Comparison Between Two Solar Drying Techniques of Sewage Sludge: Draining Solar Drying and Drying Bed


A comparative study to the performances of the drying bed and the draining greenhouse was made in summer and winter. During each season, the first experience was conducted in the drying bed. The two other experiences were carried out in the draining greenhouse under natural and forced convection. The results show that in terms of drying time, the draining greenhouse under forced convection can be the best technique of sludge drying. For the drying curves, in the three cases of drying in summer and the case of drying bed in winter, only the decreasing speed phase (phase 2) is present. However, in both cases of drying in the draining greenhouse in winter, the curves did not follow any regular shape. Twelve models were tested to fit the drying kinetics of the sludge. For all experiences, Midilli-Kucuk model was chosen as the best model. The values of the effective diffusivity varied between 5.76 × 10−10 and 8.51 × 10−10 in summer and between 1.77 × 10−10 and 3.36 × 10−10 in winter.

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Fig. 1
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Fig. 5



Drying bed


Draining greenhouse


Dry solid content (%)

Deff :

Diffusion coefficient (m2/s)


Forced convection


Flow rate (m3/s)




Half thickness of the product sample (m)

mi :

Initial mass (kg)


Number of terms taken into account


Number of experimental points


Natural convection

R2 :

Coefficient of determination


Drying rate (kg water/kg DS.h)

Vi :

Initial volume (m3)

W0 :

Initial weight of the sample (kg)

Wd :

Weight of the dry solid which is obtained by putting the sample in an oven at 105 °C for 24 hours (kg)


Wastewater treatment plant


Sample thickness (m)


Average moisture content (kg water/kg DS)

Xeq :

Equilibrium moisture content

Xi :

Initial moisture content

Xr :

Reduced moisture content

Xr exp,i :

ith experimental reduced moisture content

Xr pre,i :

ith reduced moisture content predicted by each model


Number of constants


Density (kg/m3)






Transmission coefficient

χ2 :

Chi-square parameter


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Masmoudi, A., Ben Sik Ali, A., Dhaouadi, H. et al. Comparison Between Two Solar Drying Techniques of Sewage Sludge: Draining Solar Drying and Drying Bed. Waste Biomass Valor (2020).

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  • Sewage sludge
  • Draining greenhouse
  • Drying bed
  • Drying kinetics
  • Effective diffusivity
  • Natural and forced convection