2013-2-Godini

JRHS 2013; 13(2): 188-193

Copyright © Journal of Research in Health Sciences

Treatment of Waste Sludge: A Comparison between Anodic Oxidation and Electro-Fenton Processes

Kazem Godini (MSc)a†, Ghasem Azarian (MSc)b†, Ali Reza Rahmani (PhD)c*, Hassan Zolghadrnasab(BS)b

a Department of Environmental Health Engineering, Faculty of Health, Ilam University of Medical Sciences, Ilam, Iran

b Department of Environmental Health Engineering, Faculty of Health, Hamadan University of Medical Sciences, Hamadan, Iran.

c Department of Environmental Health Engineering, Faculty of Health and Research Center for Health Sciences, Hamadan University of Medical Sciences, Hamadan, Iran

The first and the second authors have the same contribution.

* Correspondence:Ali Reza Rahmani (PhD),E-mail: rahmani@umsha.ac.ir

Received: 16 March 2013, Revised: 04 May 2013, Accepted: 23 May 2013, Available online: 22 June 2013

Abstract

Background: Electrochemical methods, as one of the advanced oxidation processes (AOPs), have recently been applied to remove different contaminants from water and wastewater. This study compares the performance of anodic oxidation (AO) and electro-Fenton (EF) methods on waste sludge treatment.

Methods: This experimental study was performed on real sludge and the effect of operating parameters such as solution pH, operating time, current density, supporting electrolyte and hydrogen peroxide concentration were investigated in a batch reactor. For determination of oxidation and treatability of the sludge, chemical oxygen demand (COD) and total coliform (TC) removal were examined. Pb/PbO2 and iron electrodes respectively for AO and EF were applied.  

Results: Experimental data indicated for both AO and EF as the operating time and current density increased, COD removal increased. pH=4.0 and 3.0 and current density=1.75 and 2 A respectively for AO  and EF and the concentration = 57.2 mMol of hydrogen peroxide for EF were measured as the optimum amounts of these variables. The removal efficiency of COD in AO and EF process was 76% and 72%, respectively. Of course, the efficiency of EF in TC removal was better and the percentage of TC removal in 60 min for AO and EF was 99.0% and 99.9%, respectively. The amounts of consumed electrical energy for AO and EF were 8.6 and 28.0 kWh kg-1 COD, respectively.

Conclusions: AO was more effective in treatment and mineralization of waste sludge and TC removal than EF in terms of environmental economical features.

Keywords: Sludge treatment, Anodic oxidation, Electro-Fenton

Introduction

Recently advance oxidation processes (AOPs) have widely been applied to remove various pollutants particularly bio-refractory organic compounds 1,2; electro-Fenton (EF) and anodic oxidation (AO) are the two most common methods which have extensively been used to remove different pollutants from water and wastewater3-7. The two methods in which, in addition to direct oxidation of pollutants on the electrode's surface, pollutants are oxidized by indirect oxidation through free radicals in particular generated OH°, as non-selective oxidants, have high oxidation power 5-7. If the reactions are performed completely, organic matters are entirely destructed 2. In the case of AO, organic matters (R) are either directly oxidized on states oxides of the metal (MOx+1) or oxidized by means of adsorbed OH°; both mechanisms are dependent on the kind of electrode 8,9 (Eqs. 1-3).

R + MOx+1 RO + MOx

(1)

MOx + H2O MOx(OH°) + H+ +e

(2)

1/2R + MOx(OH°) 1/2ROO + H+ + e + MOx     

(3)

Hydrogen peroxide in EF is catalyzed by ferrous ion to generate OH° which is shown in Eq. 4 10,11.

Fe2+ + H2O2 Fe3+ + OH + OH°

(4)

OH° + organics H2O + production

(5)

In recent years, on account of industrial and urban development and consequently the growth of wastewater treatment plants, a huge amount of sludge is produced; sludge must be treated because it contains pathogens and chemical contaminates as well as organic matters. Biological and chemical conventional techniques have some limitations as follows: They are affected by environmental conditions such as temperature, pH and so forth 12. They require a high detention time to complete degradation and microorganisms leave intact many persistent organic contaminants13. Some methods in which chemicals are added, not only they add extra chemicals to sludge economical, but also they lead to a high volume of sludge which is not reasonable in terms of environmental and disposal situations. Moreover, adverse by-products may be generated particularly when chlorine compounds are applied 12, 14,15. However, on the other hand, in electrochemical ways the main reagents are electron and OH°. Moreover, they do not add and any adverse matter to solution and for the sake of environmental compatibility, versatility energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness 8,12,16 they are being addressed as alternative measures in sludge treatment; the AO and EF processes have enormous potential for this goal. To our knowledge, the application of the electrochemical method for treatment of sludge has not been seen before.

The object of this study was to investigate the efficiency of AO and EF processes in waste sludge treatment. In this regard, the effects of current density, operating time, supporting electrolyte, pH and hydrogen peroxide concentration were studied. These two ways were compared in terms of the removal amount of chemical oxygen demand (COD), total coli form (TC) and energy consumption.

Methods

This experimental study was carried out during September 2011 to September 2012. The used sludge was taken from the disposed sludge of the sedimentation tank of Tioran Company's wastewater treatment plant. All samples were transferred to the laboratory with polypropylene containers at the temperature of 4°C. Measured properties of raw sludge samples are as follows: COD=7150±750 (mg L-1), conductivity =1500±100 (μS cm-1), TC=15×106±1×106 (MPN/100 mL), pH=7.1±0.8. A 800 ml polyvinyl chloride electrolytic cell was used to perform electrolysis (effective volume was 500 ml). In the case of AO, Pb/PbO2 electrodes were used. In order to prepare Pb/PbO2, rod Pb were placed in sulfuric acid (10%) and for each cm2 of the electrode surface, a current density of 10 mA was passed for 90 min at the temperature of 25°C- which at the end of the time PbO2 layer was formed on the surface of the electrode (Eqs. 6 and 7) 17.

Pb + SO42 PbSO4 + 2e

(6)

PbSO4 + 2H2O PbO2 + SO42 + 4H+ + 2e

(7)

In the case of EF, iron electrodes were used. At both cases, dimensions of each electrode were 10 cm×1 cm, the electrodes were used in pairs by a monopolar arrange in the reactor (10 electrodes of each one were placed in the reactor). Effective electrode surface area was 200 cm2. The distance separating the electrodes was fixed at 1.5 cm. They were connected to terminals of a direct current power supply (Adak, ps_405. Hamadan Kit Co. Iran) which is characterized by the ranges 05 A for current and 025 V for voltage. A constant stirring speed of 300 rpm was applied during all experiments. The experimental setup of EF and AO are shown in Figure 1.

Hydroxide sodium and sulfuric acid were used to adjust pH; NaCl (4, 6, 8, 10, 12, 14 and 16 g L-1) and hydrogen peroxide (30% w/w) (22.9, 34.3, 45.7, 57.2, 68.6 and 80.1 mMol) were employed as supporting electrolyte and OH° source, respectively in EF. All chemicals were purchased from Merck Co. Germany. In this work, Fe2+ was electrically added to the solution as anode destruction. Sludge treatment was performed at the electrolytic cell with the following conditions: current density=0.5-3 A, pH=2-9 and operating time=15-120 min. A very important point to make here is that all parameters were optimized separately. So as to measure the efficiency of sludge treatment, the levels of COD and TC reduction were calculated by using Eq. 8 12.

R (%) = ((Xi Xt)/Xi) × 100                        (8)

where, Xi and Xt are the concentrations of the COD (mg L-1) and TC (MPN/100 mL) in the feed and the treated solutions, respectively.

The energy consumption (kWh kg-1 COD) was calculated from Eq. 9 18.

(9)

Where V is the average cell voltage (V), VR is the solution volume (L), C is the difference in COD in mg L1, I is the applied current (A) and t is the electrolysis time (S).

The COD and TC were measured according to the standard methods for the examination of water and wastewater 19. Conductivity was measured by a lab Hach type conduct meter and pH was measured with a lab pH-meter (from Hach Co. Germany) electrometric method.

Figure 1: The experimental setup of electro-Fenton (a) and anodic oxidation (b) processes

Results

The variables were optimized as one at a time method12. First, bufferic pHs were used to investigate the pH variable effects. In the application of EF and AO methods, the highest COD removals were achieved at pH values of 3 and 4, respectively (Table 1), which were 72% and 76%, respectively. In both processes, COD removal decreased dramatically when pH value was over 5 or less 2. In both EF and AO, TC removal occurred under acidic conditions; over 99% of TC was removed when pH solution was less than 5. Of course, the EF process was more effective in TC removal than the AO process.

A rise in current density led to a rise in COD removal. Based on Figure 2 at the current density of 1.75 A in the AO process, COD was reduced from 7440 mg L-1 to 1910 mg L-1; at current densities more than this amount, there was not a more removal efficiency in COD removal and it raised only energy consumption. In contrast, in the case of the EF process, the highest COD removal was attained at the current density of 2 A, which COD was reduced to 2174 mg L-1 and when current density exceeded this figure, the amount of COD removal decreased- at current density of 2.5 A COD reached 2311mg L-1. At this current density and more than this iron was disposed from the anode's surface as brown sediment at the bottom of the reactor, which did not participate in the reaction and it wasted.

There was no need to an additive matter at the AO process by comparison with the EF process. NaCl was applied as a supporting electrolyte in the EF process. COD removal without the supporting electrolyte was only 45%. By 11.5 g L-1 of NaCl, which was selected as the optimum amount, COD removal reached to over 72% (Table 2).

In view of the fact that in EF hydrogen peroxide was injected to the reactor manually, according to Table 3 its optimum content was obtained at 57.2 mMol. The results showed COD removal fell when hydrogen peroxide concentration was over 57.2 mMol. Therefore, other experiments were conducted at current densities of 1.75 A for AO and 2 A for EF and NaCl=11.5 g L-1 and hydrogen peroxide=57.2 mMol only for EF processes, to investigate the variables of operating time and energy consumption. As Table 4 shows, the AO process was better in COD removal than the EF process; the highest removal efficiency was achieved at operating times of 75 and 120 min, respectively for AO and EF. It must be pointed that both methods were entirely effective in TC removal (99.9%). Similarly, the use of operating times longer than the optimum amount it wasted in energy consumption and did not cause an increase in the removal efficiency and was not economical as well. After all variables were optimized, consumed energy was calculated by using equation 7 for both AO and EF processes (Figure 3). The results illustrated the AO process had both higher removal efficiency and less energy consumption compared to the EF process. The amounts of consumed energy to reach the highest content of COD removal were 8.6 and 28.5 kWh kg-1 COD, respectively for AO and EF. Of course, this amount of energy was enough to remove TC completely.

Figure 2: The effect of current density on chemical oxygen demand removal by anodic oxidation and electro-Fenton processes; charged passed= 3600 C for anodic oxidation; and charged passed=7900 C, NaCl =10g L-1 and H2O2 = 57.2 mMol for EF process

Figure 3: The energy consumption for anodic oxidation and electro-Fenton processes; operation condition: Current Density=1.75 A and pH=4  for anodic oxidation, Current Density=2A, pH=3, NaCl=11.5g L-1 and H2O2=57.2 mMol for EF

Table 1: Effect of pH on chemical oxygen demand and total coliform removal by anodic oxidation and electro-Fenton processes with NaCl =10g L-1 and H2O2 = 57.2 mMol

Table 2: The effect of NaCl on chemical oxygen demand removal by the electro-Fenton process with operation condition including: Current Density=2A, Operating Times=120 min, H2O2=57.2 mMol, and pH=3

Table 3: The effect of H2O2 on chemical oxygen demand removal by the electro-Fenton process with operation condition including: Current Density=2A, Operating Times=120 min, NaCl=11.5g L-1, and pH=3.

Table 4: Effect of operating time on chemical oxygen demand and total coliform removal by anodic oxidation and electro-Fenton processes with operation condition including: Current Density=1.75 A and pH=4 for AO, Current Density=2A and pH=3, NaCl=11.5g L-1 and H2O2=57.2mMol for EF

Discussion

Under acidic conditions, both EF and AO had better removal efficiency to mineralize and treatment sludge. Different studies have suggested pH=3 for EF 2,10,20. According to Babuponnusami et al., an increase in efficiency in acidic pHs is on account of the fall in oxidation potential of HO° with a rise in pH. HO° is a weaker oxidant at near neutral pH than at acidic pH. When pH was raised beyond 7, HO° was quickly converted into its conjugate base °O and it reacted more slowly than HO° 10.Moreover, in the case of EF, the use of high pHs caused that Fe2+ transformed into Fe3+, which precipitated as brown sediment at the floor of the reactor and wasted without entering the reaction. This resulted in a decrease in removal efficiency. Under acidic conditions (pH value between 2.5 and 3.5), the degradation of hydrogen peroxide in order to generate OH° happens better 21. However, the efficiency decreased at very low pH values (<2) which is because of the fact that hydrogen ion acts as HO° radical scavengers 10.

Obviously, a rise in current density causes a rise in efficiency in accordance with Faraday law22, which is due to acceleration in direct oxidation of sludge on electrode's surface and more production of oxidant agents in the solution oxidizing sludge indirectly. However, if it exceeds the optimum amount (1.75 and 2 A for AO and EF, respectively), it will waste energy and consequently is not economical 23,24. This efficiency decline in high current densities is because of hydrogen peroxide oxidation on the surface of electrodes and the reaction between Fe2+ and OH° which Fe3+ is generated (see Eqs. 10 and 11) 2,10,20,25; in this case efficiency decreases.

H2O2 H+ + HO°2 + e                     (10) 

Fe2+ +OH° Fe3+ +HO                                (11)

In this study, the AO process was performed by Pb/PbO2 electrodes, which are from the electrodes of dimensionally stable anodes (DSAs) and a “non-active” anode 8; the upside of using this electrode is cheapness and easy preparation. The process has a very good efficiency without using any extra chemicals. In contrast, in the EF process in order to improve the efficiency, NaCl, as a supporting electrolyte, is used to produce active chlorine that as OH° participates in organic matters oxidation (Eqs. 12 and 13)2,8. It is required to optimize the concentration of hydrogen peroxide because the use of the amount more than the optimum amount not only does not take part in the reaction, but also reacts with OH° which hydroperoxyl radical (HOº2) is generated that has low oxidizing ability  2,11. Applied hydrogen peroxide concentration depends on sludge concentration; the study Virkutyte et al. in which was found that the concentration of hydrogen peroxide is associated to wastewater quality 21. The lack of pollutant removal after an optimal hydrogen peroxide concentration may be expressed by the competing reaction between hydrogen peroxide and OH° radicals when the hydrogen peroxide concentration is in excess  26.

2Cl Cl2 + 2e                                    (12)

Cl2 + H2O HClO + H+ + Cl              (13)

There are different mechanisms involving in the two methods, but the main mechanism in both methods is sludge oxidation on the anode surface by direct oxidation and indirect oxidation through creating oxidants; in AO organic matters are oxidized by absorbed OH° and in EF they are oxidized by free hydroxyl radicals. Besides, over the reactions in the processes many intermediate reactions occur which are effective in sludge oxidation. Active chlorine ions are examples of these reactions that play a role in the oxidation8,11.

By comparison with EF, AO consumed less energy for sludge treatment and COD removal. It is owing to the fact that sludge is treated in a less time (optimum operating time=75 min) and a lower current density (1.75 A) compared to EF (optimum operating time=120 and current density=2 A); in general, the AO had a better efficiency. However, in terms of TC removal both processes have a very good efficiency. Of course, it should be pointed the performance of EF is better which is because of the use of low pHs, production of active chlorine types that play an important role in germicide and the application of higher current densities.

In conventional systems (like perchlorine, electromagnetic wave, UV, and so forth) only disinfection takes place, but electrochemical processes are multiple because in addition to sludge oxidation they can remove microbes22. Previous studies have reported that these methods are entirely effective microbe removal22,27,28. In the case of electro coagulation, microbe removal occur by means of adsorption and trapping among flocks and as well as potential difference originated from current density27. Both AO and EF, in which there is potential difference and hydroxyl radical is produced, have a noticeable efficiency in microbe removal.

There is the issue of Pb corrosion and its entrance into sludge. However it should be noted PbO2 is resistance against corrosion; in this study the pure Pb was covered by a PbO2 layer which raised the resistance.   Corrosion is usually more at high current densities, but the range of which is between in which corrosion is little. Pb remained in the sludge lower than the standard value (0.005 mg/L).  The application of the PbO2 electrode was due to the following reasons: inexpensive material, commercially available and prepared easily and rapidly and as well as it has low resistivity, good chemical stability and a large area29,30.  Moreover, this electrode belongs to of dimensionally stable anodes (DSAs) electrodes; an example from this kind of electrodes is Boron doped diamond (BDD) having higher efficiency and lower corrosion, but this electrode is more expensive than the PbO2 electrode.

Conclusion

The results of the study are indicative of the fact that either EF or AO can be applied to treat wastewater works sludge and both processes have an approximately similar efficiency even though AO has a slightly better efficiency. Moreover, the AO process does not require extra chemical matter and is less dependent on operating conditions. On the other hand, EF needs an appropriate supporting electrolyte such as NaCl and quite a few parameters are effective in treatment efficiency. Variables of current density, hydrogen peroxide content and pH value must be optimized precisely; otherwise, there is a decrease in efficiency, for example, in the case of AO the use of current density over the optimum amount does not result in a higher efficiency and in the case of EF it has reverse results and the efficiency goes down. The bottom line is that AO is preferable to EF because it results in higher mineralization, and consumes less energy; in addition to these, variables can comfortably be controlled.

Acknowledgments

We are grateful to Hamadan University of Medical Sciences for providing Research materials, equipments and fund. This project received financial support from Vice Chancellorship for research Affairs of UMSHA (project No. 901216120, 9012164938). The authors gratefully acknowledge Mrs. Zohre Berizi for her assistance.

Conflict of interest statement

The authors declare that they have no conflicts of interest.

Funding

This study was funded by Hamadan University of Medical Sciences.

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