Abstract

The efficiency of electrolysis (EC/Cl2) and photo-assisted electrolysis (EC/UV/Cl2) methods, in the presence of chloride, for the abatement of real dairy waste from a producer in the Triangulo Mineiro region of Brazil, was evaluated. A complete 23 factorial design was performed for the variables time, pH and current. After determining the ideal pH, a Central Compound Design (CCD) was performed, where the applied current (533.42 mA) and treatment time (60.45 minutes) were maximized. The effluent was subsequently submitted to prolonged EC/Cl2 and EC/UV/Cl2 treatment in order to evaluate the behaviour of specific environmental parameters over time. The EC/UV/Cl2 method was more efficient than simple EC/Cl2 treatment. The EC/UV/Cl2 method resulted in a reduction of all environmental parameters investigated to levels within legal standards for effluent discharge. A relatively low cost of treatment is obtained with Energy per Order (EEO) values of 0.89 and 1.22 kWh m−3 order−1 for the EC/UV/Cl2 and EC/Cl2 treatments, respectively. The electrochemical production of free chlorine species followed by subsequent photolysis and production of radical species can convert a simple electrochemical process into an advanced oxidation process (AOP).

INTRODUCTION

The dairy industry is present in all parts of the world and is well-known for producing effluents that present not only high organic load, but also large amounts of inorganic species such as chloride ions (Cl) (Kasmi et al. 2017). Brazil's milk production chain has undergone several transformations; in 2013, it was the 5th largest producer with around 31 billion litres of milk, with nations like China (34 billion) and the United States (91 billion) (Silva & Silva 2011). By 2017, the milk industry was in sixth place with an annual production of just over 26 billion litres (Anualpec 2017). Milk production is widely distributed, with the State of Minas Gerais being the largest producer, accounting for approximately 30% of the country's production. The dairy industry is represented in every municipality in the state, with a preponderance of small properties (Silva & Silva 2011).

Among the compounds present in dairy effluent, whey is the most important contribution to Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD) and Total Organic Carbon (TOC), and treatment for an environmentally appropriate final disposal is indispensable (Güven et al. 2008).

Dairy effluent treatments include the application of physicochemical and biological methods, with the specific objective of reducing the volume of the sludge produced (Demirel et al. 2005). Physicochemical processes are effective in the removal of emulsified compounds; however, the addition of reagents increases the costs and the removal of COD is low. Although biological methods present lower costs in comparison to physicochemical methods, this type of treatment generally displays low efficiency (Demirel et al. 2005). Economically and ecologically viable solutions and stricter requirements are continuously being defined for the treatment of industrial effluents (Kubota & Da Rosa 2013).

Electrodes composed of RuO2/TiO2 mixtures (Dimensionally Stable Anodes – DSA®), have been applied in the chlor-alkali process for years due to their electrochemical stability and catalytic activity (Souza et al. 2014a). DSA® materials combine excellent activity towards the Chlorine Evolution Reaction (CER) with mechanical stability. It has been demonstrated that electrochemical generation of chlorine, and the subsequent formation of free chlorine species (FCS), in the form of hypochlorous acid (HOCl) or hypochlorite (ClO), is an interesting alternative for in situ degradation of pollutants (Helme et al. 2010; Souza et al. 2014a). Chlorine gas is produced via the anodic formation of chlorine (Cl2) from the chloride ion (Cl) (Equation (1)). 
formula
(1)
Subsequently, FCS (HOCl/(ClO) is formed via the dissolution of Cl2 in H2O (Equations (2) and (3)). 
formula
(2)
 
formula
(3)

The equilibrium mixture of HClO/ClO is dependent on the pH of the solution, with a pKa = 7.5 at 298 K (Feng et al. 2007). Electrochemical pollutant removal coupled with FCS (EC/Cl2) is the interaction, in aqueous solution, of HClO/ClO species with organic matter. A drawback of this method is the possible formation of toxic organochlorine by-products and others such as chlorite, chlorate and perchlorate (ClO2, ClO3 and ClO4) decreasing process efficiency (Martínez-Huitle & Ferro 2006). However, other studies indicate that organochlorine intermediates can be removed during the electro-oxidation process (Alves et al. 2014; de Mello Florêncio et al. 2016). Another important fact is that FCS can undergo photolysis when exposed to UV radiation.

As given in Equation (4), such photolysis can result in the formation of the hydroxyl radical (OH), which is a highly reactive oxidative species capable of degrading various pollutants. 
formula
(4)
The use of combined processes takes advantage of not only the benefits of isolated technologies, but also their synergistic effects (Souza et al. 2014b).

In recent studies performed in our laboratory, it has been demonstrated that the combination of electrochemical degradation, in the presence of Cl, and simultaneous UV irradiation of the reaction mixture (EC/UV/Cl2) can reduce toxicity and organo-chlorine production (de Mello Florêncio et al. 2016; Pinto et al. 2017).

Other authors also demonstrate the feasibility of combining electrochemical and photochemical technology for the efficient removal of recalcitrant substances (for example see Souza et al. 2014b; Brillas & Martínez-Huitle 2015; Aquino et al. 2017).

The specific combination of UV irradiation for the cleavage of reactive chlorine species has been studied with a view to its possible application as an Advanced Oxidation Process (Xiang et al. 2016; Shi et al. 2018). The process usually involves the addition of hypochlorite to the reaction mixture and irradiation with a UV source (Xiang et al. 2016). The degradation of the pollutant Bezafibrate using UV/chlorine was shown to be ∼80%, and presented rate constants that increased linearly in a wide chlorine dosage from 0.1 to 1.0 mM, which implied that ClO• generated from the reactions of chlorine with HO• and Cl• could react rapidly with effluents (Shi et al. 2018).

By electrochemically producing the reactive chlorine species in situ the need for storage, transport and handling of chemicals is eliminated. Additionally, it has been observed that electrochemically produced chlorine can be up to four times more efficient than commercial hypochlorite for antimicrobial uses (Helme et al. 2010).

The aim of the current paper is to study the potential treatment of real dairy waste from a large-scale producer in the Triângulo Mineiro region of Brazil, using electrolysis (EC/Cl2) and photo-assisted electrolysis (EC/UV/Cl2) in the presence of chloride ions present in the raw effluent. With the electrochemical production of FCS followed by subsequent photolysis, it is possible to convert a simple electrochemical process (chlorine production) into an advanced oxidation process (AOP) as radical species can be produced.

MATERIALS AND METHODS

Wastewater

The as-received effluent was provided by a large-scale regional dairy producer (name withheld) in the state of Minas Gerais, Brazil, and was collected at the end point of the installed biological treatment process. The effluent collection was performed on days when there had been no rainfall in the previous 7 days in order to limit alterations in the composition. The principal effluent parameters (turbidity, conductivity, colour, pH, chlorides, TOC, COD and BOD5) were determined and the standard deviations calculated. The toxicity of the effluent after the EC/UV/Cl2 and EC/Cl2 treatments, as well as the energy expenditure involved, were also evaluated. All analyses were performed at pH 5, according to characteristics of dairy effluent.

Electrochemical reactor

A single compartment electrochemical reactor (volume: 250 cm3) was employed in batch mode for the experimental design section and subsequent EC/UV/Cl2 and EC/Cl2 treatments, as presented in the literature (de Mello Florêncio et al. 2016). A commercial Ti/Ru0.3Ti0.7O2 mesh (De Nora Brazil, area = 1.60 cm2) was employed as the anode and two Ti plates (area: 4.0 cm2 each) were employed as cathodes and positioned either side of the anode. The area of the cathodes is greater than the anode in order to avoid current losses at the anode. The cell was equipped with an external glass jacket to maintain the temperature constant (25 ± 1 °C). When the EC/UV/Cl2 treatment was performed, a UV source (UV Pen Ray®, λmax = 254 nm, UVP LLC, 9 W) was inserted in the reactor.

Experimental setup

The current was supplied by a constant current source (Minipa, DC Power Supply MPL 1303) which also registered the associated cell potential (Ecell – between the working and counter electrodes). The reference potential was measured by a voltmeter (Minipa, ET-2076). The temperature of the reactor was maintained constant by a thermostatic bath (Cienlab), with external circulation, at 25 ± 1 °C.

Experimental procedure

The experimental procedure consisted of the following stages:

  • i.

    A 23 initial factorial design employing only the EC/Cl2 technique;

  • ii.

    A Central Composite Design (CCD) to refine the initial factorial design and maximize removal parameters, employing only the EC/Cl2 technique;

  • iii.

    Extended electrolysis (120 min) at the maximized current using both EC/UV/Cl2 and EC/Cl2 to compare the efficiencies.

An initial 23 experimental design was performed using as variables pH (5–9), current (500–1,500 mA) and degradation time (15–45 min). Chemical Oxygen Demand (COD) removal was used as the response variable. The pH range was used because the raw effluent had a pH of ∼5 and the legal limits varied from 5 to 9 and investigation of a pH <5 would require pH adjustments that were considered impractical by the dairy plant. Two replicates were performed at all points and, from the response variables; a confidence interval of 95% was adopted. A subsequent maximization was performed using a Central Composite Design (CCD), at pH 5, where the lower (−) and higher (+) levels were modified with respect to the time (30, 45 and 60 min) and maintained in relation to the current (mA) with addition of central and axial points.

After realizing the CCD, extended degradation assays (120 min) were performed under the maximized current conditions. This was done since it is important to determine if prolonged treatment times result in any improvement in the overall removal levels studied in the CCD. Two distinct treatments were performed:

  • Electrochemically generated free chlorine combined with UV radiation (EC/UV/Cl2) and

  • Electrochemically generated free chlorine (EC/Cl2)

Analyses

Chemical Oxygen Demand (COD) was determined using the closed reflux, colorimetric method (Clesceri et al. 1999) and readings were performed using a UV-vis spectrophotometer (Perkin Elmer, λ25). Samples were analysed at the start and at the end of each degradation assay. All samples were diluted to reduce the effect of reactive chlorine species and chloride ions on the results obtained.

The Total Organic Carbon (TOC) analyses were performed using a TOC analyser (Sievers InnovOx, General Electric Company) and samples were analysed at the start and at the end of each degradation assay.

Phyto-toxicity assays were carried out based on methods using lettuce (Lactuca sativa) seeds as the test organism (Hamilton et al. 1977) as employed previously (Alves et al. 2014).

The HOCl/OCl concentrations were determined according to the method presented in the literature (Clesceri et al. 1999). Additionally, the pH, turbidity, colour and conductivity of each sample was analysed before and after each assay.

The principal characteristics of the effluent were determined by standard methods (Clesceri et al. 1999): turbidity (TECNOPON), conductivity (DDS 120 W conductivity meter), colour (Policontrol colorimeter), pH (HANNA), chlorides and BOD5 (OXITOP® respirometric method 5210D).

RESULTS AND DISCUSSION

Analysis of the effluent

The main parameters established by Federal (CONAMA 1986) and State (Copam 2014) agencies for the disposal of dairy effluents are compared to the levels determined in the as-received effluent in Table 1. Brazilian legislation is generally lax and both State and Federal laws have considerable gaps. However, it can be observed in Table 1 that the as-received effluent breaks almost all the criteria, except for pH, which is at the lower limit of acceptability.

Table 1

Main parameters established by CONAMA No. 357/2005 COPAM/CERH N°1/2008 for the disposal of dairy effluents before and after the treatments

Parameter As-received dairy effluent Resolution CONAMA N°357/2005 DN COPAM (CERH N°1/2008) 
[Cl] (mg L−1730.0 ± 3.06 – 250 
pH 5.01 ± 0.07 5 a 9 6 a 9 
COD (mg O2 L−1759.4 ± 3.01 – 180a 
TOC (mg L−1361 ± 3.01 – – 
BOD5 (mg O2 L−1363.0 ± 2.99 – 60b 
Parameter As-received dairy effluent Resolution CONAMA N°357/2005 DN COPAM (CERH N°1/2008) 
[Cl] (mg L−1730.0 ± 3.06 – 250 
pH 5.01 ± 0.07 5 a 9 6 a 9 
COD (mg O2 L−1759.4 ± 3.01 – 180a 
TOC (mg L−1361 ± 3.01 – – 
BOD5 (mg O2 L−1363.0 ± 2.99 – 60b 

aOr removal efficiency >75% and annual average ≥85%.

bOr removal efficiency >70% and annual average ≥55%.

It is also important to note that the chloride concentration ([Cl]) is ∼3 times the permitted state limit. However, this subsequently results in higher levels of electrical conductivity, which in the present case was 1,041 mS·cm−1, which would facilitate any potential electrochemical treatment. Additionally, the presence of Cl would also permit the electrochemical production of free chlorine species (FCS), which are capable of removing organic matter (Alves et al. 2014).

Initial removal studies

As described in the experimental section, to determine the best conditions for effluent abatement, an initial 23 experimental design was performed using as variables pH (5, 7 and 9), current (500, 1,000 and 1,500 mA) and degradation time (15, 30 and 45 min). Chemical Oxygen Demand (COD) removal was used as the response variable. It was found that current, treatment time and pH were statistically significant. However, in the case of pH it was observed that the actual increase in COD removal from pH = 5 to pH = 9 was <10%. As a result, after discussion with the dairy producer that provided the effluent, it was decided that it would not be worthwhile adjusting the pH of the effluent from 5 to 9 when considering the need for the addition of the necessary chemicals. In addition, the pH was observed to increase during electrolysis – probably due to the consumption of Cl and subsequent readjustment of the acid-based equilibria.

For the maximization stage, a Central Composite Design was performed where the lower (−) and higher (+) levels were modified with respect to the time (30, 45 and 60 min) and maintained in relation to the current (mA). This is because the COD removal levels in the 23 experimental design did not reach satisfactory levels, considering the relevant legislation, in the time employed. The best result, reached for the factorial design was ∼72% COD removal (500 mA, pH = 5 and t = 45 min), corresponding to a residual value of 242.28 mg O2 L−1, which is still above the stipulated limit for the disposal in water bodies (180 mg O2 L−1) in Brazil/Minas Gerais.

It can be observed from the response surface (Figure 1), that the removal of COD, as also seen in the original experimental design, displays an increase with the electrolysis time and lower currents, resulting in an observed maximum. The maximized current is given at a maximum point evidenced by the curvature of Figure 1.

Figure 1

Response surface for time and current for the Central Composite Design (CCD).

Figure 1

Response surface for time and current for the Central Composite Design (CCD).

The critical values obtained were 60.45 minutes and 533.42 mA, for time and current, respectively. As reported in the literature (Júnior et al. 2017) the maximization of the process with the treatment of CCD, quantifies relevance factors and these contribute to the best parameter of degradation to be achieved. With maximized time and current data, a maximum degradation percentage can be reached for the proposed CCD. Statistical software stipulates, within critical values, this percentage. For this maximized planning, the percentage of COD removed is 89.46%, which corresponds to a COD of 91.20 mg O2 L−1, well below that stipulated by legislation for the disposal to water bodies (180 mg O2 L−1).

These results are in agreement with previous studies that demonstrate that lower current densities are more effective at removing COD load in the EC/UV/Cl2 and EC/Cl2 treatments (de Mello Florêncio et al. 2016). At acidic pH, chlorine is present mainly in the form of hypochlorous acid (HOCl), which has a reduction potential of +1.49 V. At higher pH values, the conjugate base (OCl) has a reduction potential of +1.43 V, which may explain why the extent of degradation is greater at lower pHs (Neodo et al. 2012). In similar studies Koshti et al. 2017, show that for textile waste, degradation was more pronounced at lower currents and pH values. It can be observed from the response surface (Figure 1), that the removal of COD, as seen in the original experimental design, displays an increase with the electrolysis time and lower currents and a maximum is observed.

Extended electrolysis

After realizing the CCD, extended degradation assays (120 min) were performed under the maximized current conditions for the EC/UV/Cl2 and EC/Cl2.

Figure 2 shows how TOC (Figure 2(a)) and COD (Figure 2(b)), expressed as a percentage, vary with the degradation time for the solutions in the EC/UV/Cl2 and EC/Cl2 treatments During the 120 min of treatment, pseudo-first order kinetics, with respect to COD and TOC removal, were observed for both techniques. This behaviour is commonly seen for both advanced oxidation processes and electrochemical systems (Navarro et al. 2017) and indicates that the overall order is >1, with other contributing factors (such as [Cl] or [Cl2]) remaining constant. From Figure 2, it can be observed that TOC and COD removal are reduced by >90% by both treatments after 120 mins. However, analysis of Figure 2 shows that the time needed to reach this level of removal is much reduced for the photo-assisted process and this is exemplified by the pseudo-first order rate constants. For TOC removal, the rate-constants are 5.5 × 104 and 5.0 × 104 s−1 for the EC/UV/Cl2 and EC/Cl2 techniques, respectively. In the case of COD removal, the values are 10.0 × 104 and 6.3 × 104 s−1 for the photo-assisted and purely electrochemical techniques, respectively.

Figure 2

Comparative values of (a) TOC removal and (b) COD removal as a function of time for the (○) EC/Cl2 and ( • ) EC/UV/Cl2. Conditions: Current = 533.42 mA and t = 120 min.

Figure 2

Comparative values of (a) TOC removal and (b) COD removal as a function of time for the (○) EC/Cl2 and ( • ) EC/UV/Cl2. Conditions: Current = 533.42 mA and t = 120 min.

The COD or TOC removal can be achieved by various species in the current study: (i) Electrochemically; (ii) by electrochemically produced FCS species; (iii) by HO or (iv) direct photolysis. Electrochemically, the process can occur by the formation of higher Ru oxides as described (Martínez-Huitle & Ferro 2006). Also, intermediates of O2 evolution may form oxy-chloro species that can also destroy organic species, according to Equations (5) and (6) (Scialdone et al. 2009). 
formula
(5)
 
formula
(6)
where R is an organic species. However, the principal path for organic degradation in the EC/Cl2 treatment is probably via FCS species, as given in Equation (7) for acidic solutions. 
formula
(7)

For the EC/UV/Cl2 technique, the use of UV irradiation can promote the formation of HO (Equation (4)) and reduce the COD or TOC levels (Brillas & Martínez-Huitle 2015). Direct photolysis of the untreated effluent was attempted in the present study; however, the removal levels were very low at <2% of COD or TOC. It is also possible that direct photolysis is important for degradation of the intermediates formed in Equations (6) and (7), resulting in greater COD and TOC removals (Alves et al. 2014; de Mello Florêncio et al. 2016).

Another important parameter is the COD/TOC ratio (Figure 3(a)), which provides information on how the chemical substances present in the reaction medium become more oxidized. The lower this ratio, the more oxidized the sample is. The decrease in the COD/TOC ratio indicates that the treatment led to an increase in the oxidative susceptibility of the soluble organic matter. This is more pronounced in the case of the EC/UV/Cl2 technique due to the strong oxidizing agents such as hydroxyl and chlorine radicals, as already reported in the literature (Serna-Galvis et al. 2016). This type of behaviour evidences that the organic matter is susceptible to chemical oxidation by the EC/UV/Cl2 and EC/Cl2 techniques and that this is most apparent at t < 60.

Figure 3

Ratios of (a) COD/TOC and (b) COD/BOD5 during the (○) EC/Cl2 and ( • ) EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

Figure 3

Ratios of (a) COD/TOC and (b) COD/BOD5 during the (○) EC/Cl2 and ( • ) EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

The values of the COD/BOD5 ratio (Figure 3(b)) after degradation present values <2 and this shows that the electrochemical and photo-assisted electrochemical processes increase the biodegradability of the effluent. According to the literature (Funai et al. 2017), some AOPs increase biological treatability, since the COD/BOD5 ratio decreases during the treatment processes. The present study demonstrates that biological treatability can be improved with the EC/UV/Cl2 and EC/Cl2 treatments.

Figure 4 presents the EEO values for COD removal over 2 h, and although both methods have similar efficiencies, a greater efficiency for COD removal for the EC/UV/Cl2 technique is observed at all stages. As the removal of COD obeyed pseudo-first order kinetics, the energy consumption was analysed in terms of the Electrical Energy per Order (EEO – kWh m−3 order−1) (Bolton et al. 2001). Over 2 h of electrolysis the EEO values for the EC/UV/Cl2 and EC/Cl2 techniques were 0.89 and 1.22 kWh m−3 order−1, respectively. This results from the fact that the low-pressure UV lamp employed in this study contributes 9 W to overall energy consumption, but a synergistic effect is observed where the combined system (electrochemical and the lamp) is greater than the sum of its parts. This is in agreement with previous studies carried out by this group (Alves et al. 2014) and the EEO values are in general agreement with literature values (see for example Asaithambi et al. 2017).

Figure 4

Variation of the Electrical Energy per Order (EEO) during the (○) EC/Cl2 and ( • ) EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

Figure 4

Variation of the Electrical Energy per Order (EEO) during the (○) EC/Cl2 and ( • ) EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

An important parameter when treating effluents is the residual toxicity. This is especially true when complex effluents are under investigation and the identification of individual compounds and their degradation products is difficult. In the present study, it was necessary to investigate whether the treated solutions were less harmful than the starting wastewater after the different degradation assays.

The value for the EC50 (%) was determined for the as-received effluent and for all the treated solutions (Figure 5). From Figure 5, it can be seen that EC50 (%) values for the treated solutions for each treatment method are only slightly improved compared to the starting solution. This residual toxicity is probably due to the presence of small, toxic, chlorinated degradation products. These data are compatible with previous results, where prolonged electrolysis was subsequently capable of removing residual toxicity (Alves et al. 2014; Pinto et al. 2017).

Figure 5

Phytotoxicity during the EC/Cl2 and EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

Figure 5

Phytotoxicity during the EC/Cl2 and EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min.

The seed Germination Index (IG (%)) was determined for the initial solution and for the treated solutions at 0, 15, 30, 60 and 120 min of EC/UV/Cl2 and EC/Cl2 treatment. The phytotoxicity data for each time of electrochemical degradation and photochemical electrochemistry is shown in Figure 5.

The results show that the EC/UV/Cl2 and EC/Cl2 treatments slightly decreased the toxicity of the initial solution after 15 min and, with a longer duration of treatment, 30, 60 and 120 min, there was a significant increase in the EC50 value, principally for the EC/UV/Cl2. This agrees with previous studies performed in this laboratory (Pinto et al. 2017). The literature demonstrates (Boudriche et al. 2016) that AOPs may contribute to a decrease in the toxicity of the treated effluent, but such efficacy depends not only on the method but also on the substances present in the effluent.

Figure 6 shows an overview of the relevant parameters and their reduction after 120 min of EC/UV/Cl2 and EC/Cl2 treatment. It can be observed that all the relevant parameters studied are now within the limits stipulated by Brazilian legislation (CONAMA 1986; Copam 2014) and, once more, this is most apparent in the case of the EC/UV/Cl2 technique.

Figure 6

Comparison of environmental parameters in terms of the legal limits, as-received effluent, during the EC/Cl2 and EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min. Units: BOD5: mg O2 L−1; TOC: mg L−1; COD: mg O2 L−1 and chloride: mg L−1.

Figure 6

Comparison of environmental parameters in terms of the legal limits, as-received effluent, during the EC/Cl2 and EC/UV/Cl2 treatments. Conditions: current = 533.42 mA and t = 120 min. Units: BOD5: mg O2 L−1; TOC: mg L−1; COD: mg O2 L−1 and chloride: mg L−1.

CONCLUSION

Effluent abatement employing the EC/UV/Cl2 and EC/Cl2 treatments was able to mineralize and reduce the COD and TOC load of the dairy effluent. It was observed that the EC/UV/Cl2 method was more efficient in the removal of COD (97.80%) and TOC (94.62%) than the EC/Cl2 and method alone for COD (95.78%) and TOC (92.18%) removal. In all monitored parameters (pH, colour, turbidity, chloride content) the EC/UV/Cl2 method was more efficient than the EC/Cl2 method. The data presented, principally after the EC/UV/Cl2 treatment, demonstrate that the organic matter is susceptible to chemical oxidation, which is more pronounced over the first 60 min.

Biodegradability increased for both processes, being better for the EC/UV/Cl2 method. After 30 minutes of degradation, for the EC/UV/Cl2 method, the effluent presented COD values (120.2 mg O2 L−1) lower than those required by the legislation. In terms of energy consumption, the Energy per Order (EEO) values of 0.89 and 1.22 kWh m−3 order−1 for the EC/UV/Cl2 and EC/Cl2 treatments, respectively, are obtained.

The treatments employed decreased the phytotoxicity of the initial solution after 15 minutes, and, with a longer treatment duration of 30, 60 and 120 minutes, there was a significant decrease, mainly for the EC/UV/Cl2 method.

The efficiency of the EC/UV/Cl2 method can be attributed to the photolysis of reactive chlorine species to produce highly reactive radicals.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP – 2014/02739-6); Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

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