Screening of waste materials as adjuvants for drying sewage sludge based on environmental, technical and economic criteria

Thermal drying allows the reduction of the moisture contained in the sewage sludge (SS) formed in wastewater treatment plants (WWTP) to less than 80%. The management of the dried SS is easier and may have good properties as a soil amendment (carrying organic matter and N, P nutrients). This work aims to select the most appropriate drying adjuvant for application in full-scale and produce a material adequate for agriculture soil applications by applying two multi-criteria decision making (MCDM) methods, the MOORA and TOPSIS. The criteria weights were determined using the CRITIC method. Within the circular economy framework, four industrial wastes were considered as drying adjuvants: weathered coal fly ash (CFA), green liquor dregs (GLD), eggshell (ES), and rice husk (RH). Then, small cylinders of SS were extruded with 0.15 g adjuvant g−1 SSwb and dried at a constant temperature of 70 and 130 °C. Eleven criteria were grouped into three categories (environmental, technical, and economic). Both methods MOORA and TOPSIS showed that the use of GLD at 130 °C is the best option. Moreover, ES at 130 °C and GLD at 70 °C are also good options, and the choice should consider the need for removal of pathogenic contamination. The worst alternatives were CFA and RH at 70 °C according to both MCDM methods. The most influential criteria for decision making were the removal efficiency of pathogenic microorganisms, acid neutralization capacity (ANC) and germination index (GI). •Four waste materials as adjuvants (GLD, ES, CFA, RH) and two temperatures were tested.•The screening was based on environmental, technical and economic criteria.•Pathogens removal, buffering capacity and phytotoxicity are determining criteria.•Sewage sludge drying with GLD at 130 °C was the best alternative.•Sensitivity analysis changing ±40% weights showed GLD at 130 °C as the best option.