Characteristics of fused calcium magnesium phosphate fertilizer (FCMP) made from municipal sewage sludge and its properties

A high P-availability glass phosphate fertilizer was manufactured from municipal sewage sludge ash (SSA) in reference to the conventional method for fused calcium magnesium phosphate fertilizer (FCMP). P-availability in SSA-made FCMP was examined by varying the amount of alkali earth metals (MO, CaO and MgO) addition in the FCMP. Various techniques such as XRD, FTIR, Raman, Mössbauer spectra and 27Al MAS NMR spectra were applied to examine the FCMP structure made from SSA. It was found that the silicate glass network depolymerization due to MO addition is the main cause of P-availability improvement. MgO was more effective than CaO. Special attention was paid to Fe and Al, which is considered the main cause for the difference from conventional FCMP formular. Fe and Al in the silicate-phosphate glass structure can be taken either as a glass network modifier or as a network former, depending on the amount of MO added. Cooling rate effect on P-availability was also discussed. The heavy metal contents in the SSA-made FCMP meet the requirement for agriculture use.