| As a choice for lubricant market support and ecological environment protection,recycling used lubricating oil has received widespread attention.However,large amounts of waste lubricating-oil regeneration wastewater(WLORW)are inevitably produced in the recycling process,and challenges are faced by traditional biological treatment of WLORW.Thus,a suitable pre-treatment method is urgently needed.This paper compares some of the commonly used pretreatment methods for oily wastewater with the wastewater quality characteristics of WLORW and selects electrocoagulation technology for the pretreatment of WLORW.Single-factor experiments were designed and run to investigate the main effects of the operating conditions on the removal of oil and COD from the WLORW and to assess its ability to improve the biodegradability.An orthogonal experiment was designed and run with phosphorus and boron to analyse the primary and secondary relationships between the influencing factors on the removal of inorganic pollutants(P and B)from the wastewater and to synthesise the optimum treatment conditions.Finally,the changes in flocs,the state of particles in wastewater and GC-MS analysis were used.The removal mechanism of organic and inorganic substances in WLORW treated by electrocoagulation was derived.The main experimental results are as follows.(1)In the single-factor experiments,the main effects of each operating condition such as current density,initial p H and inter-electrode distance on the removal rate of oil and COD as well as the improvement of biodegradability in the WLORW were investigated,and the better operating conditions were determined to be a current density of 15 m A/cm~2,an initial p H of 7and an inter-electrode distance of 2 cm.The specific energy consumption for COD and oil was 1.93 k Wh/kg and 25.41 k Wh/kg respectively,and the economics of the electrocoagulation process would be further enhanced if the hydrogen produced during the process was effectively recycled.Kinetic analysis shows that the process of removing oil and COD from WLORW by the electrocoagulation process conforms to the primary reaction law.The kinetic equation for oil is:A_t=0.4855t~2-21.59t+388.62 with a correlation coefficient of0.9702;the kinetic equation for COD is:A_t=4.3687t~2-230.22t+16121.15 with a correlation coefficient of 0.9942.(2)In the orthogonal experiments of phosphorus and boron removal by electrocoagulation,the order of the influencing factors were:current density>reaction time>initial p H>inter-electrode distance,ANOVA showed that the effect of current density on the removal of both phosphorus and boron was the most significant(p<0.01).The experimental results showed that the removal rate of phosphorus was faster than that of boron,and the maximum removal rate of phosphorus was 98.30%when the treatment time reached 30 min,while the maximum removal rate of boron was 52.07%when the reaction was 40 min.(3)The analysis of floc formation during the electrocoagulation reaction under optimised operating conditions showed a significant relationship between total iron ions and zeta potential and floc formation.The growth phase of flocs can be divided into three stages:(i)floc formation,(ii)floc growth,(iii)floc fragmentation.The density variation of the particles in the wastewater shows that the largest proportion of particles in the wastewater is concentrated in the particle size range of 5-10μm and that the oil removed by electrocoagulation is mainly emulsified and dispersed oil.GC-MS analysis shows that the untreated WLORW contains a wide range of organic compounds,including ketones,ethers,phenols etc.After the electrocoagulation treatment,a large number of toxic pollutants were effectively removed from the wastewater.The phenols、alcohols and amides were almost completely removed during the treatment process.In addition,the ethers were also effectively removed(39.31%-100%).The toxicity of the wastewater was significantly reduced.The experimental results support the use of the electrocoagulation process as a pre-treatment prior to biological treatment.(4)The routes of WLORW for organic pollutants removal could be concluded as charge neutralization,adsorption bridging,sweep flocculation,and air flotation.Flocs have different removal mechanisms for different types of compounds,with charge neutralisation eliminating phenols,while ketones and ethers are mainly removed by trapping and adsorption bridging.Adsorption of flocs dominates the removal mechanism of inorganic pollutants in the pretreatment of waste lubricating-oil regeneration wastewater by electrocoagulation,including the combined action of inner and outer spheres. |
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