Faecal Sludge Characteristics
The HQ40d multiparameter was used to measure the properties of faecal sludge from septic tanks and pit latrines. The on-site measurements gave the following results: pH of 6.73 and 7.09, electrical conductivity (EC) of 3405 µS/cm and 1716 µS/cm, total dissolved solids (TDS) of 1711 mg/L and 857 mg/L, and dissolved oxygen (DO) of 0.19 mg/L and 0.1 mg/L, respectively. Inorganic pollutants such as Zn, Cd, and Pb in faecal sludge are regulated by the Tanzanian guidelines for environmental disposal, which set the limit value of 5.00 mg/L for Zn and Cd and 30.00 mg/L for Pb (TZS, 2009). The Concentrations of Zn, Cd, Pb, and Cu in the faecal sludge from septic tanks and pit latrines were 0.5 mg/L and 0.5 mg/L, 0.55 mg/L and 0.6 mg/L, 10.01 mg/L and 4.87 mg/L, and 0.33 mg/L and 0.84 mg/L, respectively, which fall within the TZS limits.
Agoro et al. (2020) reported concentrations of 0.099 mg/kg, 1.17 mg/kg, and 0.14 mg/kg for Zn, Cu, and Cd in faecal sludge, which were also below the TZS limits. Similarly, Appiah-Effah et al. (2015) reported concentrations of 2.2 mg/L, 4.0 mg/L, 0.045 mg/L, and 0.160 mg/L for Zn, Cu, Cd, and Pb in low-income areas of Ashanti Region of Ghana, which are within the acceptable range set by the US-EPA. Therefore, most domestic faecal sludge, when not mixed with industrial waste, will have acceptable concentrations of the selected inorganic pollutants within the acceptable range. Additional characteristics of the faecal sludge from septic tanks and pit latrines were 2125 mg/L and 50 mg/L for BOD5, 13600 mg/L and 161 mg/L for COD, 60 mg/L and 0.56 mg/L for total solids (TS), 44.07 mg/L and 0.54 mg/L for volatile solids (VS), and 16.7 mg/L and 0.2 mg/L for fixed solids (FS), respectively. Pathogens such as E-Coli, Salmonella, Shigella, Ascaris Lumbricoides, and Hookworms Egg were present in the raw faecal sludge from both septic tanks and pit latrines. However, after carbonization at an elevated temperature of 450 ºC, all pathogens were killed (Table 2).
Table 2 Pathogens Level of the raw Faecal sludge
Sample ID
|
Pathogens
|
E-Coli (CFU/100 mL)
|
Salmonella (CFU/100 mL)
|
Shigella (CFU/100 mL)
|
Ascaris Lumbricoides
|
Hookworms Eggs
|
Raw FS from ST
|
TNTC
|
TNTC
|
TNTC
|
Positive
|
Positive
|
Raw FS from PL
|
TNTC
|
TNTC
|
TNTC
|
Positive
|
Positive
|
Dried FS from ST
|
0
|
0
|
0
|
Positive
|
Positive
|
Dried FS from PL
|
0
|
0
|
0
|
Positive
|
Positive
|
Carbonized FS from ST
|
0
|
0
|
0
|
Negative
|
Negative
|
Carbonized FS from PL
|
0
|
0
|
0
|
Negative
|
Negative
|
NB: TNTC = Too Numerous to Count
|
Briquette making
The agglomeration process was used to produce round briquettes in a rotating drum. Molasses were slowly added to the mixture together with 2mm sieve dust (faecal sludge/sawdust/charcoal dust) until round briquettes with a diameter of about 40mm were obtained. Physical composition, proximity analysis, emission level, compatibility strength, final analysis, ignition rate, burn rate, and water boiling test were used to characterize the resulting briquettes.
Heating Value (HV)
The gross heating value (G.H.V.) and Net Heating Value (N.H.V.) of the briquettes made from different biomass materials at different mixing ratios are shown in Figure 5. The briquette burning rate depends on the higher calorific values (Huang, 2011). Sample C (100% charcoal dust as a control) had the highest G.H.V. and N.H.V. followed by Sample B (100% sawdust as a control) briquettes with a G.H.V and N.H.V. of 5228.08 Kcal/Kg and 5116.83 Kcal/Kg, respectively. The briquettes made from a mixture of biomass materials indicated the following; The mixture of the faecal sludge and Charcoal dust in a mixing ratio of 25% faecal sludge and 75% charcoal dust (Sample G), as well as that of 50% Faecal sludge and 50% charcoal dust (Sample H), had the highest heating value compared to other mixing ratios with a G.H.V and N.H.V. of 4607.43Kcal/Kg and 4504.83 Kcal/Kg, respectively. Briquettes in terms of heating values go in the following order; C>B>G>H>I>D>F>E>A. In General, the addition of charcoal dust and sawdust biomass materials has been shown to improve the heating value of the faecal sludge briquettes (Figure 5).
Density
The briquette density affects the burning speed of the briquettes. High-density briquettes last longer and give off more heat (Huang, 2011). From this study, there was little significant difference in the bulk density of the briquettes made from different biomass materials. Figure 6 shows that the faecal sludge briquettes (100% faecal sludge) had a high density, with a briquette made from charcoal dust and its mixtures having the lowest density.
Proximate Analysis
Briquette's proximate analysis includes volatile matter (%), moisture content (%), bound carbon (%) and ash content (%). According to Onukak et al. (2017), the smallest volatile components indicate a maximum energy value as shown in Table 3. Briquettes at different mixing ratios were prepared with Sample A (100% faecal sludge) showing maximum volatile content of 46.2%. Briquettes prepared from charcoal dust, sawdust and the mixture with faecal sludge had shown low volatile matter when compared to briquettes prepared from faecal sludge only. This indicates that more energy is needed to burn off the volatiles before the thermal energy is released from the briquettes. Volatiles indicate those fuel components that are easily combusted in the presence of oxygen (Onukak et al., 2017). It is the per cent mass loss, corrected for moisture when coal is heated under standard conditions without contact with air. Highly volatile substances in the fuel are undesirable as they produce more tar (Onukak et al., 2017).
Table 3 Percentage volatile matter of briquettes at different ratios
Sample ID
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
I
|
% Volatile Matter
|
46.2
|
27
|
26.3
|
39.5
|
43.9
|
46
|
29.8
|
33
|
36.9
|
The tolerance limit for the briquette's moisture content is between 8 and 12%, depending on the type of feed (Oyelaran et al., 2017). This study shows that the moisture content of both briquettes was within tolerance (Figure 7). The briquette's moisture content above the acceptance limit reduces the thermal efficiency and burning rate which will require more energy to exhume the moisture. Another disadvantage of the high moisture content is that it facilitates the growth of fungi and other microbes.
The ash content of a fuel is an important parameter that provides valuable information about the degree of clogging of the combustion medium. High ash content can significantly reduce the rate of combustion and lower the calorific value of the fuel (Onukak et al., 2017). Table 4 shows the percentage of ash content in briquettes made from various biomass materials. The results show that the lowest ash content was found in samples B and C, with values of 26.2% and 26.02%, respectively, followed by samples G and H, which had ash content values of 31.28% and 33%, respectively. exhibited. Maximum ash content was observed in Sample A, which was 100% faecal sludge, with a value of 47.31%. The ash content produced during briquette combustion can cause operational problems as it can clog air holes and reduce oxygen supply (Huang, 2011). Therefore, based on the results of this study, faecal sludge briquettes made from a mixture of 75% faecal sludge and 25% charcoal dust showed better quality and lower ash content. This suggests that the optimal blend ratio can help reduce the operational challenges associated with high ash content in faecal sludge briquettes, making them a more viable alternative to traditional fuel sources.
Table 4 Percentage of ash content at different mixing ratios
Sample ID
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
I
|
% Ash
|
47.31
|
26.2
|
26.02
|
39
|
43
|
45.5
|
31.28
|
33
|
40.7
|
Fixed carbon is directly proportional to the calorific value, and the maximum value was observed in Samples B, D, G and H with the following values; 41.6%, 40.3%, 30.4% and 27.1%, respectively. The minimum carbon content was observed in samples A, F and E (Figure 8) with a fixed carbon content of 1.65%, 2.7% and 6.4%, respectively.
Elemental Composition
To assess the quality of briquettes, the proportion of carbon and hydrogen is a crucial factor (Ajimotokan et al., 2019). The results of this study showed that Sample C, made from 100% charcoal dust, had the highest percentage of carbon at 54.7%. In comparison, Samples G and H, which were a mixture of faecal sludge and other biomass materials (sawdust and charcoal dust), had high percentages of carbon at 47.8% and 48.1%, respectively. The mixture of faecal sludge and charcoal dust resulted in briquettes with a higher carbon and hydrogen content compared to other mixtures (Table 5). Therefore, adding charcoal dust to faecal sludge resulted in briquettes with higher carbon and hydrogen levels, indicating better quality. These results suggest that using faecal sludge and charcoal dust as a mixture could be a promising approach to producing high-quality briquettes.
Table 5 Elemental composition of briquettes at different ratios
Sample ID
|
Nitrogen
|
Carbon
|
Hydrogen
|
Oxygen
|
A
|
2.19
|
24.38
|
1.455
|
9.7075
|
B
|
1.09
|
47.1
|
2.25
|
12.253
|
C
|
1.27
|
54.66
|
2.17
|
27.379
|
D
|
1.45
|
38.48
|
1.91
|
20.181
|
E
|
1.43
|
33.9
|
1.5
|
17.02
|
F
|
2.06
|
33.79
|
1.79
|
13.079
|
G
|
1.34
|
47.8
|
2
|
27.279
|
H
|
1.33
|
47.14
|
2.12
|
25.68
|
I
|
1.64
|
40.22
|
1.79
|
18.712
|
Emission Level
There was a small significant difference in CO (ppm) emission between the faecal sludge briquettes compared to regular charcoal. Acceptable indoor air quality for CO emissions is 50 ppm and 5000 ppm for CO2, but this depends on exposure time (ToolBox, 2020). In Figure 9B, the CO2 emission levels of faecal sludge (100%), briquettes and sawdust (100%) were high compared to others. This could be due to the ignition and burning time. Briquettes made from faecal sludge alone take time to light the fire, and sawdust alone will ignite faster than the rest of the briquettes. Figure 9C shows that the emission level of particulate matter (PM2.5 and PM10) was high for regular charcoal compared to the briquettes, and the lowest concentration was observed for briquettes made from 100% faecal sludge. Figure 9D and Table 6 shows the emission level of TVOC and HCHO.
Table 6 Indoor air quality
Sample ID
|
TVOC (mg/m3)
|
HCHO (mg/m3)
|
O (%)
|
Humidity (%)
|
Normal Charcoal
|
4.77
|
0.8
|
20.9
|
47
|
A
|
2.91
|
0.7
|
20.9
|
51
|
B
|
1.68
|
0.8
|
20.9
|
47
|
D
|
3.53
|
0.72
|
20.9
|
55
|
E
|
0.41
|
0.09
|
20.9
|
44
|
F
|
1.1
|
0.22
|
20.9
|
58
|
G
|
0.38
|
0.06
|
20.9
|
47
|
H
|
5.11
|
0.8
|
20.9
|
59
|
I
|
3.1
|
0.17
|
20.9
|
44
|
According to the World Health Organization (2014), in a room where there is a continuous flow of air, the emitted gases are evenly mixed throughout the room, reducing their concentration.
Ignition Time and Burning Rate
The briquettes made from faecal sludge take a long time to ignite a fire compared to the briquettes made from sawdust/charcoal dust and the mixture of faecal sludge with sawdust and charcoal dust. Sawdust briquettes (Sample B) ignited faster than the other briquettes and, due to their porous nature, did not last long compared to other materials, as shown in Figure 10 (Rotich, 1998). Its ignition time was less than 5 minutes, followed by the mixture of sawdust materials and faecal sludge materials (samples D, E and F) and the briquettes made from charcoal dust and the mixture of charcoal dust and faecal sludge (samples C, G, H and I). Faecal sludge briquettes took more than 20 minutes to ignite a fire under the same conditions. Briquettes made from faecal sludge have a long burning rate compared to briquettes made from other biomass materials, taking up to five hours to burn completely to ash. The mixture of faecal sludge and charcoal dust was the best with 75% and 25% in terms of burning rate.
Compressive Strength
The force required to break the briquettes was measured in kilo-forces per square meter using the Universal Testing Machine (8551 M.F.L. System). Figure 11 illustrates that sample A (briquettes made solely from faecal sludge) requires more force to break compared to other ratios. Briquettes made from sawdust or a mixture of sawdust and faecal sludge (Samples D, E, and F) require less force to break compared to faecal sludge alone and the mixture of faecal sludge and charcoal dust (Samples G, H, and I). Briquettes made from faecal sludge (Sample A) require a force of 77 kg/m2 to break up, while the addition of 25% charcoal dust (Sample I) requires a force of about 65 kg/m2. Adding 25% sawdust (sample F) requires a force of about 50 kg/m2 each time. Thus, the compressive strength of the briquettes was increased by the addition of faecal sludge, whereas the addition of more sawdust or charcoal dust resulted in reduced strength of the briquettes.