Migration Pathway of Nitrogen and Phosphorus in Municipal Sludge During Thermochemical Transformation

Abstract

The by-product of sewage treatment, municipal sludge component is complex and rich in nitrogen (N) and phosphorus (P), which are essential soil nutrients. However, the presence of heavy metals in sludge raises concerns about soil contamination, leading to a reduced sludge use in composting, while the thermochemical treatment ways are recognized and applied. Investigating the transformation and composition of nitrogen and phosphorus during thermochemical treatment is crucial for optimizing the utilization of nutrients in municipal sludge. The present study evaluates three thermochemical methods—incineration, hydrothermal carbonization (HTC), and pyrolytic gasification—for the treatment of municipal sludge. It systematically analyzes the nitrogen and phosphorus composition properties in the resulting products. Experimental results indicate that all solid and liquid products from the thermochemical methods contain residual nitrogen and phosphorus. HTC demonstrates a distinct advantage, achieving N and P concentrations of up to 14.03% and 11.53% in solid products and 26.74 g/kg and 64.69 g/kg in liquid products, respectively, while minimizing nutrient loss. Furthermore, heavy metals are stabilized under a weak oxidation atmosphere in reaction kettle, making HTC an ideal candidate method for sludge fertilizer production. Notably, the addition of calcium (Ca) and magnesium (Mg) during HTC promotes the formation of nitrogen- and phosphorus-rich compounds, such as Ca₅(PO₄)₃Cl and NH₄MgCl₃•6H₂O, enhancing fertilizer potential and reducing toxic heavy metals by 0.229%. The present study elucidates the pathways of N and P migration during the thermochemical conversion of sludge, providing valuable insights into the sustainable utilization of municipal sludge.

Keywords: Municipal sludge, thermochemical transformation, nitrogen and phosphorus, migration pathway