The objective of this work was to analyze the effect of eggshell (ES) as adjuvant of the drying process of sewage sludge (SS) from anaerobic digestion. Small cylinders were dried in isothermal conditions at 70 and 100 °C, until complete dehydration. Two mathematical models were used to describe the experimental data, and in both cases good fitting was achieved (R2 > 0.98 and RMSE < 0.02). The mixture SS:ES equal to 85:15 revealed a positive effect in the drying rate since the drying time was reduced. When ES was used, the average drying rate observed in the first period at 70°C was improved by 9.86%, while at 100°C almost 17% improvement was achieved. The diffusion coefficients (D eff ) were also calculated, and the improvements in this parameter were also detected. Indeed, the application of ES as adjuvant increased D eff in 9% and 16,32% at 70 and 100° C, respectively.