Study On Migration And Transformation Of Phosphorus In Sewage Sludge During Co-gasification With Biomass

As a companion product of sewage treatment,sewage sludge contains pathogenic microorganisms,organic pollutants,heavy metals,and other toxic and harmful substances,which can easily cause environmental pollution.However,sewage sludge is also rich in organic matter and nutrients such as nitrogen,phosphorus,and potassium necessary for crops.The phosphorus content reaches 3% to 5%,which has a high potential for resource utilization.As a sewage sludge resource disposal technology,pyrolysis and gasification can effectively destroy and kill pathogens in sewage sludge,and it can obtain solid-phase products rich in nutrients such as N,P,and K.In this study,based on the standards,measurements,and testing protocols(SMT),biologically available phosphorus test method and XRD method,the effects of temperature,reaction atmosphere and biomass type on the form,biological availability and migration,and transformation of phosphorus in solid sewage sludge products were explored.The following conclusions are drawn:(1)A small fixed-bed reactor was used to study the enrichment rate and bioavailability of phosphorus in solid-phase products of sewage sludge at different temperatures and reaction atmospheres.During the pyrolysis process,high temperature is not conducive to the enrichment of phosphorus in solid-phase products,and when the reaction temperature reached 900 °C,the enrichment rate of phosphorus decreased to 60.22%.The trend of phosphorus enrichment during oxygen and steam gasification was similar to that of pyrolysis,and increasing the temperature would decrease the phosphorus enrichment in solid products.However,the enrichment rate of phosphorus in the solid product of carbon dioxide gasification showed a trend of decreasing first and then increasing with the increase in temperature.At medium and low temperatures of 600～700℃,the enrichment rate of phosphorus in the solid-phase products of pyrolysis and gasification was similar.At high temperatures of 800℃～900℃,compared with pyrolysis,the enrichment rate of phosphorus in gasification solid products was higher,and the enrichment rate of phosphorus in the solid phase product of carbon dioxide gasification was the highest.For bioavailability,during pyrolysis and oxygen gasification,high temperature is not conducive to increasing the solubility of phosphorus in ammonium citrate(NAC)in solid products;in the process of carbon dioxide gasification,increasing the temperature made the solubility of phosphorus in NAC increase first and then decrease.During the pyrolysis process,increasing the temperature made the solubility of phosphorus in 2% citric acid(2% CA)in the solid phase product increase first and then decrease,reaching a maximum value of 47.64 mg/g at 700 °C,It accounted for 70.26% of total phosphorus(TP).In the gasification process,increasing the temperature is beneficial to increasing the solubility of phosphorus in 2% CA.Compared with oxygen and carbon dioxide gasification,the solubility of phosphorus in the solid phase product of steam gasification was the highest,which was 83.56 at 900 °C.mg/g,accounting for 93.42% of TP.(2)The changes in the content of phosphorus and the transformation law of phosphorus forms in the pyrolysis and gasification of sewage sludge were studied.Both pyrolysis and gasification can promote the conversion of organic phosphorus(OP)to inorganic phosphorus(IP)in sewage sludge.When the pyrolysis temperature exceeds 600°C,part of the converted IP will volatilize.Increasing the temperature has little effect on the content of IP contained in the solid product of steam gasification,but it will reduce the content of IP in the solid product of oxygen gasification.In the process of carbon dioxide gasification,increasing the temperature makes the content of IP in the solid product decrease first and then increase,reaching a maximum value of 32.79 mg/g at 900 °C.Non-apatite phosphorus(NAIP)is converted to AP in the process of pyrolysis and gasification.Increasing the temperature reduces the content of AP in the solid products of pyrolysis and oxygen gasification,while steam and carbon dioxide gasification shows the opposite trend.(3)The effect of co-gasification of sewage sludge mixed with biomass(pine sawdust PS,corn straw CS,rice straw RS)on phosphorus enrichment,bioavailability,and phosphorus species in solid products was studied.The co-gasification of sewage sludge mixed with PS,CS and RS can improve the enrichment rate of phosphorus in solid-phase products,and the enrichment rate of phosphorus in solid-phase products after co-gasification with PS was 100%.After blending CS,the solubility of phosphorus in 2% CA in the co-gasification product was increased,and the bioavailability of phosphorus in the solid phase product was improved.PS,CS and RS can promote the conversion of OP to IP and NAIP to AP,so as to increase the contents of IP and AP in the solid-phase product.