Study On The Effects Of Organic Matter On Ferric Iron-dependent Phosphorus Removal In Wastewater Treatment

In certain conditions,metal salt such as iron may chemically /physically react with dissolved phosphate in liquid to form insoluble phosphate precipitate,which can transfer the phosphate from the aqueous phase to the solid sludge phase so that separated from the aqueous phase.Compared with biological phosphorus removal,the chemical phosphorus removal process is simpler.But due to the presence of various impurities in the sewage,whether the chemical dephosphorization agent can react with phosphate and crystallize out the insoluble phosphate precipitate is a fundamental problem that the chemical phosphorus removal is facing.Organic matter in sewage has an effect on chemical phosphorus removal,but the mechanism of its influence is unknown.In this paper,the removal mechanism of ferric ion on phosphate was studied by exploring the combination mode of chemical dephosphorizer and phosphate,the removal effect of feeric ion on phosphate in sewage and the influence mechanism of organic matter on phosphorus removal were studied via lab scale experiment.The main research conducted and results obtained are as follows:1.Batch chemical coagulation test on the supernatant of sewage(taken from anaerobic tank,aerobic tank and secondary sedimentation tank of the 4th WWTP,Xi'an)were conducted.The optimal p H value was 4~5 for ferric ion to precipitate phosphate in real sewage.The higher the initial phosphorus concentration,the greater the phosphate removal rate.was.The optimal ratio of ferric ion and phosphorus was n(Fe):n(P)=1.44:1;The change of ORP was different before and after phosphorusremoval in different sections of sewage treatment plant.2.The effects of organic properties(particles,colloid and ions)on chemical phosphorus removal were investigated.And soluble organics and insoluble organics had great influence on phosphorus removal(6.1% and 16.2%,respectively).The effects of different dissolved organic compounds on phosphorus removal in practical sewage were investigated.The degree of protein's participation in iron salt phosphorus removal was weak,but humic acid and humic acid species of fulvic acid vary greatly in the process of chemical phosphorus removal,especially humic acid species had a high degree of participation in chemical phosphorus removal and had obvious changes.The effect of carboxyl group on phosphorus removal rate was far greater than that of hydroxyl group.And the content of carboxyl group was the key to the effect of organic matter on phosphorus removal efficiency of iron salt.The higher the content of carboxyl group,the greater the effect on phosphorus removal was.3.In the traditional chemical phosphorus removal system,HFO adsorption accounted for 73%±5% of the total phosphorus removal;In the process of removing phosphorus from iron salt,citric acid broke the original chemical bond balance and generated new chemical bond.Citric acid reacted with iron hydroxyl oxide(Fe-HFO)to form complexes.Citric acid affected the production of iron oxide hydroxyl.Citric acid was added to the system that generated iron and phosphorus precipitates,it separated the formed phosphate from iron adsorbate,making it impossible for(Fe-HFO)to combine with phosphate to form precipitate,thus reduced the removal rate of phosphate.