Treatment Of Biologically Pretreated Coal Chemical Wastewater By Catalytic Ozonation Integrated With A/O-MBR

The energy characteristics of “rich in coal,less in oil and shortage of gas” brings about a consequence that coal is the main consumption resource in China.Coal chemical industry will play an increasingly important role in energy transition.However,amount of coal chemical wastewater is large and the wastewater includes high concentration of pollutants.In addition,conventional biological processes are not efficient in removing these compounds.As a result,the effluent still contains lots of toxic and refractory pollutants.In view of deteriorating water environment and lacking water resources,China sets the strict discharge standard for coal chemical industry.The effective and stable advanced technology was the efficient way to achieve the safe discharge of coal chemical wastewater.This study prepared the nano MgO and characterized its physical structural characteristics and surface chemical properties.The nano MgO was applied in catalytic ozonation process to explore its efficiency for advanced treatment of coal chemical wastewater and the feasibility of anoxic-oxic membrane reactor(A/O-MBR)for wastewater treatment was evaluated.Based on the advantages of catalytic ozonation and A/O-MBR,the integrated system was set up and its efficiency for advanced treatment of coal chemical wastewater was evaluated in lab and pilot scale.The nano MgO was prepared by homogeneous precipitation method with magnesium chloride and sodium hydroxide.The optimized aging time,calcination temperature and calcination time was 24 h,500 ℃ and 2 h,respectively.By characterizing the physical structural characteristics and surface chemical properties of nano MgO,it was shown that the surface of nano MgO was irregular and contained a lot of spherical particles.With the average particle size of about 17.1 nm,the nano MgO possessed high specific surface area.In addition,surface hydroxyl density of nano MgO was 4.82 mmol/g with neutral p Hpzc and the active component of nano MgO was periclase MgO crystalline powers.The nano MgO was applied in catalytic ozonation process to investigate its efficiency in advanced treatment of coal chemical wastewater.The results showed that the addition of nano MgO into ozone system could significantly increase pollutants removal.After 90 min,the average chemical oxygen demand(COD)removal efficiency in catalytic ozonation process was about 22.3% more than that of ozonation process.The order of reaction rate constant between ozone and typical nitrogenous heterocyclic organics was: indole>quinoline>pyridine.Quinoline,pyridine and indole could not be detected in the effluent of catalytic ozonation system.In the catalytic ozonation process,nano MgO could promote the transfer of ozone from gas phase to liquid phase and improve ozone transfer efficiency.Compared with ozonation process,the ozone utilization efficiency in catalytic ozonation process increased by 31.8%.By analyzing the effect of tert-butanol and phosphate on COD removal,it was found that the surface hydroxyl group of nano MgO was the active site and the higher catalytic activity was achieved at neutral p H value.The integrated system of catalytic ozonation and A/O-MBR was set up to treat the biologically pretreated coal chemical wastewater.The refractory pollutants were transformed into easy-biodegradable substances in catalytic ozonation process,thus increasing the biodegradability and decreasing acute biological toxicity of wastewater.The effluent of catalytic ozonation process was suitable for subsequent biological treatment.With the catalytic ozonation time of 30 min and hydraulic retention time of 12 h in A/O-MBR,the integrated system possessed high pollutants removal efficiency.The average effluent concentration of COD,ammonium nitrogen and total nitrogen(TN)was 40.2 mg/L,3.6 mg/L and 14.5 mg/L,respectively.The effluent characteristics satisfied first grade of the integrated wastewater discharge standard.The integrated system possessed strong shock resistance ability with phenols stress threshold of 250 mg/L.Compared with the A/O-MBR process,the integrated system possessed higher sludge concentration and lower transmembrane pressure.During the 120 cycles,the nano MgO still possessed high catalytic activity and the final effluent was stable.Based on the results of lab-scale experiment,the pilot-scale integrated system of catalytic ozonation and A/O-MBR was set up for advanced treatment of coal chemical wastewater.The combined system presented stable and efficient performance.The Pseudomonas and Sphingobacteria were the dominant genera and attributed for organic pollutants removal and nitrogen reduction.The results showed that the effluent concentration of COD,ammonium nitrogen and TN was lower than 50.0 mg/L,5.0 mg/L and 15.0 mg/L.The performance of nano MgO was stable after continuous use of 20 d with the average COD removal efficiency decreasing from 35.2% to 27.5% and low magnesium ions left.With advantages of high performance and stable operation,the combined system possessed high value in engineering application.