Evaluation and optimization of electrocoagulation for treating Kraft paper mill wastewater

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Abstract

The performance of electrocoagulation (EC) using iron as sacrificial electrode was investigated at low current densities (0.6–1.9 mA/cm2) for treating a waste stream from a Kraft paper mill. A multi-level factorial design of experiments and response surface methodology (RSM) were applied to quantify the effects of current density and reaction time on the removal efficiency of color, COD, BOD5, tannin/lignin as well as on the enhancement of biodegradability index (BI). Increasing current density or reaction time significantly improve the treatment performance. An antagonistic effect of the two studied factors was identified, indicating that simultaneously increasing current density and reaction time does not yield added improvement in treatment performance. An RSM was developed to predict the optimum current density and reaction time within the experimental data range of this study. It was found that an optimized treatment combination exists at relatively low current density and reaction time. The optimum value for maximum pollutants removal was ∼1.5 mA/cm2 and ∼25 min. Under the optimum condition, the removals of tannin/lignin and color were >70 % and >95 %, and BI was increased to 0.41 BOD5/COD. Current density is the dominant factor for energy consumption and operating cost compared to reaction time and electrode consumption. This study demonstrated that EC can be operated at low current density to remove the bio-refractory organics effectively and increase the biodegradability index to the level suitable to biological treatment at low operating costs.

Introduction

The pulp and paper mill industries discharge 75−225 m3 effluent (20−25 m3 from pulping and 80−100 m3 from bleaching process) per ton of paper products produced [[1], [2], [3]]. The streams carry high color, chemical oxygen demand (COD), biochemical oxygen demand (BOD), suspended solids, and absorbable organic halides (AOX), fatty acids, lignin/tannins, resin acids and sulfur compounds [4]. In paper making process, the bleaching stage, particularly alkali extraction process, typically contributes most significantly to the overall pollution load although the volume is relatively low [2]. Lignin and its derivatives and polymerized tannins are non-biodegradable and the primary color sources in the effluents [[5], [6], [7]]. Tannins absorb light and heat significantly, leading to lower dissolved oxygen and negatively affecting the aquatic flora and fauna [4]. Resin acids are toxic to fish and long chain fatty acids inhibit methanogen bacteria [8,9]. Organochlorides bio-accumulate in aquatic food chain especially in the body fat of animals at higher tropic levels [10]. The discharge of colored effluents to natural water also changes the algal and aquatic plant productivity due to the reduced penetration of solar radiation and causes serious aesthetic problems [4].

Treatment technologies including physicochemical and biological treatment, ozonation, adsorption, membrane filtration and advanced oxidation have been attempted for paper mill wastewater [2]. Most facilities employed primary clarification (sedimentation or flotation), secondary treatment (aerobic and anaerobic treatment), and tertiary treatment (ultrafiltration membrane processes) [11]. Conventional biological treatment methods are insufficient due to the presence of toxic and recalcitrant substrates. The employment of chemical coagulation followed by biological treatment is somewhat effective in terms of pollutants removal but not popular due to accumulation of chemical byproducts post-treatment [12]. Ugurlu et al. (2008) reported that lignin undergoes a spatial rather than chemical change and persists albeit in a different form, so even after different physicochemical processes the problems remain unsolved [13].

Electrocoagulation (EC) has been practiced for overcoming the problems associated with treating paper mill wastewater. Three processes occur simultaneously: (a) electrolytic oxidations at electrode surfaces, (b) formation of coagulants in the aqueous phase and destabilization of particulate suspensions, and (c) adsorption of destabilized particles on coagulants to form flocs followed by water-solids separation removal by electroflotation, sedimentation or filtration [14,15]. The mechanism for metal hydroxide production during EC using iron electrodes involves formation of Fe(II) hydroxide followed by the production of Fe(III) hydroxide coagulant through Fe(II) oxidation [[16], [17], [18]]. The Fe(III) hydroxide complexes remove pollutants by complexation or electrostatic attraction followed by water-solid separation of the coagulated flocs [17].

Earlier studies have demonstrated the effectiveness of EC in removing color and organic loading from paper mill effluent. Kumar & Chhaya (2019) utilized a commercial electrode to study optimum electrocoagulation conditions (pH = 7, current density =24.8 mA/cm2, time =40 min and electrolyte dose =1 g/L Na2SO4) for 82 % COD and >99 % color removal [19]. Izadi et al. (2018) utilized a combination of parallel Al and Fe electrodes in the electrocoagulation cell and reported the optimum conditions at pH 7, electrolysis time of 60 min and voltage of 10 V for 79.5 % COD, 83.4 % TSS, 98.5 % color and 85.3 % ammonia removal [20]. Yuliani et al. (2017) compared the electrocoagulation performance of Fe and Al electrodes under optimum conditions (pH 7, 60 min reaction time, 14 V and electrode gap of 1.5 cm) and showed up to 37 % COD, 98 % turbidity and 50 % BOD removal [21]. Aghdam et al. (2016) reported 85 % COD and 78.5 % lignin removal using Fe and Al electrodes at pH 5, 60 min and 10 V [22]. Asaithambi (2016) reported 85 % COD removal using Fe electrode at 4 mA/cm2 current density, 1 cm inter-electrode gap and 120 min reaction time [23]. Sharma et al. (2014) reported a maximum COD (85 %) and color (94 %) removal at pH 7, 120 min reaction time and a current density of 1.5 mA/cm2 [24].

Although these studies successfully demonstrated the efficacy of EC for treating pulp and paper mill effluents [12,13,[25], [26], [27], [28]], they were performed on a trial-and-error basis by changing one factor at a time and did not approach system optimization from cost perspective. In addition, the synergistic and antagonistic effects of process parameters remained relatively less understood. Soloman et al. (2009) used a design of experiments (DOE) approach and reported an optimal operating point for maximizing biodegradability index (BI = BOD5/COD) [29]. Similarly, Sridhar et al. (2011) developed empirical models for quantifying individual and combined effects of current density, pH, reaction time and electrolyte concentration in color, COD and BOD removal [3]. However, the treatment performance optimization was not carried out systematically. Shankar et al. (2014) using a central composite design and reported the optimum process condition at pH 7, 75 min of reaction time at a current density of 11.5 mA/cm2 and an inter-electrode distance of 1.5 cm for maximum COD (77 %), TOC (78.8 %) and color (99.6 %) removal [30]. Thirugnanasambandham et al. (2015) employed current density ranging from 15 to 35 mA/cm2 and reported 84 % COD removal [31]. These studies applied high current density (5−35 mA/cm2), leading to wasted energy, heating of waste streams and a decrease of operation efficiency [32]. The performance of EC at low current density (<5 mA/cm2) has not been investigated, although Mollah et al. (2001) noted that the electrodes with larger surface area are required to achieve a workable rate of metal dissolution at low current density [33].

This study investigates the performance of EC at low current density using Fe as sacrificial electrode. A Response surface methodology (RSM) is applied with two-factor, three-level central composite design (CCD) by using MINITAB (version 16) statistical software to quantify the individual and interaction effects of the process parameters, as well as to predict the optimum values of current density and reaction time within the experimental range of this study.

Section snippets

Wastewater characteristics and chemicals

The studied stream was collected from the exit of primary clarifier at a Kraft paper mill located in southeast Texas, USA. The stream characteristics are shown in Table 1. The biodegradability index (BI = BOD5/COD) of 0.26 suggests the presence of non-biodegradable organics inappropriate for aerobic biological treatment. The stream contains moderate levels (33 mg/L) of tannin/lignin with 85 % of COD in the dissolved phase.

Experimental method

An EC reactor of with a 2-L treatment volume and an anode area of 160 cm2

Process performance

The pollutants removal efficiency and resulted BI at low, medium, and high current density and reaction time is given in Table 3. Increasing current density from 0.6 to 1.9 mA/cm2 for a given time, the pollutants removal increased as expected. This is because current in the electrochemical cell determines the coagulant dosage rate, bubble production rate, and size of flocs which influence the treatment efficiency of EC [37]. At 0.6–1.3 mA/cm2 current density, the treatment time was increased to

Conclusions

In this study, the performance of EC using iron as sacrificial electrode was investigated at low current density (0.6–1.9 mA/cm2). The results of factorial experiments showed that reaction time and current density had a significant effect in color, tannin/lignin and COD, and BOD5 removal at 95 % confidence interval. It was demonstrated that electrocoagulation can be operated at low current density and relatively short reaction time to remove bio-refractory organics (tannin/lignin) in paper mill

Declaration of Competing Interest

None.

CRediT authorship contribution statement

Dipendra Wagle: Formal analysis, Investigation, Writing - original draft. Che-Jen Lin: Conceptualization, Methodology, Visualization, Project administration. Tabish Nawaz: Validation, Formal analysis. Heather J. Shipley: Writing - review & editing.

Acknowledgements

This study was supported in part by MeadWestvaco (now WestRock) Evadale TX facility (Project No: MWV0001). The authors would like to thank Pengchong Zhang and Sophia Yang for their assistance in the selected laboratory analysis. The assistance of Robert Sasser in supplying the waste stream samples is greatly appreciated.

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