EFFICIENT P-RECOVERY FROM THERMOCHEMICAL CONVERSION OF SEWAGE SLUDGE

Abstract

Sewage sludge is a by-product of wastewater treatment plants that contains high concentrations of essential nutrients including phosphorus (P). Considering limited P-resources, its recovery from the sludge is a sustainable method for phosphorus recycling. However, the recovery process is limited by the presence of toxic heavy metals available in the sewage sludge. Among various methods to efficiently recover phosphorus, incineration of sewage sludge spiked with chlorine-donor compounds has been useful for toxic heavy metal removal in the ash. In this study, effect of addition of single and mixed Cl-donor salts to the sewage sludge, prior to incineration, have been analyzed in a muffle furnace and bench scale bubbling fluidized bed (BFB) reactor. Additionally, Ca(OH)2 was used in combination with Cl-salts to gain a broader understanding of the process and improve Cr and Ni removal. Initially, the sewage sludge and ash samples were characterized by thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray diffraction (XRD). Phosphorus content in the residual ash was 11% by weight. Further, samples of sewage sludge were doped with CaCl2, NH4Cl, NaCl, KCl, and MgCl2 and their mixtures. Presence of salts led to a decrease in Cr (by 57%), Ni (by 59%), Cu (by 57%), Pb (by 95%), Cd (by 93%), As (by 83%), and Zn (by 98%). It was observed that CaCl2, KCl and MgCl2 were more efficient than NH4Cl and NaCl in decreasing the heavy metal content in the ash. Moreover, incineration of sewage sludge in the presence of double salt mixtures was less efficient than incineration with single salt. However, substitution of CaCl2 in a KCl and CaCl2 combination with Ca(OH)2 improved Cr and Ni removal by 10-15% compared to both single and combined salts. Observations also showed better efficiency of the muffle furnace in heavy metal removal as compared to the BFB reactor.