Effect of Magnesium Additives on Phosphorous Recovery during Sewage Sludge Combustion and Further Improvement of Bioavailable Phosphorous

Abstract

Sewage sludge (SS), a solid waste taking up a large amount of public resources, contains abundant phosphorous and urgently needs appropriate recovery, but incineration, the existing popular SS treatment method, fails to reuse phosphorous as a feasible product due to the poor phosphorous bioavailability of SS ash. Based on the mono-combustion of SS, magnesian minerals comprising of magnesium oxide were doped with SS to carry out the behavior of magnesium in phosphorous capture and its sensitivity to subsequent thermochemical modification. Five percent MgO improved phosphorous capture, and its effectiveness was disturbed by sulfur at 900 °C. The more H2O that was pumped into the atmosphere, the more phosphorous was captured by 5% MgO. The capacity of MgO in phosphorous capture was inferior to that of CaO. The utilization efficiency of MgO for phosphorous capture was inferior to that of CaO. A total of 7.2% MgO succeeded in recovering 97.46% phosphorous with 5% H2O at 900 °C. A total of 15.06% hydromagnesite merely promoted 1.85% and 5.13% of the phosphorous relative enrichment factor (RE) in SS ashes without or with 5% H2O, respectively, whereas it recovered 90.21% phosphorous with 10% H2O, supposing a potentiality in phosphorous capture for the direct combustion of wet SS. However, having been improved by magnesium, the bioavailability of phosphorous in SS ash remained extremely limited. Thus, thermal modification by K2CO3 was applied, where the limited bioavailability of phosphorous in the SS ashes was remarkably alleviated; although, SiO2 and sulfate were the main disturbers and led to the production of K2MgSiO4 and K2Mg2(SO4)3. The effective constituents were KMgPO4 and K3CaH(PO4)2 in the final mixed fertilizer. The obtained mixed fertilizer might be suitable for application on acidic soils.