E-Fenton Process Research Articles

Effective degradation of tetracycline in aqueous solution by an electro-Fenton process using chemically modified carbon/α-FeOOH as catalyst.

This study applied an electro-Fenton process using chemically modified activated carbon derived from rubber seed shells loaded with α-FeOOH (RSCF) as catalyst to remove tetracycline residues from aquatic environment. Catalyst characteristics were evaluated using SEM, EDS, XRD, and XPS, showing successful insertion of iron onto the activated carbon. The effects of the parameters were investigated, and the highest treatment efficiency was achieved at pH of 3, Fe: H2O2 ratio (w/w) of 500:1, catalyst dose of 1g/L, initial TCH concentration of 100mg/L, and electric current of 150mA, with more than 90% of TCH being eliminated within 30min. Furthermore, even after five cycles of use, the treatment efficiency remains above 90%. The rate constant is calculated to be 0.218min-1, with high regression coefficients (R 2 = 0.93). The activation energy (Ea) was found to be 32.2kJ/mol, indicating that the degradation of TCH was a simple reaction with a low activation energy. These findings showed that the RSCF is a highly efficient and cost-effective catalyst for TCH degradation. Moreover, the use of e-Fenton process has the advantage of high efficiency, low cost thanks to the recyclability of the catalyst, and environmental friendliness thanks to less use of H2O2.

Simultaneous degradation of NSAIDs in aqueous and sludge stages by an electron-Fenton system derived from sediment microbial fuel cell based on a novel Fe@Mn biochar GDC

Electro-Fenton (e-Fenton) reactions boosted using microbial fuel cells (MFCs) can degrade contaminants with high efficiency, the treatment is generally performed in the aqueous phase and the sludge pollution is ignored in such MFCs. In this study, a sediment MFC e-Fenton (SMFC-e-Fenton) system was proposed for the degradation of nonsteroidal anti-inflammatory drugs (NSAIDs) in both aqueous and solid phases. A Fe@Mn-codoped biochar gas diffusion cathode was fabricated to produce hydroxyl radicals without any additional power input. The studied NSAIDs of ibuprofen (IBU), naproxen (NPX), and diclofenac (DCF) exhibited total degradation efficiencies (including aqueous and sludge phase) of 99.1%, 98.5%, and 80.6%, respectively, in the SMFC-e-Fenton system. Compared with sludge anaerobic digestion, the residual amounts of IBU, NPX, and DCF reduced from 10.2%, 24.6%, and 32.2% to 2.7%, 4.4%, and 8.5%, respectively, in the sludge phase. Moreover, the electricity generation and sludge reduction improved owing to the e-Fenton process. Using the SMFC-e-Fenton system, the effects of NSAIDs were reduced and the richness and diversity of the microbial community are maintained, improving the overall system performance. These results demonstrate that the SMFC-e-Fenton method is a high-efficiency, economic and environmental friendly technology for simultaneously treating NSAIDs in the aqueous and sludge phases.

Rapid in situ regeneration of phenol-saturated activated carbon fiber by an electro-permonosulfate-ozone process: Performance, operators and mechanism

In this study, a new method that encompasses electricity, permonosulfate (PMS), and ozone (E-PMS-O3) was proposed and applied in the regeneration of phenol-saturated activated carbon fiber (ACF). Compared with traditional regeneration technology, the E-PMS-O3 process simultaneously regenerated exhausted ACF in situ and mineralized the desorbed pollutants effectively (78.67%) with relatively low energy consumption (4.338 kWh m−3). Furthermore, the E-PMS-O3 regeneration process only required 2 h, which was shorter than other well-documented electro-advanced oxidation processes (EAOPs), such as the E-O3 process lasting for 3 h, the E-persulfate process for 6 h, and the E-Fenton process for 6 h. Possible pathways in the regeneration process include direct electron transfer, ozone oxidation, PMS oxidation, non-radical reactions, and reactive oxygen species (ROS) oxidations. Two main ROS — hydroxyl radical (O•H) and sulfate radical (SO4•−) were probed and their contribution ratio was approximately 1:1. An external electric field visibly enhanced the recovery adsorption capacity of ACF and accelerated the decomposition of PMS. Interestingly, additional experiments revealed that adding two different oxidants (PMS/O3) provided a better protection effect on the properties of ACF against oxidation of oxidants and ROS. It's important to mention that the maximum concentration of byproducts remained below 5 mg g−1, which effectively minimized the potential for subsequent pollution. In the end, multiple regeneration cycles demonstrated that the regeneration efficiency was nearly 60% even after 6 cycles. Hence, the E-PMS-O3 process was promising for further exploration and discussion when applied in regeneration of exhausted AC materials.

Degradation of methylene blue by an E-Fenton process coupled with peroxymonosulfate via free radical and non-radical oxidation pathways

This paper reports a combined advanced oxidation process to degrade methylene blue and investigates its oxidation mechanism and degradation pathway.

Facile Synthesis of n-Fe3O4/ACF Functional Cathode for Efficient Dye Degradation through Heterogeneous E-Fenton Process

In order to put forward an efficient and eco-friendly approach to degrade dye-containing industrial effluents, an n-Fe3O4/ACF nanocomposite was synthesized using the facile precipitation method and applied as a functional cathode for a heterogeneous electro-Fenton (E-Fenton) process. In particular, optimal initial pH value, current density, pole plate spacing, and electrode area were confirmed through systematical experiments as 5.73, 30 mA/cm2, 3 cm, and 2 × 2 cm2, respectively. Under such optimal reaction conditions, 98% of the methylene blue (MB) was degraded by n-Fe3O4/ACF after 2 h of E-Fenton treatment. In addition, n-Fe3O4/ACF could still decolor about 90% of the methylene blue (MB) for five rounds with some reductions in efficiency. Furthermore, results of electrochemical impedance spectroscopy and heterogeneous E-Fenton performance tests indicated that the loading of metal material Fe3O4 could enhance the overall electron transport capacity, which could accelerate the whole degradation processes. Moreover, the rich pores and large specific surface area of n-Fe3O4/ACF provided many active sites, which could greatly improve the efficiency of O2 reduction, promote the generation of H2O2, and shorten the reaction length between •OH and the pollutant groups.

Three-Phase Three-Dimensional Electrochemical Process for Efficient Treatment of Greywater.

Water shortages around the world have intensified the search for substitute sources. Greywater can serve as a solution for water requirements. Compared to two-dimensional electrochemical processes for water treatment, the addition of particle activated carbon enhances the conductivity and mass transfer or the adsorption of pollutants in a three-dimensional (3D) electrochemical process. The large specific surface areas of these particles can provide more reactive sites, resulting in a higher removal efficiency. In this study, the treatment of greywater by the electro-Fenton (E-Fenton) method was carried out in a 3D electrolytic reactor. The effects of the operating conditions, such as electrode spacing, applied voltage, treatment time, and activated carbon loading, on the efficacy of the E-Fenton process were investigated, and the corresponding optimum conditions were found to be 7 cm, 9 V, 2 h, and 10 g. The results showed that CODCr removal of greywater treated using the 3D electrochemical process was 85%. With the help of the Box–Behnken experiment design and the response surface methodology, the parameters were optimized to determine the optimal conditions. The results of the response surface analysis were consistent with the experimental results. The above findings illustrate that the proposed three-phase 3D electrochemical process is feasible for the efficient treatment of greywater.

Construction of Novel Electro-Fenton Systems by Magnetically Decorating Zero-Valent Iron onto RuO2-IrO2/Ti Electrode for Highly Efficient Pharmaceutical Wastewater Treatment

The Electro-Fenton (E-Fenton) technique has shown great potential in wastewater treatment, while the sustainable and continuing supply of Fe2+ remains challenging. Herein, we demonstrate the construction of a novel E-Fenton system by magnetically decorating zero-valent iron (ZVI) onto a RuO2-IrO2/Ti (ZVI-RuO2-IrO2/Ti) electrode for high-efficient treatment of pharmaceutical wastewater, which is considerably refractory and harmful to conventional biological processes. By using ZVI as a durable source of Fe(II) irons, 78.69% of COD and 76.40% of TOC may be rapidly removed by the developed ZVI-RuO2-IrO2/Ti electrode, while the ZVI-RuO2-IrO2/Ti electrode using ZVI only reduces 35.64% of COD under optimized conditions at initial COD and TOC values of 5500 mg/L and 4300 mg/L, respectively. Moreover, the increase in BOD5/COD from 0.21 to 0.52 highlights the enhanced biodegradability of the treated effluent. The analysis of a simultaneously formed precipitation on electrodes suggests that the coagulation process dominated by Fe3+/Fe2+ also plays a non-negligible role in pharmaceutical wastewater treatment. In addition, the monitoring of the evolution of nitrogen elements and the formation of by-products in the E-Fenton process verifies its great capacity toward those organic pollutants found in pharmaceutical wastewater. Our study offers a practical solution for enhancing the performance of E-Fenton systems, and effectively treating refractory pharmaceutical wastewater.

Influences of CeO2 morphology on enhanced performance of electro-Fenton for wastewater treatment

As a kind of rare-earth oxide, CeO2 has been considered as a great potential material for its abundant oxygen vacancies and catalysis activity in wastewater treatment. In this study, three ceria samples with different morphological structure were prepared, and their effect on contaminates degradation in electro-Fenton (E-Fenton) system was investigated. It is found that the morphology of CeO2 has great influence on promoting the performance of E-Fenton process. The rod-like CeO2 (R–CeO2) induced E-Fenton system shows higher azo dye X3B removal and mineralization rates than cubs (C–CeO2) and octahedrons (O–CeO2), and the degradation kinetic rate constants are 0.28, 0.169 and 0.181 min−1 respectively, already surpass that of the blank one (0.120 min−1). The H2O2 generation capacity of R–CeO2 induced E-Fenton system is also superior to the others, and the corresponding Faraday current efficiency even increases to 166.2% at 2.5 min. Characterizations by scanning-transmission electron microscopy (STEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), Raman spectroscopy were conducted to disclose the mystery behind this phenomenon. The results indicate that the difference of surface oxygen vacancy density in different shaped CeO2 acts as a magic driving force for the accelerated oxygen reduction, and then leads to the enhanced degradation efficiency of E-Fenton system. This work provides a new insight into the development and application of rare-earth elements based catalyst in E-Fenton technology.

Effect of an electro-Fenton process on the biodegradation of lignin by Trametes versicolor

A synergistic effect was found between the electro-Fenton (E-Fenton) process and a white-rot enzyme (Trametes versicolor) system relative to the degradation of dealkaline lignin. The hydrogen peroxide produced by the E-Fenton process reacted with Fe2+ on the cathode to generate a large number of hydroxyl radicals. These hydroxyl radicals directly degraded various functional groups in lignin, which led to the quick initiation of lignin peroxidase (LiP) and manganese peroxidase (MnP) enzymatic hydrolysis and accelerated the progress of lignin biodegradation. In addition, the hydroxyl radicals produced by the Fenton reaction converted nonphenolic lignin into phenolic lignin, further promoting the ability of manganese peroxidase and laccase to degrade the lignin. Additionally, the Fe3+ secreted by white-rot fungi accelerated the regeneration of Fe2+ on the composite cathode, which sustained the lignin degradation system. In the synergistic system, mycelium growth was significantly improved, with the maximum growth amount reaching 2.3 g and the lignin degradation rate reaching 84.5%, the activity of the three enzymes increased with the increase of currents over 96 h. Among them, the activity of MnP increased significantly to 402 U/L.

Porous graphite felt electrode with catalytic defects for enhanced degradation of pollutants by electro-Fenton process

Etched graphite felt (EGF) was successfully fabricated using in situ CoOX etching process and used as a cathode to degrade diuron in the electro-Fenton (E-Fenton) process. The graphite felt prepared using the in situ CoOX etching process could markedly enhance the electrochemical properties with high electroactive surface area and rapid electron transfer capacity. Due to the high content of defects and porous structure, EGF electrode exhibited superior performance with high yield of hydrogen peroxide and rapid regeneration capacity of Fe2+, achieving 100% removal of diuron (20 mg/L) at −0.7 V/SCE within 50 min. In addition, the stability of the EGF cathode showed no significant decay in the performance after 35 cycles. EGF electrode had achieved good degradation effect for different pollutants, such as atrazine, amoxicillin, atenolol and 2, 4, 6-trichlorophenol. In three runs, the EGF cathode showed low Co leaching (<1 ppm). Moreover, the possible degradation pathways of diuron were proposed by analyzing the intermediates in this work. The results obtained in this work highlight the potential of EGF electrode for use as an E-Fenton cathode.

Evaluation of E-Fenton Process in Decolourization of Azo Dye (Congo Red) using Response Surface Methodology

Today there is a great focus on reusing the effluent water in the dyeing process. The objective of this research work is to effectively remove colour from textile wastewater using Electro Fenton (E-Fenton) process. E-Fenton process is the combination of Fenton and electrochemical processes. In this study decolorization of Congo red dye was investigated by E-Fenton process in a continuous reactor equipped with iron electrodes. Experiments were carried out in a reactor of working volume of 1L. The degree of decolourisation varies with the operating parameters. The optimal working conditions were maintained with an electrode potential difference of 2V, 40°C, pH 4. For Electro Fenton's process the parameters giving the highest efficiency were decolourisation percentage of 97%. The process parameters were optimized by using response surface methodology to understand the effectiveness of the interactions of variables when the maximum decolourization can be achieved. The experimental results proved that the combination of Fenton and Electrochemical process was an excellent method for treatment of textile wastewater.

Enhanced degradation of triclosan in heterogeneous E-Fenton process with MOF-derived hierarchical Mn/Fe@PC modified cathode

Mn/Fe@porous carbon (PC) was successfully fabricated through a simple carbonization of Mn-doped MIL-53(Fe) precursor and used as cathode modification in hydroxyl radical (OH) based heterogeneous electro-Fenton (hetero-EF) process for triclosan (TCS) degradation. With co-doping of bimetal (Mn and Fe) in porous carbon, Mn/Fe@PC could markedly enhance the electrocatalysis with low oxygen reduction potential, high electrochemical active area and low resistance. Mn/Fe@PC modified cathode showed high electrocatalytic activity, great stability and low energy consumption (1.92 kWh m−1 and 0.63 kWh log−1 m−3) for TCS degradation over a wide pH range. The complete TCS degradation within 120 min and a 56.9 ± 2.0% TOC removal within 240 min were achieved under the condition of current 40 mA and initial pH 3. Based on the CV curves, the more negative reductive peak of Fe and Mn revealed the kinetically beneficial regeneration of FeII/MnII/III in Mn/Fe@PC. The electron transfer between FeII/III and MnII/III/IV, together with the direct regeneration of FeII/MnII/III on the cathode, could markedly promote the utilization of H2O2, eventually leading to facilitating TCS degradation. According to the GC–MS intermediates, the possible TCS degradation pathway was deduced, including hydroxylation and dehalogenation attacked by OH. Furthermore, the TCS degradation experiments were also performed in different water matrix including river water, tap water and municipal sewage, indicating the feasibility of the TCS degradation in actual waters with Mn/Fe@PC-CP cathode in hetero-EF process. The toxicity analysis indicated that the hetero-EF process could achieve the TCS degradation and toxicity reducing.

Design of a high efficiency electro-Fenton system with Fe0@Fe3O4/ACF composite cathode for lignin wastewater treatment

Lignin is a complex phenylpropanoid biopolymer conferring mechanical strength to plant cell walls. Biorefineries, along with pulp and paper industry, generate large quantities of lignin. Lignin valorization is an essential process for an advanced, sustainable, and economical biomass-based industry. In this study, for the first time nanostructured Fe0@Fe3O4 was loaded on active carbon fiber (ACF) as an oxygen diffusion cathode to be used in a heterogeneous electro-Fenton (E-Fenton) oxidation system. This novel Fe0@Fe3O4/ACF composite cathode was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), BET surface area analysis, transmission electron microscopy (TEM). On the degradation of lignin in water, this heterogeneous E-Fenton system with the Fe0@Fe3O4/ACF cathode showed much higher activity than other E-Fenton systems with commercial zero valent iron powders (Fe0) and Iron ions (Fe3+) under same condition. On the basis of experimental results, we proposed a possible pathway of lignin degradation in this heterogeneous Fe0@Fe3O4/ACF E-Fenton process. This heterogeneous E-Fenton system is very promising to remove organic pollutants in water.

In-situ electrode fabrication from polyaniline derived N-doped carbon nanofibers for metal-free electro-Fenton degradation of organic contaminants

Electro-Fenton (e-Fenton) process is a highly efficient technology for refractory wastewater treatment. However, electrode fabrication from powdered carbon catalysts usually requires additional binders, which is expensive, non-scalable and arduous process. In this study, the electrode was in-situ fabricated using N-doped carbon nanofibers derived from carbonization of polyaniline (PANI), electrodeposited on graphite-felt substrate without any binder. The as synthesized electrode was used for in-situ electro-generation and activation of H2O2 to •OH radicals for degradation of organic pollutants without metal catalyst. Potassium hydroxide was used to tailor the functional groups and reactive surface area of the electrode. The optimized electrode showed H2O2 production of 0.306 mg cm–2 h–1 and 85% of phenol removal with 42% of mineralization in 180 min. Mainly the N functional groups (graphitic and pyridinic N) function as active sites for electro-generation and activation of H2O2 to •OH radicals, which was the main oxidant for degradation of pollutants.

Bio-Electron-Fenton (BEF) process driven by sediment microbial fuel cells (SMFCs) for antibiotics desorption and degradation

A sediment microbial fuel cell electro-Fenton (SMFC-E-Fenton) system was proposed in this study by utilizing the biological electrons produced from an SMFC to power an E-Fenton process. Antibiotics with different absorbability in both aqueous and solid phase can be removed by this system at room temperature and pressure condition, without external power or other chemical reagents. γ-FeOOH graphene polyacrylamide carbonized aerogel (γ-FeOOH GPCA) with high electrochemically active surface area (EASA), good conductivity and stable electrochemical activity were used as the cathode. After 40 h treatment, the total degradation rate of sulfamethoxazole (SMX) and norfloxacin (NOR) were 97.4 ± 2.9% and 96.1 ± 3.0%, respectively. Compared with the sludge digestion system, the residual SMX and NOR in sludge declined from 10.2 ± 1.5% to 1.1 ± 1.2% and from 31.3 ± 1.8% to 3.1 ± 1.3%, respectively. The E-Fenton process can also promote electricity production and sludge reduction efficiency of SMFC, as the maximum power density SMFC-E-Fenton system reached 472.21 ± 11.5 mW m−2 and 431.39 ± 15.6 mWm−2, respectively. 6.2 ± 0.3% and 5.7 ± 0.8% of the initial sludge was reduced while treating SMX and NOR.

Treatment of composting leachate using electro-Fenton process with scrap iron plates as electrodes

Composting process of municipal solid waste is associated with the formation of a large volume of leachate containing hazardous and bio-refractory organic compounds and toxic heavy metals. This signifies the necessity for treating composting leachates. This study investigates the effectiveness of the electro-Fenton (E-Fenton) process for the treatment of a real composting leachate. The effects of various experimental parameters including reaction time, electrical current density, pH, hydrogen peroxide (H2O2) concentration, and the distance between the electrodes on the reduction in chemical oxygen demand (COD) were examined. The E-Fenton process was optimized based on the D-optimal algorithm using response surface methodology. Under optimized treatment conditions (reaction time = 100 min, initial pH = 3.0, H2O2 concentration = 0.25 M, and electrical current = 3.0 A), 63.4% of COD, 78.7% of BOD, and 99.3% of PO4–P were reduced. During the treatment process, NO3–N concentration increased from 24.12 to 40.03 mg/L. The experiments showed that the distance between the electrodes in the studied range had no significant effects on the treatment efficiency. A reduction in the biodegradability index of BOD5/COD from 0.60 to 0.31 indicated that most of the organic matter in composting leachate were biodegradable and were oxidized by the E-Fenton process. Conducting a separate set of experiments using the standard Fenton process on the same leachate resulted in 68.4% COD removal, demonstrating that the E-Fenton method could be considered as a promising and inexpensive alternative for the conventional Fenton process to treat composting leachates.

High oxygen reduction reaction performance nitrogen-doped biochar cathode: A strategy for comprehensive utilizing nitrogen and carbon in water hyacinth

In this study, a novel nitrogen-doped biochar oxygen reduction reaction cathode-water hyacinth carbon, was prepared by ZnCl2 molten salt carbonization without additional nitrogen source, which displayed a high performance in electro-Fenton (E-Fenton) process. The BET result shows that water hyacinth carbon achieved a much larger specific surface area (829 m2·g−1) than non-melt salt carbonized one (323 m2·g−1) and graphite powder (28 m2·g−1). Furthermore, characterization by XPS and EIS shows that both pyridinic-N (43.24%) and graphitic-N (56.75%) existed in water hyacinth carbon and Warburg constant was only 0.051. Because of a high H2O2 producing yield 1.7 mmol·L−1 and corresponding current efficiency 81.2 ± 2.5% in molten salt carbonized water hyacinth biochar, a high kinetic constant 0.318 min−1 in DMP degradation was achieved, which was 4 times higher than graphite powder (0.076 min−1). The TOC removal achieved 86.8% in 30 min and the corresponding energy consumption reached a low level 60.15 kW·h·kgTOC−1.

Rapidly Enhanced Electro-Fenton Efficiency by in Situ Electrochemistry-Activated Graphite Cathode

For the development of a highly active and energy-effective electro-Fenton (E-Fenton) process, a simple and efficient electrochemical method was used to modify the nitrogen-doped graphite in situ. It showed that through ammonium nitrate electroactivation, this process could provide (i) a higher surface area, (ii) more hydrophilicity, (iii) much higher electrocatalytic activity toward the oxygen reduction reaction, and (iv) more nitrogen-containing functional groups. The reduction of oxygen to generate H2O2 was promoted, and the current efficiency of the E-Fenton process was also upgraded from 57.96 to 83.76% over 15 min. The apparent rate constant for the dye Brilliant Red X3B by electrochemisty is 0.1781 min–1, much higher than that of the Raw (0.0731 min–1). The dimethyl phthalate (DMP) and the diethyl phthalate (DEP) had the same results. Hydroxyl (·OH) radicals were identified as the main reactive oxygen species, which clearly increased after electroactivation.

Simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by electro-Fenton process using iron sacrificing electrodes

This research explored the application of electro-Fenton (E-Fenton) technique for the simultaneous decomplexation in blended Cu(II)/Ni(II)-EDTA systems by using iron sacrificing electrodes. Standard discharge (0.3 mg L−1 for Cu and 0.1 mg L−1 for Ni in China) could be achieved after 30 min reaction under the optimum conditions (i.e. initial solution pH of 2.0, H2O2 dosage of 6 mL L−1 h−1, current density of 20 mA/cm2, inter-electrode distance of 2 cm, and sulfate electrolyte concentration of 2000 mg L−1). The distinct differences in apparent kinetic rate constants (kapp) and intermediate removal efficiencies corresponding to mere and blended systems indicated the mutual promotion effect toward the decomplexation between Cu(II) and Ni(II). Massive accumulation of Fe(Ⅲ) favored the further removal of Cu(II) and Ni(II) by metal ion substitution. Species distribution results demonstrated that the decomplexation of metal-EDTA in E-Fenton process was mainly contributed to the combination of various reactions, including Fenton reaction together with the anodic oxidation, electro-coagulation (E-coagulation) and electrodeposition. Unlike hypophosphite and citrate, the presence of chlorine ion displayed favorable effects on the removal efficiencies of Cu(II) and Ni(II) at low dosage, but facilitated the ammonia nitrogen (NH4+-N) removal only at high dosage.

Combined heterogeneous Electro-Fenton and biological process for the treatment of stabilized landfill leachate

Treatment of stabilized landfill leachate is a great challenge due to its poor biodegradability. Present study made an attempt to treat this wastewater by combining electro-Fenton (E-Fenton) and biological process. E-Fenton treatment was applied prior to biological process to enhance the biodegradability of leachate, which will be beneficial for the subsequent biological process. This study also investigates the efficiency of iron molybdophosphate (FeMoPO) nanoparticles as a heterogeneous catalyst in E-Fenton process. The effects of initial pH, catalyst dosage, applied voltage and electrode spacing on Chemical Oxygen Demand (COD) removal efficiency were analyzed to determine the optimum conditions. Heterogeneous E-Fenton process gave 82% COD removal at pH 2, catalyst dosage of 50 mg/L, voltage 5 V, electrode spacing 3 cm and electrode area 25 cm2. Combined E-Fenton and biological treatment resulted an overall COD removal of 97%, bringing down the final COD to 192 mg/L.