3.1.1. Pilot-Scale Methane Production
Pilot-Scale Experiment I was performed with FB without cosubstrates. In the FB Pilot I reactor, the pH of the percolation liquid dropped below 7, and NaHCO
3 and NaOH were used to raise the pH. Eventually, methane production started when the pH increased to 7.5. The decrease in pH at the start-up phase is a typical problem for the solid digestion process [
15,
33]. The decrease in pH occurs when the easily degradable fraction of organic matter is hydrolysed and converted to volatile fatty acids (VFA). Although VFAs are intermediate products in the process, too high contents of VFAs result in a decrease in pH and the inhibition of methanogenesis.
In FB Pilot I, the BMP of the whole crop fava bean was 272 L CH
4/kg
VS (
Table A1,
Appendix A). When the specific methane production was calculated only through the inclusion of the VS of fava beans (VS of the percolation liquid excluded), methane production in the LBR reached 160 L CH4/kg
VS after 162 days. LBR methane production corresponded to 59% of the fava beans’ BMP (
Figure 2), i.e., the LBR/BMP ratio was 59%. An alternative way of comparing LBR methane production to the BMP of raw materials is including the BMP (
Table A1) and VS content (
Table 2) of the percolate in the LBR-specific methane production and LBR/BMP ratio calculation. If calculated in this way, FB Pilot I would achieve a slightly lower LBR/BMP ratio of 55% (instead of 59%), since the average BMP of the FB and percolate weighted by VS content would be 258 L CH
4/kg
VS (instead of 272), and the methane yield of the LBR would be 141 L CH
4/kg
VS (instead of 160).
In FB Pilot II, the BMP of the fava beans was 320 L CH
4/kg
VS, while the LBR methane production reached 163 L CH
4/kg
VS in 154 days, resulting in an LBR/BMP ratio of 57%. In the FB Pilot II experiment, the FB was dry and mouldy, which most likely affected methane production in the pilot reactor. If percolate VS and BMP were also included in the calculation, LBR methane production, weighted BMP, and the LBR/BMP ratio would be 169, 304, and 56%, respectively. In a laboratory study by Lehtomäki et al. [
34] with a solid retention time of 55 days, the methane production of an LBR corresponded to 20% of the grass feedstock BMP; in a combined leach-bed–UASB process, the LBR/BMP ratio was 66%.
In the last pilot-scale experiment of the current study, the co-digestion of fava beans and horse manure was studied in the parallel reactors (FB + HM Pilots I and II). The BMPs of the total solid substrates (FB + HM) in the reactors were 191 and 190 L CH
4/kg
VS, respectively. At the beginning of the experiment, there was a gas leak in Reactor 1 that decreased the methane production. After the leak had been fixed, methane production was similar in the two reactors, reaching 180 in Reactor 1 and 220 L CH
4/kg
VS in Reactor 2 in 133 days of digestion time. LBR/BMP ratios were 94 and 116%, respectively, meaning that the mix of fava beans and horse manure produced even more methane in Reactor 2 than that of the separate substrates in the BMP assays. If the VS and the BMP of the percolate were included in the calculation, the LBR/BMP ratios would be very similar (94 and 115%) because the percolate had very low VS content (
Table 2). Riggio et al. [
35] reached similar LBR/BMP ratios (85–99%) in the digestion of spent livestock bedding, but with the shorter digestion time of 60 days. The methane potentials of the whole crop fava-bean in the current study were lower than those previously reported in literature (387 L CH
4/kg
VS for fava beans [
36] and 440 L CH
4/kg
VS for fava-bean straws [
37]).
The amount of beans in relation to stem biomass could have varied between the pilot and BMP experiments, thus having an effect on biogas potential. However, the effect of the co-digestion of plant material with manure improved methane potential [
33]. Even if the methane potential of manure were relatively low compared to the plant biomass, manure contains beneficial trace elements for anaerobic microorganisms and has the buffering capacity to maintain the process pH closer to neutral [
38]. In addition, Degueurce et al. [
39] showed that the microbial community in manure was strongly involved in methane production in an LBR process. The methane potential of horse manure in this study was relatively low (48 ± 9 L CH
4/kg
VS) and is highly dependent on bedding material [
40], which was sawdust in this experiment.
The residual methane production (RMP) from the pilot-scale reactor digestates varied markedly in the different experiments and with different raw materials (
Table A1,
Appendix A). In the RMP experiments, inocula from the farm biogas reactor, mainly treating manure, were used. This might have affected the methane production of FB Pilot I and II digestates, as no manure or another cosubstrate was used in the pilot reactors. In the last experiment, the digestate RMP from the FB + HM Pilot I was markedly lower compared to previous digestates, but the digestate from FB + HM Pilot II was at the same level as that of previous ones. Although the digestates from the pilot reactors were mixed carefully, the differences in the RMP results from the last experiment may have been due to variation in the samples, as we could not measure exactly the same amount of FB and HM in a sample. The RMP from the percolation liquids decreased during the experiments (
Table A1,
Appendix A), which was expected, as the same liquid was used in all experiments with water dilution (percolation liquid having lower VS in each experiment, as shown in
Table 1).
3.1.2. Pilot-Scale Digestion Mass Balance
The mass balance was calculated for the FB + HM Pilot I experiment. After starting the experiment, 110 kg of water was added, and 58 kg of percolate and 35 kg of biogas were removed from the process. Hence, total mass increased by 4%, from 421 to 445 kg, during the process. According to the mass balance calculation, the mass of the solid material increased by 12% to 314 kg, while the mass of the percolate liquid decreased by 7% to 131 kg, indicating that part of the percolate was adsorbed to the solid material. A content comparison of the reactor and percolation tank at the beginning and end of the experiment showed that the VS mass and VS content of the solid raw material were reduced by 32 and 39%, while the VS mass and VS content of the percolate decreased by 30 and 25%, respectively (
Table 4). The total VS mass (solid material and percolate combined) decreased by 32%, while the total VS content was reduced by 39%. The VS removal of the solid substrates was slightly lower than the VS removal reported for LBR processes fed with grass (34–55%, 55 days of digestion time) [
34] and bedding-straw (45–60%, 110 days) [
41] feedstocks. A comparison of the VS removal percentage with the literature values was complicated due to the addition of water and the removal of the percolate, which were performed in the current study.
According to the mass balance calculation, the masses of total N and NH
4–N in the solid material were increased by 17 and 191%, respectively. The masses of the total N and the NH
4–N in the percolate were increased by 61 and 39%, respectively (
Figure 2). The mass of total N in the total material (solid material + percolate) was increased by 22% during digestion. The calculated increase in N could be attributed to inaccurate analyses and difficulties in sampling the heterogeneous solid biomasses.
VS and nitrogen losses during simulated storage were tested in FB + HM Pilot I. The solid digestate was stored outside for 35 days (20 October–24 November 2020) in a 1 m
3 metal reactor container without heating. The top of the container was only loosely covered with a tarpaulin, allowing for an exchange of gases between the porous digestate and air, probably resulting in (partial) aerobic composting conditions. During storage, the outside air temperature varied between −4.7 and 12.5 °C (average, 5.0 °C). The temperature of the digestate was not measured. The mass losses of VS, total N, organic N, and NH
4–N during storage were 5, 8, 1, and 29%, respectively (
Figure 3). Their contents were decreased by 8, 11, 4, and 31%, respectively, indicating that the (partially) aerobic digestate storage after a leach-bed process might cause nitrogen losses and greenhouse gas emissions. As the duration of batches in leach-bed reactors varies (see
Section 2.4.1) and also affects the amount of organic material in the digestate, this experiment was only indicative, and more research is needed to ensure the sustainability of the process. Möller et al. [
42] assumed a total N loss of 37.5% for the separated solids of the digestate during storage and handling, when it is turned several times before land application [
42]. Further information about nitrogen losses during the storage of the solid digestate was not found in the literature. For comparison, over 20% of total N was lost in 33 days in a digestate solid aerobic composting experiment by Bustamante et al. [
43].