NaHCO₃-enhanced sewage sludge thin-layer drying: Drying characteristics and kinetics

ABSTRACT

Drying sewage sludge is a highly energy-extensive process. For this reason, this work seeks to identify a reagent that can enhance the effectiveness of the drying process. In this study, drying experiments of sewage sludge were conducted at drying temperatures ranging from 100 to 160°C. NaHCO₃ was selected as the drying reagent, which was added to the sludge before drying. The thin-layer drying characteristics of the sludge and sludge/NaHCO₃ mixtures were later investigated and compared. Various mathematical models were used to simulate the sludge drying curves. It was found that adding 2 and 6% (wet basis) of NaHCO₃ to the sludge was effective in improving the moisture diffusion during the drying process, whereas the drying rate of the sludge/NaHCO₃ mixtures decreased when the addition of NaHCO₃ was further increased to 10% (wet basis). When the addition ratio was 2%, the increase in the maximum drying rate was the largest. With coefficients of determination (R²) over 0.9999, the modified Midilli model proposed in this study was observed to be the most suitable model to describe thin-layer drying of sludge relative to the other models examined in terms of R², reduced χ², root mean square error, and residual sum of squares. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, which varied from 3.700 × 10⁻⁹ to 1.085 × 10⁻⁸ m²/s over the temperature range (i.e., 100–160°C). The activation energy of moisture diffusion was determined to be 27.57 kJ/mol. Scanning electron microscope images of the dried sludge and sludge/NaHCO₃ mixtures indicated that the porosity of the sludge after drying increased with an increase in the NaHCO₃ addition ratio. Overall, the results suggested that NaHCO₃ is a suitable reagent to improve the drying efficiency of the sludge.

KEYWORDS:

• Modified Midilli
• moisture diffusion
• NaHCO₃
• sludge
• thin-layer drying