Treatment Of Petrochemical Wastewater With Synthesized Oxidized Polysilicate Aluminum And Ferric Composite Coagulant

The composition of residual COD in biological secondary treatment effluent isusually soluble refractory organics. Some new wastewater treatment technologies areneeded with the improvement of the discharge standard and the reuse of the treatedwastewater. In this work, an oxidized polysilicate aluminum and ferric coagulant wassynthesized to treat the secondary effluent of a petrochemical wastewater plant, aimingto remove the residual solubility refractory organics. The coagulant synthesisoptimization, optimal dosage and operation condition were investigated. In addition, thecoagulation kinetics and the solubility refractory organics removal mechanism were alsostudied for the preparation of application. The main results are as follows:With aluminum sulfate and ferric sulfite, ferric sulfate and aluminum sulfate,sodium silicate, oxidant and additive as raw materials, the oxidized polysilicatealuminum and ferric composite coagulant was synthesized. The results shows that theoptimal preparation condition of coagulant are: molar ratio of ferric (ferric sulfate) toaluminum (aluminum sulfate)1:1, temperature60℃, oxidant sodium chlorate, andadditive sodium hypochlorite. The synthesized coagulant is made into powder bydrying in high temperature, characterized the physico-chemical structure combined withSEM, TEM technology, and investigated the element combination of the coagulant byinfrared spectrum. The result shows that the coagulant is not the simple accumulationbetween elements, but the complex polymerization. The micro-structure of thecoagulant is irregular, and the morphology of coagulant is between microcrystal andamorphous crystal.Secondary effluent of a petrochemical wastewater plant as processing object, thecoagulant dosage, pH and slow-stirring time on the influence of COD removal wereinvestigated. The results shows that the optimum operation conditions are: the optimalcoagulant dosage100mg/L, pH7, and slow-stirring time20min. On this condition,COD removal rate reached29.3%, and the traditional PAC and PAFC coagulant in theindividual optimum operation condition for the COD removal rate of secondary effluentof petrochemical wastewater plant are3.6%and10.2%, respectively.The soluble organics removal mechanism by oxidized polysilicate aluminum andferric composite coagulant was investigated, and it was found that coagulation and oxidation have a synergistic effect in a certain extent. The TOC of secondary effluent ofpetrochemical wastewater plant had high removal amplitude in120s. The morphologyand internal structure of flocculating constituents that oxidized polysilicate aluminumand ferric composite coagulant treated secondary effluent of petrochemical wastewaterplant were observed by SEM and TEM, and it was found that polysilicate silicate is themain structure, gathering Al, Na, Si and other elements to form a complex spatialconfiguration with the fluffy structure. These materials have larger specific surface area,thus can further remove organics after oxidation. The study found that wastewater hasmore aromatic protein, followed by humic substances. Oxidation destroyed molecularstructure of protein and humic substances in a certain extent during the treatment, andcoagulation contained charge neutralization and adsorption bridging mechanisms.