Upgrading of the Fenton and electrochemicalcombined reactor (Fered-Fenton) for optimumwaste-activated sludge stabilization and energy consumption

Introduction
Activated sludge process is one of the most common methods used in wastewater treatment plants. Regarding the large volume of sludge obtained from such biological wastewater treatment processes and considerable costs of its treatment and disposal, extensive research has been and is being done on novel effective methods of sludge stabilization. In previous investigations research group introduced Fered-Fenton reactor as a novel efficient system for waste-activated sludge stabilization. Reactions (1) and (2) are the most important and effective reactions occuring in this system so that hydroxyl radical which is the main oxidation factor of organics be produced.
                                           (1)
                        (2)
In this study reengineering, optimizing and upgrading of the reactor in stabilizing waste-activated sludge obtained from urban wastewater treatment (case study: waste-activated sludge in Shahid Mahallati Treatment Plant) is focused on by investigating effective functional parameters including electrodes material, inflow organic concentrations, interelectrode distance, number of Fenton's reagent injection, sodium sulfate concentrations (the factor of electrical conductivity generation in the medium), and the ratio of electrodes surface to system volume. Besides, due to the important role of electrical energy consumption in electrochemical issues and the efforts to minimize it, analysis and estimation of electrical energy consumption in the system and its comparison to sludge aerobic digestion is discussed.
Materials and Methods
Pilot studies were conducted in 2014. Excess sludge samples were provided from return activated sludge site of Shahid Mahallati wastewater treatment plant. Pilot reactor was a Plexiglas cylinder of 0.9 liter, embedding two anodes and two cathodes. Electrodes material was selected of iron, stainless steel and graphite. Electrodes dimensions, setting depth in sludge and contact surface of each electrode with sludge are 140 60 1, 100 and 100 60 mm, respectively. Interelectrode distance varied. Stirring in reactor was done by using an electrical engine (Zheng,zs–ri, 6(V) DC, 366 rpm). Using magnetic stirrer was relinquished because of its negative effect on iron ion and its catalytic function in Fenton process. To adjust rector amperage, digital power supply (Mps,DC–3003D, 0-3 (A), 0-30 (V)) was used. Chemicals including ferrous sulfate and hydrogen peroxide (fenton's reagent), sulfuric acid and sodium hydroxide (to adjust pH), sodium sulfate (to generate electrical conductivity), and filter papers (No. 42) were provided from Merck and Whatman company, respectively.

Primarily, sludge pH was adjusted by sulfuric acid and sodium hydroxide. Then an initial sample of 50 cc was taken to measure initial VSS. Next, ferrous sulfate and hydrogen peroxide were injected to the reactor. Electrodes were set in the reactor after connecting to the power supply and current intensity was then adjusted. After 240 minutes, a secondary sample was taken from reactor depth of 80 mm and voltage was measured during the experiment. All the experiments were conducted according to standard method and each set of experiments were repeated three times to control errors.

*Corresponding author: Tel: +989121430209                                      E-mail: g.badalians@yahoo.com
 
Discussion of Results
Regarding the conducted experiments and investigations, the following results has been obtained:
- As concentrations of inflow organics increases, reactor efficiency rises gently; thus only a four-percent rise is obtained in the range of 3500-5000 mg/lit.
- Due to the highest reactor efficiency, being environmentally friendly, leaving no residual in medium and thus causing no medium contamination, availability, reasonable operation cost, not being corroded and therefore appropriate for long use, graphite is selected the best electrode material among electrodes of stainless steel, iron and graphite.
- Interelectrode distance was experimented in the range of 0.5-2.25 cm. The effect of interelectrode distance on reactor efficiency is a downward quadratic equation which the highest reactor efficiency is obtained at 1.5 cm.
-  The number of Fenton's reagent injection has been experimented up to 6 stages. As the number of injection stages increases, reactor efficiency rises thus the highest efficiency was obtained at stage 5; however, more increase in injection stages does not increase VSS removal efficiency and reactor efficiency remains constant.
- Generating electrical conductivity, adding a slight amount of sodium sulfate positively affects reactor efficiency rise and this trend continues up to 0.111 mole/lit, however more addition conduces to noticeable reduction in efficiency.
- The ratio of electrodes surface to system volume was investigated in the range of 0-266 cm²/lit. As electrodes surface increases in the range of 0-66 cm²/lit, system efficiency steeply rises from 22% to 79% (57% rise in system efficiency), however in the range of 66-100 cm²/lit a much more gentle slope is noticed (7% rise in system efficiency) and eventually in the range of 100-200 cm²/lit system efficiency rises only 2 percent. Increasing electrodes surface more than 200 cm²/lit has no effect in system efficiency (Fig. 1.).
 
Fig. 1. The relation between VSS removal efficiency and the ratio of electrodes surface to reactor volume
(pH=3, Fe²⁺/H₂O₂=0.58, current intensity: 650 mA, retention time: 240 minutes, hydrogen peroxide concentrations: 1568 mg/lit, electrodes material: graphite, interelectrode distance: 1.5 cm, number of Fenton's reagent injection:5, sodium sulfate concentrations: 0.111 mole/lit)
 
- Voltage slope steeply decreases in first 90 minutes of experiment; then from 90 to 210 minutes after the run, the slope becomes more gentle and finally in last 30 minutes becomes stable; however, electrical energy consumption increases during the experiment.
- Increasing concentrations of inflow organics results in increase of power supply voltage but decrease of electrical energy consumption for removal of 1 kg VSS (while total electrical energy consumption increases).
- Electrical energy consumption is directly and linearly related to interelectrode distance.
- As sodium sulfate concentrations increases, electrical energy consumption decreases. Initial addition of sodium sulfate decreases a great deal of electrical energy consumption (consumed electrical energy decreases 1.63 Kwh for removal of 1 kg VSS). Increasing concentrations of sodium sulfate more than a certain amount; however, makes decreasing slope of electrical energy consumption become very much more gentle (consumed electrical energy decreases 0.36 Kwh for removal of 1 kg VSS).
- Electrical energy consumption is directly and linearly related to the ratio of electrodes surface to reactor volume (Fig. 2.).
 
Fig. 2. The relation between voltage and electrical energy consumption and the ratio of electrodes surface to reactor volume
(pH=3, Fe²⁺/H₂O₂=0.58, current intensity: 650 mA, retention time: 240 minutes, hydrogen peroxide concentrations: 1568 mg/lit, electrodes material: graphite, interelectrode distance: 1.5 cm, number of Fenton's reagent injection:1, sodium sulfate concentrations: 0.111 mole/lit)
 
- Since at electrodes surface of 100 and 200 cm²/lit, there is no prominent difference in system efficiency (a two-percent system efficiency increase, only) (Fig. 1.), but there are large difference of electrical energy consumption and electrodes surface area (1.7 kWh/(kg VSS removal) difference in electrical energy consumption and 100 cm²/lit difference in consumed electrode surface area) (Fig. 2.); it's logical to ignore two percent rise in efficiency to lessen the consumption of electrical energy and electrodes surface area and consequently lower costs. Therefore electrodes surface of 100 cm²/lit is adopted for reactor operation in which condition system efficiency and electrical energy consumption are 86 percent and 1.6 kWh/(kg VSS removal), respectively. This demonstrates that Fered-Fenton process is twice as efficient as sludge aerobic digestion (one of the traditional sludge stabilization processes) while consumes half of its required electrical energy.
 
Conclusions
Following previous researches which introduced Fered-Fenton reactor as one of the effective novel methods for sludge stabilization, this study has been done to reengineer, optimize and upgrade of the reactor. In this study electrical energy consumption is also analyzed, estimated and compared to sludge aerobic digestion which is a traditional sludge stabilization method. These investigations demonstrated that under optimized conditions, %86 of VSS removal efficiency with electrical energy consumption of about 1.6 kWh/(kg VSS removal) is obtained which indicate that Fered-Fenton process is twice as efficient as sludge aerobic digestion while consumes half of its required electrical energy. This implies the application of Fered-Fenton system in action and at the same time being highly efficient.