Characteristic And Performance Of Phosphate Removal By HAP Crystallization In Seed Media Filter

With the eutrophication serious increasingly，the world water resource faces hugethreat。The nitrogen element from water plants has already reached the emissionstandard, and phosphate pollution control has become an important subject on waterresources protection. Biological and chemical methods besides adsorption haveplayed an important role in phosphorous removal, but they have some shortcomingsespecically which can’t realize the recycling of phosphate. Phosphate is an importantraw material of the modern industry and phosphate resource will run out inaccordance with the mode of mining and using phosphate. Crystallization method hasgood phosphate removal phosphate effect and can realize phosphate recovery in theform of HAP (hydroxyapatite) and MAP (magnesium ammonium phosphate). Crystalmethod can not only solve the increasingly serious eutrophication, but also canbecome a settlement on the crisis of phosphate resources. The key of crystal is thechoice of seed crystal. Steel slag and blast furnace slag have high active substancescontent such as CaO、FeO、Al2O3which can be made into hybrid seed media.In order to discuss the feasibility of two kinds of hybrid seed media treatingphosphate of low concentration by crystallization phosphate removal, the laws ofcondition control in the filter reactor model were studied. By investigating the featureof steel slag, blast furnace slag, hybrid seed media with steel slag and hybrid seedmedia with blast furnace slag for removing phosphate, the characteristics of four kindsof media were compared. The optimization parameters were searched by investigatinghe influences of the ratio of calcium and phosphate, pH and hydraulic retention timeon crystallization phosphate removal. The technical measures in seed media filter forcontinuous phosphorus removal were made through investigating the influences ofCOD, ammonia nitrogen and areation on crystallization phosphate removal.The order of phosphate removal capacity is blast furnace slag <hybrid seedmedia with blast furnace slag <hybrid seed media with steel slag <steel slag. Therecycling of phosphate can realize through the extension of reaction time and thecontrol of water conditions by hybrid seed media with blast furnace slag and hybrid seed media with blast furnace slag in accord with phosphate removal effect andanalysis of reaction products.The increase of the calcium-phosphate ratio could promote phosphate removalefficiency. When the raw water pH=7and HRT=30min and the calcium-phosphateratio improved from1to1.5, the effluent phosphate concentration of hybrid mediaseed with steel slag could be less than0.5mg/L; When the raw water pH=7andHRT=60min and the calcium-phosphate ratio improved from2.5to3, the effluentphosphate concentration of hybrid media seed with blast furnace slag could achievethe level of A of <Discharge standard of pollutants for municipal wastewatertreatment plant (GB18918-2002)>.The increase of pH could promote phosphate removal efficiency. When the rawwater pH=7, HRT=30min, n(Ca)/n(P)=1and pH improved from7to8, the effluentphosphate concentration of hybrid media seed with steel slag could be less than0.5mg/L; When the raw water pH=7, HRT=30min, n(Ca)/n(P)=2.5and pH improvedfrom7to8, the effluent phosphate concentration of hybrid media seed with blastfurnace slag could achieve the level of A of <discharge standard of pollutants formunicipal wastewater treatment plant (GB18918-2002)>.Different concentration of COD (0~100mg/L) and ammonia nitrogen (0~30mg/L) caused no any impact on crystallization phosphate removal.The aeration method can improve the pH value of raw water and the adjustmentof gas-water ratio could create suitable alkaline environment for the crystallizationprocess. When the raw water pH=7, HRT=30min and gas-water ratio improved from0to10, the effluent phosphate concentration of hybrid media seed with steel slagcould be less than0.5mg/L; When the raw water pH=7, HRT=60min and gas-waterratio improved from0to10, the effluent phosphate concentration of hybrid mediaseed with blast furnace slag could achieve the level of A of <Discharge standard ofpollutants for municipal wastewater treatment plant (GB18918-2002)>.With the rise of HRT, the phosphate removal efficiency of hybrid seed media withblast furnace slag and hybrid seed media with blast furnace slag improved because ofthe increase of pH and calcium ion concentration. Run time of hybrid media seed with steel slag and hybrid media seed with blastfurnace in their optimal conditions treating simulation wastewater was16d and14drespectively. Through the analysis of crystal products XRD, calcium phosphatecompound were formed on the surfurce of media filter after operation and hybrid seedmedia filter can realize the recycling of phosphate.