w a t e r r e s e a r c h 7 9 ( 2 0 1 5 ) 7 9 e8 7

Available online at www.sciencedirect.com

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Electrocoagulation treatment of peat bog drainage water containing humic substances € mo € b, U. Lassi a,c V. Kuokkanen a, T. Kuokkanen a,*, J. Ra a

University of Oulu, Research Unit of Sustainable Chemistry, P.O. Box 3000, 90014 Oulu, Finland University of Oulu, Thule Institute, Cewic, P.O. Box 4300, 90014 Oulu, Finland c University of Jyvaskyla, Kokkola University Consortium Chydenius, P. O. Box 567, FI-67701 Kokkola, Finland b

article info

abstract

Article history:

Electrocoagulation (EC) treatment of 100 mg/L synthetic wastewater (SWW) containing

Received 10 March 2015

humic acids was optimized (achieving 90% CODMn and 80% DOC removal efficiencies), after

Received in revised form

which real peat bog drainage waters (PBDWs) from three northern Finnish peat bogs were

13 April 2015

also treated. High pollutant removal efficiencies were achieved: Ptot, TS, and color could be

Accepted 17 April 2015

removed completely, while Ntot, CODMn, and DOC/TOC removal efficiencies were in the

Available online 5 May 2015

range of 33e41%, 75e90%, and 62e75%, respectively. Al and Fe performed similarly as the anode material.

Keywords:

Large scale experiments (1 m3) using cold (T ¼ 10e11  C) PBDWs were also conducted

Electrocoagulation (EC)

successfully, with optimal treatment times of 60e120 min (applying current densities of 60

Techno-economic analysis

e75 A/m2). Residual values of Al and Fe (complete removal) were lower than their initial

Large-scale experiments

values in the EC-treated PBDWs.

Peat bog drainage water Humic substances

Electricity consumption and operational costs in optimum conditions were found to be low and similar for all the waters studied: 0.94 kWh/m3 and 0.15 V/m3 for SWW and 0.35 e0.70 kWh/m3 and 0.06e0.12 V/m3 for the PBDWs (large-scale). Thus, e.g. solar cells could be considered as a power source for this EC application. In conclusion, EC treatment of PBDW containing humic substances was shown to be feasible. © 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Peat is one of the most important energy sources in Finland; annually, peat covers about 7% of the energy supply and 20% of district heat (Geological Survey of Finland, 2015). Globally, it is estimated that 3% of the total landmass is peatland, and the

main producers and users of peat are Finland, Belarus, Estonia, Ireland, Indonesia, Sweden, and the Russian Federation (World Energy Council, 2014). Most of these countries lie in the Northern Hemisphere, thus they have a long and cold winter season. There is an ongoing debate on whether peat energy should be produced or not, considering, e.g. its effect

Abbreviations: CODMn, chemical oxygen demand [mg/L]; DC, direct current; DOC, dissolved organic carbon [mg/L]; EC, electrocoagulation; EEC, electrical energy consumption [kWh/m3]; EMC, electrode material consumption [kg/m3]; HA, humic acids; HS, humic substances; i, current density [A/m2]; ICP, inductively coupled plasma; OC, operating costs [V/m3]; OES, optical emissions spectrometer; PBDW, peat bog drainage water; SEC, supporting electrolyte consumption [kg/m3]; SWW, synthetic wastewater; TOC, total organic carbon [mg/L]; TS, total solids [mg/L]. * Corresponding author. Tel.: þ358 50 4285 540. E-mail address: [email protected] (T. Kuokkanen). http://dx.doi.org/10.1016/j.watres.2015.04.029 0043-1354/© 2015 Elsevier Ltd. All rights reserved.

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on global warming and on the other hand its domesticity and vast reserves. However, one of the main problems of peat energy production (especially in Finland) is peat bog drainage water (PBDW), which can even be considered one of the key questions affecting the future of the entire Finnish peat industry due to the increasingly strict environmental regulations being proposed. To meet these regulations, new ideas are needed, since the effectiveness of conventional water treatment methods is often limited. PBDW is typically slightly acidic and colored, and is contains nutrients (P and N) as well as humic substances (HS) and total solids (TS). Excessive levels of nutrients are considered detrimental in natural bodies of water because oversaturation causes algae growth, leading to eutrophication (Bektas‚ et al.,  zquez et al., 2004; Zhao and Sengupta, 1998; Alvarez-Va 2014). This in turn depletes the oxygen level and hinders penetration of light into the water, thus adversely affecting organisms present in the aquatic environment and reducing  zquez et al., 2014). biodiversity (Bektas‚ et al., 2004; Alvarez-Va HS occur in soil and natural waters as residues of plant and animal decay by microbial activitydmost organic matter found in natural waters is constituted of HS (Ghernaout et al., 2009; Jones and Bryan, 1998; Seida and Nakano, 2000; Yıldız et al., 2007). One of the main components of HS in water (along with fulvic acids and humins) are humic acids (HA), which are weakly acidic aliphatic and aromatic compounds containing functional groups such as eCOOH and phenolic eOH groups  ska-Sobecka et al., 2006; (Prado and Airoldi, 2003; Seredyn Naddeo et al., 2007; O'Melia et al., 1999). These acidic compounds add a dark color to natural waters (Motheo and Pinhedo, 2000). This causes esthetic problems and further hinders the availability of light in the water. HS are heterogenous in structure, and on the whole their chemical structures are very complex with no defined physicochemical properties and a high molecular weight (several hundreds or larger) (Motheo and Pinhedo, 2000; Hesse et al., 1999). They are highly stable, and therefore are retractive to attack by microorganisms (Motheo and Pinhedo, 2000). Although they are at the end-point of nature's biodegradative and oxidative process (virtually non-biodegradable), due to their high aromatic and aliphatic residue content HS can readily be aggregated and precipitated by charge neutralization (Jones and Bryan, 1998; Yildiz et al., 2007; Ødegaard et al., 1999; Yıldız et al., 2008). Other potential methods for HS removal may include physicochemical processes, biological processes, membrane processes, etc., and especially classical methods such as usage of peatland buffer areas and/or wetlands. Electrocoagulation (EC) is a water treatment technology that has been known for over a hundred years now, but it is currently under intensive development and of commercial interest (Chen, 2004; Kuokkanen et al., 2013). In EC, so-called sacrificial anodes (commonly Al or Fe) are dissolved into water in situ, promoting coagulation, while microscopic hydrogen gas is usually generated simultaneously at the cathode, promoting flotation. The only chemical species used in EC is the electron (excluding possible pH and conductivity alteration chemicals), thus making EC a green technology. Charge neutralization of negatively charged colloids by cationic hydrolysis products and sweep flocculation

(enmeshment of pollutants in the amorphous hydroxide precipitate produced) are proposed as the main functional mechanisms of Al and Fe in EC (Karhu et al., 2012). The principal electrochemical reactions in the EC process are presented in Eqs. (1)e(5): At the anode :

AlðsÞ/Al ðaqÞþ3e 3þ

E0 ¼ þ1:66 V

(1)

FeðsÞ/Fe2þ ðaqÞþ2e

E0 ¼ þ0:44 V

(2)

FeðsÞ/Fe3þ ðaqÞþ3e

E0 ¼ þ0:04 V

(3)

2Fe2þ ðaqÞ þ 1=2 O2 ðgÞþH2 OðlÞ/2Fe3þ ðaqÞþ2OH At the cathode :

2H2 O þ 2e /H2 ðgÞþ2OH

(4)

E0 ¼ 0:83 V (5)

In principle, as presented in Eq. (3), iron may oxidize directly to Fe3þ at the anode, but this reaction is highly unfavorable (E0 ¼ þ0.04 V) compared with oxidation to ferrous (Fe2þ) iron (E0 ¼ þ0.44 V), as presented in Eq. (2). Dissolved oxygen in solution (see the sum reaction presented in Eq. (4)) may cause oxidation of the electrogenerated Fe2þ to Fe3þ (Zodi et al., 2009). The electro-generated Al3þ or Fe3þ ions undergo immediate spontaneous reactions in which corresponding hydroxides and/or polyhydroxides are produced (Mollah et al., 2004). The mass of the dissolved metal [g] can be calculated theoretically from Faraday's law (Eq. (6)), although the theoretical amount of anodic dissolution is found to be exceeded in many real EC applications (superfaradaic efficiencies). mmetal ¼

ItM zF

(6)

where I is the applied current [A], t is the treatment time of the EC process [s], M is the molar mass of the anode metal [g/mol], z is the valence number of ions of the substance (zAl ¼ 3, zFe ¼ 2), and F is Faraday's constant (96,485 C/mol). The experimental values of superfaradaic anode metal dissolution have varied case-specifically between 105% and 190% of the theoretically expected value (Terrazas et al., 2010; € Kongjao et al., 2008; S‚engil and Ozacar, 2009; Kobya et al., 2011; Yetilmezsoy et al., 2009; Mouedhen et al., 2008; Kuokkanen et al., 2015). It has been proposed that this phenomenon is due to pitting corrosion, especially in the presence of chlorine ions (Chen, 2004). In addition, when chloride is present in solution (e.g. from NaCl) and the anode potential is sufficiently high, active chlorine species (Cl2, HClO, OCl) may form. These species may then both oxidize pollutants and simultaneously chemically oxidize the anode material, and thus enhance the performance of an EC reactor. Moreover, Fe2þ can reduce organics to be transformed into Fe3þ. Superfaradaic anode dissolution is a parameter of whose effect on the EC process is rather rarely studied and should be taken into account when analyzing the economics of a given EC application, since metal material and electricity costs are regarded as the main cost components of EC. Even though the use of EC in treating various types of water and wastewater has recently been studied extensively, to the best of our knowledge there are no previous scientific papers dealing with EC treatment of PBDW. Also, scale-up studies on

w a t e r r e s e a r c h 7 9 ( 2 0 1 5 ) 7 9 e8 7

the removal of any kind of pollutant from water and wastewater by EC have been scarce. To help evaluate the feasibility and scalability of industrial application of the EC process, experiments using larger-scale equipment should also be performed already at the beginning of any given study. This also helps identify practical problems related to the particular EC system at hand. (Kuokkanen et al., 2013 and Kuokkanen et al., 2015) The aim of this work was firstly to study the effects of various operational parameters on the efficiency of the EC process in removing humic acids from synthetic wastewater (SWW) and then (primary aim) to use EC to treat real PBDWs (a novel application) containing HS. Also, scaling up the process from a laboratory-scale to a 1-m3 EC system to treat cold PBDWs (and thus prove the feasibility of the method) was an essential part of this study.

2.

Materials and methods

2.1. Laboratory-scale electrocoagulation of synthetic solutions containing humic acids The laboratory-scale EC experiments were carried out using an EC system depicted in Kuokkanen et al. (2015), applying DC power. The cleaning procedures for the glass EC cylinder and electrodes as well as the electrode weighing procedures were also similar. In each experiment, 1800 ml of synthetic humic acid solution (100 mg/L) were treated. The solution was prepared by diluting commercial humic acid (Aldrich H16752 Humic acid sodium salt) into deionized water; the solution had CODMn and DOC values of about 38e43 mg/L and 27e32 mg/L, respectively. The natural pH of the SWW varied between 7.0 and 8.0 and its color was brown. The initial pH value of the solution was adjusted using 0.1M HCl or NaOH. The pH values of the solution were also measured after each experiment. The experiments were conducted in ambient temperature of around 20  C ± 2  C unless stated otherwise. During the experiments, a magnetic stirrer was used at a rate of 250 rpm. In all the experiments with the SSW, an electrode gap of 7 mm was used and NaCl was added so that it's concentration in the solution to be treated was 0.5 g/L (to increase its conductivity). Water conductivity and temperature as well as voltage were monitored during all the experiments. All the SWW experiments were duplicated or triplicated and the results showed good repeatability.

2.2. Electrocoagulation of real peat bog drainage water containing humic substances In this work, real PBDWs from three northern Finnish peat bogs containing humic substances were also treated. The characteristics of the PBDW samples treated in this work are given in Table 1 and proved to be somewhat similar to each other. In all the experiments with the PBDWs, NaCl was added so that it's concentration in the solution to be treated was 0.5 g/L (to increase its conductivity). PBDW1 was treated on a laboratory scale applying procedures similar to those applied in the synthetic solution test runs, using both Al/Fe and Fe/Al electrode combinations.

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Large-scale EC studies on PBDW2 and PBDW3 were carried out with a large-scale batch EC apparatus (using only an Al/Fe electrode combination) in which the water volumes in the cubic EC vessel were about 950 L and 975 L for PBDW2 and PBDW3, respectively. The dimensions of the electrode plates (Al, Fe) used in the large-scale experiments were 10  400 mm  400 mm, with a submerged area of about 0.33 m2 for each electrode. The electrodes were submerged to a depth of about 10e15 cm below the water surface and had an electrode gap of 7 mm.

2.3.

Analytical procedures

The water samples were filtered through a 0.45 mm filter paper, after which a suitable strong acid (HNO3, H2SO4 or HCl) was added to them and they were refrigerated. Concentrations of trace elements were analyzed using ICP-OES. All the water analyses were conducted at an accredited test laboratory, € risto € Oy, Oulu, Finland. Ahma ympa

3.

Results and discussion

3.1. Influence of individual factors on the efficiency of humic acid removal from the synthetic solution The most essential operational variables influencing the efficiency of the EC process are: electrode materials (see 3.1.1), water characteristics including initial pH (see 3.1.2) as well as the types and concentrations of the pollutants to be removed, current density, treatment time (see 3.1.3), solution temperature (see 3.1.4), supporting electrolyte, electrode gap, etc. The efficiencies (R%) of pollutant removal can be calculated using Eq. (7): R% ¼

co  c1 * 100% co

(7)

where c0 and c1 are pollutant concentrations before and after EC treatment, respectively.

3.1.1.

Effect of electrode materials

The effect of the electrode materials on the efficiency of HA removal was investigated. Al/Fe and Fe/Al electrode combinations were used as the anode/cathode pair in these experiments. Initial pH values of 5 and ~7-8 (unmodified) were applied. Treatment time (t) in each test was 15 min and current density was 100 A/m2 (a current of 0.7 A). The results of these experiments are presented in Fig. 1. As shown in Fig. 1, both Al and Fe performed well and similarly, both removing 91e92% of the initial CODMn in 15 min under both initial pH conditions. Additionally, all the experiments yielded a 70e80% reduction in DOC in 15 min. However, with 5-min EC treatment, using an Al anode seemed to be clearly favorable, especially when a lower initial pH value was applied. A presumable reason for this is a delay in oxidation of divalent iron to trivalent, which is mainly responsible for coagulation according to the Schulze-Hardy principle (Verrall et al., 1999). Furthermore, this can be considered important, because real PBDWs tend to be varyingly acidic in nature. Thus, subsequent testing was

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Table 1 e Characteristics of the real PBDW samples used in this study. Water type PBDW1 PBDW2 PBDW3

pH

Conductivity [mS/cm]

Ptot [mg/L]

Ntot [mg/L]

CODMn [mg/L]

DOC [mg/L]

Al [mg/L]

Fe [mg/L]

TS [mg/L]

Color

6.4e7.0 6.6e6.8 6.1e6.4

~60 150e160a 85e90a

60e110 60e70 35e60

1.5e1.7 1.1 1.9e2.0

27e28 17 20e21

23e25 14e15b ~17b

~130 380e410 330e630

~4.3 3.7e3.8 5e14

n.d. ~12 ~13

Brown Brown Brown

n.d. ¼ Not determined. a At a temperature of 10e11  C. b Analyzed as TOC (total organic carbon).

conducted using Al/Fe electrode combinations and also because of the possible coloration of the water by Fe anodes.

3.1.2.

Effect of initial pH

Initial pH has been established to have a considerable influence on the performance of the EC process. Thus, its effect on the removal of humic acids from SWW was studied by adjusting the initial pH value of the solution to 4e9 (Fig. 2.). As shown in Fig. 2, there was a significant difference in the efficiencies of SWW treatment when different initial pH values were applied, with lower values bringing about better results. These results are in line with those presented on HA removal from SWW by Yıldız et al. (2008), using Al-EC, and Bazrafshan et al. (2012), using Fe-EC. They are also in line with common chemical aluminum coagulation practice, in which

pH is typically adjusted to 5e6. At this pH the solubility of aluminum is low, and it forms positively charged hydroxide sol capable of adsorbing negatively charged HA. The surface charge of the sol decreases with higher pH values, hindering its ability to adsorb and thus remove HA. Therefore, from the point of view of forming aluminum hydroxide as well as HA, charge properties probably allowed quicker removal at lower pH values. It should be noted that even though an initial pH of 4 produced the best percentages of removal of CODMn and DOC (91% and 81% in 5 min, respectively), the water remained slightly brownish after 15 min of EC treatment. However, floc was produced at the top of the EC vessel. However, this color was removed at the filtration stage of sample pretreatment. With other initial pH values, the water was always colorless at

Fig. 1 e Effect of electrode materials on the removal of humic acids from synthetic wastewater (c0 ¼ 100 mg/L, i ¼ 100 A/m2, T ¼ 20  C, 0.5 g/L NaCl).

Fig. 2 e Effect of initial pH on the removal of humic acids from synthetic wastewater (c0 ¼ 100 mg/L, anode/cathode ¼ Al/Fe, i ¼ 100 A/m2, T ¼ 20  C, 0.5 g/L NaCl); A) COD removal, B) DOC removal.

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the end of a 15-min EC run, with a clearly thicker floc formed (shortly after about 10 min of EC treatment in all cases). An initial pH value of 4 was chosen for further experimentation with the SWW.

3.1.3.

Effect of current density and treatment time

The amount of coagulant metal added to the wastewater is directly dependent on the current density i [A/m2] and treatment time applied. Therefore, their effect on the efficiency of humic acid removal from the SWW was studied. Current density can be calculated from the current I in the EC cell per effective anode area below the water surface Aeff [m2 (i ¼ I/ Aeff)].Current density values of 25 A/m2, 50 A/m2, and 100 A/m2 were applied in these experiments (15 min each) and their results are presented in Fig. 3: Surprisingly, as presented in Fig. 3, all the current densities studied were found efficient in removing HA from the SWW already in 5 min, and they yielded almost similar results. DOC removal efficiencies were also in the range of about 75e80% in 10e15 min for all the values of i studied. However, a current density of 100 A/m2 and a treatment time of 10 min were chosen as optimum values (92% CODMn and 79% DOC removal, respectively), taking into account the color remaining in the SWW (as discussed in Section 3.1.2), removal efficiency and electricity and anode metal consumption.

3.1.4.

Effect of solution temperature

The effect of the solution temperature on the efficiency of the EC process was also studied by applying initial solution temperatures of 10  C, (ambient) 20  C, and 30  C. This was done in order to pre-study whether it would be possible to remove humic substances from real PBDW also in the cold northern climate. The results showed clearly that the solution temperature had virtually no effect on the efficiency of humic acids removal from the SWW. Some of the tests were prolonged from 15 to 60 min and it was found that no further removal of CODMn or DOC could be achieved. Therefore, it was concluded that a maximum of about 90% CODMn and 80% DOC could be removed by Al-EC from this type of SWW containing HA.

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3.2. Electrocoagulation of real peat bog drainage water containing humic substances 3.2.1.

Laboratory-scale electrocoagulation experiments

Real PBDW from a northern Finnish peat bog (PBDW1) was treated using the laboratory-scale EC system. An unmodified initial pH value of 6.4e7.0 and a current density of 70 A/m2 (0.5 A) were applied in these tests. The experiments were conducted in ambient temperature using both Al/Fe and Fe/Al electrode combinations. The test was also scaled up from a volume of 1800 ml to 4500 ml (using both Al/Fe and Fe/Al) by changing the EC glass beaker; the experimental runs lasted 60 min and 150 min, respectively. The removal efficiencies achieved in these experiments for CODMn, DOC, Ptot, Ntot, and Fe are presented in Figs. 4 and 5. Four experiments were conducted in total (no duplicates), and in all of them the turbid water turned colorless and clear as a result of the EC treatment and a brown floc rose to the top of the EC vessel, as shown in Fig. 6. As observed in Fig. 4, there was no significant difference between Al and Fe anodes in terms of Ptot and Ntot removal in the 1800-ml experiments with PBDW1, and the performance of both metals was promising (treatment time of 20e30 min was found to be optimum for both, with an electrical charge of 333e500 C/L). However, Al seemed to remove phosphorus slightly more efficiently, whereas Fe was found more efficient in removing nitrogen. It should be noted that both Al and Fe were sufficient to reduce Ptot concentration to the level of the determination limit (5 mg/L) of the ICP-OES method used. From the results presented in Fig. 5 it can be seen that also in the 4500-ml experiments with PBDW1, there was no significant difference between the Al and Fe anodes in terms of removal efficiency. CODMn removal with the Al and Fe anodes was in the range of 75e79% and 85e90% in 60e90 min, respectively. The corresponding values for DOC removal were 62e65% and 71e75% in 60e90 min. About 33e40% of Ntot could be removed with the same treatment time with both Al and Fe e this is in good agreement with the 1800-ml experiments. There were problems related to the Ptot analyses in both

Fig. 3 e Effect of current density and treatment time on the removal of humic acids from synthetic wastewater (c0 ¼ 100 mg/ L, anode/cathode ¼ Al/Fe, initial pH ¼ 4, T ¼ 20  C, 0.5 g/L NaCl).

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Fig. 4 e Efficiencies of Ptot and Ntot removal from PBDW1 using Al/Fe and Fe/Al electrode combinations (i ¼ 70 A/m2, 0.5 g/L NaCl, V ¼ 1800 ml).

Fig. 5 e Efficiencies of CODMn, DOC, Ptot, Ntot, and Fe removal from PBDW1 using A) Al/Fe and B) Fe/Al electrode combination (i ¼ 70 A/m2, 0.5 g/L NaCl, V ¼ 4500 ml). In the 4500-ml scale-up tests, when Al was used as the anode the Al content of the water was found to remain unchanged until 30 min, after which it rose steadily to 1.1 mg/L in 60 min, 2.0 in 90 min, and finally to 3.8 mg/L in 150 min. However, when Fe was used as the anode, the Fe content of the water decreased to 49 mg/L in 30 min and was below the determination limits (15 mg/L) in all the samples from thereon. Thus, the scaling up and performance of EC (using either metal as the anode material) was found promising and the residual amounts of metal were found to be low with the treatment times found adequate (60e90 min, 400e600 C/L).

3.2.2.

Fig. 6 e PBDW1 before and after EC treatment (Al/Fe electrodes).

experiments, and therefore the results for the Fe anode have been left out, although the results for Al also seem to contain some inconsistencies (see Fig. 5). However, at its best (in 30 min), about 65% of Ptot was removed by using Al-EC.

Large-scale electrocoagulation experiments

Real PBDWs from two northern Finnish peat bogs (PBDW2 and PBDW3) were treated using a self-constructed large scale EC system with an Al/Fe electrode combination. Natural initial pH values and current densities of about 60 A/m2 (20 A) and 75 A/m2 (25 A) were applied in these experiments for PBDW2 and PBDW3, respectively. The experimental runs were conducted using cold water (the initial temperature of both PBDWs was about 10e11  C) and they lasted 240 min (PBDW2) and 180 min (PBDW3). It should be noted that to simulate natural conditions the water samples in these experiments were not filtered and no

w a t e r r e s e a r c h 7 9 ( 2 0 1 5 ) 7 9 e8 7

acid was added to them before refrigeration. However, the samples were shaken and left to settle for a while before the analyses. Also, in both experiments an additional reference sample was taken from the clear and settled water in the EC vessel several days after the EC test runs. It was found that the values of the measured parameters of these reference samples were nearly identical to those of the samples taken immediately after the EC test run (these are presented in Table 2). The removal efficiencies achieved in these experiments for Ptot, Ntot, CODMn, TOC, Fe, and Al are presented in Table 2. In both large-scale EC runs the turbid and dark water turned colorless and clear as a result of EC treatment. The change in color (in the web version) was similar to that presented in Fig. 6. All of the TS and most of the color (visual observation) could be removed in 30 min of EC in both tests, with all further samples being colorless. This might be related to the very fast (less than 30 min) and complete iron removal, as presented in Table 2. However, compared with the results of the laboratory-scale beaker experiments (see 3.2.1), only part of the brown floc rose to the top of the EC vessel, while the rest of the floc settled to the bottom. Using horizontal electrode setup, lower electrode placement, and lower mixing speed might have had enhanced the flotation. Interestingly, as can be seen in Table 2, the residual Al concentration of PBDW2 and PBDW3 did not increase as a result of EC treatment. In fact, Al could even be removed from the PBDWs. In the large-scale EC runs with PBDW2 and PBDW3, 60e120-min treatment times (electrical charges of 75e150 C/L and 90e180 C/L, respectively) were chosen as optimal with complete Ptot, 18e41% Ntot, 52e67% CODMn, and 42e65% TOC removal. When these results are compared with those obtained with laboratory-scale EC treatment of PBDW1 (all the PBDWs in this study had somewhat similar initial pollutant concentrations), it can be said that scaling up the EC process from laboratory-scale to 1-m3-scale showed very promising results for future practical utilization. Compared with conventional coagulation, no secondary pollution was produced, there was no need for chemicals (excluding 0.5 g/L NaCl), and the process was also feasible for cold water. However, practical problems related to continuous treatment of PBDWs in natural conditions (very large water volumes with highly

Table 2 e Efficiencies of pollutant removal from real PBDWs as a function of time (anode/cathode ¼ Al/Fe, 0.5 g/L NaCl, d ¼ 7 mm) in large-scale EC experiments. t [min] 0 PBDW2a

Ptot Ntot TOC CODMn Fe Al

0 0 0 0 0 0

PBDW3b

Ptot Ntot TOC CODMn Fe Al

0 0 0 0 0 0

a b

15

30

60

73 0 15 12 63 186

92 39 7 41 98 61

94 35 42 52 99.4 40

120

95 >95 41 43 48 55 61 67 99.4 98 38 113

>95 >95 >95 18 13 18 43 57 65 58 67 77 99.5 99.6 99.2 65 63 21

Current density of 60 A/m2, V ¼ 950 L. Current density of 75 A/m2, V ¼ 975 L.

180

>95 18 69 79 99.6 2

85

varying flow, finding sources of electricity in rural areas, cold climate, etc.) still exist and need to be discussed further in detail.

3.3.

Process economy

The operating costs (OC) [V/m3] of the EC process can simplified to consist of the electrical energy consumed (EEC) [kWh/ m3] and the mass of the anodic metal dissolved, leaving e.g. EC sludge processing out of consideration (Kuokkanen et al., 2013). In this work, OC and EEC were evaluated with Eqs. (8) and (9), respectively: OC ¼ a*EEC þ b*EMC þ c*SEC EEC ¼

U*I*t 60*V

(8)

(9)

In Eq. (8) a, b, and c are the current market prices of electricity [V/kWh], electrode materials [V/kg], and supporting electrolyte [V/kg], respectively, EMC [kg/m3] is electrode material consumption, and SEC [kg/m3] is supporting electrolyte consumption. In this work, a, b, and c were estimated to be approximately 0.09 V/kWh (in Finland in February 2015, including electrical energy, distribution of electricity, and taxes), 1.60 V/kgAl, 0.42 V/kgFe (steel), and 0.06 V/kgNaCl, respectively. In Eq. (9) U is the applied voltage [V], t ¼ treatment time [min], and V is the volume of the treated water [dm3]. Based on the initial/final concentrations of the pollutant, EEC and OC per kg of CODMn can also be readily evaluated (derived from Eqs. (8) and (9)). The economic values of the EC process for each water type treated (in optimum conditions) in this work are presented in Table 3. Based on the data presented in Table 3, it can be stated that EC is a cost-efficient alternative in treating real PBDW containing HS. Both OC and EEC values for the SWW were found to be of a similar order as those obtained for real PBDWs (0.06e0.12 V/m3 and 0.23e0.69 kWh/m3 on a large scale, respectively) and also in line with results presented in Kuokkanen et al. (2013) on the general economic values of EC. There seemed to be no great disparity between the economic values in using Al-EC or Fe-EC for this application, thus both were found to be viable options. Furthermore, the low EEC values hint at the possibility of utilizing, e.g. solar energy as the power source in rural areas.

3.4.

Other analyses

240 >95 49 35 66 99.5 1

The superfaradaic effect on Al dissolution (Fe not measured due to practical problems) was studied in each experiment conducted with the SWW. Regardless of the operational parameters, nearly all of these results showed an 18e32% increase in Al dissolution compared with the corresponding theoretic value given by Eq. (7). This is a very repeatable and narrow range and particularly well in line with the results obtained by our research group earlier for phosphate removal from various types of wastewater with EC, and in good agreement with most other values found in literature, as well (Kuokkanen et al., 2015; Terrazas et al., 2010; Kongjao et al., € 2008; S‚engil and Ozacar, 2009; Kobya et al., 2011; Yetilmezsoy et al., 2009; Mouedhen et al., 2008).

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Table 3 e OC and EEC values (in optimum conditions) for the water types treated in this work. Water type and volume SWW, 1.8 L PBDW1, 1.8 L PBDW1, 1.8 L PBDW1, 4.5 L PBDW1, 4.5 L PBDW2, 950 L PBDW3, 975 L

Anode/cathode

EEC [kWh/m3]

OC [V/m3]

EECCODMn [kWh/kg]

OCCODMn [V/kg]

Al/Fe Al/Fe Fe/Al Al/Fe Fe/Al Al/Fe Al/Fe

0.94 0.97e1.46 1.00e1.51 1.21e1.82 1.23e1.85 0.23e0.44 0.35e0.69

0.15 0.17e0.24 0.16e0.23 0.20e0.29 0.19e0.27 0.06e0.09 0.08e0.12

26 n.d. n.d. 58e83 52e73 26e43 25e44

4100 n.d. n.d. 9500e13,000 8000e10,800 7000e9000 5500e7700

n.d. ¼ Not determined.

As could be expected, in all the experiments conducted in this work, the pH of the solution changed from its initial value during the EC run. This change occurred at a much higher rate at the beginning of each test run than towards the end. This phenomenon is related to OH ion formation, as depicted in Eq. (5). In all of the SWW experiments, EC was found to bring the solution's pH towards neutral. When the initial pH value was 9, pH decreased to a final value of about 8.7, whereas it rose from 4 to about 5.2e7.4, depending on the current density used. These results were consistently reproducible. The initial pH values of the PBDWs treated in this work were in the range of 6.1e7.0. In the 4500-ml experiment with PBDW1, it was noted that using Fe as anode resulted in a greater pH increase; pH rose from 6.6 to 10.2 in 150 min, whereas it rose from 6.5 to 8.3 when using Al as the anode. The increase in pH observed was very similar when the PBDW volume treated was 1800 ml (60-min test run). In the largescale Al-EC experiments it was found that the pH values of PBDW2 and PBDW3 were 7.2e7.4 and 6.7e6.8 after 60e120min EC, respectively, after which they continued to rise to final values of 7.7 and 7.1 after 240 min and 180 min, respectively. Therefore, EC could also be used to simultaneously neutralize PBDWs, which often tend to be slightly acidic and thus have lower initial pH values than the samples used here. In every experiment in this work, the solution temperature was found to increase from its initial value. This increase was found to be repeatable and results from the ohmic drop and electrode overpotentials generating heat in the EC cell and liquid (Calvo et al., 2003). However, this increase was of a very small order of magnitude e in the large-scale EC runs the solution temperature rose 0.9  C in both experiments and 1.2  C or less in the SWW experiments. Therefore, it was concluded to have no effect on the EC process and is consistent with earlier results obtained by our research group (Karhu et al., 2012; Kuokkanen et al., 2015). In this work, current was kept constant during each experiment. Therefore, voltage had to be gradually decreased (3e13%) in every experiment with real PBDWs, with a clear tendency of Fe anodes to cause a larger decrease. Changes in solution conductivity and resistance due to changes in temperature and pollutant concentrations may account for such behavior in the EC system.

4.

Conclusions

 Laboratory-scale EC experiments were performed on a 100mg/L synthetic humic acid solution with promising results













 

over the wide pH range studied. The solution temperature was found to be of minor significance to the efficiency of HA removal. Optimum process conditions (90% CODMn and 80% DOC removal efficiencies) for SWW were found to be: anode/ cathode ¼ Al/Fe, initial pH ¼ 4, i ¼ 100 A/m2, t ¼ 10 min. Real PBDWs from three northern Finnish peat bogs containing humic substances were also treated using both Al/ Fe and Fe/Al electrode combinations (both performed similarly). High pollutant removal efficiencies were achieved: Ptot, TS, and color could be removed completely, while Ntot, CODMn, and DOC/TOC removal efficiencies were in the range of 33e41%, 75e90%, and 62e75%, respectively. Large-scale experiments (1 m3) using cold (T ¼ 10e11  C) PBDW2 (60 A/m2) and PBDW3 (75 A/m2) were conducted successfully, with optimal treatment times of 60e120 min. Residual values of Al and Fe were lower than their initial values (Fe was removed completely) in the EC-treated PBDWs. EEC and OC values in optimum conditions were found to be low and somewhat similar for all the waters studied: 0.94 kWh/m3 and 0.15 V/m3 for SWW and 0.35e0.70 kWh/ m3 and 0.06e0.12 V/m3 for PBDWs (large-scale). Thus, e.g. solar cells could be considered as a power source for this EC application. Regardless of the operational parameters, Al dissolution during EC showed an 18e32% superfaradaic increase compared with the corresponding theoretic value in SWW experiments. EC was also found to produce water with near-neutral pH from the PBDWs. EC was shown to be a feasible technology for treating real PBDW containing humic substances. However, practical problems related to continuous treatment of PBDW in natural conditions remain and need to be studied further.

Acknowledgments The authors would like to thank the RAE (Symbiosis pellet expands ecology, A32474) ERDF project, the BIOTUHKA (Utilization of bio-ash based materials as forest fertilizers, A70101) ERDF project/Luke (Natural Resources Institute Finland, Rovaniemi), Maa-ja vesitekniikan tuki ry., Rakeistus Oy and Turveruukki Oy, for financial and other support. We would € risto € Oy (Oulu) for the analytics. also like to thank Ahma ympa Expression of gratitude is presented to Authorized Translator Keith Kosola for revising the language.

w a t e r r e s e a r c h 7 9 ( 2 0 1 5 ) 7 9 e8 7

references

 zquez, L.J., Ferna  ndez, F.J., Martı´nez, A., 2014. Optimal Alvarez-Va control of eutrophication processes in a moving domain. J. Frankl. Inst. 351 (8), 4142e4182. http://dx.doi.org/10.1016/ j.jfranklin.2014.04.012. Bazrafshan, E., Biglari, H., Mahvi, A.H., 2012. Humic acid removal from aqueous environments by electrocoagulation process using iron electrodes. J. Chem. 9 (4), 2453e2461. http:// dx.doi.org/10.1155/2012/876739. Bektas‚, N., Akbulut, H., Inan, H., Dimoglo, A., 2004. Removal of phosphate from aqueous solutions by electro-coagulation. J. Hazard. Mater. 106 (2e3), 101e105. http://dx.doi.org/10.1016/ j.jhazmat.2003.10.002. Calvo, L.S., Leclerc, J., Tanguy, G., Cames, M.C., Paternotte, G., Valentin, G., Rostan, A., Lapicque, F., 2003. An electrocoagulation unit for the purification of soluble oil wastes of high COD. Environ. Prog. 22 (1), 57e65. http:// dx.doi.org/10.1002/ep.670220117. Chen, G., 2004. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 38 (1), 11e41. http://dx.doi.org/ 10.1016/j.seppur.2003.10.006. Geological Survey of Finland e Energy supply and environment. http://en.gtk.fi/energy/peat.html (accessed 26.02.15.). Ghernaout, D., Ghernaout, B., Saiba, A., Boucherit, A., Kellil, A., 2009. Removal of humic acids by continuous electromagnetic treatment followed by electrocoagulation in batch using aluminium electrodes. Desalination 239 (1e3), 295e308. http:// dx.doi.org/10.1016/j.desal.2008.04.001. Hesse, S., Kleiser, G., Frimmel, F.H., 1999. Characterization of refractory organic substances (ROS) in water treatment. Water Sci. Technol. 40 (9), 1e7. http://dx.doi.org/10.1016/S02731223(99)00633-2. Jones, M.N., Bryan, N.D., 1998. Colloidal properties of humic substances. Adv. Colloid Interface Sci. 78 (1), 1e48. http:// dx.doi.org/10.1016/S0001-8686(98)00058-X. € mo € , J., 2012. Bench Karhu, M., Kuokkanen, V., Kuokkanen, T., Ra scale electrocoagulation studies of bio oil-in-water and synthetic oil-in-water emulsions. Sep. Purif. Technol. 96, 296e305. http://dx.doi.org/10.1016/j.seppur.2012.06.003. Kobya, M., Ulu, F., Gebologlu, U., Demirbas, E., Oncel, M.S., 2011. Treatment of potable water containing low concentration of arsenic with electrocoagulation: different connection modes and FeeAl electrodes. Sep. Purif. Technol. 77 (3), 283e293. http://dx.doi.org/10.1016/j.seppur.2010.12.018. Kongjao, S., Damronglerd, S., Hunsom, M., 2008. Simultaneous removal of organic and inorganic pollutants in tannery wastewater using electrocoagulation technique. Korean J. Chem. Eng. 25 (4), 703e709. http://dx.doi.org/10.1007/s11814008-0115-1. € mo € , J., Lassi, U., Roininen, J., Kuokkanen, V., Kuokkanen, T., Ra 2015. Removal of phosphate from wastewaters for further utilization using electrocoagulation with hybrid electrodes e Techno-economic studies. J. Water Process Eng. 8. http:// dx.doi.org/10.1016/j.jwpe.2014.11.008 (in press). € mo € , J., Lassi, U., 2013. Recent Kuokkanen, V., Kuokkanen, T., Ra applications of electrocoagulation in treatment of water and wastewaterdA review. Green Sustain. Chem. 2, 89e121. http://dx.doi.org/10.4236/gsc.2013.32013. Mollah, M.Y.A., Morkovsky, P., Gomes, J.A.G., Kesmez, M., Parga, J., Cocke, D.L., 2004. Fundamentals, present and future perspectives of electrocoagulation. J. Hazard. Mater. 114 (1e3), 199e210. http://dx.doi.org/10.1016/j.jhazmat.2004.08.009. Motheo, A.J., Pinhedo, L., 2000. Electrochemical degradation of humic acid. Sci. Total Environ. 256 (1), 67e76. http://dx.doi.org/ 10.1016/S0048-9697(00)00469-1.

87

Mouedhen, G., Feki, M., Wery, M.D.P., Ayedi, H.F., 2008. Behavior of aluminum electrodes in electrocoagulation process. J. Hazard. Mater. 150 (1), 124e135. http://dx.doi.org/10.1016/ j.jhazmat.2007.04.090. Naddeo, V., Belgiorno, V., Napoli, R.M.A., 2007. Behaviour of natural organic mater during ultrasonic irradiation. Desalination 210 (1e3), 175e182. http://dx.doi.org/10.1016/ j.desal.2006.05.042. O'Melia, C.R., Becker, W.C., Au, K.-K., 1999. Removal of humic substances by coagulation. Water Sci. Technol. 40 (9), 47e54. http://dx.doi.org/10.1016/S0273-1223(99)00639-3. Ødegaard, H., Eikebrokk, B., Storhaug, R., 1999. Processes for the removal of humic substances from water d an overview based on Norwegian experiences. Water Sci. Technol. 40 (9), 37e46. http://dx.doi.org/10.1016/S0273-1223(99)00638-1. Prado, A.G.S., Airoldi, C., 2003. Humic acid-divalent cation interactions. Thermochim. Acta 405 (2), 287e292. http:// dx.doi.org/10.1016/S0040-6031(03)00196-5. Seida, Y., Nakano, Y., 2000. Removal of humic substances by layered double hydroxide containing iron. Water Res. 34 (5), 1487e1494. http://dx.doi.org/10.1016/S0043-1354(99)00295-X. € _ S‚engil, I.A., Ozacar, M., 2009. The decolorization of C.I. Reactive Black 5 in aqueous solution by electrocoagulation using sacrificial iron electrodes. J. Hazard. Mater. 161 (2e3), 1369e1376. http://dx.doi.org/10.1016/j.jhazmat.2008.04.100.  ska-Sobecka, B., Tomaszewska, M., Morawski, A.W., 2006. Seredyn Removal of humic acids by the ozonationebiofiltration process. Desalination 198 (1e3), 265e273. http://dx.doi.org/ 10.1016/j.desal.2006.01.027.  zquez, A., Briones, R., La  zaro, I., Rodrı´guez, I., 2010. Terrazas, E., Va EC treatment for reuse of tissue paper wastewater: aspects that affect energy consumption. J. Hazard. Mater. 181 (1e3), 809e816. http://dx.doi.org/10.1016/j.jhazmat.2010.05.086. Verrall, K.E., Warwick, P., Fairhurst, A.J., 1999. Application of the SchulzeeHardy rule to haematite and haematite/humate colloid stability. Colloids Surfaces A: Physicochem. Eng. Aspects 150 (1e3), 261e273. http://dx.doi.org/10.1016/S09277757(98)00858-9. World Energy Council Energy Resources e Peat. http://www. worldenergy.org/data/resources/resource/peat/ (accessed 28.08.14.) Yetilmezsoy, K., Ilhan, F., Sapci-Zengin, Z., Sakar, S., Gonullu, M.T., 2009. Decolorization and COD reduction of UASB pretreated poultry manure wastewater by electrocoagulation process: a post-treatment study. J. Hazard. Mater. 162 (1), 120e132. http://dx.doi.org/10.1016/ j.jhazmat.2008.05.015. Yıldız, Y.S‚., Koparal, A.S., Keskinler, B., 2008. Effect of initial pH and supporting electrolyte on the treatment of water containing high concentration of humic substances by electrocoagulation. Chem. Eng. J. 138 (1e3), 63e72. http:// dx.doi.org/10.1016/j.cej.2007.05.029. _ Yıldız, Y.S‚., Koparal, A.S., Irdemez, S‚., Keskinler, B., 2007. Electrocoagulation of synthetically prepared waters containing high concentration of NOM using iron cast electrodes. J. Hazard. Mater. 139 (2), 373e380. http://dx.doi.org/ 10.1016/j.jhazmat.2006.06.044. Zhao, D., Sengupta, A.K., 1998. Ultimate removal of phosphate from wastewater using a new class of polymeric ion exchangers. Water Res. 32 (5), 1613e1625. http://dx.doi.org/ 10.1016/S0043-1354(97)00371-0. Zodi, S., Potier, O., Lapicque, F., Leclerc, J., 2009. Treatment of the textile wastewaters by electrocoagulation: effect of operating parameters on the sludge settling characteristics. Sep. Purif. Technol. 69 (1), 29e36. http://dx.doi.org/10.1016/ j.seppur.2009.06.028.

Electrocoagulation treatment of peat bog drainage water containing humic substances.

Electrocoagulation (EC) treatment of 100 mg/L synthetic wastewater (SWW) containing humic acids was optimized (achieving 90% CODMn and 80% DOC removal...
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