Studies On Characteristics, Mechanism And Effluent Biotoxicity For Enhanced Treatment Of Coking Wastewater

A new coking wastewater discharge standard will be implemented in China, andthe COD and ammonia emission limits were recommended as50mg/L and10mg/L,respectively. However, the coking wastewater treatment in many plants could not evenreach the current COD and ammonia emission limits of100mg/L and15mg/L.Therefore, it could provide a technical support for the implementation of the newstandard to improve the coking wastewater treatment process and enhance the removalof pollutants. The purpose of this study is to explore an biologically/chemicallyenhanced treatment process for coking wastewater in order to meet the new dischargestandard.A combined anaerobic/anoxic/bio-zeolite–membrane bioreactor （A₁/A₂/ZB-MBR）process was used to treat real coking wastewater, and the influence of operatingparameters on the treatment efficiency was investigated. The results showed that whenHRT was73h, and the reflux ratio was2:1, and the dissolved oxygen in ZB-MBR was4-6mg/L, the effluent ammonia of5.6±4.1mg/L could reach the new emission standardstably, but the effluent COD （145±27mg/L） failed to meet the new emission standard.The recalcitrant contaminants, such as phenol, pyridine, quinoline, naphthalene,and carbazole, could be effectively removed by adding isolated strains in ZB-MBR.However, the COD removal was limited with the effluent COD of about130mg/L.Moreover, the analysis of bioaugmented effluent showed that the organic matters incoking wastewater were complex, and the concentration of each organic was low. Forexample, the concentration of octadecane was only0.38mg/L, which is proved to berelatively higher than other organics by GC/MS. Chemical treatment was conducted tofurther reduce the effluent COD. PAC coagulation could reduced the CODconcentration to about110mg/L, close to the existing emission limit. The effluent CODof Fenton and electro-Fenton oxidation was below50mg/L, which could meet the newemission limit.The microbial community structure was investigated by T-RFLP andhigh-throughput sequencing technology. The results showed that the membrane in thesystem promoted the growth of ammonia oxidation bacteria Nitrosospira, and the number of Nitrosomonas and Nitrobacter increased significantly after the addition ofzeolite in the system. They became the dominant microorganisms for ammoniatransformation in ZB-MBR. In addition, both the microbial diversity and the number ofthe main microbial groups increased after the addition of high efficiency strains.The determination of whole effluent toxicity for effluent of different enhancedprocesses showed that the acute and chronic toxicity of A₁/A₂/ZB-MBR andbioaugmentation effluent were low. However, Fenton and electro-Fenton treatmentincreased the acute and chronic toxicities of the effluent significantly, which indicatedthat chemically enhanced treatment may increase the ecological risk of the effluent to acertain extent. It is not always beneficial for water safety to select COD as the primarycontrol index.