Removal Effect Of Organics From Petrochemical Secondary Effluent By Catalytic Ozonation

Petrochemical wastewater is a typical refractory industrial wastewater with complex water quality,various high concentration pollutants.Meanwhile,high requirements have been put forward with the issue of Emission Standards of Pollutants for Petroleum Chemistry Industry（GB 31571-2015）by Ministry of Environmental Protection in 2015.However,existing treatment technologies can hardly meet the requirements of the new standards,which need to develop appropriate advanced treatment and upgrading technologies to improve the quality of effluent and reduce operating costs.In order to define the refractory components,the influence of catalytic ozonation on the removal characteristics of organics with different molecular weight fractions after ultra-filtration classification was taken into account and the results will guide to effective removal of residual refractory substances.In the meantime,to support the application of combined pretreatment-catalytic ozonation process,the effects of coagulation pretreatment and different molecular weight components on catalytic ozonation were also discussed in this study.The Petrochemical Secondary Effluent（PSE）was investigated by ultra-filtration membrane separation.The average Chemical Oxygen Demand（COD）and Dissolved Organic Carbon（DOC）of PSE were 106.0 mg/L and 28.7 mg/L respectively.The result showed that organics on the fraction of molecular weight less than 1 Kilo-Dalton（K）had the largest ration in total organics,which is higher than 90%.Secondly polysaccharides and proteins are mainly small molecules in PSE.Polysaccharides accounted for 40.59%of total polysaccharides in less than 1 K.In contrast,macromolecule of polysaccharides makes contribution to both 10-30 K and larger than 100 K.Thirdly,74.70%of the total protein remains in less than 1 K in PSE.Last but not least,the highest value of Specific Ultra-Violet Absorbance（SUVA）and the aromatic structure shows in3-5 K.According to the three-dimensional fluorescence spectroscopy combined with three-dimensional fluorescence region integration method,the area II（protein-like substances）and area IV（microbial metabolites）of each component had the largest integral standards volume,which accounted for 65.85%and 11.27%respectively.It also indicated that the main organic substances in PSE were protein-like substances and microbial metabolites.The optimum ozone dosage was 40.0 mg/L within 1 h oxidation time,and the optimum catalyst dosage was 750.0 g/L.Based on the operating parameters above,the results indicated that catalytic ozonation has obvious removal effect on DOC with molecular weight less than 3 K,while the better effect on protein removal of components with molecular weight less than 1 K,1-3 K and 10-30 K.Meanwhile,the fraction of polysaccharides showed decreased in less than 3K,10-30 K and more than 100 K.It also suggested that SUVA values increased with molecular weight less than 1 K and 10-30 K,which means can be oxidized by ozone easily.In the process of catalytic ozonation,the aggregation of small molecular organic matter or the decomposition of large molecular organic matter might lead to the increasing of DOC,polysaccharide,protein,UV₂₅₄ and standards integral volume of Three-Dimensional Excitation Emission Matrix fluorescence spectroscopy（3D-EEM）within some molecular weight components.In this study,Poly Aluminum Chloride（PAC）was used as the coagulant and the removal characteristics of dissolved organic in different molecular weight ranges at different dosages and their effects on catalytic ozonation were investigated.The results showed that when the dosages of PAC ranged from 5.0 to 50.0 mg/L,the concentration of COD and DOC increased at the beginning and then decreased when the dosage was over 25.0 mg/L.The optimized coagulant dosage was 25.0 mg/L for PAC.The concentration of COD and DOC were 85.9 mg/L and 25.3mg/L for PAC on the optimized dosage.After 2 h of coagulation and precipitation,the supernatant was taken for catalytic ozonation.When the ozone dosage was 40.0 mg/L,the COD and DOC were 35.4 mg/L and 46.8 mg/L for coagulation and 11.7 mg/L and 13.6 mg/L for un-coagulation.When the ozone dosage was 20.0 mg/L,the COD and DOC were 39.6 mg/L and13.3 mg/L for coagulation pretreatment,respectively.It has no significant difference with ozone dosage was 40.0 mg/L for un-coagulation pretreatment,which could meet the standards from GB31571-2015.Coagulation pretreatment improves the removal rate of dissolved organics with molecular weight of less than 1 K,3-10 K and 30-100 K and enhances the degree of aromatization with molecular weight of 1-5 K.The removal rates of proteins with molecular weight less than 1 K,3-10 K and greater than 30 K and polysaccharides with molecular weight less than 10 K and greater than 30 K were improved.3D-EEM combined with Fluorescence Region Integration（FRI）were analyzed.The solubility microbial metabolites with molecular weight greater than 30 K were significantly reduced after coagulation pretreatment,which was consistent with the removal effect of protein and polysaccharide.The coagulation pretreatment reduces the ozone dosage and improves the effect of catalytic ozonation on the PSE,which can save about half of ozone dosage and significantly reduce the treatment cost.