Distribution And Risk Of Polycyclic Aromatic Hydrocarbons In A Typical Process For Textile Wastewater Treatment Of The Pearl River Delta Region

Textile industry is a traditional preponderant industry of China,especially in the Pearl River Delta Region(PRDR),which still plays an important role in the economic development of this region.Generally,in an ordinary textile factory,large amount of chemicals,such as dyes,surfactants,detergents,and solvents,are used as raw materials.However,most of these chemicals,discharged with wastewater,are refractory and nearly non-biodegradable.Such an integrated process is very efficient and capable of removing most of the pollutants from textile wastewater.Nevertheless,some residual micro-organic pollutants were still contained in the discharged effluents,which could not be totally mineralized,and they even had a certain degree of biotoxicity.Therefore,exact analysis of the residual micro-organic pollutants in the textile effluents and illustrating their occurrence and distribution in a treatment process is quite important and necessary,which may be used as crucial basic data to well understand the management of textile effluents and abate the biotoxicity to the aquatic ecosystem.Referring to the list of priority pollutants for control in China,PAHs were found to be persistent and non-degradable and present in large quantities in textile wastewater.In this study,PAHs were selected as the target pollutants,and a typical textile wastewater treatment plant in the Pearl River Delta was the experimental subject.The concentrations of PAHs in the dissolved and suspended particulate matter of the six treatment processes were measured by gas chromatography coupled with mass spectrometry(GC-MS),the concentration and distribution of PAHs in the dissolved state and adsorbed state on SPM was exactly distinguished for precisely revealing their occurrence,and the risk assessment of the effluent from the treatment plant was carried out.The main research of the study is as follows.This work demonstrated that 16 PAHs were all measured in the dissolved and adsorbed state excepting acenaphthylene in suspended particulate matter,and they showed different distributions and removal efficiencies in the typical treatment process.In the influent,the total PAHs ranged from 1210.18 to 1304.93 ng/L in the dissolved state,and the PAHs with 2and 3 rings were the predominant,accounting for 83.78±3.66%,while,in the adsorbed state,it ranged from 2152.54 to 2554.18 ng/g,and the PAHs with 3 and 4 rings were the abundant,accounting for 89.41±5.31%.The concentration of total dissolved PAHs after the biological steps including AE and AN,was greatly reduced and occupied the largest ratio in its removal efficiency(75.16±3.70%),which verified that it was closely related with biodegradation/transformation and volatilization,and also indicated that the biological degradation played an major role in removing total dissolved PAHs.Throughout the treatment process,low molecular weight PAHs were more easily removed by biodegradation or volatilization,high molecular weight PAHs were more easily removed by adsorption onto the particulate matter.The ?PAHs in the AN step was lower than the other steps,whose values were 473.06(Jan.),1022.09(Feb.),934.45(Mar.),and 797.72 ng/g(Dec.)in the four sampling months,respectively,which was due to the effective degradation of the adsorbed?PAHs by the anaerobic digestion.The increment rate of?PAHs and log K_ow value in the sedimentation tank have some relationship.The risk assessment indicates that when the final effluent volume is too large,it can inhibit the growth of algal plants and the PAHs within the dissolved phase of the final effluent remain at an acceptable and safe level for human health.In general,16 PAHs showed different distributions and removal efficiencies in the typical treatment process.PAHs with low molecular weight achieved higher removal efficiency in the anaerobic digestion step,and PAHs with HMW were easier to be removed in the sedimentation step.The results verified that the number of benzene ring in PAHs was closely related to their occurrence,distribution and removal,and a risk assessment indicated the dissolved PAHs in the final effluent were still in the acceptable safety level to human health,but more HMW PAHs accumulated in the sedimentation tank might increase the environmental risk of excess sludge.