The morphological and physical changes of faecal sludge as a result of solar convection drying are presented in this section. The studied parameters were both quantitative (shrinkage, density, water activity) and qualitative (crust formation, cracking, reflectivity, colour)
4.2.1 Shrinkage
Shrinkage was due to an inward contraction of sample material resulting from inner stresses and mechanical in-equilibrium between the top and core of the sludge sample as drying progressed (Aprajeeta et al., 2015). Initial and final thicknesses of the drying sample were recorded from which final percentage shrinkage factors were deduced. Shrinkage values were considered rough estimates as the measurement method was not precise. Figure 4-14 shows the relationship between shrinkage and final moisture content for all the experiments. Shrinkage values were higher for samples with the lowest final moisture content values. This is because higher moisture reductions are associated with a greater loss of the volume that was occupied by the evaporated moisture. The reduction in volume translates into a collapse or shrinkage of the sludge samples (Lu et al., 2008). This pattern of increasing shrinkage with reducing moisture content was similar to that reported by Krokida and Maroulis (1997) on various food products. Shrinkage values for UD sludge were not influenced by the operating conditions.
Figure 4-14: Shrinkage versus final moisture content for UD and VIP sludge samples at varying drying conditions
The final shrinkages of VIP sludge samples at varying air temperature and flowrate are shown in figure 4-15. Final shrinkage was estimated at 40%, 50%, 60% at 0.5 m/s air velocity and 50%, 60% and 70% at 1 m/s air velocity, at ambient temperature, 40°C and 80°C respectively.
Shrinkage increased with increasing drying air temperature and air velocity. This trend was the result of higher air temperatures and higher air velocity that resulted in greater moisture reduction during drying (sections 4.1.2 and 4.1.3).
Figure 4-15: Shrinkage as a function of drying air conditions (temperature and velocity) for VIP sludge solar drying during sunny conditions
0 10 20 30 40 50 60 70 80
0 0.5 1 1.5 2 2.5
shrinkage (%)
moisture content (g/g db)
0.5m/s amb temp sunny VIP 0.5m/s 40°C sunny VIP 0.5m/s 80°C sunny VIP 1m/s amb temp sunny VIP 1m/s 40°C sunny VIP 1m/s 80°C sunny VIP
0.5m/s amb temp overcast VIP 0m/s amb temp sunny VIP 0.5m/s amb temp sunny UD 0.5m/s 40°C sunny UD 0.5m/s 80°C sunny UD
0.5m/s amb temp zero radiation VIP open drying sunny VIP
0 10 20 30 40 50 60 70 80 90 100
ambient 40°C 80°C
Shrinkage (%)
Air temperature
0.5m/s 1m/s
4.2.2 Density
For this study density was analysed in terms of apparent density which is the mass of the dried sample divided by its external volume (sum of the pore and solid volume).
Figure 4-16 shows the variation of density with moisture content after solar drying of sludge under the various experimental conditions. Density of dried sludge samples ranged between 1678 kg/m3 at 2.09g/g db moisture content to 1228 kg/m3 at 0.66g/g db moisture content which is similar to values reported by O'Kelly (2005) for dewatered sewage sludge. From figure 4-16 there is a general trend of decrease in density with reducing moisture content. This implied that density of sludge samples decreased as drying progressed. This trend is with exception of two experiments (1m/s amb temp sunny VIP and 1m/s 40°C sunny VIP) which could be a result of experimental inaccuracies/outliers. Decrease in density as drying progressed was an indication of general lower reduction in sludge mass as compared to volume as moisture content reduced.
There was no specific pattern between sludge density and operating parameters.
Figure 4-16: Variation of density with final moisture content for both UD and VIP sludge samples at the varying drying conditions
4.2.3 Qualitative analysis
A qualitative investigation of the dried samples was undertaken to determine the formation of cracks, crust layer, and changes in odour, colour and reflectivity of the dried samples in relation to its initial aspect. Figure 4-17 displays photographs of the initial and final aspect of VIP sludge dried at an air velocity of 0.5 m/s and at different temperatures. Initially, sludge was a deformable material with soft paste-like consistency. Its aspect was similar to the typical appearance of VIP sludge as reported in the literature (Schoebitz et al., 2014). After drying, sludge turned to a hard, crystal-like, less deformable structure with cracks on the surface Cracking was observed particularly for the samples dried at 80°C and in a lower extent for the samples dried at ambient temperature. This result could be explained by the lower moisture reduction at ambient conditions and a possible uniform moisture distribution at the sludge surface. This reduced internal stresses within the sludge structure and thus caused less cracking.
In addition, at higher temperatures the drying of sludge was at more advanced stages and thereby more cracking (S. Abbasi, 2011).
Case hardening or formation of a crust layer on the sludge surfaces was also observed. The crust formation was possibly related to internal moisture transfer limitations. Indeed, if the
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0 0.5 1 1.5 2 2.5
density (kg/m3)
final moisture content (g/g db)
0.5m/s amb temp sunny VIP 0.5m/s 40°C sunny VIP 0.5m/s 80°C sunny VIP 1m/s amb temp sunny VIP 1m/s 40°C sunny VIP 1m/s 80°C sunny VIP
0.5m/s amb temp overcast VIP 0m/s amb temp sunny VIP 0.5m/s amb temp sunny UD 0.5m/s 40°C sunny UD 0.5m/s 80°C sunny UD
0.5m/s amb temp zero radiation VIP
internal moisture transport is not fast enough to feed the external surface, a superficial desiccation can occur hardening the top surface of the sludge (Leonard et al., 2004). The crust layer was more significant for UD samples as compared to VIP (section 4.1.6).
Before drying 5 hours drying at ambient temperature
5 hours drying at 40°C 5 hours drying at 80°C
Figure 4-17: Photographs of VIP sludge samples before drying and after drying during sunny conditions, at varying air temperatures
From visual observation, initial sludge surface was shinny with a relatively high light reflectivity. This shinny surface faded gradually as drying progressed. The fading of the shiny surface was attributed to the gradual loss of surface moisture as drying progressed and this could be an indicator of a potential increase of solar irradiance absorbance of the material as drying progressed. The change in colour varied from a dark black in the wet samples to a dark brown in the dry samples. Foul odour/smell from the sludge samples was also noticed to gradually cease as drying progressed. Changes in the colour and odour of the dry sludge was in agreement to that reported in literature (Getahun et al., 2020). There was no noticeable relationship between colour and odour of sludge with the operating operators.
4.2.4 Water activity
Water activity TY indicates the amount of unbound water that is available in the sludge for microbial growth. Figure 4-18 shows the relationship between water activity and final moisture
content for all the experimental conditions. Average values from the experiments done in duplicates are presented. From the results in figure 4-18, there is a slight decrease in water activity with decreasing moisture content. However, since uncertainty bars overlap, statistically no significant difference between water activities at the varying experimental conditions. Water activity values were highest for 80°C drying air temperature and least for ambient air (23°C) drying air temperatures. This agrees with results from (Serowik et al., 2017) who reported that water activity is significantly reduced with increasing dry air temperatures. There was no specific relationship between water activity and drying air velocity.
Water activity for all dry samples was in the range 0.9374 to 0.981, which is close to 1. This result meant that moisture within all the dry samples was predominantly slightly bound. This water activity from this study were within the values reported in literature by (Getahun et al., 2020) for VIP and UD sludge at similar moisture content values.
Figure 4-18: Water activity versus final moisture content for both UD and VIP sludge samples at all the experimental conditions of weather, air temperature and airflow
0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1
0 0.5 1 1.5 2 2.5
water activity
final moisture content (g/g db)
0.5m/s amb temp sunny VIP 0.5m/s 40°C sunny VIP 0.5m/s 80°C sunny VIP 1m/s amb temp sunny VIP 1m/s 40°C sunny VIP 1m/s 80°C sunny VIP
0.5m/s amb temp overcast VIP 0m/s amb temp sunny VIP 0.5m/s amb temp sunny UD 0.5m/s 40°C sunny UD 0.5m/s 80°C sunny UD
0.5m/s amb temp zero radiation VIP open drying sunny VIP