Compositional analysis of OMSW
The content of cellulose, hemicelluloses, and lignin in the untreated and the three different treated OMSW samples are summarized in Table 1. The fiber analysis shows high lignin content, followed by cellulose, and finally hemicellulose, with a drastic decrease in cellulose content after treatment. The high cellulose content after the enzymatic hydrolysis is due to mixing remnants of multiple hydrolysis experiments (sorted by pretreatment additive but different at microwave time and temperature) some less efficient than others, leaving some cellulose.
The chemical composition of the remnants' feedstock is presented in Table 1. The VS content varied between 89.2% (untreated OMSW) and 93.9% (MA pretreated). This was an indicator of the potential for methane production since it represents the fraction of the solid material that can be converted into methane. The TS content varied between 92.1% (untreated) and 95.8% (MA).
Table 1 Characteristics of the OMSW after the different pretreatments followed by enzymatic hydrolysis, in addition to the inoculum used for the biogas production. Data are based on dry material. Each data point is an average of 2 replicates
Parameter
|
Virgin OMSW
|
Remnants after Maleic acid-MW and enzymatic hydrolysis
|
Remnants after formic acid-MW and enzymatic hydrolysis
|
Remnants after distilled water-MW and hydrolysis
|
One way ANOVA (F; p) df =3,8 in all cases
|
Inoculum
|
TS (%)
|
92.1 ± 0.39a
|
95.8 ± 0.02b
|
95.5 ± 0.01bc
|
94.7 ± 0.61c
|
64.7; <0.001
|
2.23
|
VS (%)
|
89.2 ± 0.16a
|
93.9 ± 0.10b
|
93.6 ± 0.11bc
|
92.0 ± 1.08c
|
45.8; <0.001
|
1.57
|
VS/TS (%)
|
96.9
|
98.0
|
98.0
|
97.1
|
|
70.4
|
Cellulose (%)
|
27.0 ± 1.9a
|
17.7 ± 0.1b
|
16.2 ± 0.3b
|
16.1 ± 0.8b
|
75.1; <0.001
|
NA
|
Hemicellulose (%)
|
11.1 ± 1.1a
|
7.0 ± 1.0b
|
8.8 ± 0.8bc
|
9.3 ± 0.1ac
|
11.9; <0.005
|
NA
|
Lignin (%)
|
42.9 ± 4.6a
|
54.4 ± 2.0b
|
47.5 ± 3.7ab
|
- ± 2.6ab
|
5.93; =0.02
|
NA
|
VS/Lignin
|
2.08
|
1.73
|
2.0
|
1.94
|
|
NA
|
TKN (mg·L−1)
|
NA
|
NA
|
NA
|
NA
|
|
2802
|
Different superscript letters in each row mean statistically different results using Tukey HSD Post-hoc Test; NA – not available. TS - Total Solids, VS - Volatile solids, TKN - Total Kjeldahl Nitrogen
|
2.1 Methane (biogas) production potential test
Methane production from virgin OMSW calmed after 22 days, while such from remnants of enzymatic hydrolysis after pretreatment with DW, FA, and MA samples, calmed only after 53, 53, and 82 days respectively (Fig. 2). No methane was detected in the inoculum reactor (data not shown). There were no differences in the final level of methane production for all the samples, but there were differences in the rate and the lag time (Fig. 2). The methane production ranged from 252.2 - 289.5 mL CH4/g VS (Table 3 and Fig. 2). The digestion time (DT80) describes the time needed for producing 80% of potential biogas generation. The biogas yield rate and the bio-digestibility of the substrate can be determined based on DT80 [20]. In addition, concerning the continuous bio-digester, DT80 could be used as a hydraulic retention time [21]. The AD, in the current work, stayed up to 98 days, while DT80 for FA, DW, MA, and OMSW was only 42, 42, 66, and 16 days, respectively.
The main compositions of biogas produced during AD were methane, carbon dioxide, and some traces of oxygen. Fig. 3 shows the percent of the methane produced over the time of the tested samples. The methane percentages ranged from approximately 68.5% - 72.3% for the samples MA, FA, DW, and OMSW. The lowest value was for DW while the highest value was for MA.
The carbon dioxide percentages ranged from 17.9 - 22.8. The lowest value was for MA while the highest value was for OMSW (control).
Due to the difference in the SIR (g VS substrate/g VS inoculum) in the AD tests between the untreated OMSW (SIR=6) and the OMSW bioethanol residues (SIR=12), it was difficult to compare kinetic constants. Many studies of AD tests showed that high hydrolysis rates were reached with low SIR for the same substrate [14, 22]. SIR influences were different for the various used substrates [22].
2.2 Kinetic model of methane production and fitting of Gompertz model parameters
Since the lag phase was reported in our results, the modified Gompertz model was selected to detect the cumulative methane production continuously and to perform an analysis of the effect of biomass solvent pretreatment on methanogenic activity. The modified Gompertz model was fitted to the experimental data as an important model for the study of methane production kinetics Eq. (1) [23]:
![](data:image/png;base64,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)
Where P(t) is the cumulative methane production (mL CH4/g VS) at time t, Pmax is the maximal potential of methane production (mL CH4/g VS), Rmax is the maximal specific methane production rate (mL CH4/d g VS), λ is the lag phase duration (d), and 2.7183 is the exp(1). The parameters Pmax, Rmax, and λ were obtained by fitting the modified Gompertz equation to the experimental results through non-linear regression using the Solver tool of Microsoft Office Excel software. The suitability of fit was determined by taking into consideration, both, the Residual Sum of Squares (RSS) and the R-square (R2) values.
Table 2 presented the calculated parameters (λ lag phase duration, Rmax maximum specific methane production rate, and Pmax methane production potential) which are based on the fitting model. Our results showed that the modified Gompertz model reflected well the methanogenesis of AD (all R2 values were above 0.9). The λ value can reflect the rate of reaction of the substrate in the AD system, which shows the level of adaptation of the microbes to the substrates in the AD system. Low λ values reflect quick adaptation of the microbes to the new conditions almost without a lag phase, which means that the fermentation system produces methane faster which means with less fermentation time (short lag phase). Our results showed that MA had a large λ, indicating that the microbes were affected during the fermentation process, causing a delay in methane production. When FA and DW were used for the fermentation systems, the value of λ decreased significantly, which indicated that the AD process was not inhibited and the microbes were active and started to utilize the biomass and to produce biogas with a shorter lag phase.
The explanation of the large λ could be attributed to inhibition caused by sodium. Low concentration (~350 mg·L−1) of sodium was previously demonstrated to be essential for mesophilic methanogenic bacteria [24], with 3.5 g·L−1 and higher having inhibitory effects on methane production at different temperatures [25–27]. Here we used 50 mM sodium citrate buffer for the enzymatic hydrolysis (i.e. 1.15 g·L−1 sodium), where some of it might have stuck to the remnants but this issue needs further study.
Another explanation could be that higher SIR delayed methane production, indicating a process inhibition [28]. The results of Pellera et al. [9] showed that lower SIR is more adequate for determining the methane potential of the agricultural waste. In our work, the SIR was relatively so high (SIR=12).
The highest Rmax values of 29.7 were obtained for untreated OMSW (SIR=6) and the remnants of the FA pretreatment-hydrolysis (SIR=12) was Rmax 20.9, where the lignin concentration was the lowest. When the lignin content increased, the value of Rmax decreased, indicating that lignin has additional inhibitory effects, similar to the previous study of Fernández-Cegrí et al. [29].
Besides, the Rmax and λ of DW and FA were similar with Pmax of 249.1, 256.5 ml/g VS, respectively (Table 2). These results presented in Table 2 indicate that the fermentation potential of DW and FA based on Rmax were similar. MA had a relatively high Pmax of 289.8; however, the Rmax was low, apparently due to the high λ value caused by some inhibitory factors during the biogas production.
Table 2 Kinetic parameters obtained from fitted modified Gompertz equation for untreated OMSW, and different pretreatments that followed by enzymatic hydrolysis. Each mean is an average of 3 replicates: the fitted modified curve was based on the average of the 3 replicates
|
Pmax*
(mL CH4/g VS)
|
Rmax
( mL CH4/d g VS)
|
λ( d)
|
R2
|
Virgin OMSW (1g)
|
272.5
|
29.7
|
7.3
|
0.974
|
DW
|
249.1
|
18.23
|
30.5
|
0.997
|
FA
|
256.5
|
20.88
|
31.2
|
0.9978
|
MA
|
289.8
|
12.6
|
47.5
|
0.982
|
* Pmax is the maximal potential of methane production, Rmax is the maximal specific methane production rate, λ is the lag phase duration and R2 is the correlation coefficient
2.3 The impact of pretreatment additive on methane production
The methane yields obtained through the BMP after the digestion process of untreated OMSW, MA, FA, DW, were 276 ± 15.5, 287.5 ± 20, 260 ± 17.5, 252.2 ± 20.2 mL/ gVS, respectively (Table 3). The obtained methane yield from the MA, FA, DW of treated OMSW and untreated OMSW was ca 34%, 29%, 27%, 20% (1.3, 1.3, 1.3, and 1.2fold) higher than the theoretical methane from the carbohydrate fraction, protein and lipid (Eq. 2), implying that portion of the lignin was also converted into methane and the dead bacterial remains constitute the digestive, which were similar to the results obtained by Mulat et al. [30]. The percentage of carbohydrates in treated OMSW, and after pretreatment followed by enzymatic hydrolysis was reduced to 34%, 40%, 40%, while the amount of lignin was increased to 26%, 11%, and 10% for MA, FA, and DW, respectively. The remaining carbohydrates of 66%, 60%, and 60% coming from MA, FA, DW of the pretreated OMSW, respectively, and a portion of the lignin fraction could be utilized as well as a substrate for methane production. High temperature and acidic conditions (released organic acids) during microwave pretreatment could catalyze the hydrolysis of hemicelluloses and further the degradation into lower molecular weight (LMW) compounds such as furfural. Other previous works have concluded that LMW compounds may undergo polymerization processes, forming lignin-like compounds termed “pseudo-lignin” [30, 31]. Thus, the formation of pseudo-lignin was probably the main reason for increasing the lignin fraction following pretreatment, in addition to the loss of volatiles during microwave pretreatment could also contribute to the increase in the lignin and cellulose fraction [30].
Table 3 Characteristics of the different biomass remnants and the raw material of OMSW used in the experiments. Each mean is an average of 2-3 replicates + SD
Pretreatment additive Parameter
|
OMSW
|
Maleic acid-OMSW
|
Formic acid-OMSW
|
Distilled water-OMSW
|
Lignin (%)
|
42.9 ± 4.6
|
54.4 ± 2.0
|
47.5 ± 3.7
|
47.4 ± 2.6
|
Carbohydrate (%) (cellulose+hemicellulose)
|
38.1
|
24.7
|
25
|
25.4
|
Protein (%)
|
1.56 ± 0.03
|
2.22 ± 0.07
|
1.43 ± 0.08
|
1.33 ± 0.04
|
Lipids (%)
|
9.2
|
11.35
|
10.2
|
10.2
|
BMPtheo (mL/ gVS)
|
231
|
215
|
201
|
199
|
BMPexp-model (mL/ gVS)
BMPexp (mL/ gVS)
|
272.5
276 ± 15.5
|
289.8
287.5± 20.1
|
256.5
260.1 ± 17.5
|
249.1
252.2 ± 20.2
|
Difference (mL)
|
45
|
73
|
59
|
54
|
α*
|
1.2
|
1.3
|
1.3
|
1.3
|
Increasing (%)
|
20
|
34
|
29
|
27
|
*α = Experimental production/ Theoretical production
BMPtheo= theoretical biochemical methane potential
BMPexp-model= fitted modified Gompertz model for the biochemical methane potential based on observed results
BMPexp= observed biochemical methane potential.
The OMSW was pretreated using a microwave at various conditions to improve bioethanol production during the separate hydrolysis and fermentation process. It should be noted that the enzyme solutions used also contain carbon sources and could contribute as well to methane production.
According to the results in part I of this study (data not shown), that deal with optimization of the conditions of pretreatment and enzymatic hydrolysis processes for bioethanol production, the samples that were treated with MA had the highest glucose conversion in enzymatic hydrolysis and the highest hemicellulose conversion in the pretreatment process. Therefore, the MA raw material for AD in this paper contained less cellulose and hemicellulose and more lignin compared to the FA and DW raw material. In FA and DW there were more non-degradable cellulose and hemicellulose, while in MA there was more lignin, which is difficult to be broken down. So the results showed that the lag phase in MA was longer than FA and DW. The final value of BMP showed no statistical significant differences at (p<0.05).
The theoretical biochemical methane potential formula of feedstock is given as follows [32]:
BMP = 415 × % carbohydrates + 496 × % protein + 1014 × % lipids (2)
The energy conversion efficiency and the total energy outputs of the four feedstock used for AD are summarized in Fig. 4. The energy analysis of energy output from each raw material was determined by multiplying the amount of bio-ethanol (data not shown) or bio-methane which was produced with the corresponding calorific value. According to Ouazzane et al. [33] and Wang et al. [34], the calorific values for OMSW, ethanol, and methane are 18.2 MJ/kg, 26.7 MJ/kg, and 35.9 KJ/L, respectively. The output energy was considered in the energy analysis for this study where the input energy i.e. pretreatment, fermentation, and transport were not considered.
The energy output of methane production was calculated as:
[MJ/kg VS] = MP × 0.001× HHV (3)
Where the MP is methane production [L/Kg VS], 0.001 is the conversion factor from KJ to MJ, HHV is the high heating value of methane [KJ/L], where the high heating value is 35.9 KJ/L
The energy output of ethanol fermentation was calculated as:
[KJ/Kg VS] = EP × HHV (4)
Where the EP is the ethanol production [g/kg VS], HHV is the high heating value of ethanol [kJ/g], and the high heating value was 26.7 kJ/g.
The total energy output using the FA, DW, OMSW pretreatment were 11.57, 10.80, 9.91 GJ/ton, respectively. Using the MA pretreatment resulted in the maximum bioenergy output of 13.08 GJ/ton. The total energy outputs obtained from the MA, FA, and DW were 32% and 16.75%, 9%, respectively, higher than that obtained from the OMSW. The MA showed the highest energy conversion efficiency of 71.88%.
The BMP values that were reported in the literature were about, 98-370 mL CH4/gVS, for the traditional, two-phase, and three-phase process of OMSW (Table 4). The OMSW used in this work produced 276 mL CH4/gVS. Various methane yields are due to the differences in the OMSW chemical composition, which depends on the geographical origins, olive cultivars, and the method of oil extraction processes (Table 4). The results of the combined biomethane production followed by bioethanol fermentation in the current work were compared with those of other reported values of biomethane production as a single process. Using residues of OMSW treated with microwave with MA for bioethanol production showed that the maximum fitted model methane value was 289.8 ml CH4/g VS, by using the two-phase OMSW following ultrasound pretreatment reported by Rincón et al. [15] where 393 mL of CH4/g VS was obtained. Using OMSW and olive mill wastewater mixture (1:2 ratio) with Fenton process pretreatment (H2O2/[Fe2+]) Maamir et.al. [35] reported that 224 mL of CH4/g VS was obtained.
In future research, it is recommended to operate large-scale reactors and calculate the energy balance of output/input energy.
Table 4 Comparison data of previous studies dealing with methane production from OMSW biomass
Reference
|
Substrate
|
Inoculum
|
Reactor type
|
Operating condition
|
Pretreatment
|
Methane yield
|
[36]
|
OMSW
|
Cattle Manure (CM)and Cattle Slurry (CS)
|
Batch stirred tank reactor
|
4.65% of CM, 72.10% of CS and 23.25% of OMSW ,mesophilic conditions (38°C), 55 days
|
None
|
103.4 mL/g VS
|
[37]
|
OMSW: Turkey manure
60:40
|
Turkey manure
|
Anaerobic batch operating reactors
|
8% total solid at mesophilic conditions, at 30°C for 40 days.
|
None
|
20.2 mL / g mix
|
40:60
|
23.1 mL / g mix
|
[35]
|
OMSW and OMWW mixture was in a 1:2 ratio
|
Bovine manure
|
Batch digester of 300 mL as working volume.
|
Mesophilic conditions (37 °C)
30 days
|
Fenton process (FP) pretreatment (H2O2/[Fe2+])
|
224 mL/ g VS
|
[15]
|
Two-phase OMSW
|
Industrial anaerobic reactor treating brewery wastewater and operating at mesophilic temperature
|
Reactors were continuously stirred at 500 rpm and placed in a thermostatic water bath
|
Mesophilic temperature (35 ± 2∘ C)
|
Ultrasound pretreatment
|
393 mL CH4/g VS
|
[9]
|
Olive pomace (OP)
|
Anaerobic sludge sample originating from a mesophilic anaerobic digester of the Municipal Wastewater Treatment Facility
|
250 mL reactors, consisting of conical flasks covered with rubber stoppers
|
35° C
|
Alkaline (NaOH) pretreatment, 90°C
|
242 NmL CH4/gVS
|
[14]
|
Olive pomace
|
Continuous lab-scale stirred tank reactor maintained in mesophilic conditions and feed with sewage sludge
|
Anaerobic batch
|
37° C
|
Upstream enzymatic maceration
|
274 mL CH4/ g VS
|
[16]
|
Olive pomace
|
Mesophilically operated anaerobic digester of a yeast factory
|
2500 mL anaerobic reactors
|
37° C for 30 days
|
None
|
180 mL CH4/ g VS
|
wastewater sludge
|
|
160 mL CH4/ g VS
|
Olive pomace & wastewater sludge
|
Microwave pre-treatment was applied to the sludge samples for 30 min at 175 C and 2000 kPa
|
320 mL CH4/ g VS
|
This work
|
OMSW
|
Wastewater Treatment Plant (WWTP)
|
100 mL syringe
|
mesophilic temperature (37°)
|
Microwave-assisted MA or FA followed by enzymatic hydrolysis
|
252 – 289 mL CH4 / g VS
|