3.1 Effect of mixing ratio of SFW to pre-treated P. australis at inoculum to substrate ratio of 1:4 on biogas production.
As shown in Fig. 1a, there was variation in the cumulative biogas production among the different SMRs. The reactors at SFW: pre-treated P. australis mixing ratios of 75: 25, 100:0, and 0:100, showed a bi-phase pattern of biogas production represented by the increase in biogas production at the beginning of digestion, followed by a lag period of biogas production, after which the production of biogas increase again. Which may be because rate of acidogenesis could be higher during the initial phase led to accumulation of VFAs which could have been consumed in later phase [62]. While the reactors at SFW: pre-treated P. australis mixing ratios of 50:50 and 25:75 exhibited a one-phase pattern of biogas production where biogas production ceased from the 2nd and 4th day of the digestion process, respectively. This could be attributed to the probability of low abundance or inhibition of methanogens due to the high rate of acidogenesis or VFAs in the system that caused pH drop and irreversible acidification thus system failure [31, 60, 62, 65]. The decline in the pH value of these systems also synchronized with the increase in IA/PA value to be higher than 0.3 (Table 1), which is considered a robust early indicator of system acidification [64, 66]. Furthermore, the rapid and easy hydrolysis of protein-rich substrates, such as food waste, may lead to generation of high concentrations of ammonia, which could be also inhibited methanogens, consequently reducing biogas production [67, 68].
However, the cumulative biogas production from the reactors with a mixing ratio of 0:100 was higher than that from the other mixing ratios (Table 1). This may be attributed to the fact that lignocellulosic substrates such as P. australis are degraded slower than food waste, leading to a relative balance between the production and consumption of intermediate products (such as VFAs) of the degradation process, making the digestion process more stable and thus increasing the amount of biogas produced [32, 47]. This was supported by the value of IA/PA, which was lower than 0.3, which is an indicator that system was stable at the final stage of digestion period.
On the other hand, the reactors with substrate mixing ratios of 25:75, 50:50, 75:25 and 100:0 exhibited their highest biogas production rates on the first day of digestion. The only exception to this was the mixing ratio of 0:100, which achieved its highest biogas production rate on the second day of the digestion process (Fig. 1b, Table 1). This may be due to the rapid digestion of the food waste fraction compared to that of the pre-treated P. australis [42], which made the systems acidified on the first day, and that may have resulted in the conversion of the carbonates to CO2. Similarly, the study conducted by Gandhi et al. [62] found that the digestion of SFW at ISR of 1:4 resulted in systems acidification and drop in the pH, and this was coincided with increase of CO2 concentration on the 1st day of digestion.
Table 3
The final pH reading of the digestate, the cumulative biogas production (ml/g VS added) and the peak rate of biogas production (ml/g VS added/day) for reactors with inoculum to substrate ratio (ISR) of 1:4, after 30 days of digestion period (mean ± standard deviation). The letters in parentheses indicate the results of the Games-Howell Pairwise Comparison test. Means that do not share the same letter are significantly differ at a 95% confidence level.
Substrate (food waste to pre-treated P. australis) mixing ratio (SMR) | Final pH value | IA/PA | Total volume of cumulative biogas production (ml/g VS added) | Peak biogas production rate (ml/g VS added/day) |
Inoculum (control) | 9.04 ± 0.04 | 0.19 | 9.58 ± 0.00 (D) | 9.58 ± 0.00 |
25:75% | 6.27 ± 0.01 | 12.13 | 21.58 ± 0.00 (C) | 17.14 ± 0.00 |
50:50% | 6.29 ± 0.00 | 12.87 | 33.00 ± 2.20 (B) | 24.11 ± 1.80 |
75:25% | 6.53 ± 0.02 | 6.39 | 32.99 ± 0.73 (B) | 25.06 ± 1.35 |
100:0% | 6.03 ± 0.18 | 6.86 | 32.65 ± 1.10 (B) | 21.24 ± 0.45 |
0:100% | 7.87 ± 0.02 | 0.26 | 44.46 ± 0.01 (A) | 4.76 ± 0.45 |
3.2 Effect of mixing ratio of SFW to pre-treated P. australis at inoculum to substrate ratio of 1:2 on biogas production.
In the cases where ISR = 1:2, the reactors with higher proportions of pre-treated P. australis (SFW: pre-treated P. australis mixing ratios of 25:75 and 0:100) showed higher biogas production and did not suffer from acidification effect, as shown by the pH and IA/PA values (Fig. 2a; Table 2). This could be because the slower degradation of pre-treated P. australis compared to SFW may provide sufficient time for methanogenic microorganisms to consume VFAs produced in the systems and prevent their accumulation, thus maintaining the stability of the systems. In contrast, the reactors with lesser pre-treated P. australis proportions and higher SFW proportions (SFW: pre-treated P. australis mixing ratios of 50:50, 75:25, 100:0) appear to have suffered from acidification and produced lower amounts of biogas (Table 2). This may have happened because the rate of acid formation is higher in the case of food waste due to higher concentrations of easily degradable materials such as sugars, resulting in a decline of the pH and methanogens inhibition, consequently, process disruption and/or failure [46, 69]. However, the systems at SFW: pre-treated P. australis mixing ratios of 50:50, showed a reversible acidification pattern, where the biogas production ceased on 2nd day and re-produced at day 15 of digestion, after consumption of accumulated VFAs by the methanogens.
On the other hand, it is observed from Fig. 2b that the maximum daily biogas produced from reactors with mixing ratios of 75:25 and 100:0 was recorded on day one and was significantly higher than the maximum daily biogas produced from reactors with mixing ratios of 25:75, 50:50 and 0:100, which was recorded on 1st and 2nd day respectively. This may be attributed to the high proportion of SFW in the 75:25 and 100:0 mixing ratios, which resulted in high levels of VFAs, causing a decrease in pH (Table 2), and this might have led to the release of CO2 from carbonates in the liquid phase in the reactors, thus caused high biogas production rate on the first day of the digestion process in reactors using these mixing ratios [62].
Table 4
The final pH reading of the digestate, the cumulative biogas production (ml/g VS added) and the peak rate of biogas production (ml/g VS added/day) for reactors with inoculum to substrate ratio (ISR) of 1:2, after 30 days of digestion period (mean ± standard deviation). The letters in parentheses indicate the results of the Games-Howell Pairwise Comparison test. Means that do not share the same letter are significantly differ at a 95% confidence level.
Substrate (food waste to pre-treated P. australis) mixing ratio (SMR) | Final pH value | IA/PA | Total volume of cumulative biogas production (ml/g VS added) | Peak biogas production rate (ml/g VS added/day) |
Inoculum (control) | 8.70 ± 0.05 | 0.19 | 9.62 ± 0.004 (D) | 9.62 ± 0.00 |
25:75 | 8.50 ± 0.01 | 0.20 | 82.17 ± 0.62 (A) | 20.14 ± 0.00 |
50:50 | 8.41 ± 0.01 | 0.40 | 62.52 ± 8.57(BC) | 30.20 ± 0.75 |
75:25 | 6.61 ± 0.01 | 5.57 | 52.96 ± 3.67 (C) | 52.96 ± 4.50 |
100:0 | 6.64 ± 0.07 | 5.93 | 54.00 ± 3.67 (C) | 46.58 ± 4.49 |
0:100 | 8.48 ± 0.01 | 0.21 | 69.47 ± 3.06 (B) | 10.08 ± 1.50 |
3.3 Effect of mixing ratio of SFW to pre-treated P. australis at inoculum to substrate ratio of 1:1 on biogas production.
The pattern of cumulative biogas production at ISR of 1:1 was similar for all substrate mixing ratios tested, gradually increasing from the beginning of the reaction until reaching its peak value (Fig. 3a). This may indicate that all reactors avoided inhibitive VFA accumulation [70], which may be due to the substrate concentration being optimal at this ISR so that the rate of acid production and consumption was balanced, thus resulting in no accumulation of VFAs, thus preventing drops in pH (Table 3) and acidification of the systems [71]. This was obvious from IA/PA values (Table 3), which were lower than 0.3 in all reactors. Similar results have been found by other researchers, such as Owamah et al [32] whom found better biogas production in their reactors fed with mixtures of FW and maize husks at ISRs with substrate levels lower than 1:2 with the best performance observed at ISR 1:1. Similarly, Haider et al. [72] reported that biogas production from mixtures of FW and rice husks increased when substrate portion in the ISR was decreasing below than 1:2.
The reactors with SFW to pre-treated P. australis mixing ratios of 100:0 exhibited the highest cumulative biogas production, followed by the reactors with the mixing ratio of 75:25. These were significantly higher (p < 0.05) than the cumulative biogas volumes produced from reactors with mixing ratios of 50:50, 25:75 and 0:100 (Table 3). In general, the cumulative biogas production increased significantly (p < 0.05) with an increasing proportion of SFW in the reactors. This is possibly due to the readily biodegradable nature of SFW, and its high nutrient content compared to pre-treated P. australis [31, 67].
Figure 3(b) shows that the daily biogas production in reactors with SFW to pre-treated P. australis mixing ratios 100:0, 75:25, 50:50, and 25:75 exhibited similar trends, which rose rapidly from the beginning of the digestion process to reach peak rates on the first day. While in reactors with a mixing ratio of 0:100, the daily biogas production peaked on the second day. This is possibly due to the easy degradation of SFW partition compared to reactors without SFW [73]. However, the reactors with a mixing ratio of 75:25 showed a higher biogas production rate through the digestion period compared to the reactors with mixing ratios of 50:50%, and 25:75% which make it preferable option for co-digesting of SFW with pre-treated P. australis at the ISR of 1:1, to enhance biogas production.
Table 5
The final pH reading of the digestate, the cumulative biogas production (ml/g VS added) and the peak rate of biogas production (ml/g VS added/day) for reactors with inoculum to substrate ratio (ISR) of 1:1, after 30 days of digestion period (mean ± standard deviation). The letters in parentheses indicate the results of the Games-Howell Pairwise Comparison test. Means that do not share the same letter are significantly differ at a 95% confidence level.
Substrate (food waste to pre-treated P. australis) mixing ratio (SMR) | Final pH value | IA/PA | Total volume of cumulative biogas production (ml/g VS added) | Peak biogas production rate (ml/g VS added/day) |
Inoculum (control) | 9.07 ± 0.01 | 0.22 | 9.54 ± 0.01 (F) | 9.54 ± 0.00 |
25:75 | 8.98 ± 0.03 | 0.24 | 65.07 ± 1.83 (D) | 20.63 ± 0.00 |
50:50 | 8.95 ± 0.03 | 0.23 | 72.22 ± 0.92 (C) | 26.19 ± 1.12 |
75:25 | 8.96 ± 0.02 | 0.23 | 76.15 ± 1.85 (B) | 30.14 ± 0.01 |
100:0 | 9.02 ± 0.02 | 0.24 | 82.47 ± 1.85 (A) | 37.27 ± 1.13 |
0:100 | 8.89 ± 0.01 | 0.25 | 50.01 ± 0.93 (E) | 7.94 ± 0.00 |
Overall, it is observed that the systems at ISR 1:4 with SFW: pre-treated P. australis mixing ratios of 25:75, 50:50, 75:25 and 100:0 suffered from acidification effect. While at ISR of 1:2, the systems with SMRs of 25:75 was non-acidified, whereas the systems with SMRs of 50:50 showed reversible acidification and biogas production was recovered after 13 days of cease, and only the systems with SMRs 75:25 and 100:0 undergone to continuous acidification influents. At an ISR of 1:1, the systems at all SMRs did not show any acidification pattern, and the digestion process was stable. The increase of substrate amount from ISR 1:1 towards ISR 1:4, particularly the SFW proportion, was the main reason for increasing the acidification effect on systems and decreasing biogas production. However, the systems at ISR of 1:2 with SMRs 25:75 showed higher cumulative biogas production (82.17 ± 0.62 ml/g VS added) than that produced from the systems with SMRs 75:25 at ISR of 1:1 (76.15 ± 1.85 ml/g VS added). This may be due to the doubled amount of substrate added at ISR of 1:2 compared to 1:1, and the highest proportion of substrate at SMRs 25:75 consisted of pre-treated P. australis, which is characterised by slower degradation compared to SFW, which led to producing VFAs at concentrations suitable for methanogens consumption capacity, thus enhance biogas production.