* For correspondence.
Journal of Environmental Protection and Ecology 18, No 2, 652–661 (2017)
Clean technologies – wastewater treatment
TREATMENT OF KRAFT PULP AND PAPER MILL
WASTEWATER BY
ELECTRO-FENTON/ELECTRO-COAGULATION PROCESS
A. ALTINa*, S. ALTINb, O. YILDIRIMb
aEngineering Faculty, Izmir Demokrasi University, 35 140 Izmir, Turkey
E-mail: ahmet.altin!idu.edu.tr
bDepartment of Environmental Engineering, Bulent Ecevit University,
67 100 Zonguldak, Turkey
Abstract. The present study aims to examine the electro-Fenton/electro-coagulation (EF/EC)
pro-cess for the treatment of wastewater generated from Kraft pulp and paper mills. As a result of the experimental studies, the optimum operating parameters of the EF/EC process have been identified as follows: current = 1.0 A, initial H2O2 concentration = 1000 mg/l and initial pH 2.5. It has been established that, under these conditions, the COD could be removed at 91.7%. It has been understood that the sludge produced from the EF/EC reactor possesses fairly good sedimentation characteristics and is significantly affected by the operating parameters. It has been established that the best sedi-mentation characteristics can be achieved when the current is ≥ 1.0 A, the initial pH value is between 2.3 and 3.2, and the initial H2O2 concentration is between 400 and 1800 mg/l.
Keywords: Kraft pulp and paper mill, advanced oxidation processes, Fenton,
electro-coagulation.
AIMS AND BACKGROUND
Pulp and paper industry is considered to be one of those industries that require a high volume of water consumption such as the chemical and metal industries1,2.
High water consumption, coupled with the recalcitrant organic substances origi-nating from wood such as lignin and the chemicals used during the production, brings about an extensive amount of wastewater as well3–6. It is a known fact that
such wastewater may contain 200 to 300 different organic substances and about 700 organic and inorganic substances7,8.
Electrochemical methods are known to be effectively employed for the treat-ment of toxic organic substances with low biodegradability. In order to improve the efficiency of electrochemical methods, hybrid systems where different oxidation processes (such as Fenton oxidation, photo-oxidation, electro-oxidation, etc.) are simultaneously managed within the same electro-chemical cell are frequently used. Fenton oxidation (FO) is distinguished from other oxidation methods thanks to its
high efficiency, short oxidation time, simple use, applicability for the treatment of a wide variety of substances, requirement of relatively less special equipment and sludge formation9. Known as a system where the chemicals used in the FO are
produced in a electrochemical cell and the coagulation takes place within the same reactor, the hybrid process of electro-Fenton/electro-coagulation (EF/EC) yields fairly good results in the treatment of wastewaters with high pollution load10–13.
Although such hybrid processes as EF (Ref. 14) and EC/electro-oxidation (EO) (Ref. 15) have been used for the treatment of wastewater caused by Kraft pulp and paper mill, we have yet to come across a study in the literature that has been conducted on the treatment of such wastewaters through EF/EC process. This study examines the treatability of wastewaters caused by Kraft pulp and paper mill with the EF/EC process that has never been used before for the treatment of such wastewaters. In this respect, the effects of various different operating parameters (such as treatment time, initial pH, initial H2O2 concentration, magnitude of the current being applied, etc.) on the treatment efficiency of the method and the op-timum operating conditions have been identified and the parameters that affect the sedimentation properties of the sludge formed at the end of the treatment process have been presented.
EXPERIMENTAL
The mechanism used in the study consists of three main units, namely the DC power supply (Good Will 4303), digital magnetic stirrer and EF/EC reactor (Fig. 1). Two cast iron (Fe) anodes and cathodes, each 4 × 5 cm in size, were used in the reactor.
Fig. 1. Experimental apparatus
For COD and SO4 analyses, the methods provided in the Standard methods16
were used. The H2O2 analysis, however, was conducted using iodometric method. The pH and electrical conductivity were measured by using a WTW-330i brand pH meter and WTW-315i brand conductivity meter, respectively.
In the experimental studies, the wastewater samples obtained from the plant were first put through a roughing filter in the laboratory environment. The EF/ EC reactor was fed with 0.5 l sample, and the pH value, 0.1 N H2SO4 and NaOH were adjusted to the desired values (2.5, 3.0, 3.5 and 4.0). And then a further 30% H2O2 was added to ensure that the amount was sufficient for the initial H2O2 con-centration (250, 500, 1000 and 1500 mg/l) required in the solution, and the EF/ EC experiments were launched by applying different direct current (DC) values (0.25, 0.50, 1.00 and 1.50 A). Throughout the experiments, samples were taken from the reactor at every 3rd, 5th, 10th, 15th, 20th, 25th and 30th min. Once the samples taken were centrifuged for 15 min at 5000 rpm, their COD and residue H2O2 amounts were measured.
RESULTS AND DISCUSSION
The wastewater samples used in the study were taken from the collector where the wastewater coming from both the pulp and paper units of the Kraft pulp and paper mill were accumulated, and they were kept under laboratory conditions at +4oC
until they were analysed. Prior to each experiment, the properties of wastewater were identified and the average values thus obtained are presented in Table 1. Table 1. General properties of the wastewater used
Parameter Average value Standard deviation
Conductivity (mS/cm) 9.46 ±0.71 pH 9.21 ±0.16 SS (mg/l) 901 ±86 COD (mg/l) 1286 ±251 BOD5 (mg/l) 580 ±110 SO4 (mg/l) 884 ±128
It is a known fact that the Fenton reactions occur at lower pH values. For instance, Zhang et al.16 suggested that an efficient pH range should be in between
2 and 4 to perform an effective FO. Another study conducted by Nitoi et al.18
sug-gested that the optimum pH value for the oxidation by the FO of nitroaromatic substances that are resistant to biodegradability is around 3. Ginni et al.4, on the
other hand, reported that the optimum efficiency in the treatment of paper industry wastewaters using solar photo-Fenton method is achieved when the pH value is 4. Based on the aforementioned studies, it can be argued that the pH value where the optimum oxidation efficiency is observed varies depending on the type of wastewater. Within the scope of this study, experiments were conducted with various different initial pH values (2.0, 2.5, 3.0, 3.5 and 4.0) in the treatment of Kraft pulp and paper mill wastewaters through the EF/EC process and the COD removal efficiencies thus obtained are presented in Fig. 2a.
time (min.) 0 10 20 30 CO D re m ov al e ffi ci en cy (% ) 0 20 40 60 80 100 initial pH=2.0 initial pH=2.5 initial pH=3.0 initial pH=3.5 initial pH=4.0 a initial pH 1.5 2.0 2.5 3.0 3.5 4.0 4.5 re sid ua l H2 O2 (% ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 3th min. 5th min. 10th min. 15th min. 20th min. 25th min. 30th min.
Consumption rate of H2O2 is high
b initial pH 2 3 4 5 6 fin al p H 2 3 4 5 6 7 8 3th min. 5th min. 10th min. 15th min. 20th min. 25th min. 30th min.
Effective pH zone for electro- Fenton Effective pH zone for
electro-coagulation Transition zone
c
Fig. 2. Effect of initial pH on: COD removal observed in the reactor – a; residual amount of H2O2
in the reactor – b; change of pH observed in the reactor – c (current = 1.0 A, initial H2O2 concentra-tion = 1000 mg/l)
As can be seen in Fig. 2a, the degradation rate of organic substances during the first 5 min of the treatment process appears to be very high, after that it gradu-ally declines. The reason behind this high velocity and subsequent decline is that the H2O2 inside the reactor is decomposed rapidly through the Fe2+ that is released
anodically into the environment and it forms a high amount of hydroxyl radicals. The residual amounts of H2O2 measured throughout the trial seem to support this assertion. For instance, with lower initial pH values, more than 80% of the H2O2 inside the reactor is consumed from the first 5 min onwards (Fig. 2b). Another conclusion to be drawn from Fig. 2a is that the oxidation efficiency decreases as the initial pH increases (except for pH 2.0). It was observed in the experiments that the best COD removal value (91.7%) was achieved when the initial pH was 2.5.
When the final pH values presented in Fig. 2c are considered, it can be argued that the effectiveness of FO was maintained for 15 min in the trials conducted with lower initial pH values (between 2.0 and 3.0) and that EC was more dominant in
the reactor from the first 3 min onwards in the trials with higher initial pH values (≥ 3.5).
In the EF method, the main source of hydroxyl radicals to be generated in the reactor is H2O2. However, the use of higher H2O2 concentrations makes the method very costly. For this reason, the changes in the efficiency of the method for different H2O2 concentrations were observed and the results obtained are pre-sented in Fig. 3. Since, theoretically, the amount of hydroxyl radicals produced in the system will increase in proportion with the increase in H2O2 concentration; the treatment efficiency will increase proportionally as well. As expected and presented in Fig. 3, the COD removal increased in proportion with the increase in the initial H2O2 concentration of the system. Nearly 93% COD removal efficiency were achieved for 1500 mg/l H2O2 concentration in the 10th min (Fig. 3a). The same efficiency rate can only be achieved within a 20 min treatment period for 1000 mg/l concentration. time (min) 0 5 10 15 20 25 30 C O D re m ov al e ffi ci en cy (% ) 0 20 40 60 80 100 250 mg l–1 500 mg l–1 1000 mg l–1 1500 mg l–1 a initial H2O2 (mg/l) 0 500 1000 1500 2000 se si du al H2 O2 (% ) 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 3th min 5th min 10th min 15th min 20th min 25th min 30th min
High consumption rate for H2O2 b time (min) 0 5 10 15 20 25 30 pH V al ue 2 3 4 5 6 7 8 250 mg l–1 500 mg l–1 1000 mg l–1 1500 mg l–1
Electro-Fenton process is dominant in this region c
Fig. 3. Effect of initial H2O2 concentrations on: COD removal observed in the reactor – a; residual
amount of H2O2 in the reactor – b; change of pH observed in the reactor – c (current = 1.0 A, initial pH value = 2.5)
Moreover, the residual amount of H2O2 concentration in the reactor also in-creases in case of higher H2O2 concentrations (Fig. 3b). The residual H2O2 in the solution may cause a buffering effect for pH and thereby delay the rise of pH (Fig. 3c) and help increase the effectiveness of EF in the treatment process up to 25 min. However, the EF effectiveness failed to produce a significant improvement in COD removal for higher H2O2 concentrations from the 5th min onwards. This means that some resistant organic substances still can not be decomposed and demineralised through FO. The underlying reason for this is the formation of peroxy radicals (HO2•) with lower oxidation power in extreme H
2O2 concentrations (reaction (1)).
OH• + H
2O2 → HO2• + H2O (1)
2HO2• → H
2O2 + O2• (2)
The increase in the residual amount of H2O2 in higher H2O2 concentrations, on the other hand, may be the result of a H2O2 regeneration occurring in reaction (2) (Ref. 19). For this reason, it is a good practice to shy away from very high concentrations for the purpose both of reducing the cost of the method and of avoiding the residual H2O2 concentration in discharge waters.
The weak acid properties of H2O2 creates a buffering effect in the reactor, which in turn causes it to remain within the effective pH range for EF for longer period of time (approximately 15 to 25 min). This can be observed more clearly in Fig. 3b where the residual amounts of H2O2 are shown. That being said, when the amount is increased from 1000 to 1500 mg/l, the overall treatment efficiency (92.7%) will not be changed much despite an increase in the degradation rate of organic substances. The underlying reason for this is the formation of peroxy radicals (HO2.) with lower oxidation power in extreme H
2O2 concentrations
(reac-tions (1) and (2)).
Another parameter that is influential on the costs and efficiency of the EF/EC hybrid process is the magnitude of the current being used. In this respect, the effects of direct current (0.25, 0.5, 1.0, 1.5 A) on the treatment efficiency was monitored throughout the experimental studies (Fig. 4). As can be understood from Fig. 4a, the overall COD efficiency improves in proportion with the increase in the magnitude of the current being applied. The highest treatment efficiencies were achieved in the 1.0 A magnitude. Further increase in the current did not bring about a positive impact on the treatment efficiency. In lower current applications, however, only EF appeared to be effective in the reactor (Fig. 4c). The rise of pH slows down and the formation of Fe(OH)3 is inhibited due to high residual H2O2 concentrations and the insufficiency of OH- concentration produced following
the electrolysis process. This eliminates or diminishes the effectiveness of the EC process that should develop in the reactor.
rime (min) 0 5 10 15 20 25 30 C O D re m ov al e ffi ci en cy (% ) 0 20 40 60 80 100 0.25 A 0.50 A 1.00 A 1.50 A a current (A) 0,0 0,5 1,0 1,5 2,0 re si du al H2 O2 (% ) 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 3th min. 5th min. 10th min. 15th min. 20th min. 25th min. 30th min.
High consumption rate for H2O2 b time (min) 0 5 10 15 20 25 30 pH v al ue 2 3 4 5 6 7 0.25 A 0.50 A 1.00 A
Electro-Fenton process is dominant in this region c
Fig. 4. Effect of current values on: COD removal observed in the reactor – a; residual amount of
H2O2 in the reactor – b; change of pH observed in the reactor – c (initial H2O2 concentration = 1000 mg/l, initial pH value = 2.5)
One of the biggest advantages of the FO and EC processes, compared to the traditional oxidation and coagulation processes, is the fairly good sedimentation characteristics of the flocks created as a result of the reaction and the formation of less chemical sludge2,9. In order to monitor the relationship between the operating
parameters and the sedimentation characteristics of the sludge formed as a result of the EF/EC process, a sludge volume index (SVI) parameter, which is successfully used for the purpose of assessing the sedimentation characteristics of biological sludge, was used (Fig. 5).
If the sludge volume index (SVI) is < 100 ml/g in the sludge produced from biological treatment processes, one may think that the sedimentation characteristics are good11. As can be seen in Fig. 5, the process produces sludge with very good
sedimentation characteristics featuring DC ≥ 1.0 A, initial pH value between 2.3 and 3.2, and initial H2O2 concentration between 400 and 1800 mg/l. However, one should not lose sight of the fact that the amount of sludge obtained through the
EF/EC hybrid process will increase with the increase in the applied current as per Faraday electrolysis laws and thus avoid using high currents.
initial H2O2 concentration (mg/l) 0 500 1000 1500 2000 2500 SV I ( m l/g ) 0 100 200 300 SV I ( m l/g ) 0 100 200 300 initial pH 1,5 2,0 2,5 3,0 3,5 4,0 4,5 current (A) 0,0 0,5 1,0 1,5 2,0 2,5 H2O2vs SVI Current vs SVI pH vs SVI The best settling zone
Fig. 5. Relationship between the operating conditions in EF/EC hybrid process and SVI
In this study, the best treatment efficiency was observed at: initial H2O2 concentration of 1500 mg/l, treatment time of 10 min and initial pH value of 2.5. However, in order to ensure operation at lower operating costs, it will be more cost effective to operate with 1.0 A current rating, 1000 mg/l initial H2O2 concentration and treatment time of 20 min.
CONCLUSIONS
In the present study, it was aimed to treat the Kraft pulp and paper mill wastewater by implementing the EF/EC process. According to the results obtained from the experimental studies, it was established that the hybrid process of EF/EC produced better COD removal results in the treatment of the wastewater compared to the biological treatment methods. However, the number of parameters that have a bear-ing on the efficiency of the method is rather high. For this reason, care should be taken in choosing such operating parameters as the treatment time, initial pH, initial H2O2 concentration and the magnitude of the current to be applied. The optimum operating conditions in this study were identified as follows: current = 1.0 A, initial pH = 2.5, initial H2O2 concentration = 1000 mg/l and treatment time = 20 min. It
was also established that the sludge formed under such conditions produced bet-ter sedimentation characbet-teristics. The high amount of H2O2 concentration used in optimum conditions restricts the use of this method in large scale applications. For this purpose, it will be useful to focus on process improvements intended for increasing spontaneous H2O2 formation reactions in the future studies.
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Received 6 May 2017 Revised 31 May 2017
