Total Organic Carbon Removal Rate Research Articles

Subcritical water oxidation of propham by H2O2 using response surface methodology (RSM)

ABSTRACTThis study was conducted to investigate the degradation of propham, which is a compound that pollutes water and seriously threatens human health, by subcritical water oxidation and using H2O2 as an oxidising agent. The maximum total organic carbon removal rate of propham was obtained as 73.65% at 40 min of treatment time and 60 mM of H2O2 concentration and 373 K of temperature. In addition, response surface method based on the Box-Behnken design was applied to design the degradation experiments of propham for determination of the combined effects of process variables, namely temperature, concentration of oxidising agent and treatment time. The proposed quadratic model of propham degradation, which was examined with the analysis of variance, was used for navigating the design space. The R2 and adjusted R2 values of the model were determined as 0.9921 and 0.9819 respectively. It was shown that propham was effectively degraded, thus could be removed from the water by using an environmentally friendly method.

Magnéli phases Ti O2−1 as novel ozonation catalysts for effective mineralization of phenol

Magnéli phases TinO2n−1 have been demonstrated as promising environmentally friendly materials in advanced oxidation processes. In this study, Magnéli phases TinO2n−1 have been used as catalysts for the ozonation of phenol in aqueous solution for the first time. The materials exhibited excellent catalytic ozonation activities both in phenol degradation and mineralization. When Ti4O7 was added, the reaction rate was six-fold higher than that of with ozone alone, while the total organic carbon removal rate was substantially elevated from around 19.2% to 92%. By virtue of the good chemical stability of the materials, a low metal leaching of less than 0.15 mg·L− 1 could effectively avoid the secondary pollution by metal ions. Radical quenching tests revealed O2− and 1O2 to be active oxygen species for phenol degradation at pH 5. As semiconductor catalysts, TinO2n−1 materials show electronic transfer capability. Ozone adsorbed at B-acid sites of the catalyst surface can capture an electron from the conversion of Ti(III) to Ti(IV), and is thereby broken into the active oxygen species. It was interesting to observe that TinO2n−1 exhibit better catalytic activity for phenol degradation and mineralization with lower n value. The difference in electrical conductivity can be considered as a major factor for the catalytic performances. More highly conductive catalysts show a faster electron-transfer rate and better catalytic activity. Thus, significant evidences have been obtained for a single-electron-transfer mechanism of catalytic ozonation with Magnéli phases TinO2n−1.

Polypyrrole/reduced graphene oxide aerogel particle electrodes for high-efficiency electro-catalytic synergistic removal of Cr(VI) and bisphenol A

In this paper, synthesis of polypyrrole/reduced graphene oxide aerogel (PGAs) as particle electrodes and their application in three-dimensional electrode system for simultaneous reduction of hexavalent chromium (Cr(VI)) and oxidation of bisphenol A (BPA) were implemented firstly. Electro-catalytic properties were investigated systemically in terms of the influences of pivotal factors, such as pH, the concentration ratio of Cr(VI) and BPA, voltage and the ratio of PGAs mass to solution volume (S/L). The results showed superb removal rates of 98.52 ± 1.48% for Cr(VI) and 98.00 ± 1.17% for BPA within 30 min under the optimal conditions. Furthermore, synergy of both pollutants was testified by the comparison of removal rates in electro-catalytic kinetics and their fitting in mixed solution with kobs-Cr(VI) = 0.2019 min−1, kobs-BPA = 0.1463 min−1 significantly higher than that in one-component aqueous solution (kobs-Cr(VI) = 0.0397 min−1, kobs-BPA = 0.0417 min−1). After 20 cycles of experiments, there were still high contaminants and total organic carbon (TOC) removal rates, which showed excellent reutilized ability of PGAs. More deeply, final destination of Cr(VI) was analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM−EDX). Meanwhile, intermediates of BPA oxidized with the aid of Cr(VI) and OH were analyzed and a possible degradation pathway was proposed. Importantly, Cr(VI)/BPA system provided a remarkable example of a “waste control by waste” strategy for the synergetic removal.

Degradation of Total organic carbon (TOC) and Chemical oxygen demand (COD) in Petroleum wastewater by solar Photo-Fenton process.

The aim of this study is to investigate the performance of solar photo-Fenton process (H2O2/Fe2+/Solar) to treat petroleum wastewater from Sohar oil Refinery, Oman. The effect of operating conditions such as pH, reaction time, Fenton ratio, and Fenton reagent concentrations are investigated. The obtained optimum conditions include H2O2 dosage (1 g L-1), Fe+2 dosage (0.08 g L-1), pH (3) and reaction time (180 min). TOC and COD removal rates are 64 % and 78 %, respectively. However, the photolytic process was less efficient in the petroleum wastewater treatment, achieving an 11.5% and 9% of COD and TOC removals, respectively. The solar photo-Fenton process has well efficient for petroleum wastewater treatment in the acidic conditions and more economic by free energy.

Electrochemical degradation of enrofloxacin by lead dioxide anode: Kinetics, mechanism and toxicity evaluation

In this work, the electrochemical degradation of antibiotic enrofloxacin (ENR) has been investigated by using a Ti/SnO2-Sb/La-PbO2 anode, which demonstrated good performance. The effect of the current density, pH, inorganic anions and humic acid (HA) on the reaction kinetics has been systematically studied. The increase of current density and initial Cl− concentration exerted a prominent effect on the degradation of ENR, while the presence of NO3− and HA was found to show an inhibition. The total organic carbon removal rate, defluorination rate and energy cost were 95.1%, 95.3%, and 2.1Wh/L, respectively, at a current density of 8mA/cm2. The toxicity of the reaction products of ENR towards Vibrio fischeri initially increased slightly and then rapidly reduced to non-toxicity over time. A total of seven aromatic intermediate products of ENR degradation were identified and the different structural changes to the ENR molecule included decarboxylation, substitution of –COOH by –OH group, formation of a ketone group on the piperazine ring, cleavage of the quinolone and piperazine rings, formation of an amide group, and loss of piperazinyl group. Based on the identified products, a reaction mechanism has been proposed wherein hydroxyl radicals attack and open the piperazine and quinolone rings substructures. The products formed after ring opening were further oxidized to CO2, H2O, NO3−, and F−, reaching almost complete mineralization of ENR.

Ultrathin TiO2(B) Nanosheets as the Inductive Agent for Transfrering H2O2 into Superoxide Radicals.

A reflux method to synthesize ultrathin polycrystalline TiO2(B) nanosheets (NSs) which are assembled by single crystals, and further stacked into nanoflower structures, is described. On the basis of the theoretical calculations and experiments, H2O2 can easily substitute the ethylene glycol adsorbed on the surface of TiO2(B) NSs, forming H2O2-NS due to the lower adsorption energy and the unique structural features of ultrathin TiO2(B) nanosheets. TiO2(B) NSs and the H2O2 system can be accelerated to generate superoxide radicals under heat or light and thus exhibit a great degradation property on dye molecules; the total organic carbon (TOC) removal rate was 6 times higher than that for H2O2 alone. Meanwhile, TiO2(B) NSs and the H2O2 system have a good application on the selective oxidation due to the reactive species of superoxide radicals avoiding overoxidization of benzyl alcohol. The conversion of benzyl alcohol oxidized to benzaldehyde in water solution under low temperature and atmospheric pressure was 51.13%, while the selectivity was close to 100%. We believe that the present findings will provide valuable methods for highly efficient generation of superoxide radicals and broaden their applications in catalysis.

Controlled release of iron for activation of persulfate to oxidize orange G using iron anode

Persulfate (PS) can be activated by transition metal to generate a sulfate radical and oxidize persistent organic pollutants. However, activation with excessive Fe(II) causes unnecessary self-degradation of PS. In this study, Fe(II) was slowly and continuously injected electrochemically using an iron anode to minimize the self-degradation of PS. Additionally, reaction rate was controlled by adjusting the current intensity applied to the system. Total organic carbon (TOC) was analyzed as an indicator of complete mineralization because the model pollutant, orange G (OG), produced secondary pollutants after disruption of the azo bonds. The removal rate of TOC was 1/10-th of that for OG. In addition, the effect of molar ratio of OG and PS was also studied to confirm the complete mineralization of OG.

Inhibitory effect of high calcium concentration on municipal solid waste leachate treatment by the activated sludge process.

This research focused on the inhibitory effects of Ca on the aerobic biological treatment of landfill leachate containing extremely high Ca concentrations. When the Ca concentration in leachate to be treated was more than 4500 mg l-1, the total organic carbon removal rate was significantly reduced and the processing time to achieve the same removal efficiency was 1.4 times that in the control treatment without added Ca. In contrast, the total nitrogen and ammonia nitrogen (NH4+-N) removal efficiencies were positively related to the Ca concentration, increasing from 65.2% to 81.2% and from 69.2% to 83.7%, respectively, when the dosage of added Ca increased from zero to 8000 mg l-1. During aerobic treatment, the reductions of solution Ca concentration were in the range of 1003-2274 mg l-1 and were matched with increases in the Ca content in the residual sludge. The inhibition threshold of Ca in the leachate treated by the activated sludge process appeared to be 4500 mg l-1, which could be realized by controlling the influent Ca concentration and using an appropriate sludge return ratio in the activated sludge process.

Effective natural organic matter removal in pond water by carbon nanotube membrane with flocculation/adsorption

A carbon nanotube (CNT) ultrafiltration (UF) membrane was applied to natural organic matter (NOM) removal in pond water treatment. The source water was pretreated by flocculation and/or adsorption prior to the UF process to alleviate permeate flux decline and improve NOM removal efficiency. The performance of a commercial polyethersulfone (PES) UF membrane was compared to evaluate that of the CNT membrane. The CNT membrane outperformed the PES-UF membrane. The permeate flux, total organic carbon and humic acid (HA) removal rate of the CNT membrane was observed to be 230 LMH/bar, 60%, and 80% when 30 mg/L poly aluminium chloride (PACI) flocculation was applied. This highlights that the permeate flux was three times higher with slightly higher rejection efficiency than the PES-UF membrane. In particular, severe permeate flux decline was completely overcome by the CNT membrane with 30 mg/L PACI coagulation. For powder activated carbon (PAC) adsorption, even though there was a severe permeate flux decline in the CNT membrane, almost complete HA removal (98%) was achieved when 0.5 g/L PAC adsorption was coupled. Based on the superior performance of the CNT membrane with pretreatment, the CNT membrane is suggested to be a robust system for a high concentration of organic matter pond water treatment without membrane flux decline.

A Facile Hydrothermal Route for Synthesis of ZnS Hollow Spheres with Photocatalytic Degradation of Dyes Under Visible Light

A facile hydrothermal method was employed for the synthesis of ZnS hollow spheres by using thioglycolic acid (TGA) as a capping agent under hydrothermal condition. The obtained products were characterized by X-ray powder diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). No diffraction peaks from other crystalline forms were detected, the synthesized ZnS hierarchical hollow spheres were relatively pure. The photocatalytic activities of as-synthesized samples were evaluated by the degradation of methyl orange (MO) and rhodamine B (RhB) under the condition of visible-light irradiation. The higher the initial MO and RhB concentrations, the longer it takes to reach the same residual concentration, implying that the apparent rates of MO and RhB degradation decrease with increase in the initial MO and RhB concentration. The increase of photocatalyst dosage from 0.2 to 0.6 g/L results in a sharp increase of the photodegradation efficiency from 68.50 to 92.66% after 180 min of visible-light irradiation for MO degradation, and the increase of photocatalyst dosage from 0.2 to 0.4 g/L results in a distinct increase of the photodegradation efficiency from 65.72 to 90.85% after 180 min of visible-light irradiation for RhB. The elution of intermediates generated in the photocatalytic mineralization of MO and RhB resulted in an increase in total organic carbon (TOC) level, leading to the difference between TOC removal rate and MO and RhB decolorization rates.

Solar-driven advanced oxidation processes for full mineralisation of azo dyes in wastewater

Environmental contextFull mineralisation of synthetic azo dyes in industrial wastewater is a tough job for traditional wastewater treatment technologies. There is an urgent need for the development of both sustainable and environmentally friendly technology capable of fully mineralising these azo compounds. We show that solar-driven advanced oxidation processes are capable of completely mineralising azo compounds with high utilisation of solar energy. AbstractMineralisation of synthetic azo dyes in industrial wastewater is an energy-intensive process in treatment technology. The Solar Thermal Electrochemical Process for advanced oxidation processes (STEP-AOPs) utilises solar energy and electricity for the activation and electrooxidation of organic pollutants to harmless, small and non-toxic molecules with no other energy consumption. Based on molecular structure and chemistry, the STEP-AOPs for the treatment of azo dyes in wastewater, as exemplified with a typical azo dye, methyl orange, is reported for the first time. Thermodynamic calculations of the temperature-dependent potentials of methyl orange demonstrate that Gibbs free energy decreased by 161kJmol–1 and the potential decreased by 0.019V with an increase of temperature from 20 to 80°C, which indicates that the drop in both energy and potential specifically fits the STEP-AOPs technique. Experimental results showed that the STEP-AOPs achieved a total organic carbon (TOC) removal of 95.6% for methyl orange. The TOC removal rate improved by 39.8% and the unit TOC electricity consumption decreased by 53.8% at 80°C compared with conventional methods (20°C). The mineralisation mechanism for methyl orange was a gradual shortening of the molecular chain through cleavage of the azo bond, breakdown of the benzene ring and formation of inorganic small molecules susceptible to be oxidised to non-toxic small molecules, and carbon dioxide via STEP-AOPs. The evidence shows that the STEP-AOPs is capable of mineralising azo compounds completely.

Kinetics and operational parameters for 1,4-dioxane degradation by the photoelectro-peroxone process

This study investigated the degradation of 1,4-dioxane by the photoelectro-peroxone (PEP) process, which combines conventional ozonation, UV photolysis, and electrochemical hydrogen peroxide (H2O2) generation to effectively produce hydroxyl radicals (OH) for advanced oxidation wastewater treatment. Results show that the combination of ozonation, ultraviolet (UV), and electro-generation of H2O2 could lead to significant synergistic effects that enhanced the pseudo-first order rate constant for 1,4-dioxane degradation to 33 times that of the simple linear addition of the three single processes. In addition, the PEP process could mineralize total organic carbon (TOC) from 1,4-dioxane solutions much faster than the three single processes, as well as their binary combinations (UV/O3 and the electro-peroxone (EP) process). After 45min of treatment, the UV/O3, EP, and PEP processes removed ∼70%, 37%, and 98% TOC with a specific energy consumption (SEC) of ∼0.38, 0.22, and 0.30kWh/g TOCremoved, respectively. Increasing ozone (O3) dose, applied current, and solution pH increased generally the rate of TOC removal during the PEP process. When sodium chloride (NaCl) was used as the supporting electrolyte, chlorine (Cl2) and hypochlorous acid/hypochlorite (HClO/ClO−, formed from anodic oxidation of Cl−) would react with H2O2, thus diminishing its synergistic effects with O3 and UV for pollutant degradation. Consequently, TOC removal was much less efficient when the PEP process was conducted in NaCl electrolytes than in sodium sulfate (Na2SO4) electrolytes, especially when using anodes (ruthenium and iridium oxide coated titanium, RuO2-IrO2/Ti) with higher chlorine evolution activity. These results indicate that careful optimizations of the operational parameters are critical to maximize the synergistic effects of the PEP process for pollutant degradation.

Comparative Study of Reactive Dyes Oxidation by H2O2/UV, H2O2/UV/Fe2+ and H2O2/UV/Fe° Processes

ABSTRACTThe decolorization and mineralization of two reactive dyes C.I. Reactive Blue 4 (RB 4) and C.I. Reactive Blue 268 (RB 268) were studied using various advanced oxidation processes (AOPs) such as H2O2/UV, H2O2/UV/Fe2+, and the H2O2/UV/Fe°. All processes were performed within a laboratory-scale photo-reactor setup. The experimental results were assessed in terms of absorbance (A) and total organic carbon (TOC) reduction. The main degradation products were identified by high resolution gas chromatography/high resolution mass spectrometry analyses. The results of our study demonstrated that the additions of moderate concentrations of H2O2 and Fe catalyst during the AOPs evidently increased the decolorization efficiencies within the first few minutes of the processing time (5–10 min) for both tested dyes, and prolonged irradiation does not necessarily significantly improve decolorization. On contrary, TOC removal rate increased with the processing time and with the addition of the catalyst from 40–50% up to 70–80% at defined experimental conditions. All the tested AOPs were very successful methods for RB 268 decolorization, having very complex structure and much higher molecular weight compared to the dye RB 4. This is important from both economic and ecological points of view.

Catalytic ozonation of sulfamethoxazole by composite iron-manganese silicate oxide: cooperation mechanism between adsorption and catalytic reaction.

A systematic investigation of the cooperation mechanism between adsorption and catalytic reaction during the catalytic ozonation of sulfamethoxazole (SMX) by composite iron-manganese silicate oxide (FMSO) was carried out in this work. Results showed that the total organic carbon (TOC) removal increased significantly from 27% (sole-ozonation) to 79.8% (FMSO catalytic ozonation). The presence of FMSO in the ozonation process effectively enhanced the ozone utilization efficiency and accelerated the transformation of ozone into hydroxyl radicals. The latter result was verified by the indirect method, using NaHSO3 as the reductor, and the direct electron spin resonance (ESR) determination technology. The adsorption of SMX on FMSO was minimal (1.8%). However, ozone rapidly converted SMX into various intermediates, which was exhibited by the much higher adsorption affinity on the surface of FMSO than that of SMX. The accumulation of various intermediates on the FMSO surface also increased their contact probability with the ·OH radicals generated by the ozone decomposition. The continuous interaction of intermediates with ·OH radicals could further promote the benign cycling of the release of adsorption sites and the succeeding adsorption/decomposition of ozone and intermediates on FMSO. This could be another reason for the higher and faster TOC removal rate.

Evaluating photo-degradation of COD and TOC in petroleum refinery wastewater by using TiO2/ZnO photo-catalyst.

The aim of this study is to investigate the performance of combined solar photo-catalyst of titanium oxide/zinc oxide (TiO2/ZnO) with aeration processes to treat petroleum wastewater. Central composite design with response surface methodology was used to evaluate the relationships between operating variables for TiO2 dosage, ZnO dosage, air flow, pH, and reaction time to identify the optimum operating conditions. Quadratic models for chemical oxygen demand (COD) and total organic carbon (TOC) removals prove to be significant with low probabilities (<0.0001). The obtained optimum conditions included a reaction time of 170 min, TiO2 dosage (0.5 g/L), ZnO dosage (0.54 g/L), air flow (4.3 L/min), and pH 6.8 COD and TOC removal rates of 99% and 74%, respectively. The TOC and COD removal rates correspond well with the predicted models. The maximum removal rate for TOC and COD was 99.3% and 76%, respectively at optimum operational conditions of TiO2 dosage (0.5 g/L), ZnO dosage (0.54 g/L), air flow (4.3 L/min), reaction time (170 min) and pH (6.8). The new treatment process achieved higher degradation efficiencies for TOC and COD and reduced the treatment time comparing with other related processes.

Elimination of radiocontrast agent Diatrizoic acid from water by electrochemical advanced oxidation: Kinetics study, mechanism and mineralization pathway

Diatrizoic acid (DIA) is an iodinated X-ray contrast media commonly used in hospitals. It is persistent and recalcitrant in aquatic environments and difficult to biodegrade. Thus, we studied its removal from aquatic medium by two electrochemical advanced oxidation processes: anodic oxidation and electro-Fenton using BDD anode and carbon felt cathode. The effect of two of the most important variables on, i.e., applied current intensity (both processes) and catalyst concentration (electro-Fenton) was assessed to optimize oxidative degradation of DIA and mineralization rate of its aqueous solution. 1000mA current intensity and 0.2mM catalyst (Fe2+) concentration (at pH3) were determined as optimal operating values for its effective removal from water based on the following parameters: concentration decay of DIA, total organic carbon removal rate and mineralization current efficiency. The rate constant for oxidation of DIA by OH was determined using competition kinetics method and found be 3.96×108Lmol−1s−1. Obtained results showed a quick and complete mineralization of DIA solution. In addition, the identification of aromatic intermediates, short-chain carboxylic acids and mineral ions released to the solution during electro-Fenton treatment allowed us proposing a plausible mineralization mechanism of DIA by OH generated in the process. Finally, the toxicity of treated solutions along time was assessed following the luminescence inhibition of the marine bacteria Vibrio fischeri, using Microtox® method.

Evaluation of the solar photo-Fenton process to treat the petroleum wastewater by response surface methodology (RSM)

The objective of this study was to investigate the performance of employing solar photo-Fenton to treat petroleum wastewater from Sohar oil refinery, evaluate this process by a central composite design with response surface methodology and evaluate the relationship among operating variables such as pH, H2O2 dosage, Fe+2 dosage, and reaction time to identify the optimum operating conditions. Quadratic models proved to be significant with very low probabilities (<0.0001) for the following responses: total organic carbon (TOC) and chemical oxygen demand (COD). The optimum conditions were H2O2 dosage (850 mg L−1), Fe+2 dosage (60 mg L−1), pH (3.68) and reaction time (127 min) in this method. The experimental results of the maximum TOC and COD removal rates corresponded good with the predictions, which were 59.3 and 74.7 %, respectively. This method achieved well degradation efficiencies for TOC and COD and reduced the treatment time comparing with the previous work.

Performance of an Integrated Treatment System for Clean Drinking Water Production

This study is related to the removal of organics and other pollutants from drinking water using an integrated treatment system consisting of coagulation–sedimentation, ozonation and granular activated carbon (GAC)‐enhanced sand filtration. Standard methods and instruments were employed to measure turbidity, chemical oxygen demand determined by using the permanganate method as permanganate index (CODMn), UV254, total organic carbon (TOC), ammonia and nitrite of raw and treated water under controlled laboratorial conditions. The experimental results demonstrated that the total removal rates of turbidity, CODMn, UV254 and TOC were in the range of 98–99.9, 21–71, 33–73 and 20–47%, respectively, in different units of the integrated treatment system. GAC‐enhanced sand filtration significantly removed ammonia and nitrite, and the average removal efficiency was attained to 88 and 98%, respectively. In addition, most of the organic matter concentrations were observed to be reduced in GAC‐enhanced sand filtration and only small molecular weight compounds were detected in the effluent. All these pollutant reductions were noticed in the upper 350 mm of the activated carbon column in GAC‐enhanced sand filtration. The findings from this study suggest that the integrated treatment process can efficiently remove organics and other toxic pollutants from drinking water and can be a better alternative to conventional treatment.

Preparation of iron oxide-based porous ceramsite from goethite and application for city wastewater treatment in biological aerated filters

Ceramsite is a filter medium used in biological aerated filters (BAF). We prepared iron oxide-based porous ceramsite (IPC) by calcination in an O2 atmosphere with goethite, sawdust, and palygorskite clay. The properties of the IPC were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), thermal properties (TG), X-ray fluorescence (XRF), and porosimetry. Two kinds of IPC and commercial ceramsite (CC) were used in BAFs for the treatment of city wastewater. Goethite, sawdust, and palygorskite clay were used to produce IPC that was satisfactory for city wastewater treatment. IPC BAFs had average total organic carbon, P, TN, and NH3-N removal rates of 67.92, 76.55, 43.15, and 90.71%, respectively, much better than those of CC BAFs. The uniform pore size in IPC, with its many interconnections, made it suitable for microbial growth. IPC appears suitable for use as a BAF ceramsite for simultaneous removal of nitrogen and phosphorus from city wastewater.

Evaluating the TiO2 as a solar photocatalyst process by response surface methodology to treat the petroleum waste water

The aim of this study is to investigate the performance of employing the solar photo-catalyst of TiO2 to treat petroleum wastewater from Sohar oil Refinery (SOR), evaluate the performance of employing this process by a central composite design (CCD) with response surface methodology (RSM) and evaluate the relationships among operating variables such as TiO2 dosage, pH, C0 of COD, and reaction time to identify the optimum operating conditions. Quadratic models prove to be significant with very low probabilities (<0.0001) for the following two responses: total organic carbon (TOC) and chemical oxygen demand (COD).TiO2 dosage and pH are the two main factors that improved the TOC and COD removal while C0 of COD and reaction time are the actual factors. The optimum conditions are a TiO2 dosage (0.6 g/L), C0 of COD (1600 ppm), pH (8), reaction time (139 min) in this method. TOC and COD removal rates are 15.5% and 48.5%, respectively. The predictions correspond well with experimental results (TOC and COD removal rates of 16.5%, and 45%, respectively). Using renewable solar energy and treating with minimum TiO2 input make this method to be a unique treatment process for petroleum wastewater.