Effects Of Sludge Disposal On Perfluorochemical Removal And Environmental Risk Assessment On Related Products

With the continuous development of the national urbanization level and the continuous increase of sewage treatment capacity,the sludge produced in the treatment process shows explosive growth,so finding a reasonable way of sludge disposal would become a research hotspot.Land use is one of the most effective ways of sludge resource utilization,but there are a large number of organic pollutants in sludge,which will cause secondary pollution in the process of sludge resource utilization.Perfluorinated compound(PFCs)is a new type of organic pollutant,which has deeper and more extensive harm because of its persistence in the environment.Therefore,this paper studies the sludge containing PFCs,focusing on the removal effect of PFCs in sludge by aerobic composting process,the change of PFCs content in soil after land use of sludge composting products,and the environmental risk assessment of soil and groundwater after applying PFCs-containing sludge composting products to land.The main conclusions are as follows:1.The changes of PFCs concentration and pollution characteristics of sludge from three sewage treatment plants in Shenyang during composting were analyzed by high performance liquid chromatography-tandem mass spectrometry.The results showed that PFCs and its short-chain substitutes were widely detected in sludge.The total PFCs concentration of sludge produced by plant A,B and C in the process of composting was between 73.6-103 ng/g dw,11.2-39.7 ng/g dw,51.2-72.3 ng/g dw,respectively.The main PFCs pollutant in the composting sludge products of the three sewage treatment plants is perfluorooctane sulfonic acid(PFOS),followed by perfluorooctanoic acid(PFOA)and its short-chain substitute perfluorobutyric acid(PFBA).After high temperature aerobic fermentation,the short carbon chain PFCs removed about 10%-30%;the long carbon chain PFCs had no obvious removal effect,which was inferred to be caused by the different degree of transformation of the precursors between different PFCs.2.The leaching and adsorption capacity of 10 kinds of PFCs with different carbon chain length in soil were tested by leaching experiment.It is found that short-chain PFCs,has good water solubility and strong permeability,and the sedimentation rate is fast in deep soil layer,about 50 ng/(g d);long-chain PFCs is poor in water solubility and permeability,and is enriched in shallow soil layer,and the settling rate is about 20-30 ng/(g d).The shorter the carbon chain is,the faster the infiltration speed is and the easier it is to settle.The adsorption capacity of soil to perfluorosulfonic acid PFCs is better than that of perfluorocarboxylic acid,and short-chain PFCs is not easy to be adsorbed by soil.3.In this paper,PFBA,PFOA,PFOS and PFHxA in sludge composting products were predicted by HYDRUS-1D software as typical pollutants.The conclusions were detailed as follows:the main effects of four kinds of PFCs on soil environment are concentrated in the quaternary silt layer with a depth of 2.0-4.5 m.The maximum concentration appeared at the depth of 2.5 m,and then decreased gradually.Different depths are affected differently by the initial concentration of the upper interface,and the closer to the bottom,the less affected by the initial concentration.The blocking effect of different soil lithology on PFCs is different,the blocking effect of sand is the smallest,that of silt is the second,and that of clay is the largest.The attenuation rate of four kinds of PFCs in soil water is calculated by software,and the concentration attenuation rate of short-chain PFCs in soil water is less than half of that of long-chain PFCs,.The accuracy of conclusion 2 is verified by theoretical calculation.4.Using Visual-Modflow software,PFBA,PFOA,PFOS and PFHxA were used as typical pollutants in sludge composting products to predict groundwater environment.With the passage of time,the pollution migration distance and area of each PFCs gradually increased.The central concentrations of the four kinds of PFCs ranged from 0.0430 to 0.0930 mg/L,and the concentrations increased from 26.6 to 54.4 ng/L within 100-1000 days.It is predicted that within 10 years,the maximum halo area of PFBA,PFOA,PFOS and PFHx A can reach 1.02km~2,0.98 km~2,1.13 km~2 and 1.08 km~2,respectively.