Simulation Research On Distribution And Environmental Impact Of PCDD/Fs From Hazardous Waste Incinerators

Due to advantages of harmlessness,volume reduction and weight reduction,hazardous waste incineration technologies have been widely used in China.However,secondary pollutants would be produced from hazardous waste incineration processes,the most concerned pollutants are PCDD/Fs.In order to establish an effective early warning mechanism for the environmental impact of dioxin emission from incinerator,it is necessary to understand the dioxin levels of hazardous waste incineration plants and its surrounding environment media,and the impacts of incinerator operation on the surrounding environment.In this study,2 hazardous waste incineration plants located in Zhejiang and Chongqing are selected.Research methods includes concentration monitoring,model calculation(gas-solid distribution model,diffusion and deposition model,health risk assessment model)and statistical analysis.The main contents of this research are studying the discharge characteristics of PCDD/Fs from hazardous waste incinerators under normal and abnormal operation conditions,acknowledging the distribution characteristics of PCDD/F in the surrounding environment media(soil and atmosphere),investigating the key parameters in dioxin atmospheric diffusion and settlement simulation,analyzing the environmental assessment and health risk assessment of PCDD/F emitted from two incineration plants.The following main conclusions were obtained:The PCDD/F emission levels of HWI-A and HWI-B were 0.49 and 0.40 ng I-TEQ Nm-3 respectively,which met the national emission standards.The ratio of D/F in the flue gas was<1,which belonged to the typical characteristics of the dioxin fingerprint of hazardous waste flue gas.Temperatures at the end of the second combustion chamber stayed at 420℃-540℃ for a long time,which were the optimum temperature range for precursor synthesis of PCDD/Fs.Large amounts of PCDD/F were generated at this area and led to a higher emission level.The estimated amount of annual PCDD/F emissions under normal operating conditions is 49.53 mg I-TEQ(0.5 ng I-TEQ Nm-3).Three shutdown-startup processes could generate approximately 32.6%of that.The baseline level of PCDD/F in the surrounding soil of HWI-A was 7.35 ng I-TEQ kg-1,which was at a high level.It is found that PCDD/Fs levels in the North and West directions of the incineration plant increased gradually though 8 years continuously monitoring,which was met the wind rose distribution.The soil PCDD/F fingerprints tended to close to the dioxin fingerprints of the incineration sources.However,the principal component analysis found that PCDD/Fs emitted from HWI-A had less impacts on the surrounding soil.Enzyme-linked immunosorbent assay(ELISA)was applied to screen the TEQ values of dioxin in soil around HWI-B,to identify the high resolution detection points of soil.In addition to the abnormal point S1,the average PCDD/F level in soil around HWI-B was 2.90 ng I-TEQ kg-1,and dioxin settlement or fly ash escape may be causes of the high dioxin concentration at the highest value of S1(227.45ng I-TEQ kg-1).The dioxin fingerprint of the HWI-B is similar to that of the surrounding soil dioxin.The principal component analysis shows that the hazardous waste incineration plant and the cement plant might have some impacts on the surrounding soil.The ambient PCDD/F levels around HWI-A are 0.091-1.479 pg I-TEQ m-3,the mean value is 0.525 pg I-TEQ m-3,which is at the high level.The TEQ profiles of 2,3,7,8-PCDD/Fs in the atmosphere are similar at whole year.The main concentration contributors are OCDD,OCDF and 1,2,3,4,7,8-HpCDF isomers,which varied with time and space,and the main TEQ contributor is 2,3,4,7,8-PeCDF.TSP and PM 10 were used as carriers to simulate the PCDD/F dispersion and deposition at the surrounding environment.It is found that average annual ground concentration and total settlement concentration are always TSP greater than PM10,which indicates that the particle size distribution coefficient has great influence on the diffusion and settlement concentrations,and the settlement strength of coarse particles is greater than that of fine particles.Though comparing the predict values with the measured values,it is found that simulated value obtained from TSP as a dioxin carrier is closer to the actual value.The gas-solid distribution coefficient of 100%and 80%has great influence on the total settlement concentration of dioxin,and the settlement concentrations of the former are 1.5-1.8 times higher than those of the latter.It is suggested to use the actual atmospheric gas-solid phase distribution coefficients of dioxin to simulate the diffusion and settlement of dioxin in the surrounding atmosphere.The gas-solid distribution coefficient calculated through Mader and Pankow method was also recommended to substitute the actual gas-solid phase distribution coefficient.AERMOD and CALPUFF models were used to simulate the dioxin diffusion from HWI-A,results showed that concentration at maximum landing point simulated by two models were at the same order of magnitude 10-2 pg-TEQ m-3.Compared with CALPUFF,AERMOD is more sensitive to regional scale change.The predicted dioxin values at 6 monitoring sites around HWI-A obtained through AERMOD and CALPUFF were lower than those actually measured values,and simulation values through CALPUFF deviates more from the measured values.Considering the fitting of monitoring data,the conservatism of the environmental impact assessment,the complexity of the model and the range scale of evaluation,it is concluded that the AERMOD model is more suitable for the simulation of the migration and diffusion of dioxin pollutants in the surrounding atmosphere of the incineration plant.The high level points of dioxin ground concentration from HWI-B simulated though AERMOD coincide with the trend of the valley,which shows that the effect of topographic conditions on the dioxin diffusion distribution is greater than that of meteorological conditions,when under complex terrain conditions.The maximum value of soil settlement around HWI-B after one year(9.7 × 10-6 pg g-1)is about 10 times higher than that of HWI-A(1.1 × 10-6 pg g-1),but the scale of dioxin settlement is about 1/3 of HWI-A.Under normal operation,the maximum settlement concentrations of soil in the HWI-A and HWI-B study areas(5×5 km)are 2.034 ×10-5 and 1.78 × 10-4 ng I-TEQ kg-1 after operating for a long time(over 25 years),which is far below the standard of 4 ng I-TEQ kg-1 in Canada and China’s agricultural land.The effect of its long-term operation on the surrounding soil is relatively limited.The maximum carcinogenic risk of rural residents and urban residents around HWI-A and HWI-B was 10-8 and 10-7,respectively,which were less than the strictest Alaska standard(10-6).However,the carcinogenic risk of rural residents and urban residents of HWI-A had increased nealy 15 times after considering the start-up and shut-down conditions,maximum values were up to 8.455 × 10-7 and 6.341 × 10-7.Thus,operation conditions of waste incinerators have great influence on health risk results.Thus,it is recommended to considering start-up and shut-down conditions when assessing the environmental impact of PCDD/Fs from hazardous waste incinerators.