3.1. Biogas and methane production
The evolution of the biogas production was monitored in different digesters in order to assess the AD of stems and leaves of MU. Figure 2 shows the total cumulative volumes of biogas (Fig. 2A) produced and that of methane (Fig. 2B) obtained after purification of the biogas by the absorption of CO2 by the KOH (Fig. SI1) versus time. Obviously, it is shown that no biogas production was detected for the blank samples or negative control (T-). After 128 days of AD, the total cumulative volume of biogas produced was observed to be 501.5 ± 8.5 mL for the positive control (T+), with a total volume of methane of 421.0 ± 13.0 mL. This corresponded to 84.0% of the biogas produced. The detection/or non-detection of biogas for (T+)/or (T-), respectively, enabled us to highlight the methanogenic activity of inoculum as well as the methanogenic potential of substrate. More importantly the fact that the biogas was not detected in (T-) make us sure and confident that all the biogas were produced from the AD of substrates. The total cumulative volume of biogas produced were evaluated to be 1144.5 ± 40.0 mL and 921.0 ± 31.0 mL for stems and leaves of MU, respectively. The cumulative amount of methane was 946.0 ± 18.0 mL for the stems of MU. That represented about 83.0% of methane in biogas. By contrast the amount of methane was 770.0 ± 16.0 mL for leaves of MU, representing about 84.0% of methane in the biogas.
The delay observed before the biogas production starts could be attributed to the concentration of organic matter in the substrates (VS). Indeed, the biogas production in the digester containing higher concentration of organic matter like the stems rapidly produced (10 days) the biogas after inoculation, whereas the digester with lower content of organic matter lately produced (20–30 days) the biogas after incubation (Fig. 2A, B; Table 1). Although the pH conditions were suitable in the different digesters, that is, between 6.5 and 7.2 (Appels et al. 2008; Turovskiy and Mathai 2006; Zhang et al. 2013; Peng et al. 2014).
Table 1
Physical and chemical characterization of the substrates and the inoculum
Components | Substrates | Inoculum |
MU Leaves | MU Stems | CM |
Dry weight (%) | 80.8 ± 0.0 | 17.1 ± 0.4 | 21.6 ± 0.2 |
Organic matter (% DW) | 85.2 ± 0.3 | 91.0 ± 0.2 | 16.1 ± 0.4 |
TOC (mg/g DW) | 357.5 ± 14.3 | 530.5 ± 15.4 | 486.4 |
TKN (mg/g DW) | 50.5 ± 0.8 | 23.8 ± 03 | 26.1 |
P (mg/g DW) | 2.2 | 180.2 | 27.6 |
K (mg/g DW) | 21.5 | 142.3 | 10.5 |
C/N | 7 | 22 | 19 |
C/N/P | 163/23/1 | 227/10/75 | 180/9/10 |
The physical and chemical analysis of the substrates (Table 1) showed that the MU stems contained lower amount of nitrogen, and high contents of carbon and mineral elements (K and P) compared to MU leaves. Moreover, the C/N ratio was in the optimal range (C/N: 20–30) for a good AD (Banks and Heaven 2013; Borja and Rincón 2017; Yen and Brune 2007; Akunna 2015). Biogas production containing 84% from cane sugar, which is free of nitrogen revealed that the inoculum contained nutrients capable to trigger the methanization.
Although the C/N ratio of the substrate has high influence on the AD process (Banks and Heaven 2013; Borja and Rincón 2017; Yen and Brune 2007; Akunna 2015), we also noticed that the proportion of methane in the biogas depended not only on the C/N ratio of the substrates, but also on that of the total composition in the digester. This was proved by the fact that the methane yield recorded by cane sugar (0.526 m3 of CH4/ kg.VS) was around that theoretically expected (0.692 m3 of CH4/kg.VS) (SI3). This methane yield was achieved after 50 days of AD and stayed nearly steady after 128 days like that of the MU leaves (Fig. 2).
The range of concentration of K required for its stimulating effect of the AD under mesophilic conditions is 200–400 mg/L (Appels et al. 2008; Turovskiy and Mathai 2006). The methanization of leaves of MU could be stimulated by the K because its concentration was determined to be 287 mg/L. The half maximal inhibitory concentration of K of the AD is IC50 ~ 2900 mg/L (Appels et al. 2008; Turovskiy and Mathai 2006). However, with a K content of about 1900 mg/L for the stems of MU, which is by far lower than the IC50, the methanation of this substrate was not affected by the concentration this mineral. The same effect was observed for phosphorus.
The C/N or C/N/P ratio in the leaves of MU alone and in the overall composition of digester before AD were not in the range recommended (C/N: 20–30; C/P/N: 150/4/1) (Banks and Heaven 2013; Borja and Rincón 2017; Yen and Brune 2007; Solarte-Toro et al. 2018). For the stems of MU, only the C/N ratio was in the range commonly recommended but not their C/N/P (Table 1, 2).
Due to their difficulty to solubilize induced by their composition rich in cellulosic fiber (Han et al. 2011; Reed et al. 1982; Klinpratoom et al. 2015), the production of methane of the stems of MU continued to increase after 128 days of AD (Fig. 2). This is justified that the TOC concentration of 10.9 mg/g for stems in their liquid residues compared to 73.3 mg/g for leaves (Table 2).
The conversion rate of the CS in biogas was about 70%, but those of two substrates were approximatively 19% for the leaves and 16% for the stems according Table 2.
Table 2
C/N/P ratio and K quantity of different samples in the digester before and after AD
Substrates and inoculum | Before AD | After AD |
C/N/P | K(mg) | C/N/P | K(mg) |
Inoculum alone | 186/10/11 | 13 | 186/10/11 | 13 |
Positive control | 293/10/10 | 13 | 218/10/10 | 13 |
MU leaves | 600/75/10 | 156 | 517/75/10 | 156 |
MU stems | 216/10/65 | 962 | 187/10/65 | 962 |
The analysis of secondary bioactive metabolites or bioactive substances in the leaves and stems of MU showed the presence of saponins and catechic tannins (Table 3). However, their presences did not affect the methanization of the leaves and stems. Indeed, it is known that some plants or their extracts with high concentrations of secondary bioactive metabolites such as saponins, tannins, essential oil, organosulphur compounds, flavonoids and many other metabolites have potential to inhibit methane production (Mambanzulua et al. 2015b; Patra and Saxena 2010; Mambanzulua et al. 2015c, Beauchemin et al. 2008). The methonogenesis is inhibited at concentrations of secondary bioactive metabolites above 0.3 g/L (Mambanzulua et al. 2015b, c). This behavior indicated that that concentrations of saponins and catechic tannins in the leaves and stems would be less than 0.3 g/L.
Table 3
Bioactive substances in leaves and stems of MU
Components | leaves | stems |
Saponins | + | + |
Flavonoids | - | - |
Alkaloids | - | - |
Anthraquinones (bound quinones) | - | - |
Catechic tannins | + | + |
Gallic tannins | - | - |
Anthocyanins | - | - |
Leuco-anthocyanins | - | - |
+ : sustrate contains the component - : sustrate do not contain the component |
The methane yields of our two substrates were therefore 0.156 ± 0.003 m3 CH4/kg VS and 0.136 ± 0.003 m3 CH4/kg VS for stems and leaves of MU respectively (SI3). These methane yields are in the range of those found by Amon Thomas et al. (2006) and stipulated by Gunaseelam (2004) for the vegetable wastes, whose the range is between 0.12 to 0.43 m3 of CH4/kg VS depending in the chemical composition of the substrate.
This difference of the biogas or methane yields for the stems and leaves could be due to the difference between their organic matter and C/N ratio.
The yields of the methane obtained par Mambanzulua et al. (2015b) after 100 days of AD of the leaves at the concentration of 13 g/L was 0.023 m3 CH4/g VS at 30 °C. In this study, after 100 days of AD, the methane yield obtained in the same concentration leaves at 25 °C was about 6-fold higher. That could be explained by the fact that methanogenic activity of CM used in this work is approximately 6-fold superior than that of the anaerobic sludge which came from in origin activated sludge. It is reported in the literature that the CM is more active than activated sludge which is inoculum from anaerobic reactor. The methanogenic activity of the CM is 3–13 mg CDO-CH4/g VS by contrast that of the activated sludge is not exactly known. However, the latter is classified in the range of 1–2 mg CDO-CH4/g VS and it depends on the features of the granular sludge, type of the substrate, environmental conditions, and the test procedure (Hussain and Dubey 2017; Ashekuzzaman and Poulsen 2011; Shin et al 2001).
3.2. Energetic potentials of biogas
The calorific value of biogas is proportional to its methane content. This can be determined thanks to Lower Heating value (LHV) and the proportion of methane in the biogas produced (Borja and Rincón 2017; Chen et al. 2008; Abbasi et al. 2012). The proportions of methane in the biogas produced from the stems and leaves of MU were 83% and 84%, respectively. These correspond to the calorific values evaluated at 7.820 kWh/m3 and 7.914 kWh/m3, respectively. These energy values found are within the range of the biogas calorific value reported in the literature (4.726 kWh/m3 − 9.452 kWh/m3) (Li et al. 2017; Borja and Rincón 2017; Chen et al. 2008; Abbasi et al. 2012).
However, in KCP, the MU is the first most consumed vegetable and the second most planted (Mambanzulua et al. 2015a, 2015b; Mulaji 2017; Bell et al. 2000). KCP produces annually about 3 million tons of wastes. These wastes contain 1.65 million of organic wastes, which represents 55% (Ministère Provincial d’Environnement 2019). Previous research has shown that these organic wastes are essentially composed (94%) of vegetable wastes of which the majority is leaves (Mambanzulua et al. 2015a, 2015b; Mulaji 2017). Considering that the latter consisted mainly of leaves or stems, the annual energetic potential of vegetable wastes produced in the KCP will therefore be estimated at 1.362 ± 0.028 109 kWh for leaves (SI4A) and 0.337 ± 0.006 109 kWh for the stems (SI4B). Knowing that the LHV of charcoal is about 8.229 kWh/kg (Solarte-Toro et al. 2018; Jenkins 2015; Gary 2010), the annual energetic potential obtained would cover the energy needs for the KCP households corresponding to the use of charcoal, evaluated with 166 103 tons for the leaves, and with 41 103 tons of charcoal for the stems (SI4). The substitution of the use of charcoal by biogas in KCP households, will allow them to spare from the problems of air pollution which causes 4 million premature deaths each year in the world, with more than 600 000 in African countries following the use of solid biomass energy (Ifegbesan et al. 2016; WHO 2018). The Table 4 reports the energy amounts in the resulting biogas production from the AD of leaves and stems of MU during 100 and 128 days.
Table 4
Energy amounts in the resulting biogas production from the anaerobic digestion of leaves and stems of MU during 100 and 128 days.
Samples concentrations | Methane yields for 100 days (L/g VS) | Methane yields for 128 days (L/g VS) | Energies for 100 days (kWh/g VS) | Energies for 128 days (kWh/g VS) |
1.6 g CS/L | 0.519 ± 0.020 | 0.526 ± 0.016 | 4.888 ± 0.188 10− 3 | 4.958 ± 0.153 10− 3 |
13.3 g leaves MU/L | 0.136 ± 0.003 | 0.136 ± 0.003 | 1.277 ± 0.027 10− 3 | 1.277 ± 0.027 10− 3 |
13.3 g stems MU/L | 0.148 ± 0.002 | 0.156 ± 0.003 | 1.399 ± 0.023 10− 3 | 1.468 ± 0.028 10− 3 |
3.3. Fertilizing potential of the digestates
The fertilizing pontential of the digestates of leaves and stems of MU is primarily due to the availability of the mineral elements (N, P, K) previously retained in the complex structures of these wastes, owing to their mineralization through AD. The N, P and K concentrations in the liquid digestate for the stems were determined to be 2.1, 85.0 and 117.3 mg/L respectively. By contrast, the N, P and K concentrations in the liquid digestate for leaves were 13.7, 4.4 and 110.0 mg/L respectively.
We evaluated the fertilizing potential of the digestates utilizing their C/N ratio. We noticed that the C/N ratio of Leaves digestates has fallen to 5. The digestate with a C/N ratio of 5 is recognized to be favorable for vegetable crops and fruit trees soils (Mambanzulua et al. 2015b; Mulaji 2017; Hawke and Summers 2003; Glowacka et al. 2020). The C/N ratio of the digestats resulting from the AD of stems was 10. A C/N ratio of 10 is considered optimal for the organisms, soil conditioning and could improve the soils hydraulic conductivity (Mambanzulua et al. 2015b; Hawke and Summers 2003; Wrap 2016; Lessard and Bihan 2003; Sparling et al. 1999).
It has been shown that one can spread until 30 tons of dry matters of digestate by hectare per year on an acidic poor soil (Mambanzulua et al. 2015a; Glowacka et al. 2020; Wrap 2016; Sparling et al. 1999). Therefore, with the 222 103 and 1.039 106 tons of digestates that we can produce from methanization of 1.55 106 tons of stems and leaves of MU, respectively, produced in Kinshasa per year, one could fertilize 7,400 hectares for stems and 39,966 hectares for leaves (SI5A, SI5B).