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Article

Anaerobic Digestion of Pig Slurry in Fixed-Bed and Expanded Granular Sludge Bed Reactors

by
Jurek Häner
1,2,*,
Tobias Weide
1,2,
Alexander Naßmacher
1,2,
Roberto Eloy Hernández Regalado
1,2,3,
Christof Wetter
1,2 and
Elmar Brügging
1,2
1
Faculty of Energy Building Services Environmental Engineering, Münster University of Applied Sciences, Stegerwaldstr. 39, 48565 Steinfurt, Germany
2
Institute Association for Resources, Energy, and Infrastructure, Münster University of Applied Sciences, Stegerwaldstr. 39, 48565 Steinfurt, Germany
3
Faculty of Agriculture and Environmental Sciences, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany
*
Author to whom correspondence should be addressed.
Energies 2022, 15(12), 4414; https://doi.org/10.3390/en15124414
Submission received: 10 May 2022 / Revised: 1 June 2022 / Accepted: 7 June 2022 / Published: 17 June 2022
(This article belongs to the Special Issue Biogas Production-Converting Waste to Energy and Bio-Products of Added Value)
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Abstract

:
Anaerobic digestion of animal manure is a potential bioenergy resource that avoids greenhouse gas emissions. However, the conventional approach is to use continuously stirred tank reactors (CSTRs) with hydraulic retention times (HRTs) of greater than 30 d. Reactors with biomass retention were investigated in this study in order to increase the efficiency of the digestion process. Filtered pig slurry was used as a substrate in an expanded granular sludge bed (EGSB) reactor and fixed-bed (FB) reactor. The highest degradation efficiency (ηCOD) and methane yield (MY) relative to the chemical oxygen demand (COD) were observed at the minimum loading rates, with MY = 262 L/kgCOD and ηCOD = 73% for the FB reactor and MY = 292 L/kgCOD and ηCOD = 76% for the EGSB reactor. The highest daily methane production rate (MPR) was observed at the maximum loading rate, with MPR = 3.00 m3/m3/d at HRT = 2 d for the FB reactor and MPR = 2.16 m3/m3/d at HRT = 3 d for the EGSB reactor. For both reactors, a reduction in HRT was possible compared to conventionally driven CSTRs, with the EGSB reactor offering a higher methane yield and production rate at a shorter HRT.

1. Introduction

Various types of power plants have been developed that do not use fossil fuels in an effort to reduce greenhouse gas (GHG) emissions [1]. Among them, agricultural biogas plants digest animal manure with different co-substrates (e.g., energy crops, harvest residue, food waste, municipal biowaste) to increase the methane yield (MY) [2]. Anaerobic digestion (AD) of animal residues reduces GHG emissions (e.g., methane, nitrous oxide) that would occur during storage and further utilization [3]. The energetic potential of animal manure is high yet unused in various countries [4,5]. In Germany, based on Brosowski et al. [6], it can be calculated that only 16% of the theoretical biomass potential of liquid pig manure is used. Biogas plants that use animal residues such as manure or pig slurry (PS) as the substrate often employ continuously stirred tank reactors (CSTRs) [2,7,8,9], which have hydraulic retention times (HRTs) of more than 30 d [10,11,12,13,14]. Reducing the HRT below the doubling rate of the active biomass in the CSTR could wash out the microorganisms and cause the process to destabilize or fail [15]. In contrast, biomass-enriched reactors such as the upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB), and fixed-bed (FB) reactors have solid retention times decoupled from the HRT. However, Schmidt et al. [12] conducted experiments on reducing the HRT at a constant loading and showed that a CSTR remained stable up to an HRT of 3 d, whereas the FB reactor used for comparison remained stable up to an HRT of 1.5 d. Certainly, UASB, EGSB, and FB reactors have not been applied to the AD of liquid manure at a commercial scale because these reactor types are not recommended for treating wastewater with a high content of solids [16]. These reactors are more frequently applied to treat brewery and malt effluents, pulp and paper effluents, and (petro)chemical effluents [17]. However, previous studies have considered using agricultural residue as a feedstock for AD in UASB, EGSB, and FB reactors. Ülgüdür et al. [18] treated digestate with a FB reactor to exploit the residual biogas potential and stabilize the feedstock. The substrate was provided by a full-scale anaerobic digester operating with 90% laying hen manure and 10% cattle manure. At an HRT of 1.3–1.4 d, a reduction of the chemical oxygen demand (COD) of 57–62% and a biogas yield of 395–430 Lbiogas/kgVSadded was obtained. Terboven et al. [19] used a stirred FB reactor with disks as the carrier material for flexible biogas production by variable feeding under mesophilic and thermophilic conditions. Using sugar beet silage as the substrate, MYs of 449–462 L/kg VSadded at an organic loading rate (OLR) of 4 gVS/L/d were obtained.
Rico et al. [20] and Bergland et al. [21] achieved the AD of PS in UASB reactors with different pretreatments of the substrate. The latter proved the suitability of the process and reactor type for treating this substrate through experiments, where the HRT was reduced from 42 h to 1.7 h with a methane yield of 2.4 L per liter feed. However, propionate accumulation was observed at the lowest HRT. Further research on this combination of reactor and substrate has focused on the influence of different seed sludges for UASB reactors and filtration of the manure. Using granular sludge as the inoculum and filtered PS as a substrate, Rico et al. [20] reported an MY of 233 L/kgCOD at an HRT of 3 d. Reducing the HRT from 3.0 d to 1.5 d increased the methane production rate (MPR) to 3.5 m3/m3/d. EGSB reactors are a modified version of the UASB reactor that was developed to intensify hydraulic mixing and substrate sludge contact [22]. Lee and Han [23,24] used EGSB reactors as part of a multistage system for treating PS. The system presented in these studies includes auto-thermal aerobic digestion, a coagulation process, and centrifugation as pretreatment, an organic acid tank and EGSB reactor as the main treatment, and sequencing batch reactors as the post-treatment at laboratory and pilot scale. The reported OLRs of the EGSB ranged from 2.0–6.0 kgCOD/m3/d at a constant HRT of 1 d with a methane production of 0.16–0.34 m3/kg COD removed. The objectives of this study were as follows:
  • Prove the applicability of bioreactors that predominantly utilize granular biomass (EGSB reactor) and biofilm biomass (FB reactor) for the AD of filtered PS;
  • Reduce the HRT to less than that of conventional CSTRs to demonstrate more efficient techniques for agricultural waste utilization;
  • Compare the performances of FB and EGSB reactors at different operating points;
  • Quantify the effect of the OLR on the methane production rate under the constraint of minimizing the HRT.
To achieve these objectives, two semi-technical reactor systems were operated at different operating points (OPs) with previously filtered PS as the substrate. Dealing with the remaining solid content of the substrate for the EGSB and FB reactors is considered to be a challenge in the experiments. This study took place over a period of 179 days; the same PS was used in both reactors to facilitate the comparison.

2. Materials and Methods

2.1. Substrate Properties and Pretreatment

The PS used in the experiments originated from a farm in Germany and was pretreated by a separation process with a 100 µm sieve (Klass Wendelfilter, KLASS Filter GmbH, Eresing, Germany) to reduce the solids in the substrate. No further pretreatment was performed. However, the substrate quality fluctuated naturally due to the uneven sampling intervals and storage duration at the farm. Table 1 list the substrate properties.

2.2. Reactor Setup

The experiments were performed in two semi-technical plants, as shown in Figure 1. Each plant comprised a storage tank (V = 100 L) and discharge tank (V = 60 L) with a FB reactor (R1) or EGSB reactor (R2). The storage tanks were cooled to θ = 4 °C and kept in a nitrogen atmosphere to counteract premature aerobic degradation and promote consistent substrate conditions. The storage tanks were refilled from the same larger storage container (V = 1000 L) that was also cooled to θ = 4 °C but was not kept in an inert atmosphere. R1 and R2 were fed by eccentric screw pumps (NM008BY, Erich Netzsch GmbH & Co., Holding KG, Selb, Germany) that were controlled by a potentiometer, frequency converter, and time switch. Before entering a reactor, the substrate passed a heat exchanger that pre-tempered it to approximately θ = 25 °C. Both reactors operated under ascending flow and mesophilic conditions, and they were tempered by a water jacket connected to a thermostatic heater (θ = 40 °C). R1 was made of stainless steel, had a volume of V = 34 L, a ratio of height to diameter of 6.4, and was filled with random packing (Hiflow® Rings 20-4 (ceramic), RVT Process Equipment GmbH, Marktrodach, Germany). It was inoculated by the liquid phase from the sludge of a municipal wastewater treatment plant and the digestate of a biogas plant, where maize silage and pig manure were used as a substrate. R2 was also made of stainless steel, had a volume of V = 27.8 L, a ratio of height to diameter of 4.86, and was inoculated with a granular sludge from a large-scale EGSB reactor in Germany, treating wastewater from cookie and pastries production. The initial bed height was about 60% of the reactor height. The intermixing of R1 and R2 and the bed expansion in R2 were achieved using an eccentric screw pump (SPRF05/1/6.3, Seepex GmbH, Bottrop, Germany) to partially recirculate the discharge with a flow of 18 L/h for R1 and 30.2 L/h for R2. R1 and R2 differed in geometry, and a three-phase separator was installed in the head area of R2. For both setups, a siphon was installed between the outlet and discharge tank so that the entire gas volume could be captured.

2.3. Execution of Experiments and Analytical Methods

R1 and R2 were operated for 674 days in total, and 179 days of operation were evaluated in this study since, during this period, the same filtered PS was used as a substrate in both reactors and thus, a comparison is possible. Table 2 present the analyzed parameters and measurement points, and intervals during the operation of the reactors. The sampling points correspond to the labels in Figure 1. The analytical methods are described below. The gas was quantitatively measured using a drum gas meter (TG 0.5, Dr.-Ing. RITTER Apparatebau GmbH & Co., KG, Bochum, Germany) at normal temperature and pressure conditions. To determine the gas quality, the parameters CH4-/CO2-portion and O2/H2S-content were measured using an infrared sensor and electrochemical sensor (Multitec 540, Hermann Sewerin GmbH, Marketing, Germany), respectively. The dry matter was determined according to DIN EN 12880. Similarly, the organic dry matter was determined according to DIN EN 12879. The COD and biochemical oxygen demand after 5 days were determined using cuvette tests (LCK014/LCK914; LCK555, Hach Lange GmbH, Düsseldorf, Germany). The volatile organic acids (VOA) and total inorganic carbonate buffer (TIC) were determined via potentiometric titration using an auto-titrator (AT1222, Hach Lange GmbH, Düsseldorf, Germany). The sample was previously centrifuged (t = 10 min, rpm = 2000 min−1), and the supernatant was utilized for the determination [25]. Phosphorus pentoxide, potassium oxide, magnesium oxide, calcium oxide, and sulfur were determined via inductively coupled plasma optical emission spectrometry according to the DIN EN ISO 11885 standards [26]. The measurement of ammonium nitrogen was performed as per the standards of DIN 38406-5 [27], and total nitrogen was determined according to VDLUFA [28].

2.4. Calculations

The MPR, COD degradation efficiency, and MY in COD-specific terms (COD as reference measure) were calculated in the steady-state to evaluate different operating conditions [29]. Methane yields of >350 L/kgCOD were considered outliers since the theoretical biochemical methane potential of one kilogram of COD is equivalent to 350 L of methane [30]. Concerning the anaerobic co-digestion of food and vegetable waste, Shen et al. [31] operated a CSTR with a coefficient of variation (Cvar) of 4.1–5.8% for daily biogas production in the steady-state. Weide et al. [32] operated an EGSB reactor for dark fermentation at a single HRT until the Cvar for the hydrogen production rate was <10%. Jafarzadeh et al. [33] assumed a hydraulic steady-state when the variation in the effluent COD concentration was less than ±10% for a period equal to 10 times the HRT and at least 14 days. Because an operating period equal to 10 HRTs could not be considered in the present study, the steady-state was assumed when the Cvar for MPR was <10% for a period of at least 14 d; this is similar to the assumption used by Karim et al. [34] with respect to the biogas production rate (BPR). Other studies considered steady-state AD accounting only for the operating period to be at least two or three HRTs, respectively [35,36]. This approach also applies to OPs with shorter HRTs in this study. The determination of linear regression models for MPR as the response variable and OLR as the independent variable was performed using the analysis tool of OriginPro 2021b (OriginLab Corporation, Northampton, MA, USA). Different data splits of 70/30, 80/20, and 90/10 between calibration and validation were employed. The best model was selected, taking into account R2 from both splits and the normality of the residuals [37].

3. Results

3.1. Description of the Entire Data Basis and the Operating Conditions

During the evaluation period, different operating conditions were considered with each setup, as listed in Table 3. R1 had HRTs of 13.3 ± 1.5 d (OP1) to 1.8 ± 0.1 d (OP6) corresponding to OLRs of 1.70 ± 0.43 kgCOD/m3/d to 13.52 ± 1.73 kgCOD/m3/d. These results match the HRT and OLR reported by Singha and Prerna [38] or Ülgüdür et al. [18]. Due to plugging between the foam trap and gas meter, R2 was taken out of operation for 15 days from day 118. R2 was then restarted and set to an HRT of 9.7 ± 1.4 d, which was then reduced to 3.1 ± 0.1 d to match the conditions prior to the plugging. Therefore, R2 had HRTs of 13.9 ± 1.4 d and 3.1 ± 0.4 d corresponding to OLRs of 1.55 ± 0.33 kgCOD/m3/d and 7.83 ± 0.16 kgCOD/m3/d, respectively. These loadings are low compared with the OLR of 30 kgCOD/m3/d that is possible with this type of reactor and reported by various authors [39,40]. However, animal manure is not commonly used as a substrate for this reactor in full-scale applications and can be difficult due to low organic loads and high nitrogen concentrations [17,41,42]. This issue can be addressed by co-digesting animal manure with industrial wastewater and residues, such as tofu whey, a protein-rich residue resulting from the production of tofu, or wastewater from starch production [43,44]. In this way, industrial residues that have a low pH value, for example, and agricultural residues with high nitrogen contents, are made usable at the same time.
The remaining dry matter of the pretreated substrate was 1.9 ± 0.3 wt.%, which is above the recommendations of Van et al. [45] and Kougias et al. [7] for EGSB and FB reactors. Nevertheless, Cruz-Salomón et al. [17] state that a substrate with a total suspended solid content of <8% can be used in EGSB-reactors, and Uddin et al. [46] report, that a solid content of <2% is acceptable for fixed-bed reactors. At the same time, no clogging inside the reactors was observed over the entire operating period and it concluded that the pretreatment of the substrate was sufficient.
Figure 2 show the pH, VOA/TIC, daily biogas production and COD-specific OLR of R1 throughout the evaluation period. The pH remained nearly constant at 7.4 ± 0.1. The VOA/TIC increased from 0.25 to 0.45 on day 118, but this did not appear to affect the pH. VOA/TIC continued to increase, reaching nearly 0.6 on day 122. Thereupon, the VOA/TIC decreased during the rest of the evaluation period to 0.27. Lossie and Pütz [47] recommend a VOA/TIC ratio of 0.3–0.4 to ensure the stability of the process and avoid overloading. A VOA/TIC greater than 0.6 is assumed to heavily overload the digester. The rise in VOA/TIC occurred 2 days after a new sample of PS was taken and was attributed to the degradation of the fresh substrate. However, the VOA/TIC then steadily decreased without any counteraction and was stable at OP6. The rise in VOA/TIC on day 118 was also observed for R2, but the reactor was then taken out of operation due to plugging. When the reactor was started up again, and measurements were continued, the VOA/TIC was equal to the readings prior to the pause. Because the same substrate was used for R1 and R2, this explanation seems plausible. The biogas production started at 1.49 L/d and then remained stable from day 10 to the end of OP1 at about 22.41 ± 2.16 L/d. The biogas production increased with the OLR, as shown in Figure 2, and reached the maximum at OP6 with 147.06 L/d.
Figure 3, as with Figure 2, show the pH, VOA/TIC, daily biogas production, and COD-specific OLR of R2 throughout the evaluation period. The pH was 7.91 ± 0.05. The VOA/TIC had a range of 0.11–0.58. After R2 was restarted, a VOA/TIC of 0.15 ± 0.03 was recorded. Similar to R1, the biogas production and OLR seemed to be correlated in R2. After R2 was taken out of operation for 15 days, the biogas production was not as high as prior to the pause.

3.2. Evaluation of the OPs

After the steady-state criterion was applied, OP3–6 for R1 and OP1, OP3, and OP4.1 for R2 were selected for further consideration. Figure 4 show the COD-specific MY, COD degradation efficiency, methane concentration, and MPR at these OPs to characterize the reactor performance during continuous operation [48]. For R1, reducing the HRT and thereby increasing the OLR decreased the MY from 262 L/kgCOD at OP3 to 218 L/kgCOD and 226 L/kgCOD at OP5 and OP6, respectively. Similarly, the COD degradation efficiency was initially 73% at OP3 and then decreased to 53% at OP6. The lowest methane concentration was observed at OP4, with an average value of 72%. An increase in the OLR resulted in an increase in the MPR, which occurred because more substrate became available. These results are supported by reports by several groups on AD using FB and EGSB reactors [17,18,26,36]. The MPR ranged from 0.87 m3/m3/d at an HRT of 7 d to 3.00 m3/m3/d at an HRT of 2 d. For R2, the MY was 292 and 283 L/kgCOD at OP1 and OP4.1, respectively. The COD degradation efficiency was 76% at OP1 but decreased to 73% at OP4.1. This tendency is similar to the results observed for R1. The MPR ranged from 0.43 m3/m3/d at OP1 to 2.16 m3/m3/d at OP4.1. The evaluation of the OPs shows that the highest MPR in R1 and R2 was observed at the lowest MY and COD degradation efficiency.

3.3. Process Comparison

To compare the performances of the two reactors and explore the significance of this work, Table 4 present the results of continuous experiments on AD in CSTR, UASB, EGSB, and FB reactors. Rico et al. [20] reported on the digestion of filtered PS in UASB reactors, and an observed MYs of 233–247 L/kgCOD, which is slightly above the MYs calculated at OP5 and OP6 for R1. This is a plausible result because the two studies used equivalent substrates and similar loadings. However, R2 achieved a higher MY than the previously mentioned reactors. This may be because R2 had a relatively high HRT corresponding to a low OLR at OP1-OP4.1 [17]. Similarly, Fang et al. [49] found that the EGSB reactor produced biogas from potato juice more efficiently than the UASB reactor under stable conditions. Following the MY, MPR fits into the comparison between the results of Rico et al. [20] and this study. However, neither R1 nor R2 was able to achieve an MPR of 3.5 m3/m3/d as in the aforementioned study, which can be attributed to the fact that the OLR in the case of R2 was not increased as in the studies of Rico et al. [20]. Another comparison of the MPR of R1 is possible with the results of Kalyuzhnyi et al. [50], who reported an MPR of 3.18 m3/m3/d at a maximum OLR of 12.39 kgCOD/m3/d, which is higher than the MPR observed in R1-OP6, but yet supporting the results of this study. This comparison between FB and UASB reactor reflects the results of Parawira et al. [51], who reported that a UASB reactor had a higher MPR than the FB reactor when the OLR was the same, although potato leachate was used as the substrate.
In this study, OP3 and OP4 featured similar operating conditions for OLR and HRT. For R2, the MY was 267 L/kgCOD, and the MPR was 0.91 m3/m3/d at OP3, which is attributed to the low methane concentration in this OP. However, MY and MPR were higher than at R1 in this OP. When the HRT was reduced to 3.5 d in R1 and 3.2 d in R2, R2 also showed a higher MPR with a higher MY of 283 L/kgCOD. These results indicate that the EGSB reactor is more suitable than the FB reactor for digesting PS in these conditions. However, the HRT for R2 was not reduced as much as for R1. The experimental results for R1 showed that stable operation was possible even at an HRT of 1.7 d. These results indicate that experiments on further reducing the HRT in EGSB and FB reactors for the AD of PS should be performed because neither reactor showed signs of process failure.
Compared with currently operating large-scale and laboratory-scale CSTRs for treating animal residues, which typically have an HRT of ≥15 d, EGSB and FB reactors can be used to reduce the HRT for the AD of PS [10,11,12,13,14,52]. This reduction in HRT, in turn, leads to a higher OLR and a higher MPR, shown by the comparison between the results of Reguiero et al. [53] and this study in Table 4. The CSTR in the aforementioned study was operated with filtered pig slurry at an OLR of 0.6–2.0 kgCOD/m3/d and showed MPRs between 0.08–0.47 m3/m3/d. The performances of R1 and R2 at HRT < 15 d demonstrate that EGSB and FB reactors are suitable for the energetic utilization of PS and substrates with similar compositions.
Table
Table 4. Process performance and methane potential of different reactor configurations and substrates.
Table 4. Process performance and methane potential of different reactor configurations and substrates.
ReactorVolumeSubstrateTemperatureOLRHRTMPRMYCOD Degradation EfficiencyReference
(−)(L)(−)(°C)(kgCOD/m3/d)(d)(m3/m3/d)(L/kgCOD)(%)(-)
CSTR9.2Filtered pig slurry350.6–2.010–200.08–0.47--[53]
UASB1Filtered pig slurry35 ± 114.31.53.5246-[20]
UASB2.6Liquid fraction of pig manure3512.391.193.18-75[50]
UASB0.84Potato leachate371.5–7.013.2–2.90.1 ± 0–1.4 ± 0.2-92 ± 4.2–93 ± 5.3[51]
FB0.7Potato leachate371.5–7.013.2–2.90.1 ± 0.02–0.7 ± 0.1-91 ± 4.2–98 ± 1.5[51]
FB34Filtered pig slurry403.7–13.56.6–1.80.87–3.00217–26253–73R1
EGSB27.8Filtered pig slurry401.5–7.814.3–3.20.43–2.16267–29273–76R2

3.4. Influence of the Organic Loading Rate on the Methane Production Rate

Owing to the strong correlation between the OLR and HRT, Figure 5 show their relationship with the substrate used in this study. The results are similar to the curves presented by Friehe et al. [54].
The previous qualitative description of the relation between MPR and OLR was extended by linear fitting. Table 5 present linear regression models where the OLR is the main influencing parameter of AD in EGSB and FB reactors [17,42], and the MPR is the response variable. Yu et al. [55] and Jafarzadeh et al. [33] reported similar observations for the AD of soybean wastewater with an anaerobic filter and the AD of petrochemical wastewaters in a hybrid UASB/anaerobic filter. The latter achieved a coefficient of determination (R2) for BPR versus OLR of 85.2% and BPR−1 versus OLR−1 of 90.3%, which is below the R2 values for R1 and R2. The models provided in Table 5 are suitable for interpolative predictions within the evaluated ranges of parameters and can thus be used as a basis for economic efficiency considerations. Extrapolative predictions can only be made to a limited extent because the response variables can change considerably outside the ranges observed in the experiments of this study [56].
The established models for R1 and R2, average values from the selected OPs, and the results of Rico et al. [20] and Kalyuzhnyi et al. [50] are shown in Figure 6 in relation to the OLR. The models for R1 and R2 show that the EGSB offers a higher MPR above an OLR of 4.32 kgCOD/L/d, which corresponds to an HRT of 5.9 ± 1.8 d. Below this OLR, similar or lower MPRs are expected for the EGSB compared to the FB reactor. The average values calculated in the evaluation of the OPs are in agreement with the models for R1 and R2, indicating a consistent approach.
Moreover, the results of Kalyuzhnyi et al. [50] and Rico et al. [20] are congruent with the model’s extrapolation for R2, which considered a maximum OLR of 10.9 kgCOD/L/d. This behavior is evident since EGSB and UASB reactors are similar high-rate reactors, and similar substrates were used in both studies [57]. The latter used pretreated pig slurry, which was filtered through a 100 µm textile filter in a lab-scale reactor with a volume of V = 1 L, made of plexiglass at mesophilic temperatures of 36 ± 1 °C. The COD concentration of the filtered slurry used was 21.5 ± 1.3 g/L. Kalyuzhnyi et al. [50] prepared the influent of the UASB reactor with a working volume of 2.6 L by diluting the raw slurry with tap water (1:2-10) and settling the suspended solids for 12 h. The prepared manure had a COD concentration of 14.7 g/L, which is below the concentration of the substrate used in this study. However, a maximum OLR of 12.39 gCOD/L/d could be applied at an HRT of 1.19 d.
In addition, the comparison between the FB reactor and the bibliography, as described in Section 3.3, becomes evident. Based on the model for R1, it can be deduced that the fact that the FB reactor did not achieve an MPR of 3.18 m3/m3/d or 3.5 m3/m3/d is not due to the lower OLRs, but to the characteristics of the substrate used or the reactor-system itself. This result is supported by Parawira et al. [51], who found the UASB reactor to be more efficient than the FB reactor for the treatment of potato leachate.

4. Discussion

The overall plant concept of high-rate anaerobic digestion of liquid agricultural residues contains a pretreatment of the substrates (e.g., pig slurry, cow manure) to reduce the solid content from the raw slurries [58]. Thereby, two output streams are obtained, one rich in solid material (solid phase) and the other one with a decreased solid content (liquid phase). The low solid content in the liquid phase enables the AD in high-rate anaerobic reactors such as FB or EGSB reactors, as investigated in this study. The solid phase appears to be a suitable substrate for biogas production in CSTRs, due to a relatively high DM content [59]. The individual treatment of the liquid and solid materials resulting from the pretreatment allows different plant configurations in full scale. Hence, possible advantages in the plant operation as a consequence of the gains in flexibility are attractive for the biogas market. However, there are also obstacles as well as new research questions that need to be answered before making this technology market available.
Using the liquid phases, which correspond to about 90% of the total mass [59], in high-rate reactors with low HRTs of 2–15 d, the total reactor volume required for the energetic utilization of the residues can be reduced. Similarly, the solid material is used in correspondingly smaller CSTRs, since the liquid phase with high water content can be used under the process conditions investigated in this study [14]. However, installing two reactors and appropriate facilities for pretreatment may lead to higher investment costs, compared to the conventional approach of building one reactor at which the raw slurries are digested. At the same time, the plant concepts strongly depend on the chosen commercialization strategy for the produced energy and the amount of available substrate in a certain range. Therefore, a techno-economic assessment, based on the results of this study, addressing these issues is currently in progress. In this study, the main influencing parameters on the economic efficiency will be evaluated, and possible bottlenecks will be identified. Furthermore, different business cases under European and German energy legislation include small manure-based biogas plants with an installed electrical capacity of <150 kW, the extension of existing biogas plants that are at the end of their first funding period and are now applying for a second funding period, or the upgrading of the biogas produced to biomethane at high-rate anaerobic systems are also going to be evaluated, due to the recent impulse that the biogas market is receiving since the start of the ongoing European energy crisis [60].
In a broader perspective, the plant concept and investigations on high-rate anaerobic digestion presented here can facilitate an energetic use of pig manure and other residual materials, contributing to some of the sustainable development goals established by the United Nations in 2015: (no. 2) Zero Hunger, (no. 7) Affordable and Clean Energy, and (no. 13) Climate Action [61]. Achieving these goals can be supported by bioenergy production based on substrates that do not compete with food production, which replace fossil fuels and avoid GHG emissions that otherwise would occur from storage and application of the manure as a fertilizer [62]. Furthermore, the more widespread employment of manures or agroindustrial residues helps to fulfill the energy potential of non-used biomass due to low organic loads while increasing the positive environmental impact by means of cradle to cradle approach [63,64]. Consequently, the introduction or application of high-rate technologies in the biogas sector allows for the treatment of high-liquid content substrates, e.g., liquid manure, wastewater, or liquid phase of any substrates that are mostly left untreated due to lower economic gains.
In order to evaluate the feasibility of treating liquid manure or liquid mixtures from agro-industrial substrates, pilot-plant experiments are currently being carried out in an EGSB reactor with a volume of V = 564 L. Moreover, experiments with the resulting solid phase of the solid–liquid separation are also conducted at different scales.

5. Conclusions

This study evaluated the applicability of EGSB and FB reactors to the AD of filtered PS. Both reactors demonstrated a shorter HRT than conventional CSTRs, according to the reviewed bibliography, and the EGSB reactor offered a better MPR at shorter HRTs, increasing the technical feasibility of the AD of PS. Hence, enabling the treatment of liquid manures of which large quantities remain untreated. Furthermore, linear regression analysis described the influence of the OLR on the MPR for both reactors with an R2 larger than 90%. Thus, the resulting models can be applied as a prediction tool for the process performance within the parameters and conditions investigated in this study and showed that the EGSB reactor is more efficient at OLRs > 4.32 gCOD/L/d (corresponding to HRT = 5.9 ± 1.8 d). Nevertheless, the highest MPR was observed at the lowest MY and COD degradation efficiency, which indicates a conflict between maximizing methane production and optimizing substrate use. For practical application, a broader approach considering the impact of these parameters and their interaction with the economic profitability of possible treatment schemes is recommended. Further aspects (e.g., legal framework, amount and handling of the substrate) will be considered in future research, as well as the utilization of the solid phase that is produced during pretreatment.

Author Contributions

Conceptualization, J.H. and T.W.; Data curation, J.H.; Formal analysis, J.H.; Funding acquisition, E.B.; Investigation, J.H.; Methodology, J.H. and R.E.H.R.; Project administration, T.W. and A.N.; Resources, J.H. and T.W.; Software, J.H.; Supervision, C.W.; Visualization, J.H.; Writing—original draft, J.H.; Writing—review and editing, J.H., T.W., R.E.H.R. and C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding from the Germany-Netherlands INTERREG program, which made this work possible with the research project entitled “Grüne Kaskade–Hochlastvergärung” [funding code: 151073].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the laboratory team, especially Jens Brüninghoff, for conducting most of the experimental work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of the experimental setup.
Figure 1. Schematic of the experimental setup.
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Figure 2. pH, VOA/TIC, biogas production, and COD-specific OLR of R1 over the experimental period.
Figure 2. pH, VOA/TIC, biogas production, and COD-specific OLR of R1 over the experimental period.
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Figure 3. pH, VOA/TIC, biogas production, and COD-specific OLR of R2 over the experimental period.
Figure 3. pH, VOA/TIC, biogas production, and COD-specific OLR of R2 over the experimental period.
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Figure 4. COD-specific MY, COD degradation efficiency, methane concentration, and methane production rate at OPs in R1 and R2.
Figure 4. COD-specific MY, COD degradation efficiency, methane concentration, and methane production rate at OPs in R1 and R2.
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Figure 5. Correlation between the OLR and HRT.
Figure 5. Correlation between the OLR and HRT.
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Figure 6. Linear regression models, average values from the selected OPs and results of Rico et al. [20] and Kalyuzhnyi et al. [50] in relation to the organic loading rate.
Figure 6. Linear regression models, average values from the selected OPs and results of Rico et al. [20] and Kalyuzhnyi et al. [50] in relation to the organic loading rate.
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Table
Table 1. Substrate properties of separated pig slurry.
Table 1. Substrate properties of separated pig slurry.
ParameterUnitValue
pH(-)7.93
Dry matter(wt%)1.9 ± 0.3
Organic dry matter(wt%)61.1 ± 2.8
Chemical oxygen demand(gCOD/L)25.48 ± 7.72
Biochemical oxygen demand after 5 days(g/L)13.43 ± 4.25
Total nitrogen(wt%)0.28
Ammonium–nitrogen(wt%)0.22
Phosphorus pentoxide(wt%)0.061
Potassium oxide(wt%)0.22
Magnesium oxide(wt%)<0.05
Calcium oxide(wt%)<0.1
Sulfur(wt%)0.04
Table
Table 2. Analyzed parameters, intervals, and sampling points.
Table 2. Analyzed parameters, intervals, and sampling points.
ParameterIntervalSampling Point
pHEvery secondRecirculation flow
TemperatureEvery secondRecirculation flow
Effluent massEvery daySP1.3/SP2.3
Gas volumeEvery dayTG 0.5.1/TG 0.5.2
Gas qualityEvery dayGB1/GB2
Chemical oxygen demandTwice a weekSP1.1/SP2.1; SP1.3/SP2.3
VOA/TICTwice a weekSP1.2/SP2.2
Dry matter and organic dry matterEvery two weeksSP1.1/SP2.1; SP1.3/SP2.3
SP = sampling point; TG 0.5 = drum gas meter; GB = gas bag.
Table
Table 3. Hydraulic retention times and organic loading rates of R1 and R2 at OP1–6.
Table 3. Hydraulic retention times and organic loading rates of R1 and R2 at OP1–6.
ReactorOperation PointOperating Conditions
Hydraulic Retention TimeOrganic Loading Rate
(d)(kgCOD/m3/d)
R1OP113.3 ± 1.51.70 ± 0.43
OP2--
OP37.2 ± 1.63.32 ± 0.96
OP43.5 ± 0.37.13 ± 0.88
OP52.4 ± 0.212.26 ± 1.52
OP61.8 ± 0.113.52 ± 1.73
R2OP113.9 ± 1.41.55 ± 0.33
OP29.7 ± 1.43.03 ± 0.51
OP37.4 ± 0.63.57 ± 0.56
OP4.13.2 ± 0.67.60 ± 1.54
OP4.23.1 ± 0.47.83 ± 0.16
Table
Table 5. Linear regression analysis data.
Table 5. Linear regression analysis data.
ReactorRelationshipOLRminOLRmaxR2DatasplitR2 Validation
R1MPR = 0.188·OLR + 0.325152.516.890.78%90/1092.66%
R2MPR = 0.245·OLR + 0.078531.010.990.20%80/2094.34%
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Häner, J.; Weide, T.; Naßmacher, A.; Hernández Regalado, R.E.; Wetter, C.; Brügging, E. Anaerobic Digestion of Pig Slurry in Fixed-Bed and Expanded Granular Sludge Bed Reactors. Energies 2022, 15, 4414. https://doi.org/10.3390/en15124414

AMA Style

Häner J, Weide T, Naßmacher A, Hernández Regalado RE, Wetter C, Brügging E. Anaerobic Digestion of Pig Slurry in Fixed-Bed and Expanded Granular Sludge Bed Reactors. Energies. 2022; 15(12):4414. https://doi.org/10.3390/en15124414

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Häner, Jurek, Tobias Weide, Alexander Naßmacher, Roberto Eloy Hernández Regalado, Christof Wetter, and Elmar Brügging. 2022. "Anaerobic Digestion of Pig Slurry in Fixed-Bed and Expanded Granular Sludge Bed Reactors" Energies 15, no. 12: 4414. https://doi.org/10.3390/en15124414

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