al(OH)3(s) solids (see section 3.2.1) that resulted in particle agglomeration, non-transformed positively charge aluminum hydroxide species such as Al(OH)2+ could also contribute to organic macromolecules aggregation by charge neutralization. These positively
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charge species could form neutral complexes with negatively charged organics, resulting in neutral colloidal entities [50].
3.3. UF performance
Selection of a suitable membrane for specific membrane application is an important phase
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in a process development procedure. This requires the understanding of the influence of different
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parameters in order to get highest possible permeate flux and rejection of targeted components in
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the feed solution. Permeate flux and solute rejection depend on the type of membrane used, feed
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concentration, transmembrane pressure, feed pH, etc.
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Figure 7.a gives the variation of permeate flux for RC 30 kDa membrane when treating
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PPW as a function permeate volume. Error bars represent the standard deviation of the permeate flux measurements. Permeate fluxes of virgin membrane, membrane after first cleaning and
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membrane after second cleaning are shown. As can be seen, all three permeate flux curves indicate a decrease in the permeate flux relative to the initial flux. Initially, the permeate flux is found to
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decline rapidly followed by a more gradual decrease over time. Avula et al. [4] report a rapid flux decline (~40% decrease in initial flux) in the first 20 min of UF experiment with a continuous
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gradual flux decline till the end of process when treating PPW. Bialas et al. [12] report the same trend using a 30 kDa molecular weight cut-of UF membrane. The initial period of rapid flux decline is probably due to initial deposition inside the membrane pores. Since a large number of particles in raw PPW are smaller than the membrane pore size, they can plug the membrane pores
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and be trapped within the membrane structure [52,53]. The region of more gradual flux decline could be mainly due to deposition of foulants on the membrane surface and formation of a cake layer [54]. In addition, increase in feed solution viscosity due to increase in feed solution
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concentration leads to a gradual decrease in permeate flux. Membrane was regenerated using the four-step cleaning method (see section 2.5) twice.
As can be seen in Figure 7.a, membrane cleaning was able to reasonably recover the initial permeate flux. However, a more sever flux decline was observed after membrane cleaning compared to the permeate flux of the virgin membrane, mainly due to incomplete foulant removal
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during cleaning. Comparing the permeate flux after the first and second cleaning indicated a more
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rapid drop in flux after each subsequent use. This suggests that while the cleaning protocol
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developed here is able to regenerate almost the original DI water flux, some irreversible fouling
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occurs. The presence of adsorbed species lead to more rapid flux decline during subsequent PPW filtration.
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The rejection of contaminates from feed PPW by UF are calculated as removal efficiency
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and are presented in Figure 7.b. Removal efficiency is calculated using Eq. (7)
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𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =
𝐶𝐹 −𝐶𝑃 𝐶𝐹
ꓫ 100
(7)
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where CF and CP are the concentration in the feed and bulk permeate after removal of 500 mL permeate, respectively. Figure 7.b gives the removal efficiency results for the virgin membrane
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as well as membrane after cleaning. As can be seen, COD and BOD were removed by up to 82 and 87%, respectively. On average, ~95% of TSS and FOG were removed. Proteins were removed by up to 55%. Only limited removal of TDS was achieved. The results indicate that unlike permeate flux, rejection performance is almost similar for virgin and regenerated RC 30 kDa membrane. 19
SC RI PT U N A M D TE EP CC A Figure 7. a) Variation of permeate flux as a function of permeate volume when treating PPW for virgin and regenerated RC 30 kDa membrane, and b) rejection of contaminates from raw PPW by UF. 20
3.4. Membrane filtration after pretreatment (EC-UF) EC was used to pretreat PPW prior to UF experiments in order to mitigate fouling. Figure 8.a shows the variation of permeate flux for raw and EC pretreated PPW as a function of collected
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permeate volume. Comparing Figures 6 and 8.a, 5 min EC reaction time followed by 6 h sedimentation resulted in significant removal of TSS, FOG and BOD which consequently, led to higher initial permeate flux as well as lower flux decline during UF. Similar to this observation, Choi et al. [55] report significant fouling reduction using in-line coagulation prior to UF. Gradual flux decline observed for EC pretreated PPW in Figure 8.a indicates the foulant deposition on the
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membrane surface could be the main reason for the slight decrease in permeate flux over the period
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of permeate collection [54]. This is not unexpected given the significant removal of contaminates
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in pretreatment step (EC) as well as the change in particle size distribution of EC pretreated PPW
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compared to raw PPW (see section 3.2.3). As can be seen in Figure 4.b, majority of particles in
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EC pretreated PPW are larger than the membrane pore size due to particles agglomeration in EC;
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thus, remainder of suspended particles and organic macromolecules in EC pretreated PPW are less likely to enter the membrane pores during UF.
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Figure 8.b illustrates the performance of EC-UF compared to individual UF for
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contaminates rejection in terms of removal efficiency. Removal efficiency is calculated using Eq. (7) where CF and CP are the concentration in the feed and bulk permeate. Almost a complete
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removal of TSS and FOG is achieved in EC-UF. Significant increase in removal of COD, BOD, proteins and TKN is observed when pretreating PPW via EC prior to UF. COD and BOD were removed by up to 92 and 98%, respectively. Proteins were removed by up to ~90%.
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SC RI PT U N A M D TE EP CC A Figure 8. a) Variation of permeate flux for raw and EC pretreated PPW as a function of collected permeate volume and b) rejection of contaminates from PPW by individual UF as well as EC followed by UF. 22
3.5. Long-term experiments Long-term filtration experiments were performed in order to investigate the performance of individual UF as well as combined EC-UF. Figure 9.a illustrates the variation of permeate flux
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as a function of time. Raw and EC pretreated PPW (5 min reaction time) were used separately as feed solutions. A rapid flux decline was observed for raw PPW. As can be seen, permeate flux was reduced by over 91% for raw PPW after one day of testing. This permeate flux dropped to almost zero after 3 days. For the pretreated PPW, permeate flux was reduced from an initial value of 106 to 46 L m-2 h-1 after one day. A gradual flux decline continued from day one to day seven.
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At the end of day seven, 22 L m-2 h-1 permeate flux was measured, and a total of 19.8 L water was
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recovered. While EC leads to higher permeate fluxes, it is the increase in water recovery versus
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the additional cost of the EC step that will determine the economic viability of the process [32]. Intermittent cleaning was utilized to enhance the water recovery in a long-term EC-UF
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experiment. In this experiment, the membrane was regenerated using the cleaning method
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discussed earlier (see section 2.5) at 24 h intervals. Cleaning was repeated 6 times. Results are shown in Figure 9.b. Cleaning could successfully recover the permeate flux over the period of the
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experiment. As can be seen, recovered water volume was increased by over 30% in a period of 7
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days. A total of 29.7 L water was recovered after 8 days experimental time. In an optimized
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process, cost of membrane cleaning must be compared with additional volume of recovered water. In this work, no attempt was made to optimize the EC sedimentation time (6 h). In an
actual practice, it is likely that the sedimentation time will be determined by factors such as available land area, maximum volume of the sedimentation tanks and integration into the overall wastewater treatment process. It is likely that EC sedimentation time of less than 1 h will be required. This may require skimming the floc as it forms at the top surface rather than waiting for 23
it to age, densify and settle to the bottom. This in turn could affect the optimal EC reaction time. Further development of a continuous EC process will be essential. While the effect on EC performance of only a limited number of parameters has been considered here, a more detailed
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study which considers the rate of coagulant dissolution into feed water, either by increasing the reaction time, current density or voltage will need to be considered.
A DAF system can also aid
in physical separation of the EC sludge from the pretreated water. In some poultry processing plants, two sequential DAF systems are used to treat the PPW prior to discharge. EC can be installed prior to the second DAF. Hence, the sludge separation can take place in the second DAF
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instead of a sedimentation tank. Nevertheless, our work indicates the feasibility of a combined
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EC-UF process for treatment of PPW and highlights the possibility of recovery and reuse of a
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portion of the PPW.
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Figure 9. Long-term filtration experiments with a) raw and EC pretreated PPW without membrane
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cleaning, and b) EC pretreated PPW utilizing intermittent cleaning. Selection of a process for recycling and reuse of PPW depends on the quality of the
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recovered permeate as well as the related costs. The concentration of contaminates in raw PPW as well as the permeate quality for long-term UF and EC-UF experiments are shown in Figure 10.
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Comparing the UF and EC-UF permeates, higher contaminate removal is observed for EC-UF. Similar permeate quality is reported for EC-UF process with and without intermittent membrane cleaning. Almost a complete removal of TSS and FOG is obtained in the integrated EC-UF.
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Proteins are reduced to less than ~45 mg L-1. COD and BOD are removed to below ~70 and ~40
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mg L-1, respectively.
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Figure 10. Concentration of contaminates in raw PPW as well as the permeate quality for long-
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term UF and EC-UF experiments
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4. Conclusions
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Here we have investigated the application of EC pretreatment prior to UF for treating high
strength PPW. We report significant membrane fouling of the UF membrane due to high levels of O&G, proteins and TSS in the feed water that results in rapid flux decline. However, we show that the membrane fouling can be greatly mitigated using EC. We show that for an EC reaction time of 5 min equivalent to 0.15 kWh m-3 energy consumption for EC, followed by 6 h 26
sedimentation, over 85% reduction of O&G, BOD and TSS results. This reduction leads to higher initial permeate flux as well as lower flux decline during UF. In addition, higher contaminate rejection is obtained in EC-UF process compared to individual UF.
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Selection of a membrane-based process for reuse of PPW depends on a number of factors including: the quality of the recovered permeate, cleaning requirements and the related costs. Pretreatment of PPW streams will be essential if membrane-based separation processes such as UF are to be used to treat these highly impaired waters. As we show in this study, pretreatment
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(here EC) enables the UF process to operate more efficiently for longer periods. We also show
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that intermittent cleaning can increase the water recovery to a great extent. Here, the recovered
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water volume was increased by over 30% in a week of experimental time when employing
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membrane cleaning after each 24 h.
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Acknowledgements
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Funding for this work was provided by Tyson Foods Inc. through the National Science
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Foundation Industry/University Cooperative Research Center for Membrane Science, Engineering
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and Technology, the National Science Foundation (IIP 1361809) and the University of Arkansas.
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S2213-3437(18)30401-9 https://doi.org/10.1016/j.jece.2018.07.022 JECE 2517
To appear in: Received date: Revised date: Accepted date:
2-4-2018 19-6-2018 8-7-2018
Please cite this article as: Sardari K, Askegaard J, Chiao Y-Hsuan, Darvishmanesh S, Kamaz M, Wickramasinghe SR, Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.07.022 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.
Electrocoagulation followed by ultrafiltration for treating poultry processing wastewater
Kamyar Sardari1, John Askegaard2, Yu-Hsuan Chiao1, Siavash Darvishmanesh1, Mohanad
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Kamaz1, S. Ranil Wickramasinghe11
Ralph E. Martin Department of Chemical Engineering, University of Arkansas,
Fayetteville, AR, USA Tyson foods, Inc., Springdale, AR, USA
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Graphical abstract
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Corresponding author: Tel: +1 479 276 479; Fax: +1 479 575 8475; E-mail: [email protected]
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Highlights Electrocoagulation-ultrafiltration used for treating poultry processing wastewater.
Fouling was the main obstacle when operating ultrafiltration with poultry wastewater.
5 min electrocoagulation significantly reduced fouling on ultrafiltration membrane.
Integrated system achieved higher permeate flux and contaminate rejection.
Intermittent cleaning led to 50% increase in water recovery in a 7 days period.
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Abstract
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Ultrafiltration (UF) is an emerging technology of interest for the treatment of highly impaired
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industrial wastewaters. Here we focus on treating poultry processing wastewater (PPW) using UF.
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Although UF suffers from severe flux decline due to membrane fouling when treating PPW, we show that the flux decline could be significantly reduced using a pretreatment step.
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Electrocoagulation (EC) is studied as the pretreatment method. EC is shown to be effective in
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removing suspended solids and organic compounds which foul the membrane during UF. Higher EC reaction times result in higher contaminate removal. We show that for an EC reaction time of
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5 min, equivalent to energy consumption of 0.15 kWh m-3, over 85% reduction of fats, oil and
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grease and total suspended solids results. Compared to individual UF, the EC-UF process results in lower flux decline and enhanced contaminate rejection. Finally, we run long-term EC-UF
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experiments, with and without membrane cleaning, to study the feasibility of the combined process.
Key words: membrane, flux, fouling, pretreatment, membrane cleaning, water reuse.
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1. Introduction Water is extensively used in the food industry [1]. In 2016, the poultry industry within the United States (US) processed approximately 8.9 billion broilers, using an average of 26.5 L of
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water per bird and produced over 60 billion gal of poultry processing wastewater (PPW) requiring treatment [2,3]. PPW contains proteins, fats, carbohydrates, feather, blood and skin, resulting in much higher biochemical oxygen demand (BOD) and chemical oxygen demand (COD) compared to municipal wastewater [4,5]. Several unit operations have been reported in the literature for the treatment of PPW including biological treatment involving aerobic and anaerobic systems [6–10].
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Dissolved air floatation (DAF) is the most popular treatment method currently utilized in US
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poultry processors with approximately 80% of slaughter plants employing the technology [11].
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While in most cases, the DAF-treated PPW is sent to a biological wastewater treatment plant for
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be recycled and reused [2,12,13].
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further removal of contaminants and discharge into environment, at least a portion of PPW could
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In a poultry processing plant, live birds are slaughtered, scalded, de-feathered, eviscerated, cleaned and chilled [8,14]. PPW effluent (at the end of a poultry processing plant) is a combination
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of different wastewater streams, originating from the process steps listed above. The actual composition of PPW depends on the type of system used, the method of operation and the
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processing loads [15]. Here, we focus on the wastewater effluent coming from the chilling process. In the chilling process, the birds are chilled in water containing antimicrobial agents [16,17]. The
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main contaminants in chiller water are blood, oil, grease and fat particles, with the total suspended solids (TSS) in the range of 200 to 600 mg L-1 [14]. Membrane filtration is an established technology for treatment of highly impaired wastewaters [18,19]. Reverse osmosis (RO) and nanofiltration (NF) can remove the majority of pollutants
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including dissolved organics. However, their operational costs are high due to their high energy requirement. Ultrafiltration (UF) is a low-pressure and cost-effective option in terms of higher permeate flux compared to NF and RO [20–22]. UF has been widely used in the food processing
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industry for the past 20 years [23]. UF provides a variety of advantages over conventional treatment technologies (such as diatomaceous earth filtration and dissolved air flotation (DAF)) including no added chemicals, low energy consumption and small footprint. In addition, UF can provide an absolute barrier for pathogen removal which could be crucial for recycling PPW. In this work, we investigate the application of UF for PPW recycling and reuse.
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Similar to all membrane separation processes, UF suffers from membrane fouling, which
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causes a dramatic drop in the efficiency of the water recovery, due to accumulation of separated
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particles both on the membrane surface and inside the membrane pores [12,24–26]. Pretreatment
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as well as membrane cleaning are the main strategies for fouling minimization and membrane regeneration. A number of researchers have focused on membrane fouling control as well as single
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reagent and multiple-step cleaning of fouled membranes [23,27]. In a previous study, we have
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pretreatment [5].
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shown that the membrane cleaning process can only provide moderate flux recovery without using
Numerous pretreatment processes have also been considered prior to membrane filtration [28].
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Chemical coagulation, adsorption, pre-oxidation and pre-filtration are among the most popular pretreatment methods prior to membrane filtration [29]. Here, we focus on electrocoagulation
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(EC) as a pretreatment step prior to UF. Development of a viable UF process for treating PPW will require the inclusion of a viable pretreatment step. Here we consider the use of EC prior to UF.
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EC is an emerging technology in water and wastewater treatment, as it combines the benefits of coagulation, flotation and electrochemistry [30]. In EC, sacrificial electrodes are utilized to release coagulant counter ions into solution using electricity. The following electrode reactions
At the anode:
𝑛+ 𝑀(𝑠) → 𝑀(𝑎𝑞) + 𝑛𝑒 −
At the cathode:
2𝐻2 𝑂 + 2𝑒 − → 2𝑂𝐻 − + 𝐻2
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occur at the anode, cathode and consequently, in the solution [31]: (1) (2)
where M is the electrode metal material (usually Al or Fe). Analogous to chemical coagulation,
U
the metal and hydroxide ions form various monomeric species such as M(OH)(n-1)+ , M(OH)2(n-2)+
N
and also polymeric species such as M6(OH)15(6n-15)+ [32,33]. As the solution ‘ages’, polynuclear
A
complexes develop and amorphous M(OH)n(s) forms in the solution, as given by the following
M
general scheme, according to complex precipitation kinetics [34]: 𝑛+ 𝑀(𝑎𝑞) + 𝑛𝑂𝐻 − → 𝑀(𝑐𝑜𝑚𝑝𝑙𝑒𝑥) → 𝑀(𝑂𝐻)𝑛(𝑠)
(3)
D
In solution:
TE
Metal complexes eventually transform to solid M(OH)n(s) with a large surface area that can adsorb organic compounds, trap suspended particles and form flocs. Finally, M(OH)n(s) flocs (with
EP
adsorbed organics and colloidal particles) will polymerize and deposit according to the following reaction [35]:
CC
𝑥 𝑀(𝑂𝐻)𝑛 → 𝑀𝑥 (𝑂𝐻)𝑥𝑛
(4)
A
A number of researchers have looked at EC though not EC combined with UF for treatment
of PPW [36–39]. While the fundamental chemical basis for chemical coagulation (e.g. alum or ferric chloride coagulation) and EC are similar, EC has gained significant attention from many researchers [40–42]. EC also has the potential of treating oily wastewaters, where the presence of an electric current can contribute to the electro-coalescence of oil droplets [43]. 5
In this study, we investigate the application of PPW pretreatment via EC prior to UF for fouling mitigation and stable water recovery. We focus on chiller wastewater that is produced during chilling operations. The chiller step is one of the last unit steps prior to cooking operations.
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An antibacterial agent is added to the water to suppress bacterial growth. We design and develop an EC system as a pretreatment operation prior to UF. In addition, we investigate the impact of membrane cleaning on UF water recovery from raw and EC pretreated PPW. Finally, due to lack of studies on long-term operation of EC and UF for PPW treatment in the literature, we perform
U
long-term EC-UF experiments with and without intermittent membrane cleaning.
A
N
2. Materials and Methods 2.1. PPW samples
M
PPW samples were obtained from Tyson Foods Inc. (Springdale, AR) facilities, processing
D
200,000 broilers per day. All samples were stored in 4⁰ C prior to testing in order to minimize
TE
bacterial growth. All samples were analyzed at the Food, Safety and Research Laboratory, Tyson Foods Inc. Following wastewater parameters were measured: BOD, COD, TSS, total dissolved
EP
solids (TDS), fats, oils and grease (FOG), proteins, total Kjeldahl nitrogen (TKN) and pH. In
CC
addition, the particle size distribution of the raw and EC pretreated PPW samples were determined using a laser diffraction particle size analyzer (Beckman Coulter, LS 13 320, Brea, CA).
A
2.2. EC setup
Figure 1 is a schematic diagram of the established EC setup. Five aluminum electrodes
(6061 aluminum alloy, Sapa, Rosemont, IL) with effective surface area of 180 cm2 per electrode were placed in a 1 L polycarbonate reactor. The electrodes were arranged to have 9 mm spacing between them. The spacing between each two electrodes was selected wide enough in order to 6
prevent short circuiting of the used (curved) electrodes. The first and last electrodes were connected to a DC power source (Hewlett Packard, Palp Alto, CA). All EC experiments were carried out in batch mode. For each experiment, 1 L raw PPW was placed in the EC reactor, the
SC RI PT
voltage was maintained at 9.0 V and current was recorded every 20 seconds. Different reaction times were investigated. After EC, pretreated samples were transferred to a separatory funnel for sludge sedimentation/flotation. After 6 h of sedimentation, the clear portion of the sample was recovered and the deposited sludge as well as floating skimmings were wasted. Used aluminum electrodes were soaked in a 1 mM sulfuric acid solution for 30 min and
A
CC
EP
TE
D
M
A
N
U
rinsed with DI water several times before storage.
Figure 1. Schematic diagram of the EC setup
7
2.3. Membrane Regenerated cellulose (RC) membrane with embedded polypropylene support and 30 kiloDalton (kDa) nominal molecular weight cut-off was used. The membrane samples were
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provided by Pall Corporation (New York, NY) and received in rolled flat sheets. 2.4. UF Experiments 2.4.1. Filtration setup
Tangential flow filtration (TFF) experiments were conducted in order to evaluate the water recovery efficiency of the UF process. Figure 2 illustrates the experimental set up. For TFF
U
experiments without EC, the feed was passed through a 300 µm stainless steel mesh in order to
N
remove any large particulate matter. Then, the PPW feed was placed in a 1 L feed tank. Initially
A
the permeate outlet was closed and feed was recirculated through the membrane module by means
M
of a peristaltic pump (Masterflex I/P, Cole Parmer, Vernon Hills, IL). The membrane module consisted of a Minitan S tangential flow filtration cell (MilliporeSigma, Billerica, MA). The
TE
D
membrane surface area available for filtration was 33 cm2. The feed flow rate was 1.2 L min-1 and the feed pressure was 70 kPa. The permeate side pressure was essentially atmospheric pressure.
EP
After 5 min of operation, once steady state had been reached, the permeate outlet was opened and permeate was collected in the permeate tank which was placed on a computer-connected analytical
CC
balance (Mettler Toledo, Columbus, OH). Permeate flux was calculated based on the rate of
A
permeate collection in permeate tank. 2.4.2. Membrane pre-compaction Prior to testing, each membrane was first pre-compacted. The feed tank was filled with de-ionized (DI) water and the feed pump was started while the permeate outlet was closed. The feed flow rate was 2.3 L min-1 and feed pressure was 200 kPa. DI water was recirculated through
8
the membrane module for 5 min after which the permeate outlet was open. The permeate side was essentially atmospheric pressure. After a steady state flux was observed, DI water pressure was reduced to 70 kPa and the permeate weight was recorded in 1 min intervals over a period of 5 min
SC RI PT
for DI water flux calculation. Then the DI water in the feed tank was replaced with PPW and filtration commenced. UF experiments were conducted till 500 mL of permeate was collected. The permeate weight was recorded in 1 min intervals. The permeate flux was then calculated. 2.4.3. EC –UF
EC, see section 2.2, was used to pretreat the PPW prior to UF. The RC 30 kDa membrane
U
was tested as described above using 1 L of PPW pretreated using EC. The variation of flux as a
N
function of permeate volume was determined. Finally, a long-term filtration experiment was
A
conducted using the RC 30 kDa membrane with raw and EC pretreated PPW. RC 30 kDa
M
membrane was first compacted with DI water using the method described in section 2.4.2. Then,
D
10 L feed PPW was placed in the feed tank and the experiment was continued over a week. Fresh
A
CC
EP
the feed tank.
TE
feed PPW was intermittently added to the feed tank to maintain a volume of greater than 5 L in
9
SC RI PT U N A
2.5. Membrane cleaning
D
M
Figure 2. Schematic diagram of the filtration (TFF) setup
TE
Since the permeate flux decreases during filtration due to membrane fouling, membrane
EP
cleaning was performed in order to recover the initial permeate flux. The cleaning cycle consisted of four steps: cleaning with a detergent, rinsing with DI water, cleaning with acid and rinsing with
CC
DI water. (1) 0.2% Triton X-100 in DI water (pH adjusted to 9.0 with 0.1 M sodium hydroxide at room temperature) was placed in the feed tank and pumped through the membrane module at a
A
flow rate of 1.2 L min-1 and 70 kPa pressure for 30 min. Next, the feed tank was filled with DI water which was pumped through the membrane module under the same conditions for 30 min. The feed tank was filled with DI water containing citric acid (pH adjusted to 4.0). Again, the feed was recirculated for 30 min through the membrane module under the same conditions. Finally, DI water was placed in the feed tank and recirculated through the membrane module for 30 min, again 10
under the same conditions. After completion of the cleaning procedure, the membranes were retested with feed PPW. 3. Results and discussion
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3.1. Wastewater characterization
Ten PPW samples were collected over a period of three months at Tyson Food Inc. facilities and were analyzed for the level of contaminations. Measured pH of the PPW samples were in the range of 6.2 to 7.5 with an average of 6.7. Figure 3 represents the other characterization results of
U
the ten PPW samples. As can be seen, the value of all wastewater parameters is relatively higher
N
than average domestic wastewater. BOD in the range of ~275 to 500 mg L-1, COD in the range of
A
~800 to 1050 mg L-1, FOG in the range of ~100 to 190 mg L-1 and TSS in the range of ~160 to
M
280 mg L-1 were observed. Asselin et al. [44], Del Nery et al. [45] and Basitere et al. [15] report much higher values (more than twice) for BOD, COD, TSS, TKN and FOG in their tested PPW
D
compared to the values we report in Figure 3. However, PPW tested by them is obtained from the
TE
effluent wastewater of the poultry processing plant, where the PPW is a combination of different wastewater streams; whereas the PPW in this work is obtained from the chilling process. Figure
EP
3 shows proteins in the range of ~125 to 200 mg L-1, which is almost half of the value reported by
A
CC
Lo et al. [46].
11
SC RI PT U N A
M
Figure 3. Characterization of collected PPW samples from Tyson Foods Inc. facilities processing
D
200,000 birds per day.
TE
Results of particle size distribution analysis for raw PPW is shown in Figure 4.a. The size range of the particles in the raw PPW are within 0.05–100 µm with an average size of 0.084 µm
EP
(number-based) and 36.24 µm (volume-based). The volume-based size distribution diagram is skewed to right compared to the number-based diagram mainly due to presence of a small number
CC
of larger particles which in terms of the total volume of particulate matter are significant. Majority
A
of the total particle volume comes from the particles around 36.24 µm, whereas individual particles in the range of 0.084 are more abundant.
12
SC RI PT U N A M D TE EP CC A Figure 4. Particle size distribution of particulate matter in a) raw PPW sample, and b) EC pretreated PPW sample after sedimentation.
13
3.2. EC performance 3.2.1. Removal mechanism In this study, EC was investigated as the pretreatment step prior to membrane filtration. According to Eqs. (1) and (2) (see
SC RI PT
Aluminum electrodes were used in the EC reactor.
introduction), Al3+ and OH- ions were produced at the anode and cathode, respectively. Aluminum and hydroxide ions react and form a variety of monomeric (e.g. Al(OH)2+) and polymeric (e.g. Al6(OH)153+) species in the solution that eventually transform into insoluble Al(OH)3(s) solids. For pH in the range of 6 to 8, which is the case for tested PPW here, formation of amorphous Al(OH)3(s)
U
prevails [47]. Due to their large surface area, Al(OH)3(s) particles adsorb organic compounds, trap
N
suspended particles and form agglomerated flocs. Formed Al(OH)3(s) flocs could be easily
A
separated from the solution by sedimentation or flotation. Here, EC pretreated PPW samples were
M
transferred to a separatory funnel for sludge deposition.
Thus, flocs were separated by
D
sedimentation following a 6 h sedimentation time. Figure 5 shows digital images of a PPW sample
TE
during different stages of EC pretreatment. While the overall reaction mechanism of EC may seem ordinary, the actual series of reactions that undergo to form aluminum hydroxide solids is much
EP
more complex and require in-depth investigation.
CC
Figure 5.a shows raw PPW and Figure 5.b shows EC pretreated PPW after 5 min reaction time. As can be seen, the original yellow color of the raw PPW was transformed to grey after 5
A
min EC which indicates the formation of Al(OH)3(s) solids [48]. Figure 5.c illustrates the recovered EC pretreated PPW after 6 h sedimentation (deposited sludge and floating skimmings were wasted). As can be seen, the majority of agglomerated flocs are removed. However, the efficiency of physical floc separation can be enhanced by employing a floatation process (e.g. DAF) after the EC [49]. Although removal mainly takes place by Al(OH)3(s) agglomeration and sedimentation, 14
the remainder of positively charged aluminum hydroxide species such as Al(OH)2+ can also help with contaminate removal [32]. These species can contribute to destabilization of the organic macromolecules by charge neutralization and their consequent agglomeration as neutral colloidal
M
A
N
U
SC RI PT
entities and finally, physical separation by sedimentation [50,51].
Figure 5. Digital image of a) raw PPW, b) EC pretreated PPW after 5 min reaction time, and c)
D
recovered EC pretreated PPW after 6 h sedimentation.
TE
3.2.2. Contaminate removal efficiency
EP
The efficiency of pollutant removal by EC process depends on variety of operating parameters including: the material of the electrodes used, current density, voltage, reaction time,
CC
etc. Here, we conduct all EC experiments at steady voltage of 9.0 V (autonomous current change) and vary the reaction time. The effects of EC on removal of different contamination factors are
A
given in Figure 6. Removal efficiencies are calculated using Eq. (5): 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =
𝐶𝑝𝑝𝑤 −𝐶𝑟𝑤 𝐶𝑝𝑝𝑤
ꓫ 100
(5)
where, Cppw and Crw are the concentration in the raw PPW and recovered water, respectively. As it can be seen in Figure 6, EC has significantly decreased the amount of impurities in PPW and 15
could be an alternative pretreatment method. In Figure 6, it can be seen that increasing the reaction time above 5 min does not significantly increase the removal efficiency of impurities after 6 h sedimentation time. After 5 min reaction time, up to 94% of FOG, 87% of BOD, 59% of COD
SC RI PT
and 84% of TSS are removed. Kobya et al. [38] report the same level of COD and FOG removal from their tested PPW after 5 min EC reaction time, employing aluminum electrodes, 11 mm electrode spacing and constant current density of 150 A m-2. Here we maintained a steady voltage of 9.0 V throughout EC experiments, resulting in an average of ~30 A m-2 current density. TDS was not significantly removed by EC and is not shown in Figure 6 (TDS removal less than 5%).
U
Due to the generation of ionic species during EC, the current was decreased to ensure a
N
constant voltage of 9.0 A. The electrical energy consumption per volume during EC was
M
𝑉∗𝐼∗𝑡 𝑉𝑟
(6)
D
𝐸=
A
calculated using Eq. (6) [34]:
TE
where V is applied voltage, I is average current, t is reaction time and Vr is volume of feed water. Voltage was fixed at 9.0 volts for all experiments and the current provided by DC power source
EP
was recorded through the experiments. Figure 6 represents the energy consumption of EC system as a function of reaction time in the secondary vertical axis. Higher removal was achieved for
CC
longer reaction times which resulted in higher energy consumption. Increasing the EC reaction time from 1 to 5 min resulted in over 24% increase in BOD and FOG removal as well as over 14%
A
increase in TSS removal after 6 h sedimentation. However, increase in reaction time over 5 min did not significantly increase the removal of contaminants after sedimentation. Consequently, 5 min was chosen as the EC reaction time for EC-FO experiments (see section 3.4). For 5 min reaction time, 0.15 kWh m-3 is estimated as the EC energy consumption using Eq. (6).
16
SC RI PT U N A M
Figure 6. Removal efficiency of wastewater parameters as well as EC energy consumption as
D
functions of reaction time. Voltage was fixed at 9.0 V and the current from the DC power source
TE
was recorded every 20 seconds in order to determine energy consumption. TDS removal is not
EP
shown in the figure due to minimal removal in EC process. 3.2.3. Particle size distribution
CC
Results of particle size distribution analysis for EC pretreated PPW after 6 h sedimentation
are shown in Figure 4.b for the recovered water. The size range of the particles in the EC pretreated
A
PPW are within 0.05–100 µm, similar to raw PPW, with majority of the total particle volume coming from the larger particles (~25.70 µm). However, the average particle size of EC pretreated PPW is different from the raw PPW. As can be seen, the number-based size distribution for pretreated PPW is skewed to right compared to raw PPW, denoting agglomeration of particles
17
after EC. Aside from Al(OH)3(s) solids (see section 3.2.1) that resulted in particle agglomeration, non-transformed positively charge aluminum hydroxide species such as Al(OH)2+ could also contribute to organic macromolecules aggregation by charge neutralization. These positively
SC RI PT
charge species could form neutral complexes with negatively charged organics, resulting in neutral colloidal entities [50].
3.3. UF performance
Selection of a suitable membrane for specific membrane application is an important phase
U
in a process development procedure. This requires the understanding of the influence of different
N
parameters in order to get highest possible permeate flux and rejection of targeted components in
A
the feed solution. Permeate flux and solute rejection depend on the type of membrane used, feed
M
concentration, transmembrane pressure, feed pH, etc.
D
Figure 7.a gives the variation of permeate flux for RC 30 kDa membrane when treating
TE
PPW as a function permeate volume. Error bars represent the standard deviation of the permeate flux measurements. Permeate fluxes of virgin membrane, membrane after first cleaning and
EP
membrane after second cleaning are shown. As can be seen, all three permeate flux curves indicate a decrease in the permeate flux relative to the initial flux. Initially, the permeate flux is found to
CC
decline rapidly followed by a more gradual decrease over time. Avula et al. [4] report a rapid flux decline (~40% decrease in initial flux) in the first 20 min of UF experiment with a continuous
A
gradual flux decline till the end of process when treating PPW. Bialas et al. [12] report the same trend using a 30 kDa molecular weight cut-of UF membrane. The initial period of rapid flux decline is probably due to initial deposition inside the membrane pores. Since a large number of particles in raw PPW are smaller than the membrane pore size, they can plug the membrane pores
18
and be trapped within the membrane structure [52,53]. The region of more gradual flux decline could be mainly due to deposition of foulants on the membrane surface and formation of a cake layer [54]. In addition, increase in feed solution viscosity due to increase in feed solution
SC RI PT
concentration leads to a gradual decrease in permeate flux. Membrane was regenerated using the four-step cleaning method (see section 2.5) twice.
As can be seen in Figure 7.a, membrane cleaning was able to reasonably recover the initial permeate flux. However, a more sever flux decline was observed after membrane cleaning compared to the permeate flux of the virgin membrane, mainly due to incomplete foulant removal
U
during cleaning. Comparing the permeate flux after the first and second cleaning indicated a more
N
rapid drop in flux after each subsequent use. This suggests that while the cleaning protocol
A
developed here is able to regenerate almost the original DI water flux, some irreversible fouling
M
occurs. The presence of adsorbed species lead to more rapid flux decline during subsequent PPW filtration.
D
The rejection of contaminates from feed PPW by UF are calculated as removal efficiency
TE
and are presented in Figure 7.b. Removal efficiency is calculated using Eq. (7)
EP
𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =
𝐶𝐹 −𝐶𝑃 𝐶𝐹
ꓫ 100
(7)
CC
where CF and CP are the concentration in the feed and bulk permeate after removal of 500 mL permeate, respectively. Figure 7.b gives the removal efficiency results for the virgin membrane
A
as well as membrane after cleaning. As can be seen, COD and BOD were removed by up to 82 and 87%, respectively. On average, ~95% of TSS and FOG were removed. Proteins were removed by up to 55%. Only limited removal of TDS was achieved. The results indicate that unlike permeate flux, rejection performance is almost similar for virgin and regenerated RC 30 kDa membrane. 19
SC RI PT U N A M D TE EP CC A Figure 7. a) Variation of permeate flux as a function of permeate volume when treating PPW for virgin and regenerated RC 30 kDa membrane, and b) rejection of contaminates from raw PPW by UF. 20
3.4. Membrane filtration after pretreatment (EC-UF) EC was used to pretreat PPW prior to UF experiments in order to mitigate fouling. Figure 8.a shows the variation of permeate flux for raw and EC pretreated PPW as a function of collected
SC RI PT
permeate volume. Comparing Figures 6 and 8.a, 5 min EC reaction time followed by 6 h sedimentation resulted in significant removal of TSS, FOG and BOD which consequently, led to higher initial permeate flux as well as lower flux decline during UF. Similar to this observation, Choi et al. [55] report significant fouling reduction using in-line coagulation prior to UF. Gradual flux decline observed for EC pretreated PPW in Figure 8.a indicates the foulant deposition on the
U
membrane surface could be the main reason for the slight decrease in permeate flux over the period
N
of permeate collection [54]. This is not unexpected given the significant removal of contaminates
A
in pretreatment step (EC) as well as the change in particle size distribution of EC pretreated PPW
M
compared to raw PPW (see section 3.2.3). As can be seen in Figure 4.b, majority of particles in
D
EC pretreated PPW are larger than the membrane pore size due to particles agglomeration in EC;
TE
thus, remainder of suspended particles and organic macromolecules in EC pretreated PPW are less likely to enter the membrane pores during UF.
EP
Figure 8.b illustrates the performance of EC-UF compared to individual UF for
CC
contaminates rejection in terms of removal efficiency. Removal efficiency is calculated using Eq. (7) where CF and CP are the concentration in the feed and bulk permeate. Almost a complete
A
removal of TSS and FOG is achieved in EC-UF. Significant increase in removal of COD, BOD, proteins and TKN is observed when pretreating PPW via EC prior to UF. COD and BOD were removed by up to 92 and 98%, respectively. Proteins were removed by up to ~90%.
21
SC RI PT U N A M D TE EP CC A Figure 8. a) Variation of permeate flux for raw and EC pretreated PPW as a function of collected permeate volume and b) rejection of contaminates from PPW by individual UF as well as EC followed by UF. 22
3.5. Long-term experiments Long-term filtration experiments were performed in order to investigate the performance of individual UF as well as combined EC-UF. Figure 9.a illustrates the variation of permeate flux
SC RI PT
as a function of time. Raw and EC pretreated PPW (5 min reaction time) were used separately as feed solutions. A rapid flux decline was observed for raw PPW. As can be seen, permeate flux was reduced by over 91% for raw PPW after one day of testing. This permeate flux dropped to almost zero after 3 days. For the pretreated PPW, permeate flux was reduced from an initial value of 106 to 46 L m-2 h-1 after one day. A gradual flux decline continued from day one to day seven.
U
At the end of day seven, 22 L m-2 h-1 permeate flux was measured, and a total of 19.8 L water was
N
recovered. While EC leads to higher permeate fluxes, it is the increase in water recovery versus
M
A
the additional cost of the EC step that will determine the economic viability of the process [32]. Intermittent cleaning was utilized to enhance the water recovery in a long-term EC-UF
D
experiment. In this experiment, the membrane was regenerated using the cleaning method
TE
discussed earlier (see section 2.5) at 24 h intervals. Cleaning was repeated 6 times. Results are shown in Figure 9.b. Cleaning could successfully recover the permeate flux over the period of the
EP
experiment. As can be seen, recovered water volume was increased by over 30% in a period of 7
CC
days. A total of 29.7 L water was recovered after 8 days experimental time. In an optimized
A
process, cost of membrane cleaning must be compared with additional volume of recovered water. In this work, no attempt was made to optimize the EC sedimentation time (6 h). In an
actual practice, it is likely that the sedimentation time will be determined by factors such as available land area, maximum volume of the sedimentation tanks and integration into the overall wastewater treatment process. It is likely that EC sedimentation time of less than 1 h will be required. This may require skimming the floc as it forms at the top surface rather than waiting for 23
it to age, densify and settle to the bottom. This in turn could affect the optimal EC reaction time. Further development of a continuous EC process will be essential. While the effect on EC performance of only a limited number of parameters has been considered here, a more detailed
SC RI PT
study which considers the rate of coagulant dissolution into feed water, either by increasing the reaction time, current density or voltage will need to be considered.
A DAF system can also aid
in physical separation of the EC sludge from the pretreated water. In some poultry processing plants, two sequential DAF systems are used to treat the PPW prior to discharge. EC can be installed prior to the second DAF. Hence, the sludge separation can take place in the second DAF
U
instead of a sedimentation tank. Nevertheless, our work indicates the feasibility of a combined
N
EC-UF process for treatment of PPW and highlights the possibility of recovery and reuse of a
A
CC
EP
TE
D
M
A
portion of the PPW.
24
SC RI PT U N A M D
TE
Figure 9. Long-term filtration experiments with a) raw and EC pretreated PPW without membrane
EP
cleaning, and b) EC pretreated PPW utilizing intermittent cleaning. Selection of a process for recycling and reuse of PPW depends on the quality of the
CC
recovered permeate as well as the related costs. The concentration of contaminates in raw PPW as well as the permeate quality for long-term UF and EC-UF experiments are shown in Figure 10.
A
Comparing the UF and EC-UF permeates, higher contaminate removal is observed for EC-UF. Similar permeate quality is reported for EC-UF process with and without intermittent membrane cleaning. Almost a complete removal of TSS and FOG is obtained in the integrated EC-UF.
25
Proteins are reduced to less than ~45 mg L-1. COD and BOD are removed to below ~70 and ~40
M
A
N
U
SC RI PT
mg L-1, respectively.
D
Figure 10. Concentration of contaminates in raw PPW as well as the permeate quality for long-
EP
TE
term UF and EC-UF experiments
CC
4. Conclusions
A
Here we have investigated the application of EC pretreatment prior to UF for treating high
strength PPW. We report significant membrane fouling of the UF membrane due to high levels of O&G, proteins and TSS in the feed water that results in rapid flux decline. However, we show that the membrane fouling can be greatly mitigated using EC. We show that for an EC reaction time of 5 min equivalent to 0.15 kWh m-3 energy consumption for EC, followed by 6 h 26
sedimentation, over 85% reduction of O&G, BOD and TSS results. This reduction leads to higher initial permeate flux as well as lower flux decline during UF. In addition, higher contaminate rejection is obtained in EC-UF process compared to individual UF.
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Selection of a membrane-based process for reuse of PPW depends on a number of factors including: the quality of the recovered permeate, cleaning requirements and the related costs. Pretreatment of PPW streams will be essential if membrane-based separation processes such as UF are to be used to treat these highly impaired waters. As we show in this study, pretreatment
U
(here EC) enables the UF process to operate more efficiently for longer periods. We also show
N
that intermittent cleaning can increase the water recovery to a great extent. Here, the recovered
A
water volume was increased by over 30% in a week of experimental time when employing
M
membrane cleaning after each 24 h.
D
Acknowledgements
TE
Funding for this work was provided by Tyson Foods Inc. through the National Science
EP
Foundation Industry/University Cooperative Research Center for Membrane Science, Engineering
A
CC
and Technology, the National Science Foundation (IIP 1361809) and the University of Arkansas.
27
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