Treatment Of Phenol Wastewater By Fenton-two-stage Contact Oxidation Process

With the development of China’s economy,the emission amount of industrial wastewater is increasing within decade.The water pollution has arisen roar attention.Phenol wastewater,originating from plastic,pharmaceutical,paper,printing and dyeing industries,is a typical high concentration,toxic and hard-to-degrade organic wastewater.Therefore,many new combined treatment processes have emerged for phenol wastewater treatment at present,among which most of the hard-to-degrade organic wastewater is currently treated by biological methods.However,the toxicity in phenol wastewater will inevitably affect the biodegradation ability,which in turn affects the final treatment effect.Fenton oxidation that favored for its advantages of strong oxidation,short treatment time and no secondary pollution,which displayed great potential as a pretreatment process in synergy with biological treatment technology.The organic components in phenol wastewater are complex,among which COD,NH4’-N and phenol are the priorities of wastewater treatment.In this experiment,Fenton oxidation was applied for pretreatment to convert the hard-to-degrade organic substances into easily degradable molecular substances in this experiment.And then the biological contact oxidation process is used to further degrading processes.Our study applied a combined Fenton and two-stage biological contact oxidation process to treat phenol wastewater,and we also discussed the operating conditions for the optimal treatment of phenol,COD,NH4’-N and TN,so as to lay the ground for the treatment of phenol wastewater within engineering practice.The main results are as follows:(1)Fenton oxidation pretreatmentWith phenol wastewater as the pretreatment object,the optimal working conditions of oxidation reaction were determined by orthogonal and single-factor tests at the influent phenol,BOD and COD concentrations of 91～105 mg/L,60～70 mg/L and 457～562 mg/L.When the ratio of H2O2 to Fe2+ mass is 2.5:1,pH=4 and the reaction time is 30 minutes,the average removal rate of phenol and COD can reach 81.99%and 84.31%,and the average concentrations of phenol,BOD and COD in the final effluent are 18.4 mg/L,51 mg/L and 88 mg/L.The Fenton oxidation process oxidized large organic compounds such as phenols in the wastewater to small molecules,and the B/C increased from the initial 0.13 to 0.58 after the reaction.The wastewater becomes biodegradable,which is more conducive to the adsorption and degradation of pollutants by subsequent biological methods.(2)Start-up of two-stage biological contact oxidation process with hanging filmA certain amount of activated sludge is inoculated into the biological contact oxidation tank,and the water is continuously fed and aerated when the hydraulic load is low,and then the influent flow rate is gradually increased to the design flow rate(10L/h).The water temperature at start-up is 18-22℃,pH=6,and the DO concentration of the biological contact oxidation tank is 2.5-3.5 mg/L.The HRT of the primary oxidation tank is 6 hours,and the organic load is within 1.8 kgCOD/(m3·d);the HRT of the secondary oxidation tank is 9 hours,and the organic load is within 0.9 kgCOD/(m3·d).The influent COD,NH4+-N and TN were 457～562 mg/L,51～79 mg/L and 70～100 mg/L respectively.With the extension of the hanging time,the removal rates of COD,NH4+-N and TN by the reactor were measured to reach 82.82%,96.06%and 71.09%respectively.After stable operation of the system,a layer of yellow-brown biofilm was adhered to the surface of the elastic packing of the reactor,indicating that the film was successfully installed.A total of 27 microbial phyla were detected by high-throughput sequencing,with Proteobacteria(46.52%)and Bacteroidetes(15.41%)as the dominant phylum;A total of 58 phyla were detected,with Sphingobacteriales(12.08%),Planctomycetales(9.12%)were detected as the dominant phylum.(3)Fenton-two-stage contact oxidation processAfter the start-up of biofilm formation is completed,parameters such as HRT,organic load and dissolved oxygen are controlled to further optimize the operating conditions.The experimental results showed that the removal rates of phenol and COD in the Fenton oxidation section were 81.99%and 84.31%respectively,and the removal rates of COD,NH4+-N and TN in two-stage biological contact oxidation section were 79.44%,97.17%and 71.69%respectively.The final removal rates of phenol,COD,NH4+-N,TN and phenol in wastewater were 95.92%,96.70%,97.36%and 74.58%respectively,with the combined Fenton-secondary biological contact oxidation process,which is a reasonable treatment process for phenol wastewater.(4)Fenton reaction kinetic equationThe main factors affecting the degradation of phenol wastewater by Fenton’s method include the initial H2O2 concentration,the initial Fe2+ concentration and the initial phenol concentration,while keeping the external conditions constant.A kinetic model was developed for the oxidative degradation of phenol by Fenton’s method based on the chemical reaction kinetics.The kinetic equation for the oxidative degradation of phenol by the Fenton method was obtained by measuring the phenol content in the wastewater with the reaction time and calculating the relevant graphs:①v=-dC/dt=3.5 ×10-6R0.3744S0.8505C2.4469(S<40.17mg/L);②v=-dC/dt=1.5×10-4R0.3744S(-0.1692)C2.4469(S≥40.17mg/L)。In summary,the combined Fenton-two-stage contact oxidation process has a good degradation effect on phenol wastewater,which can provide reliable technical support for the practical application of the project.