Kinetics of electro-oxidation of ammonia-N, nitrites and COD from a recirculating aquaculture saline water system using BDD anodes

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 2 5 e1 3 4

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Kinetics of electro-oxidation of ammonia-N, nitrites and COD from a recirculating aquaculture saline water system using BDD anodes V. Dı´az a, R. Iba´n˜ez a, P. Go´mez b, A.M. Urtiaga a, I. Ortiz a,* a b

Dpto. Ingenierı´a Quı´mica y QI. ETSIIyT, Universidad de Cantabria, Av. de los Castros s/n, 39005 Santander, Spain APRIA Systems S.L., Polı´gono Trascueto s/n, 39600 Camargo, Spain

article info

abstract

Article history:

The viability of the electro-oxidation technology provided with boron doped diamond

Received 15 June 2010

(BDD) electrodes for the treatment and reuse of the seawater used in a Recirculating

Received in revised form

Aquaculture System (RAS) was evaluated in this work.

31 July 2010

The influence of the applied current density (5e50 A m
2) in the removal of Total Ammonia

Accepted 10 August 2010

Nitrogen (TAN), nitrite and chemical oxygen demand (COD) was analyzed observing that

Available online 25 August 2010

complete TAN removal together with important reductions of the other considered contaminants could be achieved, thus meeting the requirements for reuse of seawater in

Keywords:

RAS systems.

Aquaculture saline water reuse

TAN removal, mainly due to an indirect oxidation mechanism was described by a second

BDD anode

order kinetics while COD and nitrite removal followed zero-th order kinetics. The values of

Electro-oxidation

the kinetic constants for the anodic oxidation of each compound were obtained as

Nitrogen compounds

a function of the applied current density (kTAN ¼ 7.86  10
5$exp(6.30  10
2 J);

COD

kNO2 ¼ 3.43  10
2 J; kCOD ¼ 1.35  10
2 J). The formation of free chlorine and oxidation byproducts, i.e., trihalomethanes (THMs) was followed along the electro-oxidation process. Although a maximum concentration of 1.7 mg l
1 of total trihalomethanes was detected an integrated process combining electrochemical oxidation in order to eliminate TAN, nitrite and COD and adsorption onto activated carbon to remove the residual chlorine and THMs is proposed, as an efficient alternative to treat and reuse the seawater in fish culture systems. Finally, the energy consumption of the treatment has been evaluated. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Aquaculture is a value-added economic activity worldwide. This sector has become a necessity to meet the demand for fish and seafood. Aquaculture operations require large quantities of feed water that is contaminated with three primary contaminants that are often regulated: organic matter, nutrients, such as phosphorous and nitrogen, in the forms of Total Ammonia

Nitrogen (TAN), nitrite and nitrate and solids. Bacteria and pathogens are also waste products that must be controlled (Davidson et al., 2008). Moreover, legal regulations concerning the discharge of effluents from fish farms have become increasingly strict, therefore, one of the challenges facing the aquaculture industry is the treatment of the culture water. Traditional aquacultural activities are performed in large ponds with low fish density. But this way of culture requires

* Corresponding author. Tel.: þ34 942 20 15 85; fax: þ34 942 20 15 91. E-mail address: [email protected] (I. Ortiz). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.08.020

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large amounts of land and water resources. Due to increasing cost and limited suitable water supplies, the traditional aquacultural activities are becoming relatively uneconomic. An emerging recent approach is to employ high density aquaculture which requires significantly less amount of land and water resources than conventional methods. High density aquaculture operations that mainly consist of Recirculating Aquaculture Systems (RAS) based fish tanks where the fish tank effluent is biologically treated and the water is recycled back to the rearing tanks, are becoming key solution for the control of contaminants present in the closed culture and keep the quality of water needed for large-scale fish production. In all aquaculture systems a rapid TAN accumulation caused primarily by fish excretion and decomposition of uneaten feed, is found. Being this problem especially important in the RAS as an increase in fish density in a limited aquatic space leads to a more rapid degradation of the water quality. Besides, in the recirculating systems, nitrite is also found as an intermediate of the ammonia nitrification. Moreover, biofiltration is significantly less effective in saline conditions than in freshwater and it is subject to biological upsets and wide fluctuations in performance as a consequence of the dynamic properties of this system. Since unionized ammonia and nitrite are toxic to fish even at low concentrations, the introduction of new technologies is necessary to avoid the accumulation of these toxic compounds and achieve an effective removal, thus allowing the reuse of the treated water in the fish culture systems and reducing environmental problems and operating costs. The treatment of fish culture water for removal of ammonia and nitrite has been investigated by numerous researchers based on the following methods: selective ion exchange (Miladinovic and Weatherley, 2008), flocculation (Ebeling et al., 2005), sand beds (Palacios and Timmons, 2001) and membrane bioreactors (Sharrer et al., 2007; Pulefou et al., 2008), among others. In recent years, oxidation of nitrogen compounds by chemical methods, e.g. ozone (Krumins et al., 2001; Tango and Gagnon, 2003), UV (Bronk et al., 2000) and breakpoint chlorination (Potts and Boyd, 1998) have received significant attention. Although these methods have been proven to be capable of removing nitrite and ammonia, they also have several limitations, such as, high cost of operation and instrumentation and the frequent use of reagents (Sun and Chou, 1999). Recently, electrochemical oxidation has been considered as a promising alternative for the treatment of polluted waters containing nitrogen compounds due to its advantages such as minimal generation of secondary wastes, easy operation and remote control. Besides, this treatment is not subject to failure due to the presence of high salinity and variation in wastewater flow (Vijayaraghavan et al., 2008). In recent literature, relevant works can be found on the application of this technology to the mineralization of organic compounds and ammonia contained in wastewaters (Szpyrkowicz et al., 2001; Szpyrkowicz and Radaelli, 2006; Urtiaga et al., 2009; Li and Liu, 2009). In the context of aquaculture saline water, electrochemical oxidation has not received much attention yet. However, the electrochemical treatment of seawater presents several advantages, as high salinity ensures an excellent electric conductivity that could reduce the energy consumption and the

high chloride concentration improves the indirect oxidation through the electro-generation of strong oxidants like hypochlorous acid. Additionally, the electrochemical treatment avoids the handling of toxic chlorine gas used in the breakpoint chlorination process, since the oxidants are electrolytically generated in situ, and this generation can be controlled by the electrolysis operation conditions (Alfafara et al., 2004). Several authors have studied the influence of the operating conditions of the electrochemical treatment like pH, conductivity, initial concentration or applied current density in the oxidation of nitrite, ammonia and/or organic matter using synthetic freshwater (Lin and Wu, 1996; Abuzaid et al., 1999), prepared seawater (Lin and Wu, 1997; Sun and Chou, 1999; Lee et al., 2002; Wijesekara et al., 2005) or raw wastewater collected from fish farms (Katayose et al., 2007; Vijayaraghavan et al., 2008). This work is focused on the viability and kinetics of the application of electrochemical oxidation to enhance the seawater quality of a Recirculating Aquaculture System by removing TAN, nitrite and COD offering an alternative to the biological filter currently operated. Formation of by-products, total trihalomethanes and free chlorine and the energy consumption have also been evaluated. This study treats seawater collected from the inlet of the biological filter situated in a high density hatchery. A laboratory set-up provided with Boron Doped Diamond (BDD) electrodes has been used. Although, there are no previous references that concern with electrochemical oxidation of seawater from a hatchery using BDD electrodes, this material presents many advantages compared to other anodic materials reported in literature, such as, graphite, titanium alloys or platinum (Rychen et al., 2003; Chen et al., 2003; Cabeza et al., 2007; Chatzisymeon et al., 2009; Polcaro et al., 2009; Pe´rez et al., 2010).

2.

Materials and methods

2.1.

Materials

Seawater from a sea bream hatchery located in Cantabria (Northern coast of Spain) was employed in this work. A previous characterization of the Recirculating Aquaculture System (RAS) installed in this fish farming, allowed to determine that the TAN concentration ranged daily in the interval of 0.05e8.00 mg l
1. This wide range of concentration is due to the fish metabolism and the hatchery production requirements. Laboratory scale experiments were performed using the hatchery water doped with ammonium chloride to give an initial TAN concentration of 8 mg l
1. The initial physicochemical characteristics (measured according to analytical methods described in the following section) of the seawater used in all experiments are shown in Table 1. Similar characterization was found in the literature about water collected from a high density fish hatchery (Taparhudee et al., 2008). The high conductivity allows the direct application of electrochemical techniques.

2.2.

Laboratory-scale experimental setup

The experiments were performed in a two-compartments electrochemical cell (DiaCell 201 PP) supplied by Adamant

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 2 5 e1 3 4

Table 1 e Initial physicochemical characterization of seawater used in this study. Parameter pH Redox Potential (mV) Conductivity (mS cm
1) Salinity (&) Temperature ( C) COD (mg O2 l
1) TAN (mg l
1) Nitrite (mg l
1) Nitrate (mg l
1) Chloride (mg l
1) Sulphate (mg l
1) Total Trihalomethanes (mg l
1) Free chlorine (mg l
1)

Average value 6.9
31 51.1 32.2 24.7 54.8 8.00 80.00 403.20 26,167 35,000 2.25 0.00

Technologies. Monopolar circular Boron Doped Diamond (BDD) on silicon anode and cathode, with a surface area of 70 cm2 each, an interelectrode gap of 1 mm and a bipolar electrode coated with diamond on both sides and placed between monopolar electrodes were inserted in the cell. Fig. 1 shows the basic layout of the laboratory scale plant. Galvanostatic conditions were applied by using an Agilent power supply 6554A (with a maximum output of 9 A and 60 V). The range of the applied current density, J, was between 5 and 50 A m
2. A volume of 2 L of seawater was treated in each experiment. The feed tank was refrigerated in order to maintain the working temperature at 25  2  C. A magnetic pump was used to recirculate the feed from the tank through the cell, at a flow rate of 6 l min
1 per compartment. This value of flow rate was selected according to the technical specifications of the electrochemical cell, which depend on the number of compartments and the distance between anode and cathode. The experiments were finished when TAN concentration was lower than 0.06 mg l
1 corresponding the detection limit of the analytical method used in this work.

2.3.

127

The pH was measured with a Crison pH 25 pH meter and the conductivity and the salinity were measured with a Crison CM 35 conductivity meter. TCOD was determined by heat of dilution COD procedure (Ruttanagosrigit and Boyd, 1989) employing mercuric sulphate to remove chloride interference. The concentration of TAN, nitrite, nitrate and chloride in solution was measured spectrophotometrically by using a Spectroquant Pharo 100, (Merck Company) according to Standard Methods (APHA, 1998): 4500-NH3-D, 4500-NO2-B, 4500-NO3-B and 4500-Cl-E, respectively. Sulphate was measured using ion chromatography (Dionex 120 IC, with an IonPac AS9-HC Column). Additionally, characterization of electrochemical oxidation by-products, such as, chlorine and trihalomethanes (THMs), was performed. Total and free chlorine were analyzed using a pocket chlorimeter (HI 95734, Hanna Instruments Company) according to DPD (N, N-diethyl-p-phenylenediamine) method. The THMs, chloroform (CHCl3), bromodichloromethane (CHBrCl2), bromoform (CHBr3) and dibromochloromethane (CHBr2Cl) were determined employing Direct Aqueous Injection Gas Chromatography with electron capture detection (HP 6890 Series GC Systems, with Headspace injector 7694 Agilent), following the Standard Methods 6232C (APHA, 1998), LiquideLiquid Extraction gas chromatographic method with modifications of the GC column and temperature heating rate. An HP-1 methyl siloxane column (30 mm  0.53 mm i.d., with 2.65 mm thickness) was selected in order to perform the analysis, and temperature was as follows: 50  C heating at 10  C min
1 up to 150  C and isothermal conditions for 5 min. All analytical determinations were performed immediately after sampling and were done by replicate, except THMs measurement. For this determination, samples were stocked at 4  C within 48 h after sampling.

3.

Results and discussion

3.1.

Kinetics of TAN oxidation

Analytical methods

Seawater samples were withdrawn at regular time intervals from the feed tank using a syringe and they were characterized by means of the following procedures.

Fig. 2 shows the influence of the applied current density on TAN removal in the J range 5e50 A m
2. For better comparison of the results, dimensionless data have been plotted, since the initial TAN concentration in the feed water slightly varied

Fig. 1 e Experimental Setup: 1, refrigerated feed tank; 2, pump; 3, two-compartments electrochemical cell; 4, power supply.

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1.00 0.90

TAN/TAN0

0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

0

10

20

30

40

50

60

70

80

90 100 110 120 130

Time (minutes) Fig. 2 e Influence of the applied current density (A J [ 5 A mL2; - J [ 10 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2) on the evolution of TAN/ TAN0; [TAN]0 z 8.0 mg/l.

around the value of 8 mg l
1. It can be observed in the Fig. 2 that increasing values of J enhanced the removal of TAN, as also shown in literature (Lin and Wu, 1997; Lee et al., 2002; Cabeza et al., 2007; Anglada et al., 2009; Li and Liu, 2009). Fig. 2 shows that a value of the current density of J ¼ 5 A m
2 fails to significantly remove TAN. This fact can be explained according to the works of Cabeza et al. (2007). In that work, the authors demonstrated that if ammonia and COD have to be eliminated simultaneously, current densities higher than the Jlim,COD have to be applied, being Jlim,COD, defined as the current density value that implies null accumulation of oxidizable substances at the surface of the anode. In this work, the value of Jlim,COD is 13.09 A m
2. On the other hand, in the range 10e50 A m
2 complete TAN removal (remaining concentration below detection limit of 0.06 mg l
1 of TAN) was achieved. At J ¼ 10 A m
2, a value of current density close to the Jlim,COD, complete TAN removal was achieved after 115 min. As reported by Deng and Englehardt (2007), the rule of competition between the removal of COD and the removal of ammonia seems to be that the removal of ammonia is favored when indirect oxidation is dominant, whereas the removal rate of COD takes priority under direct anodic oxidation. Ammonia oxidation takes place due to an indirect oxidation through the electro-generation of hypochlorous acid, according to a mechanism analogous to the breakpoint chlorination reactions (Szpyrkowicz et al., 2001; Lee et al., 2002; Li and Liu, 2009; Anglada et al., 2010). Besides, some authors (Alfafara et al., 2004; Wijesekara et al., 2005) consider other oxidants, such as hypobromous acid as well as hypochlorous acid, although HBrO is not as efficient as HClO. Taking into account that in the normal pH range of pond water (6e7.5), more than 98% of TAN is in the form of NHþ 4, being HOCl the major component of free chlorine in this pH interval, the overall reaction occurring between HClO and NHþ 4 is shown by equation (1).


þ 2NHþ 4 þ 3HClO/N2 þ 5H þ 3Cl þ 3H2 O

(1)

A second order kinetics has been proposed by Xu et al. (2008) and Anglada et al. (2009) to describe the ammonia oxidation

by free chlorine. Szpyrkowizc and Radaelli also proposed a second order to describe the kinetics of decolourisation via indirect electro-oxidation. A zero-th order kinetics (linear profiles) has been proposed when working with high values of the applied current density, whereas curved evolutions with an initial delay are observed when working at lower values of the applied current density (Cabeza et al., 2007). In the present work, the experimental data regarding TAN disappearance versus time shown in Fig. 2 were fitted to equation (2): d½TAN ¼
k$½TAN$½HOCl dt

(2)

where, k is the rate constant (l mg
1 min
1) and [HOCl] is the concentration of hypochlorous acid (mg l
1). Assuming that the rates of chlorine disappearance due to i) cathodic reduc
tion of active chlorine, ii) anodic oxidation to ClO3 and, iii) homogeneous reaction with TAN were much lower than the chlorine production rate in the anode, given by the high chloride concentration present in seawater, the variation of chlorine concentration with time (until saturation) can be described by equation (3): d½HOCl 4 A J ¼ dt nFV

(3)

where F is the Faraday constant (96485 C mol
1) and 4 is the current efficiency for chlorine evolution, which depends on the applied current density, mass transport rate coefficient and chloride concentration (Anglada et al., 2009). In this work, it is assumed that the chloride concentration remains constant throughout the oxidation process and that free chlorine evolution is the main anodic reaction. Consequently, the substitution of the integrated form of equation (3) into equation (2), and subsequent integration gives equation (4): ln

½TAN k4AJt2 ¼
k0 t2 ¼
4FV ½TAN0

(4)

Values of the apparent rate constant, k0 (min
2) presented in Table 2 were obtained from the slopes of the logarithms of [TAN]t/[TAN]0 vs. the square of the electrolysis time, for the different applied current densities. The values of the correlation coefficients were high enough in all cases (Table 2). The apparent rate constant of TAN (k0 ) increases exponentially with the applied current density according to equation (5), obtained from the fitting of the data given in Table 2.

2  R ¼ 0:997 k0 ¼ 7:86  10
5 $exp 6:30  10
2  J

(5)

Combining equations (4) and (5), equation (6) was obtained:

Table 2 e Values of k0 for TAN oxidation at the different applied current densities. J (A m
2) 10 20 30 50

k0 (min
2)

R2

1.55 e
4 2.78 e
4 4.71 e
4 1.92 e
3

0.991 0.953 0.965 0.979

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Equation (6) is the second order kinetic model developed to describe the kinetics of TAN removal along the electrooxidation treatment of seawater from a hatchery as a function of the operation variable, J, in the range 10e50 A m
2 and considering indirect oxidation mediated by electrogenerated active chlorine. A parity graph of the dimensionless TAN concentration for all the performed experiments is shown in Fig. 3. Good agreement between experimental and simulated data is observed, because 80% of the results of Csim fall within the Cexp  15% range.

3.2.

Kinetics of nitrite oxidation

Nitrite concentration along electrochemical experiments was measured according to the procedure described in the previous section. The experiments were performed with an average initial nitrite concentration of 80.0 mg l
1, as received from the hatchery. Fig. 4 shows the normalized nitrite concentration profiles during the electrochemical treatment of seawater at different applied current densities, in the range 5e50 A m
2. More than 90% of the initial nitrite concentration was removed in the whole range of applied current densities, by keeping the adequate operating time. It is observed that the nitrite removal rate is highly affected by the applied current density. Concerning nitrite removal, a mechanism based on the indirect oxidation reaction between nitrite and the oxidant, HClO, generated at the anode, has been reported in literature (Sun and Chou, 1999) as shown by equation (7):



þ NO
2 ðaqÞ þ HClOðaqÞ /NO3 ðaqÞ þ ClðaqÞ þ HðaqÞ

(7)

Few works have been found in literature describing the influence of the applied current density on the kinetics of nitrite electro-oxidation (Lin and Wu, 1996, 1997). Sun and

1.0

0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

0

40

80

120

160

200

240

0.8

280

Time (minutes) Fig. 4 e Influence of the applied current density (A J [ 5 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2) on the evolution of L L ½NOL 2 =½NO2 0 ; ½NO2 z80:0 mg=l.

Chou (1999) used the ButlereVolmer equation to define the effect of current density (J ) on nitrite removal kinetics. In that work, the nitrite oxidation rate was expressed according to equation (8):   d NO
2 ¼
dt

  ! kK NO
2   1 þ K NO
2

(8)

Where k and K are reaction rate constants expressed in mol L
1 min
1 and L mol
1, respectively. In the present study equation (8) has been used to describe the behaviour of nitrite in our system. For an initial nitrite concentration of 80 mg l
1 and employing a value of K of 2.48  104 l mol
1 obtained by Sun and Chou (1999) for a similar system, the resulting K½NO
2  value is 43.13, being 1 < < K½NO
2  and therefore, equation (8) can be simplified to equation (9). As shown in equation (9), the nitrite oxidation rate is described by a zero-th order expression, in the range of variables under study. Lin and Wu (1997) also described the nitrite oxidation by means of a zero-th order kinetics.   d NO
2 ¼k
dt

0.9

Simulated Value

1.00

(6)

NO2-/NO2-0


 ½TAN ln ¼
7:86  10
5 $exp 6:30  10
2  J $t2 ½TAN0

(9)

The integrated form of equation (9) gives equation (10):

0.7

  NO
k 2    $t ¼ 1
k0 $t ¼ 1
 NO
NO
2 0 2 0

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Experimental Value Fig. 3 e Parity graph: dimensionless TAN concentration at the different applied current densities (- J [ 10 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2).

(10)

Values of the apparent rate constant, k0 (min
1) reported in Table 3 were obtained from the slopes of the dimensionless nitrite concentration vs. the electrolysis time (Fig. 4) for the different applied current densities. In all cases correlation coefficients higher than 0.980 were obtained (Table 3). Table 3 also includes the values of k (mg l
1 min) calculated from the values of k0 and the initial nitrite concentration (80 mg l
1). It is observed, that an increase of the current density leads to a significant increase in the nitrite removal specific rate. The kinetic constant of nitrite removal (k) increases linearly with the applied current density according to equation (11). Similar relationship between k and J was reported in the works of Lin and Wu (1997).

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1.00

Table 3 e Values of k0 and k for nitrite oxidation at the different applied current density. 2

J (A m
2)

k (min
1)

R

5 20 30 50

3.44 e
3 8.99 e
3 1.32 e
2 2.11 e
2

0.995 0.998 0.997 0.987

½NO
2 0
1

(mg l ) 80.0

0.90 0.80

k

(mg l


1

min)

0.272 0.720 1.056 1.688

COD/COD0

0

0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

k ¼ 3:43  10
2  J




 R2 ¼ 0:998

(11)

A parity graph of the dimensionless nitrite concentration for all the performed experiments is shown in Fig. 5. Good agreement between experimental data and simulated values obtained from equations (10) and (11) is observed, since more than 90% of the results of Csim fall within the Cexp  10% range. The values of the zero-th kinetics constants of nitrite oxidation working with real seawater from a hatchery are higher than those obtained by Lin and Wu (1997) who oxidized nitrite from synthetic seawater samples, using graphite anodes and operating at current density between 68 and 182 A m
2.

3.3.

Kinetics of COD oxidation

Chemical Oxygen Demand (COD) of pond waters may be used as an index of the organic matter concentration. In the presence of organic matter, the oxidants (HOBr and HOCl) generated during the electrolysis process will also be consumed by oxidation of organic matter (Westerhoff et al., 2004; Wijesekara et al., 2005). Fig. 6 shows the influence of the applied current density in the range 5e50 A m
2 on COD removal starting from an average initial COD concentration of 54.8 mg l
1. COD removal follows a linear trend with time in the whole range of J

40

80

120

160

200

240

280

Time (minutes) Fig. 6 e Influence of the applied current density (A J [ 5 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2) on the evolution of COD/COD0 COD0 z 54.8 mg lL1.

considered, concluding that the higher the current density, the faster the removal of COD. Similar behaviour on COD removal with time, operating at galvanostatic conditions has been reported in the works of Vijayaraghavan et al. (2008); treating raw aquaculture wastewater from a shrimp farm with a initial COD concentration of 1730 mg l
1 at 372 and 745 A m
2 and using a graphite anode, achieved a residual COD concentration at the end of experiments around 50 mg l
1. The initial COD concentration (54.8 mg l
1) used in the present work is representative of the sea bream production on hatcheries. Comninellis and co-workers (Panizza et al., 2001) developed one of the most cited models to describe the electrochemical oxidation of organic pollutants, COD, on BDD electrodes (Can˜izares et al., 2005; Cabeza et al., 2007). In this model, two different operating regimes are defined, depending on the value of the applied current density, Jappl, and the value of the limiting current density, Jlim: a) If Jappl < Jlim, the electrolysis is under current control and the COD decreases linearly with time. b) If Jappl > Jlim, the electrolysis is under mass transport control and the COD evolution follows an exponential trend with time.

1.0 0.9 0.8

Simulated Value

0

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Experimental Value Fig. 5 e Parity graph: dimensionless Nitrite concentration at the different applied current densities (: J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2).

Nevertheless, in those matrices where chloride is the major compound like in seawater, chlorine is also formed during the electrochemical process, thus, consuming a high percentage of the applied current. In the present work where [Cl
]0 is around 500 times higher than [COD]0, most of the Jappl is employed in the oxidation of chloride. According to the latter, although the total applied current density in this work (range 5e50 A m
2) is higher than Jlim,COD (13.3 A m
2), the effective applied current density for COD oxidation is just a fraction of the total applied current density, suggesting an operating regime under current density control. As shown in the experimental data plotted in Fig. 6, a linear decrease in the normalized COD removal with time is observed, confirming the previous hypothesis. Therefore,

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1.0

equation (12) can be used to describe the evolution of COD during the electrochemical treatment at the different applied current densities.

The integrated form of equation (12) gives equation (13): ½COD k ¼1
$t ¼ 1
k0 $t ½COD0 ½COD0

(13)

Values of the kinetic constant, k, corresponding to the different values of J applied are shown in Table 4. These values were calculated from the slopes (k0 ) of the dimensionless COD concentration vs. the electrolysis time (Fig. 6) for the different values of the applied current density and the initial COD concentration (54.8 mg l
1). In all experiments, correlation coefficients higher than 0.980 were obtained. The data in Table 4 show a linear relation between k and J expressed by equation (14): k ¼ 1:35  10
2  J




 R2 ¼ 0:985

(14)

Consequently, the substitution of equation (14) in equation (13) results in equation (15): COD ¼ COD0
1:35  10
2  J  t

(15)

Equation (15) describes the evolution of COD concentration as a function of the current density, in the studied range 5e50 A m
2, during the electrochemical oxidation of such a complex matrix as seawater from a hatchery. A parity graph of the dimensionless COD concentration for all the performed experiments is shown in Fig. 7. Good agreement between experimental and simulated data is observed, since more than 90% of the results of Csim fall within the Cexp  10% range.

3.4. Formation of chlorine and organochlorinated compounds During the electrochemical oxidation of seawater, chlorine gas generated on the anode is converted into hypochlorous acid (HOCl), as shown by equations (16) and (17). The sum of the three species: dissolved gas chlorine (Cl2), hypochlorous acid (HOCl) and hypochlorite ion (OCl
) is termed free chlorine. In the normal pH range of pond water (6e7.5), HOCl is the major component of free chlorine.


2Cl /Cl2 þ 2e

(16)


Cl2 þ H2 O/HOCl þ Cl þ H

þ

(17)

Table 4 e Values of k for COD oxidation at the different applied current density. J (A m
2) 5 20 30 50

k0 (min
1) 1.32 5.78 6.91 1.24

e
3 e
3 e
3 e
2

½COD0 (mg l
1)

k (mg l
1 min)

54.8

0.072 0.316 0.379 0.679

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Experimental Value

Fig. 7 e Parity graph: dimensionless COD concentration at different applied current densities (A J [ 5 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2).

The evolution of free chlorine concentration within the electro-oxidation time is shown in Fig. 8, for an applied current density in the range 5e50 A m
2. As expected, the results show that chlorine concentration increases with electrolysis time and applied current density, in the range of J values considered. An increase in chlorine formation leads to a higher efficiency on the electrochemical oxidation when the indirect oxidation is the main oxidation mechanism. This behaviour has been also considered in the works of Lee et al. (2002), Katayose et al. (2007), Taparhudee et al. (2008) and Vijayaraghavan et al. (2008). On the other hand, an undesirable effect of free chlorine is the formation of organochlorinated compounds. Trihalomethanes (THM) are the most commonly halogenated byproducts found after water electro-oxidation. HOCl generated

9.00

Free Chlorine (mg l-1)

(12)

Simulated Value

0.8

d½COD ¼
k dt




0.9

7.50 6.00 4.50 3.00 1.50 0.00

0

10

20

30

40

50

60

70

80

90 100 110 120

Time (minutes) Fig. 8 e Influence of the applied current density (A J [ 5 A mL2; - J [ 10 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2) on the evolution of free chlorine.

132

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 2 5 e1 3 4

a

1.60

b

1.40

1.40

1.00

THM (mg l-1)

TTHM (mg l-1)

1.20 0.80 0.60 0.40 0.20 0.00

CHBr3

1.20 1.00

CHBr2 Cl

0.80 CHBrCl2

0.60 0.40

CHCl3

0.20 0 10 20 30 40 50 60 70 80 90 100

0.00

10

20

30

50

J (A m-2)

Time (minutes)

Fig. 9 e THM formation during electrochemical oxidation: (a) influence of the applied current density (- J [ 10 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2) on the evolution of TTHM concentration; (b) Individual THM concentrations at the end of each electro-oxidation experiment.

1.00

0.60 0.40 0.20 20.0

40.0

60.0

80.0

TAN eliminated (%)

100.0

Energy consumption

The technical feasibility of the electrochemical oxidation is usually evaluated in terms of the percentage removal of pollutant reached, while the economic feasibility is determined by the energy consumption. The cumulative energy consumption is shown against the percentage of TAN eliminated in Fig. 10a, where higher energy consumption is required to obtain higher TAN removal, for all the J values considered. Operation at 10 A m
2 yielded the lowest energy consumption, whereas similar profiles were obtained at 30 and 50 A m
2. Fig. 10b shows the time for total TAN removal and the corresponding energy consumption to these removals, at the different applied current densities. As shown in Fig. 10b the energy curve has a clear increase up to a value of J of 20 A m
2. This increase shows lower slopes for current densities higher

b

0.80

0.00 0.0

3.5.

Time for total TAN removal (min)

W (kW h m-3)

a

Excess of chlorine and the THMs are harmful to aquatic organisms (Lee et al., 2002; Katayose et al., 2007; Taparhudee et al., 2008). However this problem can be minimized by adsorbing these compounds onto activated carbon. This alternative is currently under investigation in our group.

140 120 100 80 60 40 20 0

0

10

20

30

40

50

0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 60

W (kW h m-3)

in the electrochemical oxidation of seawater reacts with dissolved organics resulting in the formation of chlorinated halomethanes. HOCl also oxidizes the bromide anions (Br
) present in seawater, that react readily with organic matter to form brominated halomethanes. In the present study four organic halogenated compounds have been detected: bromoform, dibromochloromethane, bromodichloromethane and chloroform. Fig. 9a shows the influence of the applied current density in the range 10e50 A m
2 on the evolution of Total Trihalomethanes (TTHM) concentration, resulting from the sum of these four organic halogenated compounds considered. The TTHM concentration increased with the increase in the available chlorine concentration, formed during electrochemical oxidation, whatever the value of the applied current density is. Fig. 9b shows the concentrations of the organic halogenated compounds at the end of each experiment. In all experiments, bromoform was the predominant by-product formed with a mean weight percentage of 85.3%, followed by dibromochloromethane (10.2%) and traces of bromodichloromethane (3.1%) and chloroform (1.4%) were also detected. The high levels of brominated by-products are attributed to the presence of bromide ions in seawater (Allonier et al., 2000; Budziak et al., 2007; Katayose et al., 2007).

J (A m-2)

Fig. 10 e Energy consumption during electrochemical oxidation: (a) Cumulative energy consumption profiles against the percentage of TAN eliminated (- J [ 10 A mL2; : J [ 20 A mL2; C J [ 30 A mL2; > J [ 50 A mL2); (b) Time and energy consumption profiles to achieve total TAN removal.

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 2 5 e1 3 4

than 20 A m
2. The calculated energy consumption to achieve complete oxidation of TAN operating at 10, 20, 30 and 50 A m
2 was 0.33, 0.61, 0.70 and 0.78 kWh m
3, respectively. These consumptions are lower than the range from 12 to 21 kWh m
3 found by Lin and Wu (1996), who oxidized nitrite using graphite anodes and operating in the range of J from 442 to 1106 A m
2. According to these results, a compromise must be reached between maximizing the oxidation rate and minimizing energy consumption.

4.

Conclusions

This work reports the electro-oxidation viability and kinetics of the organic matter and nitrogen compounds contained in the seawater of a Recirculating Aquaculture System (RAS). Seawater collected from the inlet to the biological filter operated in a high density hatchery with average initial concentrations [TAN] ¼ 8.00 mg l
1, [NO2] ¼ 80.00 mg l
1, [COD] ¼ 54.80 mg l
1 and [Cl
] ¼ 26167 mg l
1 has been oxidized in an electro-oxidation cell working in the range of current densities 5e50 A m
2, and observing that this variable exerts strong influence on the removal kinetics of the considered parameters. A second order expression correlated satisfactorily well the kinetic data of TAN removal, whereas the kinetic evolution of nitrite and COD needed zero-th order equations being the kinetic constants for the anodic oxidation dependent on the applied current density (kTAN ¼ 7.86  10
5$exp(6.30  10
2  J ); kNO2 ¼ 3.43  10
2  J; kCOD ¼ 1.35  10
2  J ). The evaluation of the energy consumption by the oxidation process showed a dependency with the applied current density up to a value of J ¼ 30  A  m
2; above this value no apparent sensitivity of the variable on the energy consumed was observed. Thus this work reports for the first time the kinetics of oxidation of TAN, nitrites and COD contained in seawater of a high density hatchery that are needed for process design. A comparison of the results with previous works using graphite anodes already referred in this work, showed a considerable improvement both on the oxidation kinetics as well as on the reduction of the consumed energy. Finally the formation of THMs during the electrochemical treatment has been evaluated, detecting a maximum concentration of 1.7 mg l
1 of total trihalomethanes. A hybrid process that couples an adsorption step onto activated carbon to the electro-oxidation cell in order to remove the generated THM and the residual chlorine is currently under operation in the hatchery facilities.

Acknowledgements Financial support of projects CTQ2008-03225/PPQ, CTQ200800690/PPQ, Consolider CSD 2006-44 (Spanish Ministry of Science and Innovation (MICINN)), 080/RN08/03.2 (Spanish MARM) and 18-04-2007 (SODERCAN, Cantabria Government) is gratefully acknowledged. The collaboration of Tinamenor S.L.

133

is also acknowledged. V. Dı´az would like to thank the MICINN for an FPI research grant.

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