Simultaneous Application of Fenton – Electrochemical Reactor for Removal of Organic Loading in Biological Waste Sludge

Introduction
            Wastewater treatment using activated sludge process (ASP) produces huge amounts of sludge that must be treated by efficient methods. Wastewater sludge handling is one of the most important economic-technical challenges of wastewater treatment projects. Conventional methods for this purpose are very time consuming, expensive and in lots of cases have limited success. In recent years the electrochemical oxidation techniques have been progressively used for water and wastewater treatment process. Many reports showed that these methods are very effective to eliminate or decrease the organic loading content of wastewater and sludge. Combination of electrochemical reactions and Fenton process has widely been studied for the destruction of organics and bio-refractory pollutants contained in industrial wastewater by highly oxidative hydroxyl radicals formed from reaction of H₂O₂ with Fe²⁺. This method offers more advantages than the chemical Fenton process due to the high efficiency of Fenton reagents (e.g., H₂O₂) utilization and lower retention time. The integrated Fenton and electrochemical reactor were used to reduce concentration of volatile suspended solids in sludge. High consumption rate of divalent iron ions is one of the disadvantages of Fenton method compared with its reduction. In this research, the electrochemical method has been used to solve this problem. The electrochemical method increases the reduction rate of divalent iron by converting the trivalent iron ions into the bivalent ones and it reduces the production of iron sludge. In this research, the effective parameters including ratio of iron to hydrogen peroxide, pH, retention time, concentration of hydrogen peroxide and the amount of electric current were investigated.
 
Materials and methods
In the present study we have evaluated the efficiency of Fenton-Electrochemical process in reduction of organic contents of Secondary Sludge of Mahallati ASP wastewater treatment plant. This plant is located in Tehran, with treatment capacity of 4800m³ per day and operates in extended aeration process. Table 1 shows the specifications of the investigated sludge. Our reagents were H₂O₂ and Fe²⁺.
 The evaluated Fenton-Electrochemical reactor has been shown in figure1. The reactor contains two grafit anodes and two grafit cathods. The electrodes dimentions were 1mm*140mm*60mm, and the distance between them is 1.5 cm. The volume of reactor has been selected 0.9 L. We used the mechanical mixer  instead of electromagnetic stirer because of the sensitivity of Fe²⁺ ions to electromagnetic materials. The temperature during the test was between 13.8 and 15.6 ˚C. All tests were conducted according to standard methods (APHA, 1999) and each part of experiments was repeated three times.
 




property


Value




pH


6.53-7.33




Temperature(˚C)


13.8-15.6




COD (mg/lit)


6051-11164




VSS ( mg/lit)


3536-7782




VS ( mg/lit)


4086-8382




TSS (mg/lit)


4088-8432




VSS/TSS


0.86-0.92




 
  
Figure 1. the evaluated EF reactor                                            Table 1. Investigated Sludge Specifications
 
Result and discution
            The removal of volatile suspended solids (VSS) as the efficiency factor haveevaluted the effects of the ration of Fe²⁺ concentration on H₂O₂ concentration ( [Fe²⁺]/[ H₂O₂]), time,  pH, H₂O₂ concentration and current density as the effective process operation parameters. To obtain the most efficient we have investigated the operational parameter serily. The firest parameter that is studied is [Fe²⁺]/[ H₂O₂] . Figure 2 showes the effect of [Fe²⁺]/[ H₂O₂] to removal of VSS. Regarding to the diagram, the removal of VSS increases sharply as [Fe²⁺]/[ H₂O₂] increases to 0.58 (optimum point) and reach its maximum in this point and is constant to about 1.015 and then decreases slightly. The second parameter is pH. It has been studied in the optimum point of [Fe²⁺]/[ H₂O₂]. Adjustment of pH has been done by 98% sulfuric acid. Figure 3 shows that the removal of VSS is constant by increasing the pH up to 3, and then starts to decrease. The maximum VSS removal (approximately 72%) was attained in pH=3.1. Oxidation efficiency essentially decreases at pH values lower than 3. Figure 4 shows the relation of VSS removal and time. The time was assessed in the maximum point of the last two parameters. According to the diagram, the VSS removal increases by time and reaches its maximum in approximately 6 hours and then is constant.
 
                    
Figure 2. VSS removal and [Fe²⁺]/[ H₂O₂] relationship   Fig.3: VSS removal and pH relationship    Fig.4: VSS removal and time relationship
   
In order to attain the best concentration of Fe²⁺ and H₂O₂, we studied the effect of H₂O₂ concentration on VSS removal. The optimum ratio of [Fe²⁺]/[ H₂O₂] can be attained from figure2. Fe²⁺concentration are also resulted by determining the optimum H₂O₂ concentration. The tests carried out in the most effective point of prior parameters and the results has been shown in figure 5. According to the diagram, the highest VSS  removal was obtained when the H₂O₂ concentration is approximately 1568mg/L. Finally, after determining the optimum point for ( [Fe²⁺]/[ H₂O₂]), detention time,  pH, H₂O₂ concentration, and the effect of Current density was evaluated. Current density improves the reduction of trivalent iron to divalent iron, i.e., it enhances the efficiency of Fenton process. According to figure 6, the maximum VSS removal  has been reached in (650-750) mA current density. The remained Fe²⁺ concentration has also been investigated. As shawn in the figure 7, approximately (40-70)% of inserted [Fe²⁺] adsorbed on the electrodes, (7-15)% was remained in the sludge layer and (23-45)% in the sludge liquor.
 
                               
 Figure 5. VSS removal and [ H₂O₂] relationship              Figure 6. VSS removal and current density relationship               Figure 7. Remained and consumed [ Fe²⁺]relationship
 
Fenton and electrochemical-Fenton were compared on an equal basis. According to the figure 8, the efficiency of electrochemical-Fenton is 76%, while the efficiency of Fenton is 22%. Figure 9 shows the current density of energy consumption. By increasing the current density rate, energy consumption increases exponentially.
 
                                                                   
Figure 8. VSS removal efficiency of Fenton and Fenton-electrochemical regarding detention time              Figure 9. Current density of energy consumption in Fenton-                                                                                                                                                                        electrochemical
 
Conclusions
Even if in the recent decades many efforts have been done in order to enhance usual sludge treatment or commence novel more efficient methods, lots of researches have focused on techno-economical issues carefully. The key purpose of the researches is to eliminate the organic load of sludge and improve its quality in order to be reused in various applications like fertilizer in agriculture. In this study we have evaluated a combined Fenton-Electrochemical process with the aim of stabilizing excessive biological sludge of municipal wastewater treatment plant. For this purpose, several determining parameters have been selected and investigated in a lab-scale pilot. The results showed that the system has preserved its stability in a wide range of pH (3-7), also the organic load of the sludge decreased noticeably (maximum decreasing efficiency of the sludge was approximately 81%). Comparison of the results of Fenton or other usual systems and combined Fenton-Electrochemical process showed that the later system is appropriate and efficient method to get better quality of stabilized sludge and operational conditions.