Hydrothermal Carbonization Of Municipal Sewage Sludge:Invesgation On The Transformation Release And Recovery Of Phosphorus In Hydrochar

Phosphorus?P?is a non-renewable nutrient element in nature and plays an important role in human activity.Sewage sludge?SS?is considered to be the largest secondary resource of P for receiving most P in the wastewater.Therefore,the retrieval of P from SS using different treatment techniques to alleviate the depletion of P resources represents an important research area.This subject carried out the following research work of P in excess sludge from municipal wastewater treatment plants.First,the effect of different feedwater pH values on the P transformation during the hydrothermal carbonization?HTC?of the sludge was exp lored.The experiments set three temperatures at 200°C,230°C and 270°C and set five pH?3,5,7,9 and 11?at three temperatures,respectively.The extraction protocol of the Standards,Measurements and Testing?SMT?Programme was applied for the P fractionation in the SS and the hydrochars,and the speciation dynamics of P were determined by X-ray diffraction?XRD?and scanning electron microscopy-energy dispersive spectroscopy?SEM-EDS?.The results show that The HTC significantly accumulated P in the hydrochar and promoted the transformation of OP into IP and of NAIP into AP.The different feedwater pH environment has a significant effect on the fate of P during the HTC of the SS.An acidic feedwater significantly decreased the contents of TP,OP,and AP at the expense of increased proportions of IP and NAIP;In contrast,larger amounts of TP,OP,and AP and lower amounts of IP and NAIP were formed when the SS treatment occurred at high feedwater pH values.The combination of the P in different states and the metal cations?e.g.Al and Ca?changed with the feedwater pH,which resulted in the transformation of different P species.The calcium bound phosphate was dissolved and released by the action of hydrogen ion under acidic conditions and the free phosphate interacted with aluminium and iron to form insoluble phosphates;the phosphates were ion-exchanged with the hydroxide in a basic environment and were released into the process water.Calcium underwent cation exchange on the surface of the solid particles,produced a synthesis of surface complexes with the dissociative phosphate in the process liquid,and was then adsorbed on the solid surface.Secondly,The effect of different leaching conditions?the type of acid,acid concentration,extraction time and solid/liquid ratio?on the extraction of P from char?hydrochar,pyrochar?producted from?hydro?thermal treatments of SS was investigated.The elemental and mineral composition analyses of char?hydrochar,pyrochar?before and after acid extreation were conducted by the X-ray fluorescence?XRF?and XRD.The results show that P extraction yield from char?hydrochar,pyrochar?when using oxalic acid is higher than when using inorganic acid?sulphuric acid,hydrochloric acid and nitric acid?.P extraction yield increased with an increase in a certain range of the type of acid,acid concentration,extraction time or solid/liquid ratio.leaching hydrochar and pyrochar with 1 mol·L^-1 oxalic acid at 1:50 solid/liquid ratio and 80?for 60min resulted in 93.6%and 97.3%P extraction,respectively.Thirdly,HAP method was used to recycle the P in char?hydrochar,pyrochar?supernatant after optinal extraction condition.The effect of initial pH and Ca²⁺/PO₄^3--P on P recovery was studied.The morphology and composition of the recovered precipitates were analysed by SEM and XRF.The results show that P in SS was effectively recovered by the HAP reaction.Pi precipitation rate reached over 94.4%and 96.4%,TP precipitation rate reached over90.3%and 93.4%,and SS phouphorus recovery rate was 84.5%and 90.9%under the optimized conditons of Ca²⁺/PO₄^3—P of 1.6 and pH at 8.