Electrochemical treatment of chemical oxygen demand in produced water using flow‐by porous graphite electrode

Produced water is the largest wastewater stream generated in the oil and gas industries. In this study, experiments were carried out using a bench‐scale electrochemical cell using flow‐by porous graphite electrode, for oxidation of organic matter in produced water which was collected from natural gas processing field (real sample). The effect of anodic current density and influent feed flow rate on chemical oxygen demand (COD) removal efficiency, and energy consumption were investigated. The maximum removal efficiency of 66.52% was obtained for a flow rate of 50 mL/min, current density of 1.41 mA/cm² and pH of 7.3 for an influent COD of 2845 mg O₂/L. The energy consumption at these conditions was 2.12 kWh/kg_COD.     

Non‐steroidal anti‐inflammatory pharmaceutical wastewater treatment using a two‐chambered microbial fuel cell

The two‐chambered microbial fuel cell (MFC) was designed and used for studying the efficiency of the real wastewater treatment from a non‐steroidal anti‐inflammatory pharmaceutical plant as well as from synthetic wastewater containing diclofenac sodium (DS). The removal of the contaminants was expressed regarding chemical oxygen demand (COD) removal, as measured by spectrophotometry experiments. Moreover, the effect of two different types of the cathode on current characteristics and COD removal was investigated. This research showed that the Pt‐coated Ti cathode could lead to higher efficiency of both power density and COD removal. In this case, the results indicated that the maximum power density (P_max) was 20.5 and 6.5 W/m³ and the maximum COD removal was 93 and 78% for MFCs using real and synthetic wastewater, respectively.     

Optimization of hydroxyl radical production using electro‐Fenton method for chemical oxygen demand reduction in diluted palm oil mill effluent

Palm oil mill effluent (POME) is a well‐known highly polluting wastewater due to its extremely high contents of organic matter, suspended solids and nutrients. In this study, we used electro‐Fenton method to treat POME by optimizing OH? generation from hydrogen peroxide (H₂O₂) under low voltage input (1.5–6.0 V). A set of electro‐Fenton system was set up using stainless steel as the anode and graphite as the cathode. Four parameters namely retention times, concentrations of H₂O₂ as well as FeSO₄ catalyst and applied voltages were studied. The results were reflected in the form of removal efficiency of chemical oxygen demand (COD). The optimum conditions to degrade organic matter in POME were found to be in 4 h retention time with the respective H₂O₂ and FeSO₄ catalyst concentrations of 0.05 and 0.10 M, and the power input of 1.5 V. Under such conditions, the maximum COD removal efficiency achieved 94%. The electro‐Fenton treatment was found to have higher efficiency than the conventional Fenton treatment. Without the electrolysis, the COD removal efficiency of the conventional Fenton treatment was only 48%.     

Electrochemical sulfide removal from synthetic and real domestic wastewater at high current densities

Hydrogen sulfide generation is the key cause of sewer pipe corrosion, one of the major issues in water infrastructure. Current abatement strategies typically involve addition of various types of chemicals to the wastewater, which incurs large operational costs. The transport, storage and application of these chemicals also constitute occupational and safety hazards. In this study, we investigated high rate electrochemical oxidation of sulfide at Ir/Ta mixed metal oxide (MMO) coated titanium electrodes as a means to remove sulfide from wastewater. Both synthetic and real wastewaters were used in the experiments. Electrochemical sulfide oxidation by means of indirect oxidation with in-situ produced oxygen appeared to be the main reaction mechanism at Ir/Ta MMO coated titanium electrodes. The maximum obtained sulfide removal rate was 11.8 ± 1.7 g S m⁻² projected anode surface h⁻¹ using domestic wastewater at sulfide concentrations of ≥30 mg L⁻¹ or higher. The final products of the oxidation were sulfate, thiosulfate and elemental sulfur. Chloride and acetate concentrations did not entail differences in sulfide removal, nor were the latter two components affected by the electrochemical oxidation. Hence, the use of electrodes to generate oxygen in sewer systems may constitute a promising method for reagent-free removal of sulfide from wastewater.     

Optimization of phenol removal from wastewater by activation of persulfate and ultrasonic waves in the presence of biochar catalyst modified by lanthanum chloride

In this paper, the effects of phenol concentration, pH, catalyst dose, persulfate concentration, temperature and contact time on the phenol removal from wastewater by activation of persulfate (S₂O₈^?2) in the presence of biochar modified by lanthanum chloride and ultrasonic waves (US) are optimized. Experimental design and optimization were carried out by response surface methodology. The optimum conditions for the maximum phenol removal were obtained pH of 4, phenol concentration of 86 mg/L, catalyst dose of 43 mg/L, persulfate concentration of 86 mg/L, temperature of 41 °C and contact time of 63 min. The optimum phenol removal from synthetic wastewater was attained 97.68%. Phenol removal by the mentioned system was fitted with the first‐order kinetic model. The combination of the ingredients of ‘S₂O₈^?2/US/Biochar‐LaCl₃’ system had a synergistic effect on the phenol removal.     

Field analysis of PSA O2–ozone production process for water recycling

The use of an oxygen generator based on pressure swing adsorption technology (PSA) has been investigated as an alternative method for supplying oxygen for onsite ozone production. During the investigation period of 1040 h, the oxygen purity from the PSA process fluctuated within a range of 90.5–93 O₂% (v/v). Using the working ratio of 2.8 mol of O₂ to 1 m³ of raw water, the PSA process in series with a corona discharge ozone generator yielded an oxidation–reduction potential of ca. 200 mV and a concentration of 22.8 mg/L dissolved oxygen. The average efficiency of the raw water treatment was as follows: 47% chemical oxygen demand removal, 78% biological oxygen demand removal, 36% sulphide removal and 34% colour removal. The operating cost of the PSA O₂ generation was reduced 3 times compared with the cost of high‐pressure oxygen cylinders.     

Electro-oxidation of pistachio processing wastewater using Ti/Pt mesh-type anodes in batch system

In this study, the treatment of pistachio processing wastewater (PPW) by electro-oxidation method was investigated. Ti/Pt-plated electrodes were used for the anode material, and stainless steel electrodes were used for cathode material. Experimental studies were carried out in batch mode. Stirring speed, supporting electrolyte species and concentration, initial pH value, current density, temperature and dilution ratio were selected as experimental parameters effecting removal efficiency. In Ti/Pt electrode experimental studies on the optimum conditions, chemical oxygen demand (COD), total organic carbon (TOC) and total phenols (TP) removal efficiencies were obtained, respectively, as 99.98%, 70.74% and 100%, and energy consumption value was obtained as 297.5 kW-h/m³ (12.39 kW-h/kg COD, 51.29 kW-h/kg TOC and 64.68 kW-h/kg TP). As a result of the experimental studies, the PPW can be treated by electro-oxidation. Given the results of removal efficiency and energy consumption values, it was concluded the electro-oxidation using Ti/Pt anode very appropriate treatment of PPW.     

Optimization of electro-oxidation and electro-Fenton techniques for the treatment of oilfield produced water

Electrochemical advanced oxidation processes are the most promising methods for destroying and degrading organic and inorganic pollutants present in produced water effluents. This study presents the electro-oxidation process using graphite electrodes and electro-Fenton process using iron electrodes for the treatment of real produced water. The effect of operating parameters such as current density on chemical oxygen demand (COD) removal efficiency was addressed. The result showed that electro-Fenton process was more efficient than electro-oxidation process where it gave 98% as maximum COD removal efficiency with energy consumption of 1.9 kWh/dm³ at H₂O₂ concentration of 12 mM, current density of 10 mA/cm², temperature of 25°C, pH of 3, and treatment time of 80 min compared with 96.9% as maximum COD removal efficiency with energy consumption of 3 kWh/dm³ at pH of 6, current density of 10 mA/cm², temperature of 40°C, and reaction time of 80 min when using electro-oxidation process. These results demonstrated that electrochemical technologies are very promising methods for the treatment of produced water from oil/gas industry, so it can be safely disposed of or effectively reused for injection and irrigation.     

Combined biological treatment of high‐sulphate wastewater from yeast production

The wastewater from baker's yeast production contains above‐average concentrations of organic pollutants (25 000 mg/L total chemical oxygen demand, TCOD), nutrients (1500 mg/L N_tot, 100 mg/L P_tot) and sulphate (2900 mg/L SO₄^2?). Baker's yeast wastewater with a flow rate of 190 m³/day was treated in a mesophilic anaerobic/anoxic continuous stirred tank reactor (CSTR) system. At the expense of the reduction of trimethylglycine (or betaine‐component of sugar‐beet molasses) to other nitrogen‐containing compounds, it was possible to re‐oxidize the sulphides to elemental sulphur, remove them from the wastewater and increase biogas production. Therefore, the average removal efficiency in the anaerobic/anoxic system was 79% by TCOD, 100% by SO₄^2? in which the concentration of sulphides in the effluent did not exceed 50 mg/L. The application of this combined anaerobic/anoxic system to a full‐scale treatment plant supported biogas production up to 1300 m³/day, and the purification of wastewater was feasible without the use of granular sludge.     

Treatment of chromium‐bearing wastewaters with constructed wetlands

Chromium can have detrimental effects on most organisms, including humans. The present study focused on the effect of treating chromium‐bearing, organic‐rich wastewater on constructed wetland performance. Eight laboratory‐scale systems were used to study organic matter and chromium removals, with three features tested in duplicate: media with proven chromium sorption capacity, vegetation presence and intermittent influent loading. A set of two wetlands acted as the base design. Average BOD₅ and chromium removal rates were 13.4 g/m² day and 135 mg Cr/m² day, respectively. From an influent chromium concentration of 5 mg/L, <0.5 mg/L could be detected at the outlet during the study in all systems. Chromium removal efficiencies were statistically different for all systems (P<0.01), except when comparing the base design against the intermittent feed systems (P>0.05). Overall treatment performance of all systems was remarkably good in spite of significant chromium content and salinity.     

Development of an expanded-bed GAC reactor for anaerobic treatment of terephthalate-containing wastewater

An expanded-bed granular activated carbon (GAC) anaerobic reactor was developed to treat terephthalate-containing wastewater. Terephthalate inhibits biological anaerobic degradation of terephthalate and methane production when present at a concentration of more than 150 mg/L. In the GAC anaerobic reactor developed here, degradation of terephthalate and other organic compounds occurred smoothly and stably with removal and methane fermentation ratios of more than 90% under a chemical oxygen demand (COD) loading rate of 4 kg COD/(m³ d) and a terephthalate loading rate of 1 kg terephthalate/(m³ d).     

Removal of phenolic compounds from raw industrial wastewater by Achromobacter sp. isolated from a hydrocarbon‐contaminated area

The discharge of raw industrial wastewaters, specifically coking wastewater, represents a severe environmental problem. In this work, a phenol‐degrading aerobic strain isolated from a hydrocarbon contaminated site, Achromobacter sp. C‐1, was tested for degrading raw coking wastewater to explore its potential for use in biological treatment. Initially, phenol degradation was reached after 24 h of inoculation in synthetic wastewater [600 mg/L of phenol]. The maximum specific degradation rate was 0.436 h^–1 found in the concentration 300 mg/L. In a raw industrial wastewater containing a mixture of phenols as carbon source [phenol 370 mg/L, m‐cresol 100 mg/L and o‐cresol 60 mg/L], 90% biodegradation of a mixture of phenols was achieved after 80 h of inoculation. Following the biodegradation process to remove the colour from the wastewater, polishing was performed by activated carbon adsorption, resulting in a clear wastewater (without colour and contaminants) ready for industrial reuse purposes. These results provided useful information about use of the phenol‐degrading bacteria for bioaugmentation in industrial wastewater treatment improving the quality of final wastewater. The quality of the resulting wastewater was confirmed by mass spectrometry analysis. This work shows the biodegradation process could be a cost‐effective and promising solution for the treatment and reuse of phenolic wastewater.      

On‐site wastewater treatment using reactive filter media

The aim of this study is to reduce the phosphate concentration in treated wastewater using a small amount of the reactive filter media, Filtralite P. Biologically treated wastewater was passed through a filter with 215 g of Filtralite P. In the laboratory, the phosphate removal efficiency was 51% at a flow speed of 0.87 m/h. Under real conditions, in an experimental stand filled with 0.5 m³ of Filtralite P, the phosphate removal efficiency was 85% and the removal efficiency of total suspended solids was 57% after a 5‐month period. The residual phosphate concentration in the filtrate from the experimental stand was 1 mg/L of PO₄‐P after the 5‐month period. The experimental filtration plant was buried in the ground, and it did not freeze and worked well under winter conditions. The results of this study can be useful in the design and development of tertiary wastewater treatment plants in view of their sustainability potential.     

A comparative study on treatment of wastewaters with various biodegradability and various pH values using single‐chamber microbial fuel cells

This work assessed the performance of a single‐chamber microbial fuel cell (MFC) with various substrates. Primary settled domestic wastewaters were used to simulate wastewaters of high biodegradability; while phenol‐based wastewaters and benzene‐based wastewaters were used to simulate wastewaters of low biodegradability. Experiments were performed at initial pH values of 6, 7 and 8. The maximum voltage production, power density and removal of substrate were obtained using primary settled domestic wastewater, whereas the lowest values were obtained using phenol‐based wastewater. The maximum chemical oxygen demand removal efficiency, phenol removal efficiency and benzene removal efficiency were 80.8, 63.3 and 77.8%, respectively. The performance of the MFC was enhanced by increasing the influent pH. The lowest coulombic efficiencies were obtained from phenol‐based wastewater and benzene‐based wastewater, which indicated that electrogenic bacteria were not the primary microorganisms responsible for the biodegradation of low biodegradable wastewater.     

Fenton氧化处理炼化废水中苯酚的实验研究

通过单因素实验探索了Fenton氧化炼化废水中苯酚的最佳工艺条件。实验结果表明,Fenton试剂处理苯酚废水时,最佳氧化反应条件为:pH=3.5,反应温度为20℃,H2O2投加量为12mL·L-1,反应时间为30min,FeSO4·7H2O投加量为600mg·L-1。因此用Fenton氧化法处理含苯酚废水是一种非常有效的方法。     

Soil aquifer treatment of artificial wastewater under saturated conditions

A 2000 mm long saturated laboratory soil column was used to simulate soil aquifer treatment under saturated conditions to assess the removal of chemical and biochemical oxygen demand (COD and BOD), dissolved organic carbon (DOC), nitrogen and phosphate, using high strength artificial wastewater. The removal rates were determined under a combination of constant hydraulic loading rates (HLR) and variable COD concentrations as well as variable HLR under a constant COD. Within the range of COD concentrations considered (42 mg L⁻¹-135 mg L⁻¹) it was found that at fixed hydraulic loading rate, a decrease in the influent concentrations of dissolved organic carbon (DOC), biochemical oxygen demand (BOD), total nitrogen and phosphate improved their removal efficiencies. At the high COD concentrations applied residence times influenced the redox conditions in the soil column. Long residence times were detrimental to the removal process for COD, BOD and DOC as anoxic processes and sulphate reduction played an important role as electron acceptors. It was found that total COD mass loading within the range of 911 mg d⁻¹-1780 mg d⁻¹ applied as low COD wastewater infiltrated coupled with short residence times would provide better effluent quality than the same mass applied as a COD with higher concentration at long residence times. The opposite was true for organic nitrogen where relatively high concentrations coupled with long residence time gave better removal efficiency.     

Electrochemical oxidation of trace organic contaminants in reverse osmosis concentrate using RuO2/IrO2-coated titanium anodes

During membrane treatment of secondary effluent from wastewater treatment plants, a reverse osmosis concentrate (ROC) containing trace organic contaminants is generated. As the latter are of concern, effective and economic treatment methods are required. Here, we investigated electrochemical oxidation of ROC using Ti/Ru_0.7Ir_0.3O₂ electrodes, focussing on the removal of dissolved organic carbon (DOC), specific ultra-violet absorbance at 254 nm (SUVA₂₅₄), and 28 pharmaceuticals and pesticides frequently encountered in secondary treated effluents. The experiments were conducted in a continuously fed reactor at current densities (J) ranging from 1 to 250 A m⁻² anode, and a batch reactor at J = 250 A m⁻². Higher mineralization efficiency was observed during batch oxidation (e.g. 25.1 ± 2.7% DOC removal vs 0% removal in the continuous reactor after applying specific electrical charge, Q = 437.0 A h m⁻³ ROC), indicating that DOC removal is depending on indirect oxidation by electrogenerated oxidants that accumulate in the bulk liquid. An initial increase and subsequent slow decrease in SUVA₂₅₄ during batch mode suggests the introduction of auxochrome substituents (e.g. -Cl, NH₂Cl, -Br, and -OH) into the aromatic compounds. Contrarily, in the continuous reactor ring-cleaving oxidation products were generated, and SUVA₂₅₄ removal correlated with applied charge. Furthermore, 20 of the target pharmaceuticals and pesticides completely disappeared in both the continuous and batch experiments when applying J ≥ 150 A m⁻² (i.e. Q ≥ 461.5 A h m⁻³) and 437.0 A h m⁻³ (J = 250 A m⁻²), respectively. Compounds that were more persistent during continuous oxidation were characterized by the presence of electrophilic groups on the aromatic ring (e.g. triclopyr) or by the absence of stronger nucleophilic substituents (e.g. ibuprofen). These pollutants were oxidized when applying higher specific electrical charge in batch mode (i.e. 1.45 kA h m⁻³ ROC). However, baseline toxicity as determined by Vibrio fischeri bioluminescence inhibition tests (Microtox) was increasing with higher applied charge during batch and continuous oxidation, indicating the formation of toxic oxidation products, possibly chlorinated and brominated organic compounds.     

Operation of a horizontal subsurface flow constructed wetland – Microbial fuel cell treating wastewater under different organic loading rates

The aim of the present work is to determine whether a horizontal subsurface flow constructed wetland treating wastewater could act simultaneously as a microbial fuel cell (MFC). Specifically, and as the main variable under study, different organic loading rates were used, and the response of the system was monitored. The installation consisted of a synthetic domestic wastewater-feeding system and a pilot-scale constructed wetland for wastewater treatment, which also included coupled devices necessary to function as an MFC. The wetland worked under continuous operation for 180 d, treating three types of synthetic wastewater with increasing organic loading rates: 13.9 g COD m⁻² d⁻¹, 31.1 g COD m⁻² d⁻¹, and 61.1 g COD m⁻² d⁻¹. The COD removal efficiencies and the cell voltage generation were continuously monitored. The wetland worked simultaneously as an MFC generating electric power. Under low organic loading rates, the wastewater organic matter was completely oxidised in the lower anaerobic compartment, and there were slight aerobic conditions in the upper cathodic compartment, thus causing an electrical current. Under high organic loading rates, the organic matter could not be completely oxidised in the anodic compartment and flowed to the cathodic one, which entered into anaerobic conditions and caused the MFC to stop working. The system developed in this work offered similar cell voltage, power density, and current density values compared with the ones obtained in previous studies using photosynthetic MFCs, sediment-type MFCs, and plant-type MFCs. The light/darkness changes caused voltage fluctuations due to the photosynthetic activity of the macrophytes used (Phragmites australis), which affected the conditions in the cathodic compartment.     

An energy-efficient two-stage passively aerated trickling filter for high-strength wastewater treatment and reuse

This work aimed to assess the technical and energetic feasibility of a passively aerated laboratory-scale trickling filter, configured as a two-stage system, to produce urban wastewater (UWW) reusable in agriculture. The trickling filter was fed continuously with high-strength UWW at four hydraulic retention times (HRTs), that is, 10, 5, 2 and 1 day, corresponding to organic loading rates (OLRs) of 0.1, 0.2, 0.5 and 0.9 kg COD/m³/d, respectively. The results revealed a good performance in organic load removal and nitrification at the four HRTs. The trickling filter showed high organic pollutant removal efficiencies of up to 93%, 94% and 98% for chemical oxygen demand (COD), BOD₅ and total suspended solid (TSS), respectively, as well as high ammonia nitrogen removal above 96% at the shortest HRT of 1 day. All physicochemical parameters were significantly lower than the allowable limits set out in ISO 16075 for category C (non-food crop irrigation) irrigation water. The reuse of treated UWW in irrigation led to germination indexes and growth parameters of triticale (Triticosecale Wittm.) almost equal to those obtained using tap water. Energy use was found to be about 0.2754 kWh/m³ of treated wastewater, making it competitive with trickling filter plants reported in the literature. The simplicity and energy efficiency of the developed trickling filter system, combined with its capacity for almost full nitrification, make it appealing for sewage treatment in small communities in developing countries.     

Simultaneous removal of carbon and nutrients from an industrial estate wastewater in a single up-flow aerobic/anoxic sludge bed (UAASB) bioreactor

Simultaneous removal of carbon and nutrients (CNP) in a single bioreactor is highly significant for energy consumption and control of reactor volume. Basically, nutrients removal is dependant to the ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD). Thus, in this study the treatment of an industrial estate wastewater with low BOD₅/COD ratio in an up-flow aerobic/anoxic sludge bed (UAASB) bioreactor, with an intermittent regime in aeration and discharge, was investigated. Hydraulic retention time (HRT) of 12-36 h and aeration time of 40-60 min/h were selected as the operating variables to analyze, optimize and model the process. In order to analyze the process, 13 dependent parameters as the process responses were studied. From the results, it was found, increasing HRT decreases the CNP removal efficiencies. However, by increasing the BOD₅ fraction of the feed, the total COD (TCOD), slowly biodegradable COD (sbCOD), readily biodegradable COD (rbCOD), total nitrogen (TN), and total phosphorus (TP) removal efficiencies were remarkably increased. Population of heterotrophic, nitrifying and denitrifying bacteria showed good agreement with the results obtained for TCOD and TN removal. The optimum conditions were determined as 12-15 h and 40-60 min/h for HRT and aeration time respectively.