Effect Of Calcium-based Additives On Phosphorus Transformation Of Municipal Sludge During Thermal Treatment

Phosphorus is an irreplaceable nutrient element,which mainly exists in phosphate rocks in nature.Phosphorus becomes a non-renewable resource due to its unidirectional fluidity.The sewage sludge from sewage treatment plants contains abundant phosphorus resources,and there are also toxic and harmful substances such as heavy metals and pathogenic bacteria in the sewage sludge.Therefore,the rational treatment and disposal of sludge and the resource utilization of sludge are hot issues in the current research.The thermal treatment method is a common method for sludge disposal.Thermal treatment not only changes the original physical and chemical properties of sludge,but also affects the form of phosphorus in the sludge,and the form of phosphorus is a basic factor that affects the bioavailability of phosphorus and subsequent product recycling.Additives are also one of the factors that affect the phosphorus form in the thermal treatment process In this research,municipal sewage sludge was used as raw material to collect the solid phase products of pyrolysis and hydrothermal carbonization of the sludge.The effects of CaCl₂on the phosphorus content and form in the sludge during the two thermal treatment methods were studied,and the following conclusions were drawn:（1）After pyrolysis and hydrothermal carbonization of sludge,more than 80%of P in sludge is enriched in pyrochar and hydrochar,in which the concentration of total phosphorus（TP）increases.Phosphorus in sludge mainly exists in the form of inorganic phosphorus（IP）.Pyrolysis and hydrothermal carbonization make part of organic phosphorus（OP）further transform into inorganic phosphorus（IP）.Therefore,IP content in pyrochar and hydrochar increases while OP content decreases.（2）Pyrolysis and hydrothermal carbonization results in the conversion of non-apatite inorganic phosphorus（NAIP）to apatite inorganic phosphorus（AP）,and the content of AP increases.Phosphorus-containing compounds in sludge are homogenized and mixed and released during pyrolysis and hydrothermal carbonization.Since（PO₄）^3-has a stronger affinity with Ca²⁺than Al³⁺and Fe³⁺,more（PO₄）^3-is combined with Ca²⁺,resulting in the transformation of part of Fe-P and Al-P to Ca-P,resulting in the increase of AP concentration in solid products and the decrease of NAIP concentration.In the hydrothermal carbonization,the introduction of CaCl₂ will intensify the transformation,and the best conversion effect will be achieved when the mass fraction of CaCl₂ reaches 20%.However,during the pyrolysis process,the addition of CaCl₂ will not only aggravate the transformation,but also reduce the content of AP and IP in the sludge pyrolysis charcoal.（3）Under the condition that the reaction temperature of hydrothermal carbonization is 200℃,when the mass of CaCl₂ accounts for 20%of the total mass of the sample,the AP concentration and the percentage of IP in the hydrochar reach the maximum.The soluble phosphorus in the hydrothermal carbon is in 2%citric acid（CA）solution The solubility in sludge is also maximized,and the bioavailability of phosphorus in the hydrochar is the best.When the pyrolysis temperature is 600℃and CaCl₂ is not added,the contents of AP and IP in the pyrochar reach the maximum,and the solubility of the soluble phosphorus in the pyrolysis carbon in the 2%CA and neutral ammonium citrate（NAC）solution also reaches the maximum.At this time,the bioavailability of phosphorus in pyrochar is the best.（4）After pyrolysis and hydrothermal carbonization,the bioavailability of phosphorus in pyrochar and hydrochar is improved,and the heavy metal content in pyrochar and hydrochar meets the national standard of B-class sludge product.Therefore,the two kinds of char derived under these conditions have the potential of direct land application.In conclusion,pyrolysis and hydrothermal carbonization will enrich phosphorus of sewage sludge in solid phase products of sludge and have a positive effect on the bioavailability of them.CaCl₂has different effects on the content,form and bioavailability of phosphorus in the sludge during the two thermal treatment methods.