Distribution Characteristics,mass Transfer And Risk Assessment Of Volatile Organic Compounds In Coking Wastewater Treatment Processes

In the treatment process of wastewater treatment plants(WWTPs),there will be a certain degree of secondary pollution,mainly in two aspects:on the one hand,with the operation of mixing,aeration,and evaporation in the treatment process,volatile organic compounds(VOCs)escape from the sewage into the air,causing air pollution;on the other hand,some toxic and harmful pollutants,such as heavy metals and hydrophobic polycyclic aromatic hydrocarbons(PAHs),are adsorbed and enriched in the activated sludge,which become secondary pollutants.Therefore,WWTPs are not only the pollution treatment unit,but also the pollution producing unit.The secondary pollution problems in the process of wastewater treatment,such as the removal and dissipation of VOCs,need to be solved.The hazards of the secondary pollution must be recognized on the basis of the health risk sssessment and environmental pollution assessment.Previous studies focused on odor emission from municipal WWTPs,but did not study and discuss industrial wastewater,especially coking wastewater.So far,the emission characteristics and laws of VOCs in coking WWTPs have not been understood.Based on the distribution characteristics of VOCs in water phase and gas phase of coking wastewater biological treatment process(A/O/O),the equivalent and total emissions of VOCs in coking wastewater treatment industry in China were estimated for the first time.The health risk and environmental pollution degree of VOCs emission in coking wastewater treatment process were evaluated.It was pointed out that cancer existed in the staff of coking wastewater treatment project for a long time.Distribution behavior of VOCs in water phase,atmospheric environment and activated sludge and the fate of VOCs in coking wastewater treatment process were clarified.The influencing factors of VOCs emission were discussed,and the Countermeasures of in-situ pollution control were proposed to reduce VOCs emission.The main conclusions are as follows:(1)According to the distribution characteristics of VOCs in water phase and gas phase during coking wastewater A/O/O process,the emission of VOCs in coking wastewater treatment industry was estimated.It was found that 17 kinds of gaseous VOCs were detected in each treatment unit,mainly benzene series,halogenated hydrocarbons and chlorobenzene compounds;among the gaseous VOCs,the concentration of benzene was the highest,reaching 180.49?g m^-3.The concentration range of gaseous VOCs was 28.56-857.86?g m^-3,and the order was:raw water tank>anaerobic tank>amination removal tower>anterior aerobic tank>posterior aerobic tank>discharge tank,which was related to the process characteristics;the total emission rate of VOCs in the WWTP was 1773.42 g d^-1,the estimated annual emission of VOCs was 0.65 t,and the emission equivalent was 1.18 g m^-3.According to 3.4×10⁸ m³ coking wastewater produced in China every year,it can be estimated that the annual emission of VOCs in coking wastewater treatment industry is about 402 t.(2)According to the research of VOCs concentration level,distribution behavior and mass transfer process in gas phase,water phase and sludge phase,it was found that there was a significant correlation between benzene series concentration in each phase and between benzene series and total benzene series concentration;with the treatment of wastewater,COD and TOC in wastewater gradually decreased,concentration of VOCs in the aqueous phase gradually decreased,and concentration of VOCs in the gas phase also decreased;the concentration of total benzene series in coking wastewater reached 397.19?g L^-1,and the concentration of benzene series in aqueous phase decreased with the process.The fate of VOCs mainly includes volatilization,sludge adsorption,biodegradation,and efflux with effluent.The total influent mass load of benzene series was 594.30 g d^-1,66.47 g d^-1(11.18%)was removed with effluent,and123.28 g d^-1(20.74%)was removed with the discharged sludge.Volatilization and degradation account for 68.07%and the total removal rate of benzene series was88.82%.The influencing factors of VOCs emission in wastewater treatment process include VOCs concentration in aqueous phase,aeration rate,physical and chemical properties of VOCs,water temperature,residence time,etc.In situ pollution control measures include reducing aeration rate as much as possible,sealing and collecting treatment units with high load discharge capacity,improving treatment efficiency to reduce VOCs concentration in wastewater and so on,so as to realize VOCs emission reduction.(3)According to the maximum incremental reactivity(MIR)method,the ozone formation potential(OFP)was estimated,and the SOAP method was used to estimate the secondary aerosol formation potential.Data showed that the OFP of the wastewater treatment area was 81.49-2960.49?g m^-3,and the average OFP in this study(1136.27±154.11?g m^-3)was higher than the air quality guideline of 100?g m^-3 proposed by the World Health Organization(WHO).The six compounds that contributed most to ozone generation were m-xylene(36.0%),toluene(20.8%),p-xylene(13.5%),o-xylene(10.6%),styrene(6.8%),and benzene(5.3%).The total VOCs concentration of the coking WWTP was one order of magnitude higher than that of the municipal WWTP,but the total of OFP level at each unit of the coking WWTP is equivalent to that of the municipal WWTP.Among gaseous VOCs,styrene,benzene,toluene,m-xylene,p-xylene,and o-xylene contributed the most.(4)Health risks caused by VOCs emissions from coking wastewater treatment units were evaluated.In each unit of the WWTP,the carcinogenic risk associated with gaseous VOCs ranged from 3.0×10^-5 to 7.8×10^-4,and the total risk was 3.26×10^-3.The risk of benzene was higher than the public health risk level(1×10^-6)recommended by US EPA.Therefore,benzene series from the coking WWTP had obvious carcinogenic effect on human health.The non-carcinogenic risk of benzene series in raw water tank was the highest,and the non-cancer risk(HR)of benzene was 3.008,which was more than 1,and this was a confirmed non-cancer risk.The results of health risk assessment showed that the staff who worked in coking WWTPs for a long time had benzene exposure risk,including cancer risk and non-cancer risk.