Study On The Recovery Mechanism And Its Interfering Factors Of Different Phosphorus Fractions In The A2N-IC System

Phosphorus（P）is a non-renewable resource indispensable for biological maintenance of cells and life activities.The Anaerobic-anoxic/nitrifying-Induced crystallization system（A2N-IC）combines biological removal and chemical recovery of P,which enables the simultaneously achievement on the control of eutrophication and P recovery.It has dual economic and environmental benefits and meets the needs of sustainable development.However,with the rapid development of industry and synthetic biotechnology,the existence of non-active phosphorus（NRP）in wastewater cannot be ignored.NRP has poor reactivity and does not follow the migration and transformation rules of reactive phosphorus（RP）.NRP,as well as heavy metals and complex organic compounds in sewage may be crucial causes of the decline of P recovery efficiency in pilot-scale experiment.Therefore,exploring the recovery mechanism of different P fractions in A2N-IC system as well as its influencing factors is helpful to supplement a new understanding of P recovery in A2N-IC system and provide better ideas for future P recovery.This research is based on the previous achievements of our research group.A variety of typical P fractions were selected to explore the recovery mechanism.Effects of crystallization operation conditions and dissolved organic matter on P recovery were analyzed as well.The main conclusions are as follows:（1）In the A2N unit,there were significant differences in the adsorption and conversion of NRP between anaerobic and aerobic biological treatments.Tripolyphosphate（TPP）containing phosphoric anhydride bonds and 5-adenosine monophosphate monohydrate（AMP）containing phosphate ester bonds are more easily hydrolyzed to orthophosphate under the action of activated sludge than ethylenediamine tetramethylene phosphonic acid（EDTMP）and2-phosphonic butane-1,2,4-tricarboxylic acid（PBTC）containing carbon-phosphorus bonds.The aeration process alone cannot promote the transformation of AMP but can promote the transformation of EDTMP and PBTC.In addition,the adsorption of activated sludge to TPP is concentrated within 1～2 minutes from the beginning of the reaction.The adsorption capacity of activated sludge to AMP is weak,but its conversion capacity is considerable.The adsorption capacity of activated sludge to PBTC is almost negligible,but strong to EDTMP.（2）In the IC unit,there are significant differences in the recyclability and recovery mechanism of different P fractions.RP forms hydroxyapatite（HAP）with amorphous calcium phosphate（ACP）as precursor.TPP can precipitate into solid phase by chelating with Ca²⁺.While the three typical organophosphates（AMP,EDTMP and PBTC）are not easy to be conversed or enter the solid phase.When TPP and RP coexists,Ca²⁺will preferentially complex with TPP,which will negeatively affect the recovery of RP.（3）Operating conditions will affect the recovery of RP and TPP.Both the increase of Ca/P molar ratio and initial p H value can promote the removal of RP and TPP.However,the increase of initial p H value will lead to the decrease of HAP crystallization rate and greatly prolong the induction time.P balance analysis at HAP and ACP stages showed that it was necessary to ensure that the reaction time reached the induction time to facilitate efficient P recovery.Considering the P removal rate,induction time and the P recovery rate,the Ca/P molar ratio of 2.0 and p H of 8.5 are a set of operating parameters suitable for both RP and TPP recovery.（4）Fulvic acid（FA）,sodium alginate（SA）and bovine serum albumin（BSA）can all affect RP recovery.Crystallization kinetics analysis showed that FA,SA and BSA could decrease the HAP crystallization rate,and the order of inhibition strength was SA>FA>BSA.P equilibrium calculation showed that FA could stably inhibit the removal and recovery of RP,while low concentrations of SA and BSA could slightly promote the removal of RP.Also,all the three organic matters could change the micro morphology,crystallinity and purity of HAP,but could not change the main phase of the recovered products.Furthermore,FA,SA and BSA can undergo chemisorption with HAP.The decrease in the concentration of FA,SA and BSA indicated that FA was more likely to chemisorb on ACP surface,SA seemed to have the same adsorption ability on ACP surface and HAP surface,while increasing the initial concentration of BSA was not conducive to the chemisorb on crystal surface.（5）The impact of FA,SA and BSA on TPP recovery is obviously different.FA and SA will not affect the recovery rate of TPP,while BSA will promote TPP recovery.Characterization analysis showed that FA only changed the color of CPP,making it yellowish-brown.SA made CPP transform from the white loose structure of ball clusters into a compact,smooth and shiny regular geometry.BSA brought no significant effect on the morphology of CPP.This study revealed the recyclability and sensitivity of several typical P fractions in A2N-IC system,and described the effects of several practical factors on the recovery efficiency of P and and quality of recoverd products,providing ideas for further improvement and development of A2N-IC system.