Study On Application Of Catalytic Ozonation In Advanced Treatment Of Refractory Organics

The discharge of municipal and industrial wastewater is the main source of water pollution. The secondary effluents of these wastewater treatment plants are usually unable to meet the wastewater discharge standards, owing to refractory organics contained in the industrial wastewater or mixed in municipal wastewater. Further improvement of the effluent quality is necessary before discharging. Therefore, the aim of this study is to provide a catalytic ozonation approach for advanced treatment of refractory organics remained in the secondary effluents of municipal and industrial wastewater treatment plants.Six types of catalysts for ozonation were compared, including wood activated carbon, coconut shell activated carbon, activated coke, coal activated carbon, γ-Al2O3 particles and Mn O2 particles, through catalytic ozonation batch experiments with oxalic acid as a model substance in ozonation process. The results indicated that the catalytic effect of wood granular activated carbon(GAC) was the best with TOC removal of 31.6%. The impact of particle size and pore size of GAC on the catalytic ozonation effects was also studied. The results illustrated that the catalytic ozonation effect was the best when 70~100 mesh activated carbon was used, with TOC removal efficiency of approximately 92.47%. Meanwhile, the pore structure of activated carbon also affected the catalytic ozonation effects. When the activated carbon with iodine values of 500~800 mg/g and 1000 mg/g were used to degrade oxalic acid by catalytic ozonation, TOC removal rates were around 50% and 29.2% respectively, which indicated TOC removal rate was related to the iodine value.On the basis of the results of batch experiments, an activated carbon catalytic ozonation-ceramic membrane continuous flow reactor was set up. The effects of hydraulic retention time(HRT) and GAC dosage on oxalic acid degradation were studied. The results showed that TOC removal efficiency was the best(~72%) with the GAC dosage of 15 g/Lreactor and HRT of 50 min, ozone dosage is 125 mg O3/Lwater. The reactor was respectively utilized to treat the secondary effluents of municipal wastewater and coal gasification wastewater. On one hand, the average removal rates of chroma, UV254 and TOC were 91.2%, 82.6% and 80.4%, respectively, when the secondary effluents of municipal wastewater were treated with HRT of 50 min. On the other hand, when the secondary effluents of coal gasification wastewater were treated, the average removal rates of chroma, UV254, TOC and COD were 91.9%, 75.1%, 34.6% and 54.5%.Moreover, the impact factors of membrane fouling were investigated using the activated carbon catalytic ozonation-ceramic membrane reactor. The results implied that aeration helped to control membrane fouling. The amount of activated carbon deposited on the membrane surface was limited. The wastewater with high concentration of small molecular substances or TOC could more easily cause membrane fouling. Whereas the addition of ozone might help to reduce membrane fouling. And the chemical cleaning can significantly recovered the membrane flux.