3.1 Biomass characteristics post-pretreatment
The volatile solids of the milled Sargassum spp. biomass with particle sizes of 505µm (61.02 ± 0.44%) and 107 µm (62.07 ± 0.40%) were not significanlty different (p = 0.130). However, the volatile solids of the chopped biomass (71.02 ± 0.40%) was higher (p < 0.005) than those from the milled and crushed biomass. Similarly, the ash content of the chopped biomass (28.98 ± 0.23%) was lower (p < 0.001) than those of the milled biomass. The difference between the biomass with particle sizes of 505µm (39.00 ± 0.30%) and 107 µm (37.93 ± 0.20%) is considered to be not quite statistically significant (p = 0.052).
3.2 Granulometric analysis of Sargassum spp.
The crushed biomass (S2) reduced its monomodal size distribution all within the range detectable by the analysis tool (Fig. 2a), although it appears very wide. The comminution by the milling (S3) was more efficient, as the particle size distribution shifted towards lower sizes (see Fig. 2a). For both, S.2 and S.3, characterized by laser diffraction, the surface to volume mean, dSV, drops drastically from 86 to 15 µm, as more fine particles are produced by the last type of the comminution pretreatment (milling, S3). In fact, the particle size distribution widens in amplitude, as signaled by the increase of the parameter span indicated in Table 1, and also it becomes a bimodal curve (see Fig. 2a). The sample now appears characterized as a mixture of two parts, one is of finer particles that represent more than 40% and the rest of bigger ones. This aspect is relevant because, increasing the fines presence in a solid sample increases the superficial area, but also introduces agglomeration problems[25], so the bigger particles of the sample help to prevent this.
It is evident that S3 (Fig. 2c) has the highest distribution frequency within the range of finer particles (from 0 to 45 µm), while sample S2 predominantly consists of particles in the range of 200 m to 1000 µm. In the sample S3, the size classes bigger than 1mm disappeared resulting in the presence of fine particles due to the milling process. This aspect is crucial, because there is an useful mix between fine content and coarse particle that reflects into a macroscopic behavior of S3: high superficial area with agglomeration[25].
Table 1 Granulometric analysis and biomethanation of Sargassum spp. at different particle sizes due to mechanical pretreatment.
|
Granulometric Analysis
|
|
Pretreatment
|
Span [-]
|
dSV [µm]
|
dV [µm]
|
Methane yield
(Nml/g VS)
|
Chopped (S1)
|
-
|
-
|
50000.0 ± 0.5
|
17.17 ± 2.14 a
|
Crushed (S2)
|
2.088
|
86.0 ± 0.1
|
505.0 ± 0.1
|
79.68 ± 2.77 b
|
Milled (S3)
|
7.730
|
15.0 ± 0.1
|
107.0 ± 0.1
|
65.08 ± 2.18 c
|
3.3 Effect of particle size on the biomethane production
After the 30 day-period, the methane yield increases with the decrease of the particle size (Fig. 3). The methane yield goes from 17.17 Nml/g VS to 79.68 Nml/g VS when the average size of the sample goes from 50000 µm to 505 µm, and from 17.17 Nml/g VS to 65.08 Nml/g VS when the average size of the sample go from 50000 µm to 107 µm. The methane yield (Nml/g VS) for the chopped biomass (S1) were significantly lower (p < 0.0005, Table 1) than those for the mechanically pretreated biomass (S2 and S3). Similarly, the methane yield of the biomass with particle sizes equal to 107 µm (S3) is higher than (p = 0.0279) that with particle size equal to 505 µm (S2). However, there is no difference (p = 0.06156) between the methane yield of S2 and S3.
A low methane yield from biomass S.3 compared to S.2 could be related to the excessive reduction of particle size. Sample S.3 is characterized by an average particle size of 107.0 ± 0.1 µm. The granulometric analysis of sample S.3 highlighted the predominant presence of fine particles in the range between 0–45 µm. These fine particles are subject to the formation of aggregates and consequently, the contact area between the algae cells and the microorganisms that operate the anaerobic digestion process decreases [25]. Furthermore, the excessive decrease in particle size increases the presence of volatile fatty acids which inhibit the methanogenesis process[26]. In a work by Izumi et al. conducted on food waste it is reported that when the size of the food waste sample was excessively reduced (0.7 mm) the methane yield did not increase due to the accumulation of volatile fatty acids[11].
3.3 Biomethanation kinetics
The parameters from the First kinetic model are shown in Table 2. The parameters from the modified Gompertz model were calculated. The maximum methane yields (Table 3), A (Nml /g VS), are in agreement with the experimental methane yields (Table 1). The methane yield for S2 was higher ( p = 0.0114) than that from S3. Similarly, the methane production rate, u (Nml /g VS.day), is higher ( p < 0.004) after mechanical pretreatment, and the biomass wih smallest particle size (S3) experienced a reduction in the rate (p = 0.0145) compared to S2.
The lag phase m, of the sample S1 indicates that less than 3 days are required before the methane production starts. There was not significant difference ( p > 0.074) between chopped (S1) and milled (S2, S3) biomass in terms of lag phase. The correlation coefficient (R2) for each experimental unit was above 0.980, indicating that the model fits the experimental data well.
Table 2
Kinetic parameters of the First kinetic model
Pretreatment
|
A (Nml /g VS)
|
k (day− 1)
|
R2
|
Chopped (S1)
|
18.919
|
0.107
|
0.951
|
Crushed (S2)
|
160.529
|
0.025
|
0.982
|
Milled (S3)
|
167.248
|
0.018
|
0.981
|
Table 3
Kinetic parameters of the Modified Gompertz model
Pretreatment
|
A (Nml /g VS)
|
u (Nml /g VS.day)
|
m (day)
|
R2
|
Chopped (S1)
|
17.677 a
|
1.75 a
|
1.57
|
0.988
|
Crushed (S2)
|
86.085 b
|
4.495 b
|
2.315
|
0.996
|
Milled (S3)
|
72.823c
|
3.526c
|
2.573
|
0.993
|
The different letters within the column means there is a significant difference between the values at the 0.05 level using unpaired t-test
|
The two kinetic models fit the experimental data well. The modified Gompertz model shows more accurate results, while the first order kinetic model is less accurate in estimation. The difference between the experimental A value and the A value obtained using the modified Gompertz model for system S.1, S.2, S.3 are 0.35%, 6.86%, and 9.24%, respectively. While the differences between the experimental A value and the A value obtained using the first order kinetic model for system S.1, S.2, S.3 are 9.20%, 50.36%, and 61.08%, respectively.
The parameter u (NmL · g− 1VS · day− 1) indicates the maximum biogas production rate that can be obtained in each system. Sample S.2 shows the highest value of parameter u.
The parameter m indicates the period required to start the production of biomethane. Sample S.1 shows a delay period equal to 1.749 days, sample S.2 a delay period equal to 2.322 days while sample S.3 a delay period equal to 2.568 days. For samples characterized by finer particle size the delay time is longer than for sample S.1.
The hydrolysis rate constants (k) of the different sample S.1, S.2, S.3 have been determined from the first-order model and they are equal to 0.107, 0.025 and 0.018 (day− 1). The first order kinetic model, as already highlighted in other works, does not fit the data precisely. In fact, according to this model, the greater the k, the faster the degradation. On the other hand, from the results obtained, samples S.2 and S.3 show a higher biomethane yield than sample S.1 which instead shows a higher k value.
The experimental data were fitted with the two models are shown in Fig. 3.
3.4 Energy analysis
Table 4 shows the data obtained from the energy analysis. For sample S2 it is estimated that it is possible to produce 37.03Nm3/ton of biomethane and therefore the energy produced is equal to 455.5 kWh. To obtain the particle size desired of sample S2 (505 µm), the sample was crushed with a mixer. A container with a capacity of 1L can contain 91.1g of sargassum biomass as it is, therefore its density is equal to 91.1g/L. Considering a mixer with a power of 2200W and a capacity of 20L, to blend 1 ton you need to use the blender 555 times for 1 minute for a total of 555 minutes (about 9 hours). Therefore, the energy needed to mix 1 ton of Sargassum spp. is equal to 19.8kWh, which corresponds to 4.3% of the energy produced. For sample S3 it is estimated that it is possible to produce 32.88Nm3/ton of biomethane and therefore the energy produced is equal to 404.4 kWh. To obtain the granulometry of sample S3 (107 µm), the sample was milled with a grinder, in this case it takes twice as long and therefore a consumption of about 39.6 kWh is estimated, corresponds to 9.8% of the energy produced. In contrast, sample S.1 was cut with knives, therefore, no mechanical energy was used, it is estimated that it is possible to produce 9.62 Nm3/ton of biomethane and therefore the energy produced is equal to 118.3 kWh.
It can be concluded that if on one hand the pre-treatment involves an energy expenditure, on the other hand it allows to obtain a higher yield and therefore does not negatively affect the energy balance. In particular, for sample S.2 the best results are obtained in terms of yield of biomethane produced and useful energy. Furthermore, as already observed, sample S.3 shows a higher reaction rate than sample S.2. This means that the reaction produces a greater quantity of methane in a shorter time, and therefore also the production cost, in terms of energy, is lower.
Table 4
Energy analysis parameters. BMP (Nm3/t) is the biochemical methane potential, Ep (kWh) is the energy produced, Ec (kWh) is the energy consumed, and Ed (kWh), is the difference between Ep and Ec.
Pre-treatment
|
BMP (Nm3/t)
|
Ep (kWh)
|
Ec (kWh)
|
Ed = Ep - Ec
|
Chopped (S1)
|
9.62
|
118.3
|
-
|
118.3
|
Crushed (S2)
|
37.03
|
455.5
|
19.8
|
435.7
|
Milled (S3)
|
32.88
|
404.4
|
39.6
|
364.8
|