Treatment Of Bio-treated Coking Wastewater By Combination Of Coagulation And Micro-bubble Ozonation

Coking wastewater is a typical refractory industrial wastewater from coking industry,which has the characteristics of high pollutant concentration,large chroma,complex components and high toxicity.Traditional biological treatment is difficult to effectively treat it to meet the discharge standard.Therefore,advanced treatment of Bio-treated coking wastewater(BTCW)is usually required.Since coagulation can economically and efficiently remove colloid,suspended solid and part of organic matters in wastewater,it is usually used as a pretreatment for advanced treatment.In recent years,micro-bubble ozonatiom oxidation(OMB)can overcome the shortcoming of low solubility of ozone in water and insufficient selectivity of oxidation during ozonation alone,which can further enhance the removal efficiency of pollutants.Accordingly,the present study focused on the treatment effect of BTCW using the coagulation pretreatment combined with the ozonation alone or OMB oxidation,and explored the reaction mechanism of the pollutants in BTCW using some characterization techniques.The effects of coagulant pretreatment on the removal of organic matters in BTCW were investigated.The effects of coagulant type,coagulant dosage,initial p H,reaction temperature and coagulant Polyacrylamide(PAM)on the removal of organic matters in BTCW were investigated.Polyferric sulfate(PFS)and Polyaluminium chloride(PAC)could remove 73.3%and 53.0%COD under the optimal conditions,respectively.Coagulant PFS is less affected by the p H change of the solution.As for the mechanism on the removal of organic matters by coagulation,it was found that the UV absorbance of the solution after coagulation at the band of 190-370 nm was significantly reduced,indicating that the content of PAHs and nitrogen-containing heterocyclic compounds in the wastewater was reduced.In addition,the contents of aromatic proteins,humic acids and fulvic acids were also significantly reduced,and the removal efficiency of organic matters by PFS was much better than that of PAC coagulant.From the perspective of molecular weight of organic matters,PAC has a better performance on the removal of organic matters between the molecular weight of less than 2k Da and 10-20k Da,while PFS has a better perfomance on the removal of organic matters for each component of molecular weight compared to PAC.Secondly,the treatment effect of ozonation alone for BTCW after coagulation pretreatment was investigated,and the influence of ozone dosage,initial p H and reaction temperature on the removal effect of organic matters in BTCW was studied.The initial p H9,T 25 ~oC and ozone dosage of 10 mg/min were selected as the experimental conditions for these experiments.The COD removal efficiency increased with the increase of p H and temperature.The COD removal efficiency was 19.0%under optimal conditions.The results of UV-vis scanning showed that the UV absorbance of the solution at the band of 190-370nm after ozonation alone was significantly decreased,and the content of PAHs and nitrogen-containing heterocycle compounds in the system were also decreased.The HPSEC results showed that the organic structure of large molecules was destroyed and decomposed into small molecules,resulting in a significant decrease in molecular weight during ozonation alone.The content of long-chain alkanes and benzenes decreased while that of amides and halohydrocarbons increased after ozonation.Organics such as furic acid and humic acid are effectively degraded by ozonation alone.Finally,the treatment effect of OMB system for BTCW after coagulation pretreatment was investigated,and the effect of ozone dose,initial p H and reaction temperature on the degradation efficiency of organic matters in BTCW was studied.The removal efficiency of COD increases with the increase of initial p H and reaction temperature.The removal efficiency of OMB process reached the maximum in 10 min,which is 5 min earlier than that of ozonation alone.The COD removal efficiency of BTCW by OMB was almost twice as effective as higher than that of ozonation alone.The results of various analysis showed that the degradation of organic matter in OMB system was more thorough compared to that of ozonation alone,especially for the long chain alkanes and polycyclic aromatic hydrocarbons(PAHs)with high concentration in wastewater.The degradation efficiency of furic acid in the OMB process is faster than ozonation alone.Combined with the analysis of ozone consumption and COD removal,it was found that the OMB process had a faster oxidation rate and a higher ozone utilization efficiency.