Improvement of Thermal Dehydration and Agronomic Properties of Products Obtained by Combining Sewage Sludge with Industrial Residues

Abstract

Sewage sludge (SS) formed in wastewater treatment plants can contain organic matter and nutrients in concentrations that are valuable for amending poor soil. This work aims to compare the performance of six residues not only as drying adjuvants of SS but also as improvers agents in agronomic properties of the final product. According to criteria in line with circular economy and industrial ecology, the selected adjuvants were weathered coal fly ash (CFA), bottom biomass ash (BBA), green liquor dregs (GLD), lime mud (LM), eggshell (ES), and rice husk (RH). The main physicochemical properties of these materials were determined. Then, small cylinders with and without 0.15 g adjuvant/g SSwet basis were shaped at room temperature and dried at 130 oC in a moisture analyzer to evaluate the drying kinetics. The profiles were modeled, and the main parameters determined. Furthermore, several relevant parameters for agronomic applications of the final products were determined, such as pathogen contamination, acid neutralization capacity (ANC), and oxygen uptake rate (OUR). Adding BBA to SS (named as SS_BBA product) promoted the highest diffusion coefficient (2.55·10−7 m2/s) and drying rate (41.6 g H2O/kg min). On the other hand, SS_ES showed a positive impact in almost all agronomic parameters, with an ANC of 0.166 g CaCO3/g, OUR of 10.18 mmol O2/kg organic matter h, and germination index of 29.1%. In general, the results indicate that several residues can be used to improve not only sludge drying but also the final properties of the product for soil applications.