Characterization Of Heavy Oil Refinery Wastewaters And Comprehensive Evaluation Of The Treatment Processes

The refining of heavy oil deteriorates the quality of refineries wastewaters.More toxic and harmful pollutants discharge into regional environments and result in potential ecological risks.Heavy oil refining wastewaters（HO-RWWs）are characterized by complex mixtures of numerous pollutants.Lack of understanding of HO-RWWs characteristics and their transformation behaviors along treatment processes,the guidance for wastewater treatment plants（WWTPs）optimization in heavy oil refineries are often insufficient.In this study,the characteristics of highly-polluted HO-RWWs in heavy oil refineries were analyzed,and the pretreatment processes were developed.The pollutants transformation behaviors along WWTPs were investigated,and the suggestions for efficiencies improvement were proposed.The characteristic pollutants in effluents of WWTPs in typical heavy oil refineries were analyzed,and their potential impacts on ecological environments were evaluated.The results are expected to benefit the treatment and management of wastewaters generated in heavy oil refineries.（1）The characterization of highly-polluted point source HO-PRWs.Heavy oil electric desalting wastewaters（HO-EDWs）are the key highly-polluted point source wastewaters and present high pollutants contents with complicated compositions,strong emulsion stability and high acute biotoxicity,compared to light oil-EDW,resulting in severe impacts on WWTPs.Large amounts of biologically recalcitrant aromatic compounds,as well as heteroatomic compounds such as CHO,CHOS and CHON species,extensively distribute in HO-EDWs.Additionally,highly dispersed colloidal,micron-sized particles and polar compounds promote the emulsification and stabilization of HO-EDWs.The pollutants in Liaohe heavy oil EDW（LHO-EDW）and Karamy Fengcheng heavy oil EDW（KHO-EDW）show different compositions and acute biotoxicity.The organic nitrogen compounds(e.g.,anilines and N_2-3O_x,N₁O_xS₁ species)in KHO-EDW most probably contribute to the elevated levels of acute biotoxicity.The pretreatment of HO-EDWs should be focused on breaking the emulsion stability,removing polar pollutants,as well as reducting acute biotoxicity.（2）Influences of enhanced pretreatment processes on HO-EDWs.Mechanisms of two enhanced pretreatments,coagulation and coagulation-ozonation,were investigated for improving the quality of HO-EDWs based on comprehensive characterization.Stability of HO-EDWs was broken by electrical neutralization of aluminum sulfate hydrolysis products.Most types of aromatic compounds,aliphatic acids and O_x polar compounds were removed by sweep flocculation and adsorption bridging of flocs.Coagulation notably improved the biodegradability,but still kept acute biotoxicity in coagulated HO-EDWs.Most species of remained aromatic compounds,phenols,aliphatic acids,anilines and naphthenic acids with high carbon numbers and ring numbers were further degraded by ozonation.The biodegradability was further improved and the acute toxicity was substantially reduced.Coagulation-ozonation enhanced pretreatment can effectively improve the quality of HO-EDWs,thus guaranteeing the efficiency of refinery WWTPs.（3）Evaluation for main processes of heavy oil refinery WWTPs.The pollutants transformation behaviors along main processes of two typical heavy oil refinery WWTPs were investigated and the suggestions for efficiencies improvement were proposed.CAST and A/O are the key units for removing polltants contents（COD,total nitrogen and ammonia nitrogen）.However,the hydrolysis acidification units failed to improve the biodegradability.Low molecular weight compounds（e.g.,phenols,carbonyls,and volatile fatty acids）and compounds with low unsaturation degree were dominantly degraded.Some macromolecular compounds（e.g.,amides,long-chain aliphatic acids,heterocyclic esters and long-chain alkanes）and compounds with high unsaturation degree(e.g.,O_3-4,O_3-4S₁ and N₁O₃ species)remained in the effluents of two main processes.Repairing the hydrolysis acidification units and adding advanced oxidation units to futher degrade and mineralize the residual pollutants can provide a more comprehensive guarantee for meeting the discharge standards.（4）The characterization of effluents from heavy oil refinery WWTPs.The characterization and potential environmental impacts of the effluents from three typical heavy oil WWTPs were investigated.Effluents from heavy oil refinery WWTPs show high pollutants contents with complicated compositions,as well as high acute biotoxicity,compared to light oil refinery.There are 25 to 32 types of non-/weak-polar compounds（e.g.,esters,ethers,alkanes,aromatics,and amines）and 1800 to 3200 types of polar compounds（including 169 to 292 O₂-O₆ naphthenic acids with low-rings）were identified.The crude oil types,refining processes and wastewater treatment processes all influence the composition and distribution of characteristic pollutants of effluents.The effluents of KL refinery and HZ refinery contain more organic nitrogen compounds relative to LH refinery.The characteristic pollutants in the effluents of LH refinery are N_x and N₁O_xS₁ species,and N_x species in HZ refinery effluent.Removal of some toxic and harmful characteristic pollutants become more important.