Journal of Environmental Management 254 (2020) 109811
Available online 8 November 2019
0301-4797/© 2019 Elsevier Ltd. All rights reserved.
Research article
Electro-catalytic ozonation for improving the biodegradability of mature landfill leachate
Mina Ghahrchi
a,b, Abbas Rezaee
a,*aDepartment of Environmental Health, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
bSciences Research Center, Torbat Heydariyeh University of Medical Sciences, Torbat Heydariyeh, Iran
A R T I C L E I N F O Keywords:
Landfill leachate Biodegradability Ozonation Electrochemistry Homogeneous catalytic
A B S T R A C T
Landfill leachate contains complex, resistant, and diverse compounds that are considered as an environmental health problem. This study aims to investigate the efficacy of integrated homogeneous catalytic ozonation and electrochemical process for improving the biodegradability of landfill. This experimental study was conducted on real landfill leachate on the laboratory scale. The variables were current density (O3/H2O2-42.1 mA/cm2), ozone concentrations (100–400 mg/h), the initial pH (3–9), and the reaction times (1–6 h). The optimum operating condition was obtained at 1.42 mA/m2, 400 mg/h of ozone concentration, initial pH of 3, during 3 h. In the proposed integrated catalytic ozonation-electrochemical process, the chemical oxygen demand (COD) and biochemical oxygen demand (BOD) concentrations were removed to 3381.9 and 1521.8 mg/L, respectively.
Under the optimum condition, the biodegradability index increased from 0.27 to 0.45. The results showed that the electro-catalytic ozonation process has a significant effect on the biodegradability index and could improve the removal efficiency of landfill leachate treatments.
1. Introduction
Sanitary landfill is among the widely used methods for municipal solid waste disposal. The most important consequence of landfills is the production of very complex leachate (Poblete et al., 2019). According to the literature, about 70–100 L of landfill leachate are produced per ton of waste. Since landfill leachate contains various types of organic and inorganic pollutants and heavy metals, if they are not properly managed, surface water and groundwater are polluted and soil quality changed (Fernandes et al., 2015; Rasool et al., 2016). Therefore, it is essential to use reliable and efficient technologies to treat landfill leachate that are able to deal with various complicated wastewater.
Various methods have been developed to treat landfill leachate.
Although many studies have been conducted in this regard, landfill leachate treatment still remains a challenge (Fernandes et al., 2014). As leachate age increases, COD concentration and BOD5 are reduced while the pH is increased. Also, the concentration of refractory compounds, including humic substance, is higher than that of young leachate. As a result, its biodegradability index declines (Fernandes et al., 2015).
Although biological treatments such as activated sludge provide a
common, efficient, and cost-effective way for wastewater treatment, the process has some drawbacks for utilizing in landfill leachate treatment.
Low biodegradability of landfill leachate presents the need for the development of alternative technologies with the aim of effectively reducing the amount of pollutants. Hence, landfill leachate requires a pre-treatment process to increase the efficiency of the biological system (Renou et al., 2008). In this regard, the mentioned physical and chem- ical methods such as filtration and coagulation have shown some drawback.
Electrocoagulation (EC) is one of the methods that are considered for the treatment of water and wastewater. EC is a process of producing coagulants in situ using the electrical decomposition of electrodes such as iron or aluminum and producing metal ions in the anode and hydrogen gas in the cathode (Darvishi Cheshmeh Soltani et al., 2013;
Loloi et al., 2016). The advantages of this method are easy management and maintenance, no need for controlling the addition of chemicals, and effective and rapid reduction in organic matter with a common effi- ciency (Fernandes et al., 2015; Hoseinzadeh and Rezaee, 2015). Anode corrosion, sludge sedimentation, and the relatively high initial invest- ment cost are some disadvantages of this process (Holt et al., 2005;
Abbreviations: AOP, Advanced oxidation process; BOD, Biochemical oxygen demand; COD, Chemical oxygen demand; EC, Electrocoagulation; ORP, Oxidation- reduction potential; SOP, Single Ozone process.
* Corresponding author.
E-mail address: [email protected] (A. Rezaee).
Contents lists available at ScienceDirect
Journal of Environmental Management
journal homepage: http://www.elsevier.com/locate/jenvman
https://doi.org/10.1016/j.jenvman.2019.109811
Received 3 January 2019; Received in revised form 9 October 2019; Accepted 29 October 2019
Safari et al., 2015). Moreover, EC process is not effective on the destruction of some resistant pollutants and landfill leachate. Single ozone process (SOP) involves some negative points such as a high cost, and high energy requirement for ozone production (Nawrocki and Kasprzyk-Hordern, 2010). However, AOPs such as O3=H2O2, H2O2=UV, catalytic ozonation Fenton, electrophoton, photocatalysis processes, and ultrasonic that act through hydroxyl radical production have overcome these limitations. AOPs can be effective in treating and removing various toxic and biodegradability resistant or low biodegradable pollutants and improving biological degradability of wastewater (Kalantary et al., 2015).
Over the past two decades, several studies have been conducted on the use of oxidation-based technologies. These studies, in addition to removing pollutants and reducing the organic charge, have also improved the biodegradability index of landfill leachate. These works have reported the improvement of BOD5/COD status and removal of COD from leachate of young municipal waste using Fenton process. The BOD5/COD is an index for evaluating the biodegradability of the leachate. If the index is roughly in the range of 0.35–0.45, the biode- gradability of the compounds in the wastewater is moderate and can be easily decomposed by biological methods at higher ratios about 0.5 (El-Gohary and Kamel, 2016). For ratios lower than 0.35, conventional biological methods for treating this wastewater are not very effective (Kurniawan et al., 2006; Fernandes et al., 2014). It has been shown that when the BOD5/COD ratio increased from 0.58 to 0.64, the COD removal efficiency was increased from 55.9% to 89.4%. However, higher process efficiency was reported for landfill leachate treatment using Fenton pre-treatment (Yilmaz et al., 2010). In another study, the evaluation of the advanced oxidation process based on ozone and Fenton as the pre-treatment for mature leachate treatment showed that ozone combined with hydrogen peroxide had an acceptable yield, was able to remove COD by 72%, and increased BOD5/COD the ratio from 0.01 to 0.24. Nevertheless, produced landfill leachate is still considered as low biodegradable leachate. In addition, the remaining hydrogen peroxide can be mentioned as a problem (Cortez et al., 2011).
In the present study, due to the low biodegradability of old landfill leachate, EC and SOP processes were integrated with the aim of effective homogenous catalytic ozonation process and improving biodegrad- ability index of landfill leachate. Regarding the importance of electrical conductivity for the current applied in EC process and its use as an index for degrading compounds in the landfill leachate, and to have knowl- edge of the predominant reaction of oxidation or reduction in the re- action medium. We also have investigated the changes in electrical conductivity and oxidation-reduction potential (ORP). It is expected that the obtained results are used as a reference for the improving the biodegradability of landfill leachate.
2. Materials and methods
The landfill leachate was obtained from a landfill from Tehran, Iran.
Sample collection and preservation were performed in accordance with the Standard Methods for the of Water and Wastewater (APHA, 2005).
The characteristics of the landfill leachate are presented in Table 1. The landfill leachate COD and BOD5 were 11,387.2 �146.12 and 3110.1 �34.46 mg/L, respectively. The initial pH of landfill leachate
was 9.3 �0.26. The collected leachates were examined under the EC, SOP and EC/SOP processes. The characteristics of the utilized reactor in the experiment are presented in Table 2. In the electrochemical process, a power supply (model APS3005S-3D, China) was utilized for supplying the current and voltage. With the current flow, the oxidation-reduction reaction occurred in the anode and cathode, respectively. The treatment process started with the production of iron hydroxide particles. The capacity of the utilized ozone generator was 400 mg/h. EC/SOP process, by simultaneously performing the EC and SOP processes, was employed to treat the landfill leachate (Fig. 1). All experiments were carried out at room temperature (20 �2 �C). At the end of each experiment in the EC and SOP process, COD concentration, electrical conductivity, ORP and in the EC/SOP process, the concentration of COD and BOD5 were measured. Sulfuric acid and sodium hydroxide (Merck, Germany) was used for pH adjustment.
3. Results
3.1. Effect of current density
In this study, the effect of current densities (0.35–1.42 mA/cm2) on COD, BOD5, and BOD5/COD ratio was studied. As shown in Fig. 2, in both EC and EC/SOP processes, with increasing current densities, COD concentrations were reduced. The COD concentrations for EC and EC/
SOP processes at a current density of 1.42 mA/cm2 were 9143 and 7525 mg/L, respectively. Also, the biodegradability of landfill leachate in the EC/SOP process at a current density of 1.42 mA/cm2 was increased to 0.35.
The changes in electrical conductivity and ORP indicate their fluc- tuations in each process (Fig. 3).
3.2. Effect of ozone concentration
The effect of various concentrations of ozone (100–400 mg/h) with a flow rate of 0.1, 0.14, 0.17, and 0.19 L/min on the SOP and EC/SOP process were studied. The final COD in SOP process at concentrations of 100, 200, 300, and 400 mg/h were obtained 9187, 8751, 8322, and 7834 mg/L, respectively. Meanwhile, it was 8051.7, 7525.6, 6910.7, and 5635.4 mg/L in the EC/SOP process, respectively (Fig. 4). In the EC/
SOP process, increasing ozone concentration has a direct effect on reducing BOD5 and improving biodegradability. As can be seen, BOD5/ COD at concentrations of 100–400 mg/h increased from 0.34 to 0.39. In the SOP process, electrical conductivity increased from 4.87 to 5.4 mS/
cm and oxidation-reduction potential increased from 276 �8.9–27 mV (Fig. 5). In EC/SOP reactor, electrical conductivity increased. In com- parison, the ORP was increased toward positive values.
3.3. Effect of initial pH
Regarding the importance of pH in the efficiency of treatment pro- cesses, we studied the effect of the initial pH values of 3, 5, 7, and 9 on the variable parameters. The results in Fig. 6 indicate that each of EC, SOP, and EC/SOP processes at an operating pH had a specific maximum COD concentration reduction. In addition, in the EC/SOP process, the
Table 1
Characteristics of landfill leachate utilized in the experiments.
Parameter Value
COD (mg/L) 11,387.2 �146.12
BOD5 (mg/L) 3110.1 �34.46
pH 9.3 �0.26
EC (mS/cm) 4.8 �0.13
ORP (mV) 276 �8.9
Table 2
Characteristics of the reactor used in each experiment.
Parameter Value
Height 30 cm
Internal diameter 8 cm
Type of electrode Iron
Effective surface of electrodes 70cm2
Distance of electrodes 1 cm
Electrode array Paralled, monopolar
Inlet wastewater volume 400 mL
minimum BOD5 concentration (i.e., 1562.146 mg/L) and the maximum BOD5/COD ratio (0.439) was also achieved at pH value of 3. As shown in Fig. 7, in the EC process, electrical conductivity is lower than raw leachate electrical conductivity, while it is higher at neutral pH, at which ORP is also more negatively affected. In the SOP process, elec- trical conductivity and ORP increased with increasing pH while at a natural pH, they reached the maximum levels of 5.53 mS/cm and 110 mv, respectively. In comparison, in the EC/SOP process, electrical conductivity and ORP reduced with increasing pH to neutral levels.
3.4. Effect of reaction time
The reaction time is considered an important factor in treatment processes, which is economically significant. Hence, the time of 1 to 6 h was studied and COD and concentration and /COD ratio were measured.
According to the results shown in Fig. 8, the minimum COD concen- trations in EC, SOP, and EC/SOP processes after 6 h of reaction time
were 8463.95, 5956.62, and 2973.146 mg/L, respectively. In EC/SOP process, the minimum concentration of (1343.9 mg/L) and the maximum /COD ratio (0.452) were obtained after 6 h of reaction time.
Electrical conductivity and ORP, which were measured over time, indicate a reduction in these values in the EC process and increase in the SOP and EC/SOP processes (Fig. 9). However, its rate was reduced at the higher time. The obtained results in Fig. 10 shown the SOP, EC and EC/
COP efficiency for COD and BOD5 removal.
4. Discussion
4.1. Effect of current density
In the EC/SOP process, the presence of ozone together with the dissolved iron produced in the reactor not only provides the pollutant oxidation by ozone but also the conditions for homogenous catalytic ozonation, according to Eqs. (1)–(3) (Song et al., 2008):
Fe2þþ O3→ðFeOÞ2þþO2 (1) FeO2þþH2O→Fe3þþ HO�þ OH (2) Fe3þþ O3→FeO2þþ HO�þ O2þHþ (3) Fig. 1. Schematic of the reactor used in the EC/SO process.
0.329
0.335
0.342
0.357
0.315 0.32 0.325 0.33 0.335 0.34 0.345 0.35 0.355 0.36 0.365
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000
0.35 0.7 1.07 1.42
BOD5/COD
Concentra�on (mg/L)
Current density (mA/cm2)
COD-EC COD-EC/SOP BOD5- EC/SOP BOD5/COD- EC/SOP
Fig. 2.Effect of various current density on BOD5/COD ratio (operating conditions: ozone concentration 200 mg/h, initial pH: 7, reaction time: 2 h Fig. 3. Effect of various current density on electrical conductivity and oxidation-reduction potential (operating conditions: ozone concentration 200 mg/h, initial pH: 7, reaction time: 2 h).
As mentioned earlier, AOP methods have high potential to improve biodegradability. Given that in raw leachate the BOD5/COD ratio is about 0.27, this wastewater is considered as low biodegradable waste- water. Hence, the application of biological methods will not be effective for its treatment. However, with the pre-treatment of the leachate by EC/SOP method, the BOD5/COD ratio in the current density of 1.42 mA/
cm2 increased to 0.357. Regarding the changes in electrical conductiv- ity, its reduction in EC process can be attributed to the coagulation of soluble ions during the process. On the other hand, the increase of ozone concentration in EC/SOP process can be attributed to the fact that degrading high molecular weight organic compounds by ozone and hydroxyl radicals and their conversion to ions and shorter chained compounds generally increase the system electrical conductivity (Nad- deo et al., 2009). With increasing the current density values, due to higher concentrations of iron soluble ions and effective production of radical hydroxyl through the COP process, electrical conductivity also was increased. The oxidation-reduction potential of the EC process, contrary to EC/SOP process, has become less negative due to providing the conditions for reduction and consumption of oxygen, which does not have a consistent trend. In this regard, it has to be noted that ORP values 0.34
0.357
0.37
0.389
0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
100 200 300 400
BOD5/COD
COD,BOD5(mg/L)
Ozone cocentra�on(mg/hr)
COD-SOP COD- EC/SOP BOD5- EC/SOP BOD5/COD- EC/SOP
Fig. 4.Effect of various ozone concentrations on BOD5/COD ratio (operating conditions: current density: 1.42 mA=cm2; initial pH: 7, reaction time: 2 h).
Fig. 5.Effect of various ozone concentrations on electrical conductivity and oxidation-reduction potential (operating conditions: current density: 1.42 mA=cm2; initial pH: 7, reaction time: 2 h).
0.439
0.4
0.389 0.393
0.36 0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.44 0.45
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000
3 5 7 9.3
BOD5/COD
Concentra�on(mg/L)
Ini�al pH
COD- EC COD- SOP COD-EC/SOP
BOD5- EC/SOP BOD5/COD- EC/SOP
Fig. 6.Effect of initial pH on BOD5/COD ratio (operating conditions: current density: 1.42 mA=cm2; ozone concentration: 400 mg/h, reaction time: 2 h).
are sensitive to the amount of dissolved oxygen in the system and vary over time for various reasons such as temperature and mixing effects. In the EC/SOP process, for obtaining the predominant oxidation reactions, the ORP values have increased in the system.
4.2. Effect of ozone concentration
Landfill leachate contains a humic substance that has less tendency to react with ozone and substances that are more aliphatic and less ar- omatic (Wang et al., 2004). Therefore, single ozonation cannot signifi- cantly reduce COD removal efficiency. The process efficiency was also improved with integrated EC and SOP processes and effectively imple- menting COP process. Landfill leachate treatment with ozonation and its combination with hydrogen peroxide was reported by Cortez et al.
(2010). They found that with increasing ozone concentration, COD removal efficiency was improved. The maximum COD removal effi- ciency at the concentration of 112 mg/L after 1 h of reaction time was
23%. However, with the addition of 600 mg/L of hydrogen peroxide, COD removal efficiency was increased to 63%. Their results show that in the absence of hydrogen peroxide, BOD5 concentration was increased due to the breakdown of large resistant molecules into smaller mole- cules; in this condition, BOD5/COD ratio also was improved. But, with the addition of hydrogen peroxide, BOD5 was reduced and the BOD5/- COD ratio was increased from 0.01 to 0.17, probably due to the effective production of hydroxyl radicals during the process. In the present study, by performing EC/SOP process, iron ions in the system were effective on degradability of ozone and radical hydroxyl production through ho- mogenous catalytic ozonation process (Eqs. (1)–(3)). As a result, these hydroxyl radicals significantly reduced COD content and with degrading complex compounds present in the leachate, also BOD5 declined. Under these conditions, the biodegradability index also was improved and increased to 0.39 at a concentration of 400 mg/L of ozone.
The changes in electrical conductivity in both SOP and EC/SOP processes have a rising trend, indicating the degradability of the Fig. 7.Effect of initial pH on electrical conductivity and oxidation-reduction potential (operating conditions: current dencity: 1.42 mA=cm2; ozone concentration:
400 mg/h, reaction time: 2 h).
0.42
0.439
0.449 0.45 0.451 0.452
0.4 0.41 0.42 0.43 0.44 0.45 0.46
0 2000 4000 6000 8000 10000 12000
1 2 3 4 5 6
BOD5/COD
Concentra�on(mg/L)
Reac�on �me(hr)
COD- EC COD- SOP COD- EC/SOP
BOD5- EC/SOP BOD5/COD- EC/SOP
Fig. 8.Effect of reaction on BOD5/COD ratio (operating conditions: current dencity: 1.42 mA=cm2; ozone concentration: 400 mg/h, Initial pH (EC:7, SOP: 9.3, EC/SOP:3).
compounds in waste leachate. Nevertheless, considering the greater ef- ficiency of EC/SOP process due to the COP process, destruction of resistant pollutants in this process and electrical conductivity is higher.
Increasing the ORP is also rational in both processes. In general, ozone reacts directly and indirectly through oxidation route. In indirect oxidation, ozone is degraded and radical hydroxyl is produced according to Eqs. (4) and (5) (Von Gunten, 2003):
O3þ H2O→2HO:2 (4)
O3þ HO:2→HO:þ2O2 (5) Therefore, as oxygen is generated in the system, ORP increases during SOP process. Furthermore, in the EC/SOP process, electro- chemical process-dependent mechanisms consume oxygen while SOP and COP mechanisms produce oxygen. As a result, while ORP is also increased in this process, it is less than the values of the SOP process. In other studies that used ozone, system ORP was increased (Tango and Gagnon, 2003).
4.3. Effect of initial pH
Generally, at the range of neutral pH, most of Fe3þ ions formed through the electrochemical process generate iron hydroxide flakes that can rapidly coagulate and remove polluting molecules. The electrical conductivity at this pH has reached its minimum value, probably due to a more effective coagulation and reduction in the ions present in the leachate. However, the ORP is also affected by pH and irrespective to the process type, the oxidizing substance and its concentration are reduced with increasing pH value. In the electrochemical process, the anode is oxidized and the cathode is reduced. Due to the prevailing conditions of the reduction, the reduction in ORP is more pronounced at pH levels that the electrochemical process occurs effectively. Regarding the SOP pro- cess, with increasing pH and subsequently increasing the concentration OH as the primer of ozone degradability, the rate of ozone degradability was increased. According to Eqs. (6) and (7), this increase leads to the formation of much more potent and active secondary oxidants than ozone molecule, especially hydroxyl radicals (Chaturapruek et al., 2005):
O3þ OH →HO2 þ O2 (6)
O3þ HO2 →intermediateproduct O�3 ; O�2 ; OH��
(7) The radical oxidation potential of hydroxyl is equal to 2.33 V, while ozone oxidation potential is 2.07 V. As a result, for alkali pH value, the reduction in COD concentration is more than acidic conditions. Tizaoui et al. reported that the maximum removal efficiency of COD at the initial pH 8.7 was equal to 27% (Tizaoui et al., 2007). However, at natural pH, the biodegradability and oxidation processes are more efficient. With integrating of EC and SOP processes, the reduced COD concentration was improved and peaked at an initial pH of 3. As shown in Fig. 6, the minimum BOD5 concentration and the maximum BOD5/COD ratio were obtained at the same pH value. In general, due to the greater solubility of iron ions at this pH, homogeneous catalytic ozonation provides the effective formation of hydroxyl radicals under acidic conditions (Nawrocki and Kasprzyk-Hordern, 2010).
4.4. Effect of reaction time
The optimum time is an important factor in a treatment process.
Moreover, the optimum time is affected in economic cost of the process.
In this study, the reaction time of 3 h was obtained as the optimal re- action time. The same results in reducing electrical conductivity by electrocoagulation process was reported for removal of heavy metals.
Morover, in our last study for reducing nitrate using electrocoagulation we have reported that the ORP decreases over time (Hoseinzadeh and Rezaee, 2015).
5. Conclusions
According to the obtained results, integrated EC/SOP process showed high efficiency in the reducing of COD concentrations compared to the EC and SOP processes and resulted in an increase in the biode- gradability index of landfill leachate. In the optimum condition, the BOD5/COD ratio increased from 0.27 to 0.45. The ORP and electrical conductivity results showed that, according to the type of process and the experiment conditions, the parameters are varying. In the EC pro- cess, the reduction condition was dominant while in the processes of SOP and EC/SOP, the ORP system was increased, probably due to the oxidation reactions. Furthermore, electrical conductivity was increased due to the decomposition of the compounds in the landfill leachate in Fig. 9.Effect of reaction time on electrical conductivity and oxidation-reduction potential (operating conditions: current dencity: 1.42 mA=cm2; ozone concen- tration:400 mg/h, reaction time: 2 h).
the EC/SOP process and reached 6.6 at optimum conditions, suggesting the proper functioning of the treatment process.
Acknowledgements
The authors gratefully acknowledge the financial and technical support provided by Tarbiat Modares University, Tehran, Iran.
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