Ecological Risk Assessment And Resource Utilization Of Pyrolysis Residue Of Oily Sludge

Oil sludge was a kind of toxic and hazardous solid waste with a large number of pathogenic bacteria,heavy metals,salts and PCBs radioactivity generated in the process of oil development,refining,storage and transportation and disposal.After pyrolysis treatment,the solid residue（commonly known as pyrolysis residue）had a small amount of remaining heavy metals,petroleum hydrocarbons,polycyclic aromatic hydrocarbons and other hazardous components.Because its impact on the environmental effect was still unclear,the toxic ecological risk of pyrolysis residue must be conducted before the resource utilization.In this study,the oily sludge from the sludge storage field of Changqing Oilfield in Wuqi County,Yan’an City,Shaanxi Province,China was pyrolyzed to obtain a solid residue with a gray-black appearance and an oil content of 2%.The X-ray diffractometer phase structure analysis showed that the main mineral components were quartz,calcite and albite.The determination of p H,water content,oil content,heavy metal content and other physicochemical properties showed that the contaminant content in the residue was in accordance with the limits of each specification and did not belong to the hazardous waste with leaching toxicity characteristics,which was in accordance with the landfill standards and was not flammable.Combined with the results of acute experiments on mice and zebrafish by biological toxicity testing methods,it was found that the pyrolysis residues were not acutely toxic.Plant cultivation revealed that the pyrolysis residues showed certain effects on the growth of alfalfa,seed amaranth and vetiver grass.When the proportion of pyrolysis residue in soil was 1:7,the effect on crops was relatively small.The single heavy metal of residue was calculated by the geo-accumulation index and potential ecological risk evaluation method as no pollution,and the comprehensive ecological risk was low potential risk.Based on the results of residue toxicity ecological risk evaluation,vetiver grass was planted in different ratios of pyrolysis residue,municipal sludge and nutrient soil.Vetiver grass was found to grow in all ratios,which could effectively degrade the total water-soluble salts and total alkalinity in the matrix,as well as increase the activities of three soil enzymes:sucrase,urease and peroxidase.Among them,the plant height,leaf width,average root length and number of tillers of vetiver grass under pyrolysis residue:municipal sludge:nutrient soil V3（1:1:1）treatment ratio substrate were not significantly different from those of blank CK control,and the enrichment ability of Cu,Zn,Cd,Cr and Ni was stronger,that was considered as the best ratio for green planting application of oil-containing sludge residue and municipal sludge.The study of different masses of pyrolysis residues instead of fly ash incorporated into concrete building materials revealed that the compressive strength of concrete specimens with different mix ratios after standard curing met the design values.Among them,the compressive strength of the specimens with 1:1 mass ratio of residue to fly ash was increased compared to the blank concrete specimens without addition.After 56d of natural erosion in 5%Na₂SO₄ solution,the concrete specimens showed not very obvious changes in appearance with sulfate crystallization on the surface,and both the mass change rate and compressive strength showed a trend of first increase and then decrease.Among them,the residue to fly ash mass ratio of 1:1concrete specimens showed relatively small change in mass loss,and the strength increased by17%compared with the uncorroded specimens.The test data of heavy metals,petroleum,COD,benzo（a）pyrene,chromaticity,HCHs and DDTs in the leachate of pyrolysis residue concrete met the relevant standards,so it would not cause secondary pollution to the environment.