Study On Water Quality Analysis And Pollutant Control Mechanism In Pretreatment Processes Of Coal-to-Oil Wastewater

Coal is the most abundant and economical fossil fuel.By liquefying coal and converting it into liquid fuel,it can obtain clean utilization compared with traditional coal-burning technology,reduce dependence on oil,and provide greater energy security.However,in the process of coal liquefaction,a large amount of high-concentration wastewater,mainly suspended solids,sulfides,ammonia nitrogen,petroleum,phenols,and COD,will be produced.At present,the treatment process of coal-to-oil（CTO）wastewater is mainly composed of pretreatment,biochemical treatment and advanced treatment.The pretreatment is mainly composed of oil separation,double-tower stripping,phenols solvent extraction and high-efficiency catalytic oxidation process.Aiming at the practical problem of the high concentration of suspended solids and organic pollutants in dephenolized purified wastewater（DPWW）during the pretreatment of CTO wastewater in a company,this paper studies the effect of chemical coagulation and electrocoagulation on the removal of suspended solids in CTO wastewater,a scheme to control the concentration of suspended solids in the pretreatment process is proposed,and the mechanism of ozone/hydrogen peroxide method to remove organic pollutants in DPWW is studied.Firstly,using conventional water quality analysis methods,GC-MS and FT-ICR-MS and other analysis and characterization methods,the organic and inorganic pollutant indicators of CTO high-concentration wastewater and DPWW are analyzed from both macroscopic and microscopic scales.The results show that the COD_Cr of the high-concentration wastewater is as high as 93550 mg/L,the TOC is 26800 mg/L,the organic load of the wastewater is extremely high,and it also contain a large amount of reducing substances.The BOD₅ value is 14968 mg/L,and the low B/C value indicate that the biological Refractory.The ammonia nitrogen content of the wastewater is extremely high,which is 25596 mg/L.The pretreatment process can effectively remove high concentrations of oil,sulfide,inorganic carbon,ammonia nitrogen and phenols in wastewater.The polar organic pollutants in CTO high-concentration wastewater are mainly N_xO_xS₁ species and O_x species,and the number of pollutants is as high as 1196.while the inorganic ion composition is not complicated,mainly K₂SO₄,which may contain Na₂CO₃ or Na HCO₃.The chemical coagulant PAC show better effect in the removal of suspended solids in CTO wastewater.Compared with electrocoagulation treatment,the stability of wastewater after chemical coagulation treatment is lost,the particle size distribution range is significantly smaller,and the shading rate is greatly reduced.When the dosage is1.60g/L,the turbidity of wastewater is reduced to 4.18 NTU,and the TSS is reduced to11.00 mg/L.It also has a certain removal effect on organic pollutants in wastewater,especially aromatic compounds.The COD removal rate is 13.91%.Therefore,it is proposed that a coagulation sedimentation tank can be added after the double-tower stripping,and part of the coagulation effluent can be refluxed before the double-tower stripping,so as to alleviate the blockage problem of the double-column stripping unit and eliminate the impact on the subsequent process.The introduction of hydrogen peroxide into the ozone oxidation system significantly enhances the removal of contaminants from DPWW.When the ozone concentration is 50mg/L and the dosage of hydrogen peroxide is 750 mg/L,the COD removal rate increases from 27.45%to 42.12%after 90 min.At the same time,26.33%of TOC,27.18%of biorefractory compounds and 93.95%of polar oils are removed.The direct oxidation of ozone can convert phenolic compounds into more stable saturated aliphatic acids,and also increase the intensity of species with high oxygen content in polar organic pollutants,but the pollutant removal effect is poor.However,the introduction of hydrogen peroxide makes the peroxone reaction dominated in the system to generate a large amount of·OH,which significantly enhances the removal of pollutants in wastewater.