Study On Degradation Of Phenolic Wastewater With Ozone-Based Advanced Oxidation Processes Intensified By A High Gravity Method

The organic wastewater becomes a big problem in petrochemical industry, printing and dyeing industry and pharmaceutical industry with the rapid development of China’s economy. Phenolic wastewater is a typical organic wastewater with significant impact on environment. Chemical oxidation followed by biotreatment is an efficient method for phenolic wastewater treatment. Ozone-based advanced oxidation processes, such as O3/H2O2process and homogeneous catalytic ozonation, have been used in wastewater treatment field due to the high oxidizability and no secondary pollution. However, the low solubility and utilization rate of ozone limit the application of ozone-based advanced oxidation processes.In this study, the high-gravity technology (rotating packed bed, RPB) was used to intensify the ozone-based advanced oxidation processes, including O3/H2O2, O3/Fe(II) and O3/H2O2/Fe(II)(O3/Fenton) processes. The effects of different operation conditions on phenol degradation were investigated and the operation conditions in high gravity environment were optimized using response surface method (RSM). The relative concentrations of hydroxyl radicals and phenol degradation mechanisms in different oxidation processes were compared, with an attempt to provide a new process for the rapid treatment of phenolic wastewater. This study has reached the following conclusions:(1) Single-factor method was used to investigate the catalytic ozonation of phenol with O3/Fe(Ⅱ) process in RPB. In acidic environment, the phenol degradation rate in O3/Fe(Ⅱ) process was about10%higher than that in O3process. The mass transfer coefficient was doubled and the phenol degradation rate increased0.7times in O3and O3/Fe(Ⅱ) processes when the high gravity level in RPB increased from2g to175g. According to the result of the RSM experiments, the phenol degradation rate in O3Fe(Ⅱ) process reached70%at the Fe(Ⅱ) concentration of0.58mM, O3concentration of60mg/L, liquid flow rate of10L/h, rotating speed of1500rpm respectively. The ozone utilization rate of O3/Fe(Ⅱ) process reached0.096mol/mol (phenol/O3) at the Fe(Ⅱ) concentration of0.5mM, O3concentration of30mg/L, liquid flow rate of30L/h, rotating speed of1500rpm respectively. After the phenol solution was treated by O3/Fe(Ⅱ) process, the BOD5/COD value (biodegradability) of the phenol solution increased significantly from0.2to0.56which is much higher than the requirement of biochemical treatment (>0.3).(2) The kinetics of the phenol degradation in O3/H2O2-RPB process was investigated in this study. The apparent reaction rate constant of phenol degradation and COD degradation increased1times and0.6times respectively when the high gravity level in RPB increased from8g to200g. Experimental results indicated that the phenol and COD removal rate reached96%and75%under the suitable operation conditions:H2O2concentration of1.3mM, O3concentration of60mg/L, initial pH of9, reaction temperature of25℃, and rotating speed of1500rpm.(3) The ozonation of phenol in03/Fenton process intensified by an RPB was investigated in this study. It was found that the phenol degradation in03/Fenton process was about20%higher than that in O3process and reached98%at the H2O2concentration of1.4mM. The phenol degradation rate of O3and03/Fenton processes increased0.6times and0.3times respectively when the high gravity level in RPB increased from8g to200g. The phenol degradation rate in O3/Fenton process reached98.6%under the operation conditions:Fe(II) concentration of0.17mM, H2O2concentration of1.1mM, O3concentration of50mg/L, initial pH of6, and rotating speed of1000rpm. The BOD5/COD value of the phenol solution increased from0.2to0.58after the phenol solution was treated by03/Fenton process.(4) The ratios of hydroxyl radicals concentration to ozone concentration (Ret) in O3, O3/Fe(II), O3/H2O2and03/Fenton processes were studied. It was found that the addition of catalyst could improve the formation of hydroxyl radicals; addition of H2O2and Fenton reagent could increase both the formation efficiency and amount of hydroxyl radicals, addition of Fe(II) could increase the formation efficiency of hydroxyl radicals but decrease oxidant amount (∫[·OH]dt) The Rct value in O3/Fenton process was about80times higher than that in O3process, but the total hydroxyl radical amount in03/Fenton process was only3times higher than that in O3process. Therefore, advanced oxidation processes have higher oxidation rate and low selectivity but could not increase the oxidation amount. The application of advanced oxidation processes will increase the treatment efficiency and decrease the equipment size in wastewater treatment.(5) The addition of Fe(Ⅱ) increased the amount of hydroquinone in O3/Fe(Ⅱ) process due to the reduction of benzoquinone by Fe(Ⅱ) at pH=2. There were more free radicals such as phenoxy radicals in O3/Fe(Ⅱ) process leading to the formation of some dimethyl phthalate and polymers. The intermediates concentration in O3process was lower in neutral environment than that in acidic environment. Only residual phenol and a small amount of1,2-benzodiazepines, dimethyl phthalate, and phenylacetate were determined in O3process under neutral environment. A relatively thorough phenol degradation was attained in O3/Fenton process and most intermediates were degraded to small organic acids. So there were almost no intermediates found in O3/Fenton process. It also explained why phenol solution had a significantly higher BOD5/COD after treated by O3/Fenton process than by O3process.