| Acrylonitrile as a kind of important chemical raw material was widely used in the field of manufacture of acrylic fiber, ABS industrial plastic, nitrile rubber and synthetic resin etc. Acrylonitrile production wastewater mainly contains poisonous and harmful materials such as acrylonitrile, acetonitrile, hydrocyanic acid and a large number of refractory oligomers. Most domestic producers of acrylonitrile mainly adopt biological treatment processes like activated sludge process to treat acrylonitrile wastewater, but the treatment effect is not ideal, and the effluent COD concentration is above 100 mg·L-1, which cannot meet the petrochemical industry wastewater discharge standards,50 mg·L-1. On the other hand, the biochemical process tail water also contains a certain amount of acrylonitrile and other poisonous and harmful substances. In order to meet the wastewater discharge standards, advanced treatment of biochemical tail water is needed. Hydroxyl radicals generated by advanced oxidation technology can efficiently remove refractory organic matter from tail water. Therefore, it is necessary to study this kind of advanced oxidation process to treat the tail water and carry out technology integration. In this paper, we mainly study the advanced oxidation processes of Fenton, UV/Fenton, and UV/H2O2, to investigate the treatment effect on the biologically treated acrylonitrile wastewater in an acrylic fiber plant at northeast of China, and to optimize the process parameters. The toxicity of the advanced oxidation process treated wastewater and the removal situation of the characteristic pollutants were measured, and finally the running costs of the advanced oxidation processes used were calculated. Specific results are as follows:1. Homogeneous Fenton process:The effects of Fe2+ dosage, H2O2 dosage, initial pH and reaction time on tail water’s COD removal were evaluated according to the single-factor test, and response surface methodology based on central composite design was developed to optimize the parameters of the Fenton process. The optimum reaction conditions were:Fe2+ dosage 1.02 mmol·L-1, H2O2 dosage 11.13 mmol·L-1, initial pH 3.66, and reaction time 105 min. Under the optimum reaction conditions, the COD removal efficiency was 61.1% and the COD concentration of treated effluent was lower than 50 mg·L-1, which could meet the strictest discharge standard of the petrochemical industry. The Fenton process showed high removal efficiencies on the typical pollutants. Under the optimum reaction conditions, the removal efficiencies of acrylonitrile, m-phthalonitrile, and 3-cyanopyridine were 99.5%,97.6%, and 73.7%, respectively. The degradability of these three pollutants followed the order: acrylonitrile> m-phthalonitrile> 3-cyanopyridine. Excitation-emission matrix spectra revealed that large amount of fulvic-like fluorescent substances existed in the bio-treated effluent of acrylonitrile wastewater, and the ultra-violet fulvic-like substance accounted for the largest proportion. The Fenton process could effectively remove the refractory substances in short reaction time and with a small amount of reagent dosage. The running cost of the process is 3.013 yuan/ton.2. UV/H2O2 process:The effects of H2O2 dosage, initial pH, reaction time and UV lamp power on tail water’s TOC removal were evaluated according to the single-factor test, and response surface methodology based on Box-Behnken design was developed to optimize the parameters of the Fenton process in view of the three main factors affecting the wastewater treatment. The optimum reaction conditions were H2O2 dosage 8.5 mmol·L-1, initial pH 3.77, and reaction time 81min. The degradability of these six pollutants followed the order:5-methyl-H-benzo triazole>acrylonitrile>7-azaindole>m-phthalonitrile>fumarodinitr-ile>3-cyanopyrdI ne. In comparison with the raw tail wastewater, the luminous intensity of the luminous bacteria of the advanced oxidation treated tail water decreased by 35%, showing that the advanced oxidation method has played an important role for the removal of the tail wastewater toxicity. The running cost of the UV/H2O2 process is:6.378 yuan/ton.3. UV/Fenton process:The effects of Fe2+ dosage, H2O2 dosage, initial pH, reaction time and UV lamp power on tail water’s TOC removals were evaluated according to the single-factor test, and response surface methodology based on central composite design was developed to optimize the parameters of the Fenton process in view of the four main factors affecting the wastewater treatment. The optimum reaction conditions were Fe2+ dosage 0.8 mmol·L-1, H2O2 dosage 3.88 mmol·L-1, initial pH 3.39, and reaction time 27.5 min under the irradiation of the ultraviolet lamp with the power of 10W power. Under the optimum reaction conditions, the TOC removal efficiency was close to 100%. The degradability of these six pollutants followed the order:5-methyl-H-benzotriazole>7-azaindole>acrylonitrile>m-phthalo nitrile>fumaro-dinitrile>3-cyan-opyridine. Excitation-emission matrix spectra revealed that Fenton process effectively removes main pollutants of the tail water in shorter reaction time and less reagent dosage conditions. According to the toxicity of the luminescent bacteria, it was found that:under neutral conditions toxicity test,100% of the original tail wastewater sample luminosity was weakened. At the same time, the luminous intensity of the luminous bacteria of the raw tail wastewater samples was decreased by 12% through the advanced oxidation under optimal conditions, showing that the advanced oxidation method has played an important role for this kind of wastewater treatment. The running cost of the UV/Fenton process is:20.41 yuan/ton.4. In terms of the organic pollutant removal capabilities of the advanced oxidation, there are:UV(15W)/Fenton> UV(10W)/Fenton> UV(15W)/H2O2> UV(10W)/H2O2> UV(5W)/Fenton>Fenton. |
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