Study On Pretreatment Of Coal-to-Oil Wastewater

Coal-to-oil production is a technology,which based on coal as raw material to produce diesel, gasoline, kerosene, etc. In recent years, with the rapid development of coal-to-oil industry, the amount of discharged coal-to-oil wastewater is increasing day by day, which is becoming a difficult problem to deal with. Coal-to-oil wastewater, which contains a large number of surfactant, has the characteristics of big chromaticity, high degree of emulsification, poor biochemistry, frothing easily and so on, all of which made the wastewater hard to deal with. Therefore, it is necessary to study the pretreatment of coal-to-oil wastewater to reduce some COD_Cr and harmful substances, so as to reduce the load of biological treatment, and solve the current difficult problem of treatmenting coal-to-oil wastewater, which is beneficial to protect the water environment and to promote a healthy coordinated development.The wastewater for study is obtained from a coal-to-oil enterprise in Yunnan province. The main contents include the followings: The First, three methods, for example, acidification, acidification-Fenton and chemical coagulation are used as the pretreatment method, from which the best pretreatment program on coal-to-oil wastewater had been obtained; The second, the modification of PFS, such as poly silicon sulfate (PFSS) and poly ferric sulfate (PPFS), have been produced in the experiment, so as to improve the performance of coagulant; The third, the best operating conditions for pretreatment, for example, the type of coagulant, the pH and its regulation, the dosage of PFSS, the dosage of PAM, the temperature and the stirring intensity have been studied, respectively; The fourth, Analysis the feasibility of the pretreatment, respectively, from the effluent biodegradability, economic viability and operational conditions of the analysis demonstrated.It is showed in the experiment that: compared with acidification and acidification-Fenton methods, chemical coagulation is the best method on pretreatting the coal-to-oil wastewater, for its lower operating cost and the mild operating conditions; Modified PFS has a much better property than before on the removal of COD_Cr, especially for poly silicon sulfate (PFSS), whose coagulation properties are best and the price is cheap. As a result PFSS is the best pharmaceutical modification of coagulation; The optimum conditions for pretreatment are controlling the temperature being about 30℃, adjusting pH at 10 by limewater, the dosage of PFSS being about 550mg/L and the dosage of PAM being about 12mg/L, the stir intensity being 60r/min. Under these optimum conditions, the removal rate of COD_Cr, ammonia and oil is,respectively,58.2%, 17.4% and 86.0%. The B/C of the water rises from 0.191 to 0.348, which created a favorable condition for subsequent biochemical treatment. After the total treatting process of this study, the water meet the national "Integrated Wastewater Discharge Standard" (GB8978-1996a); The calculation showed that the cost of pretreatting,with the method of chemical coagulation,a ton of coal-to-oil wastewater is 1.14 yuan, which make the method has a strong feasibility on economy; Besides, the chemical coagulation pretreatment is easy to operate.