Study On The Effects Of Chemical Removal Of Phosphate In Wastewaters From Different Treatment Unit In A Municipal Wastewater Treatment Plant And Their Removal Characteristics

Metal ions such as iron and aluminum can react with dissolved phosphate in water to form insoluble phosphate compounds under certain conditions,so phosphates from water are transferred to solid phase sludge that separated from the aqueous phase.The chemical phosphorus removal from sewage is based on this principle.the process of chemical phosphorus removal is simpler compared with biological phosphorus removal from sewage.However,due to the existence of various impurities in the sewage,whether chemical phosphorus removal agent can react with phosphate to obtain insoluble phosphate precipitate and the precipitate can be crystallized is a fundamental problem that the chemical phosphorus removal can be achieved.In this study,batch tests of chemical phosphate precipitate were conducted for the clarified solutions of wastewater from several sections(Anaerobic tank ? aerobic tank ?secondary sedimentation tank?sludge thickening tank?sludge removal tank)of Xi'an No.4 Wastewater Treatment Plant,examining the effects of different chemical phosphorus removal agents,p H,organic matter,and different ion concentration conditions on the results of chemical phosphorus removal.The influence of operating conditions on flocs during the chemical phosphorus removal and coagulation process of sewage were studied;the role of each reactant in the process of chemical phosphorus removal was further analyzed with QCM-D,aiming to guide the sewage treatment plant to improve its efficiency of chemical phosphorus removal and to enrich the understanding of the mechanism of chemical phosphorus removal.Results of the paper are shown as following:(1)The optimal p H,ratio of phosphorus removal agent and ion concentration required for achieving the greatest chemical phosphorus removal of sewage in different sections of the municipal wastewater treatment plant are different,which indicated that the influence of impurities on the chemical precipitation of phosphate was different in different water quality.On the whole,the phosphorus removal rate of ferric ion was higher than that of aluminum ion,indicating that the combination of iron salt and phosphate was firmer and less affected by water quality.In the process of chemical phosphorus removal from ferric salt,the change of floc size had experienced three phases: lag phase,rapid growth phase and stable growth phase.The resulting flocs were easily broken by the action of shear force;after the shear force became weak,the broken flocs could re-aggregate,but the size of the re-agglomerated flocs was much smaller than that before.(2)The organic substances in the sewage were involved in the chemical precipitation of phosphates,affecting the type,crystallinity and surface structure of the precipitated products.The types of chemical phosphorus removal deposits formed were more complex and the crystallinity was reduced,the amount of crystals was also less,the surface of the precipitate appeared a rough villous material,also the density of fluffy substances was small,which affected the compactness of the precipitated product when organic matter exists.(3)The concentration of Na+ in the sewage had an effect on the Zeta potential of the chemical phosphorus removal reaction system when organic matter exists.When the Na+ concentration was 50 mmol/L,the absolute value of the potential at the steady state of the coagulation system was the lowest,and the degree of colloidal destabilization deeply,the degree of agglomeration of the colloidal particles was the largest.In the two-hour reaction process of the coagulation system,the potential was not completely stable at all times,indicating that the adsorption of phosphate by the hydrolysis products of iron salts and aluminum salts were still not fully conducted.(4)When the Na+ concentration increased from 0 to 50 mmol/L,the adsorption amount of the metal ions on the HA surface gradually increased;when the Na+ concentration continued to increase to 100 mmol/L,the adsorption amount of the metal ions on the HA surface decreased;When the chelate formed by the reaction of humic acid and metal ions reacts with phosphate,the change in the amount of adsorption shew the same pattern as the formation of humic acid-metal ion chelate,indicating that the concentration of Na+ concentration was 50 mmol/L,the adsorption capacity of phosphate was the largest;In the two reaction processes,the adsorption rate in the initial stage of the reaction was much greater than that in the later stage of the reaction,indicating that the removal of phosphate mainly depended on the adsorption rate at the initial stage of the reaction.