Investigation On Degradation Of Coking Wastewater And Enhancement Of Its Biodegradability By Gas Phase Surface Discharge Plasma

Coking wastewater is a kind of intractable wastewater which is composed of complex contaminants such as phenolic compounds, ammonia and polycyclic aromatic hydrocarbons (PAHs). Moreover, most of these pollutants are toxic, refractory, mutagenic and carcinogenic, and will result in environmental pollution if the wastewater is discharged directly into rivers. Because of the presence of refractory and biologically inhibitory organic compounds in coke-plant wastewaters, the conventional activated sludge system is not efficient in removing COD and NH3-N.Gas phase surface discharge plasma utilizes the free radical (·OH) and ozone with strong oxidation potential to oxidize and remove refractory pollutants. Gas phase surface discharge plasma is an effective method for mineralizing recalcitrant pollutants in industrial wastewaters. The use of the gas phase surface discharge plasma as a pretreatment process to convert persistent organics to intermediates that are more readily degradable in conventional biological treatment processes, could be attractive for coking wastewater treatment.In this study, the enhanced biodegradability of coking wastewater pretreated by gas phase surface discharge plasma was investigated and the impact factors including pH, air flow rate were examined. The main results are summarized as follows:(1) The influence of pH, initial concentration, and applied voltage on phenol degradation was examined.At an initial pH of about9, air flow rate of2min/L and initial phenol concentration of80mg/L, and applied voltage of7kV and frequency of7kHz, phenol removal reached100%in40min. Experimental results reveal that phenol can be effectively remove by gas phase surface discharge system.(2) The effect of initial concentration, initial pH, and air flow rate on coking wastewater degradation and the biodegradability were investigated. At an initial pH of about8.5, air flow rate of2min/L and initial COD concentration of340mg/L, and applied voltage of7kV and frequency7of kHz, COD removal reached70%in120min. Several organic compounds, including phenol,bifuran, quinoline, resorcinol and benzofuranol were removed completely as determined by GC-MS analysis. The removal of these compounds from coking wastewater depends on the properties and position of substituent groups, the removal efficiency of pollutants with ethyl and larger substituent groups is higher than that with methyl substituent. (3). NH3-N removal efficiency was about21%after2h, the concentration of NO3-increased after discharge.These date reveal that NH3-N compounds can be oxidized into NO3-Moreover, the removal of NH3-N and the generation of NO3-would benefit for further biodegradation.During oxidation, the BOD5/COD of the effluent increased significantly, the value of BOD5/COD increased from0.13to0.52after100min treatment, indicating that the biodegradability of these solutions was significantly enhanced. pH value and air flow rate also affect the biodegrability, and the optimum pH value and air flow rate are8.5and2L/min, respectively. Combining discharge pretreatment of60min and biological treatment of180min, the removal efficiency of COD and NH3-N reached75%and36%, respectively.