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Article

Study of the Effect of Temperature to Optimize the Anaerobic Digestion of Slaughterhouse Sludge by Co-Digestion with Slaughterhouse Wastewater

by
Maria Candel
,
Laura Ballesteros
,
Juana Fernandez-Rodriguez
,
Montserrat Perez
* and
Rosario Solera
Department of Environmental Technologies, IVAGRO, Faculty of Marine and Environmental Sciences (CASEM), University of Cádiz, Avenida República Saharaui, s/n, 11510 Puerto Real, Cádiz, Spain
*
Author to whom correspondence should be addressed.
Recycling 2025, 10(2), 47; https://doi.org/10.3390/recycling10020047
Submission received: 6 February 2025 / Revised: 5 March 2025 / Accepted: 12 March 2025 / Published: 19 March 2025
Browse Figures
Versions Notes

Abstract

:
The anaerobic co-digestion (ACoD) of sludge (S) and slaughterhouse wastewater (SW) can improve biomethane production and removal efficiency in terms of organic matter. The aim of this study was to determine the impact of temperature on co-digestion, considering different hydraulic retention times (HRTs). The substrates were selected at a 50:50 weight ratio and two single-stage digesters of 2 L capacity were used, operating under ideal thermophilic (55 °C) and mesophilic (35 °C) conditions. The HRTs studied were 30, 25, 20 and 15 days. The biomethane production reached 588 mL CH4/d at 55 °C and 477 mL CH4/d at 35 °C for an HRT of 15 days, indicating a significantly higher yield under the thermophilic process. The volatile solids removal efficiency ranged from 41% to 66% for the thermophilic digester and between 32% and 42% for the mesophilic digester. This further highlights the superior performance at higher temperatures.

1. Introduction

Continuous population growth, coupled with industrialization, has led to an increase in energy demand and waste generation, posing an environmental challenge. This energy demand is mostly met by fossil fuels, contributing to increased greenhouse gas emissions and global warming [1,2]. Recently, the elimination or reduction of waste has become a priority for companies, driving research into new energy sources through waste valorization to replace fossil fuels [3]. The main objective of waste valorization is to move towards a circular economy, thereby reducing the negative impact of waste as much as possible [4].
In this context, anaerobic digestion offers a sustainable solution for converting organic waste into biogas [5]. Anaerobic digestion is a biological process whereby microorganisms degrade organic matter in the absence of oxygen to obtain biogas, composed mainly of biomethane and carbon dioxide, and a biosolid that can be used for agronomic purposes [6,7,8,9,10]. Anaerobic co-digestion offers a sustainable approach to waste management by combining different organic substrates to improve biogas production, nutrient recovery, and overall process stability [11,12,13,14,15]. Other advantages of anaerobic co-digestion include the greater biodegradability of organic matter, the increased reduction of volatile solids (VS), the stabilization of heavy metals and other toxins, nutrient balance, and therefore a suitable C/N ratio [16,17,18,19,20].
In Spain, the meat sector is the fourth most important industry, accounting for 29.46% of total employment in the Spanish food industry [21,22]. Studies on the anaerobic co-digestion of different waste types, such as sewage sludge combined with other organic substrates like sugar beet waste or wine stillage, can be found in the literature [14,23].
Slaughterhouse sludge is composed of the remains of intestines, stomachs, blood, and fats. This sludge contains high protein and fat contents [24,25], making it a suitable substrate for anaerobic digestion due to its high biomethane yields [26]. However, monodigestion has presented limitations due to the ammonia generated during protein degradation, and the accumulation of volatile fatty acids (VFA) and long-chain fatty acids (LCFA) produced during lipid hydrolysis [27]. Furthermore, the large amount of solids present in these wastes leads to high hydrolysis times, which can limit the degradation rate of the process [28]. A possible solution to these problems is co-digestion with other waste and/or wastewater, such as wastewater generated in slaughterhouses. Slaughterhouses generate large quantities of wastewater with high concentrations of organic matter. These discharges are highly variable and heterogeneous because they depend on the type of species slaughtered, the sizes of the facilities and the activities carried out therein [29,30]. Slaughterhouse wastewater may contain ammonia, oil, fats, and other by-products resulting from livestock activity that are also noxious to the environment, such as detergents, cleaning products, antibiotics, and heavy metals. In addition, its pathogen content makes it damaging to human health [31]. The anaerobic co-digestion of these wastes represents an attractive technology within the framework of the circular economy. It is a low-cost and well-established technology for the treatment of organic waste. The biogas generated can be used to produce electricity or thermal energy within the agri-food industry itself, thus closing the cycle between energy consumption, food production and subsequent waste disposal [6,11,32].
This study focuses on optimizing the anaerobic co-digestion of slaughterhouse sludge and wastewater under mesophilic and thermophilic conditions. Using stirred tank reactors and a semi-continuous feeding regime at a laboratory scale, we investigated the impacts of different hydraulic retention times on the anaerobic degradation process of meat industry waste.

2. Results

2.1. Effect of HRT Under Mesophilic Conditions

The observed pH values and trends are illustrated in Figure 1, which shows the pH’s evolution throughout the study for each tested HRT. The optimum pH range for the activity of methanogenic microorganisms is 7.5–8.5. Therefore, for the anaerobic digestion process to be stable, it is necessary to control the pH evolution in each system, as this parameter is key in monitoring the anaerobic degradation of organic matter [10,33,34,35]. The pH value was quantified daily in the systems for a total time of 250 days (x-axis). During this total time, different HRTs were tested, each of which is shown, separated by a discontinuous vertical line. The change in pH detected, from the vertical line, indicates a change in the system’s conditions, specifically a change in the HRT. In the mesophilic digester, the pH remained stable during the experiment, generally within the range of 7.21 to 8.0, with an average value of 7.53. However, the pH tended to become more acidic as the tested HRT decreased. This problem often occurs in single-stage reactors, as increasing the organic loading rate can lead to an accumulation of VFA, resulting in acidification [36].
The organic matter content in the effluent was determined by measuring tCOD, sCOD, TS, VS, and VFA. Figure 2 and Figure 3 illustrate the evolution of these parameters after reaching the steady state at each studied HRT. Figure 2a depicts the effluent organic matter content, specifically tCOD and sCOD. An inverse trend was observed, whereby tCOD increased with decreasing HRT, while sCOD decreased, obtaining values of 41.06 g/L and 3.44 g/L, respectively, at an HRT of 15 days. In anaerobic digestion, HRT critically impacts effluent quality; shorter HRTs result in increased tCOD due to incomplete particulate matter degradation and the reduced removal of suspended solids, while sCOD decreases as microorganisms have less contact time to break down soluble organic compounds. Therefore, optimizing HRT is essential to balancing the degradation of both particulate and soluble matter, maximizing anaerobic digestion system efficiency [37,38,39]. Regarding solids (Figure 2b), both TS and VS exhibited a similar trend, showing a decrease in solid concentration with decreasing HRT. In the thermophilic digester, the highest values were 48.19 g/L for TS and 39.50 g/L for VS at an HRT of 30 days. This suggests that the microorganisms effectively depleted the soluble organic matter, thereby contributing to process stability.
Figure 3 illustrates the evolution of total acidity, expressed as acetic acid concentration, and the concentrations of individual volatile fatty acids. The detected VFAs were acetic, propionic, and butyric acids, with acetic acid predominating across all HRT studied, reaching a maximum concentration of 148.65 mg/L at an HRT of 30 days. The highest total acidity was also observed for the 30-days HRT. A decreasing trend in total acidity was noted for HRTs between 25 and 15 days, indicating efficient VFA degradation. Notably, the VFA concentrations remained below the inhibitory values reported in the literature [40,41]. The enhanced stability observed in mesophilic co-digestion is a result of a combination of factors, including improved buffering capacity, nutrient balance, microbial diversity, and synergistic effects, which work together to create a more resilient and robust system capable of withstanding the challenges associated with VFA accumulation. Furthermore, the results demonstrate that anaerobic co-digestion within the mesophilic range helps to decrease the detrimental effects and instability of the system, which are associated with VFA accumulation [34].
Table 1 presents the removal efficiencies of organic matter-related parameters during the anaerobic co-digestion of slaughterhouse sludge and slaughterhouse wastewater in different HRTs. The percentage removal for sCOD is not included in the table because the slaughterhouse sludge with which we worked had a low sCOD, which, during the degradation process, increased until it was higher than the initial one, giving a negative percentage value. The highest tCOD removal efficiency, 56.28%, was achieved at an HRT of 15 days, and the same occurred for the other measured parameters. The removal efficiency obtained is consistent with the values considered adequate in the literature for anaerobic degradation processes [26].
Biomethane yield depends on several factors, including reactor feed, HRT and temperature, which is the primary focus of this study. The biomethane production obtained from the co-digestion of slaughterhouse sludge and wastewater was compared across the analyzed THRs. To analyses the amount of biomethane obtained, both daily production (mL/d) and yield (mL/gVSadded) were monitored. The daily biomethane volume and yield for each experimental condition are presented in Figure 4 and Table 2. To compare the energy efficiencies of systems, it is necessary to evaluate biomethane values per gram of organic matter [40]. Biomethane production increased with decreasing HRTs, reaching a maximum of 477 mL CH4/d for an HRT of 15 days.
The biomethane volume varied across the tested HRTs, although the difference was not significant between the two shortest HRTs, where the amount obtained ranged from 368 to 477 mL CH4/d. The biomethane yield exhibited a similar trend, increasing with decreasing HRT. This parameter remained relatively stable throughout the test, ranging from 8.27 to 9.54 mL CH4/gVSadded. These values are comparatively low, indicating that there are difficulties with the anaerobic digestion of this type of waste.

2.2. Effect of HRT Under Thermophilic Conditions

As depicted in Figure 5, the thermophilic digester showed stable pH values throughout the experimental process, without requiring the addition of external agents to correct it. This stability is attributed to several key factors inherent in thermophilic anaerobic digestion: firstly, the high temperatures accelerate microbial metabolic rates, leading to the rapid consumption of VFA, thereby preventing pH drops; secondly, the enhanced solubility of inorganic carbon at higher temperatures strengthens the bicarbonate/CO2 buffering system; and lastly, thermophilic conditions foster a more specialized microbial community that efficiently manages pH fluctuations [42,43]. The pH of the thermophilic process was generally higher than that observed in the mesophilic process. Values ranged between 7.49 to 8.2, with an average of 7.81. In this case, as the tested HRT decreases, the pH does not tend to become as acidic as it does in a mesophilic digester.
The organic matter content in the effluent of the thermophilic digester is presented in Figure 6 and Figure 7. The tCOD levels in the thermophilic range are slightly lower compared to those in the mesophilic digester for all HRTs, with the exception of an HRT of 30 days, where a value of 42.32 g/L was obtained (Figure 6a), compared to 40.65 g/L at 35 °C. Conversely, the sCOD trend differs from that observed in the mesophilic digester, exhibiting an increase with decreasing HRTs (Figure 6a). The highest sCOD concentration, 21.82 g/L, was recorded for an HRT of 15 days. These results could justify a second mesophilic stage to remove the solubilized organic matter generated in the thermophilic digester. As shown in Figure 6b, TS and VS concentrations decreased as the studied HRT decreased; however, an increase in TS concentration to 36.34 g/L was observed at an HRT of 20 days. This deviation from the general trend can be explained due to variations in substrate composition when dealing with real samples, as well as the increased instability when working at higher temperatures, such as 55 °C [44,45].
Acetic acid was the predominant VFA in the thermophilic process (Figure 7). The elevated acetate concentration in the thermophilic digester is attributed to the higher organic loading rate, as reported in the literature [44,45]. We observed an increase in propionic acid concentration throughout the study; this could have occurred due to the variation in influent characteristics. The data suggest that acetogens and hydrogenotrophs under thermophilic conditions are more sensitive to environmental changes. At 15 days of HRT, the thermophilic process was able to compensate for variations in the influent because it worked at an optimal organic loading rate, and allowed a reduction in the amount of propionate. Although total acidity decreased during the process, an increase was observed at an HRT of 15 days. These data do not agree with the trend that total acidity should follow; however, one reason for this may be the use of real samples, which inherently vary in composition.
For the thermophilic reactor, removal efficiencies were generally higher than those observed in the corresponding mesophilic reactor (Table 3), with the exception of tCOD at an HRT of 15 days (43.04%). An increase in the removal of total solids and volatile solids was observed when the reactor temperature was elevated, with removal rates of 67.73% and 65.84%, respectively, at an HRT of 15 days. The differences in solid compound degradation capacity between thermophilic and mesophilic temperature ranges became more significant with decreasing HRT [46,47].
As depicted in Figure 8, the average biomethane content in the thermophilic process was higher, approximately 70%, compared to the mesophilic process. Under both conditions, the biomethane volume increased proportionally to the decrease in HRT. At 15 days of HRT, the biomethane volumes obtained were 588 mL CH4/d and 477 mL CH4/d for the thermophilic and mesophilic processes, respectively. The average biomethane yield in the thermophilic process was significantly higher than in the mesophilic digester, with values of 23.02 mL CH4/gVSadded and 9.54 mL CH4/gVSadded, respectively, at the optimal retention time (Table 4). Compared to the mesophilic reactor, the thermophilic reactor produced a greater amount of biomethane per gram of volatile solid added, indicating a higher efficiency in converting intermediate products to biomethane. Based on the analysis of the single-stage anaerobic co-digestion of sludge and slaughterhouse wastewater, an HRT of 15 days in the thermophilic regime was determined to be the most efficient condition.

3. Discussion

AD at large industrial scale is predominantly implemented using single-stage digesters under mesophilic conditions, aiming for operational stability. However, this configuration faces inherent challenges, such as the need to simultaneously optimize conditions for diverse microbial groups and the long operation time, which increase operational costs [48]. As Chen et al. [49] pointed out in their review, the inhibition of the AD process is a critical factor affecting biogas production. The accumulation of VFA when the organic loading rate increases can significantly disrupt the performance of mesophilic reactors, limiting biomethane production [50,51,52]. In contrast, AD in the thermophilic range offers accelerated substrate hydrolysis, which is particularly valuable for raw materials with high organic loads [44]. High temperatures facilitate cell lysis, making cellular components available for bioconversion. However, the results obtained in the study reveal an increase in sCOD in the thermophilic digester, consistent with findings in the literature indicating greater instability due to accumulation of VFA in this temperature range. Specifically, the thermophilic system exhibited a total acidity of 268 mgAcH/L, compared to 27 mgAcH/L in the mesophilic system at an HRT of 15 days. This instability is attributed to the reduced diversity and robustness of methanogens, with fewer species identified in thermophilic digesters, and the increased sensitivity of acetogens and syntrophic methanogens to metabolites at high temperatures [45,53,54,55].
The findings confirm that thermophilic AD improves biomethane production (Figure 5) and organic matter removal efficiency (Table 3). However, the mesophilic process demonstrated greater pH stability (Figure 2) and reduced VFA accumulation with shorter HRT (Figure 4). The greater data dispersion in the mesophilic digester, possibly due to the instability induced by the high temperature and the use of real samples, reinforces the superior consistency of the mesophilic system. To leverage the advantages of both temperature ranges and mitigate the risks of instability and VFA accumulation, temperature-phased anaerobic co-digestion (TPACoD) could be considered in future research. This strategy could optimize hydrolysis in the thermophilic phase and stabilize methanogenesis in the mesophilic phase [56]. As reviewed by Tian et al. [57], TPACoD offers significant advantages in biogas production [58].

4. Materials and Methods

4.1. Characterization of the Substrates, the Feed and the Inoculum

For this study, the effluent from a 5 L semi-continuous mesophilic anaerobic digester was used as the inoculum. At the time of sampling, the digester was operating under stable conditions with ca. 20 days HRT within the mesophilic temperature range (35 °C). It was fed with sewage sludge from the Guadalete wastewater treatment plant (WWTP), located in Jerez de la Frontera (Cádiz).
The substrates used for anaerobic co-digestion were sludge (S) and slaughterhouse wastewater (SW). Both substrates were collected from Montesierra S.L. slaughterhouse, located in Jerez de la Frontera, Cadiz, Spain. Upon collection, they were stored cold at 4 °C to prevent biodegradation until use. All substrates were characterized before use. The reactor feed for the experiments was a 50:50 mixture of S and SW, a ratio selected to dilute the sludge and based on the results from previous studies [5,14].
Table 5 presents the main physico-chemical characteristics of the substrates, feed and inoculum. Both substrates exhibited a basic pH. The slaughterhouse wastewater was notable for its low total chemical oxygen demand (tCOD) and soluble chemical oxygen demand (sCOD), which were only 4.77 g/L and 2.93 g/L, respectively. It also displayed very low concentrations of total solids (TS) and volatile solids (VS)—1.98 g/L and 1.97 g/L, respectively. The slaughterhouse sludge provided a significantly higher tCOD of 106.16 g/L; however, the sCOD remained low at 12.57 g/L. The TS and VS concentrations for this substrate were higher, reaching 120.69 g/L and 94.64 g/L, respectively. These parameters were balanced by mixing the substrates in a 50:50 ratio of sludge and slaughterhouse wastewater, resulting in a mixture with total and soluble chemical oxygen demands of 55.59 g/L and 7.21 g/L, respectively. The total volatile fatty acids (TVFA) concentration was higher for slaughterhouse wastewater (159.02 AcH/L) than for sludge (8.59 AcH/L). After mixing, this parameter was balanced, resulting in a value of 86.52 AcH/L. Conversely, the inoculum followed a similar trend to slaughterhouse wastewater, showing a low sCOD concentration of 3.04 g/L, and low TS and VS concentrations of 8.06 g/L and 5.56 g/L, in turn. However, it exhibited a much higher TVFA concentration, more similar to the feed of 89.12 AcH/L. The carbon-to-nitrogen (C/N) ratio significantly influences the stability of the anaerobic digestion process. A C/N ratio between 30 and 40 is considered optimal for anaerobic bacterial growth in this process. Slaughterhouse sludge exhibits a C/N ratio exceeding this optimal range, whereas slaughterhouse wastewater falls within it. These values for both substrates justify the use of anaerobic co-digestion to achieve an ideal C/N ratio [16,20].

4.2. Operating Conditions

Two continuous stirred tank reactors (CSTR) were used for this experiment. The digesters had a total volume of three liters, with a working volume of two liters. The agitation of each reactor was set at 30 rpm. Each digester was equipped with a 5-liter Tedlar bag for daily biogas collection and a heating plate at its base for temperature control. The biogas composition was measured using a gas syringe. The first digester was operated in the mesophilic range (35 °C) and the second digester in the thermophilic range (55 °C). Each digester, made of stainless steel, was fitted with a glass lid containing ports for mechanical stirring, temperature control, feed inlet, effluent outlet, and chemical agents for pH control (Figure 9). The digesters were initially loaded with an 80:20 mixture of substrates and inoculum, a ratio considered optimal for process initiation [33]. Various HRTs were investigated, with each HRT maintained for a period equivalent to three times its duration. This parameter was progressively reduced once biomethane production had stabilized to determine the optimal HRT for maximizing VS and tCOD removal, as well as biomethane production and yield. The HRTs tested were 30, 25, 20, 18 and 15 days.

4.3. Analytical Methods

An initial characterization of the substrates, feed and inoculum was performed to determine pH, tCOD, sCOD, TS, VS and TVFA, and to monitor the reactor’s performance, the physicochemical parameters of the digestate, as well as volume and composition of biogas. The determination of TS, VS, tCOD, sCOD and C/N ratio followed the Standard Methods APHA-AWWA-WPFC [59]. pH was measured using a HACH sensION + pH. VFAs were quantified by gas chromatography, employing a Shimadzu GC-2010 gas chromatograph equipped with a flame ionization detector (FID) system and a Nukol capillary packed column [59]. Acetic, propionic, butyric, butyric, isobutyric, valeric, isovaleric, caproic and heptanoic acids were analyzed. The concentrations of these acids were expressed in mg/L, and the total VFA content was calculated as mgAcHequivalent/L. The daily biogas volume and composition were measured. Biogas volume was directly measured using a Ritter TG1 gas flow meter and KNF Laboport gas suction pump. Biogas composition was determined by gas chromatography using a Shimadzu GC-2010 gas chromatograph with a thermal conductivity detector (TCD) and a Supelco Carboxen 1010 Plot column. H2, CO2, CH4 and O2 were analyzed. The gas chromatography conditions were as follows: split, 100; constant pressure at the injection port, 70 kPa; initial temperature, 40 °C for 2 min, followed by a ramp of 40 °C/min to 200 °C, held for 1.5 min; detector temperature, 250 °C; injector temperature, 200 °C. Helium was used as the carrier gas (266.2 mL/min), synthetic air (120 mL/min) and hydrogen (80 mL/min) were used as flame gases, and helium (8 mL/min) was used as the auxiliary gas for compensation [10,33].

5. Conclusions

This study highlights a critical trade-off in the anaerobic co-digestion of slaughterhouse waste; while mesophilic anaerobic co-digestion demonstrated superior process stability, thermophilic digestion exhibited greater efficiency in terms of biomethane production (577 mL CH4/d vs. 477 mL CH4/d) and the removal efficiencies of total solids (67.73 g/L vs. 52.24 g/L) and volatile solids (65.84 g/L vs. 41.67 g/L). Future research should address the challenge of bridging this gap. Additionally, a deeper understanding of the microbial communities driving thermophilic digestion is needed to predict and mitigate process fluctuations. Consequently, it can be concluded that anaerobic digestion under mesophilic and thermophilic conditions displays a variable performance depending on the HRT employed. Optimizing hydraulic retention time (HRT) remains a key factor, and future studies should explore dynamic HRT strategies that adapt to influent variability. Furthermore, the scalability and economic viability of thermophilic co-digestion at industrial levels require thorough evaluation to ensure sustainable waste management practices.

Author Contributions

Conceptualization, M.C., L.B., J.F.-R., M.P. and R.S.; methodology, M.C. and L.B.; software, M.C. and L.B.; validation, M.C.; formal analysis, M.C.; investigation, M.C. and L.B.; resources, M.P. and R.S.; writing—original draft preparation, M.C.; writing—review and editing, J.F.-R., M.P. and R.S.; visualization, J.F.-R., M.P. and R.S.; supervision, J.F.-R., M.P. and R.S.; project administration, M.P. and R.S.; funding acquisition, M.P. and R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Innovation and the State Research Agency, and co-funded by the European Union. This work is part of project PID2021-123174OB-I00, funded by MCIN/AEI/10.13039/25890110 EU.

Data Availability Statement

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

Acknowledgments

The authors are grateful to the Ministerio de Ciencia e Innovación for the pre-doctoral contract associated with the project, PRE2022-104937.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ADAnaerobic Digestion
ACoDAnaerobic Co-Digestion
TPACoDTemperature-Phased Anaerobic Co-Digestion
WWTPWaste Water Treatment Plan
CTSRContinuous Stirred Tank Reactor
FIDFlame Ionization Detector
VFAVolatile Fatty Acids
TVFATotal Volatile Fatty Acids
LCFALong-chain Fatty Acids
SSlaughterhouse Sludge
SWSlaughterhouse Wastewater
HRTHydraulic Retention Time
tCODChemical Oxygen Demand Total
sCODChemical Oxygen Demand Soluble
TSTotal Solids
VSVolatile Solids

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Figure 1. Temporal pH evolution of the S-SW fed digester under mesophilic conditions.
Figure 1. Temporal pH evolution of the S-SW fed digester under mesophilic conditions.
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Figure 2. Measurements of tCOD and sCOD (a) and TS and VS (b) concentrations in a mesophilic system.
Figure 2. Measurements of tCOD and sCOD (a) and TS and VS (b) concentrations in a mesophilic system.
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Figure 3. Evolution of total acidity (mgAcH/L) and of the concentration of each individual acid in the different HRTs studied for the mesophilic system.
Figure 3. Evolution of total acidity (mgAcH/L) and of the concentration of each individual acid in the different HRTs studied for the mesophilic system.
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Figure 4. Evolution of biomethane production at each HRT in the mesophilic system.
Figure 4. Evolution of biomethane production at each HRT in the mesophilic system.
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Figure 5. Temporal pH evolution of the S-SW-fed digester under thermophilic conditions.
Figure 5. Temporal pH evolution of the S-SW-fed digester under thermophilic conditions.
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Figure 6. Measurements of tCOD and sCOD (a) and TS and VS (b) concentrations in a thermophilic system.
Figure 6. Measurements of tCOD and sCOD (a) and TS and VS (b) concentrations in a thermophilic system.
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Figure 7. Evolution of the total acidity (mgAcH/L) and of the concentration of each individual acid in the different HRTs studied for the thermophilic system.
Figure 7. Evolution of the total acidity (mgAcH/L) and of the concentration of each individual acid in the different HRTs studied for the thermophilic system.
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Figure 8. Evolution of biomethane production at each HRT in the thermophilic system.
Figure 8. Evolution of biomethane production at each HRT in the thermophilic system.
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Figure 9. Schematic of the continuous stirred tank reactor used in the experiment.
Figure 9. Schematic of the continuous stirred tank reactor used in the experiment.
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Table 1. Percentage of elimination of tCOD, ST and VS in mesophilic system.
Table 1. Percentage of elimination of tCOD, ST and VS in mesophilic system.
HRT (d)tCOD (%)TS (%)VS (%)
3032.72 ± 4.4318.03 ± 3.4031.89 ± 4.42
2537.23 ± 1.7233.84 ± 2.9632.30 ± 1.89
2039.21 ± 2.2743.75 ± 3.2937.00 ± 1.50
1556.28 ± 2.3252.24 ± 1.3341.67 ± 1.67
Table 2. Biomethane yields (mL CH4/gVSadded) for mesophilic system.
Table 2. Biomethane yields (mL CH4/gVSadded) for mesophilic system.
HRT (d)Biomethane Yields (mL CH4/gVSadded)
308.27 ± 0.04
258.46 ± 0.04
208.78 ± 0.4
159.54 ± 0.4
Table 3. Percentage of elimination of tCOD, TS and VS in the thermophic system.
Table 3. Percentage of elimination of tCOD, TS and VS in the thermophic system.
HRT (d)tCOD (%)TS (%)VS (%)
3038.33 ± 1.8846.32 ± 3.3541.45 ± 4.10
2544.03 ± 2.4847.36 ± 2.4746.73 ± 2.74
2048.68 ± 3.1455.26 ± 3.3152.89 ± 3.55
1543.04 ± 2.1467.73 ± 5.6565.84 ± 3.75
Table 4. Biomethane yields (mL CH4/gVSadded) for the thermophilic system.
Table 4. Biomethane yields (mL CH4/gVSadded) for the thermophilic system.
HRT (d)Biomethane Yields (mL CH4/gVSadded)
3011.61 ± 0.04
2513.24 ± 0.03
2016.50 ± 0.04
1523.02 ± 0.06
Table 5. The initial physicochemical characterization of substrates, feed, and inoculum used in the anaerobic co-digestion process.
Table 5. The initial physicochemical characterization of substrates, feed, and inoculum used in the anaerobic co-digestion process.
ParametersSlaughterhouse Sludge (S)Slaughterhouse Wastewater (SW)Feed Mixture (S-SW)Inoculum
pH7.21 ± 0.367.23 ± 0.367.60 ± 0.388.33 ± 0.42
tCOD (g/L)106.16 ± 0.024.77 ± 0.0455.59 ± 0.0319.08 ± 0.01
sCOD (g/L)12.57 ± 0.022.93 ± 0.067.21 ± 0.063.04 ± 0.02
TS (g/L)120.69 ± 7.621.98 ± 0.1156.52 ± 2.988.06 ± 0.18
VS (g/L)94.64 ± 5.841.97 ± 0.3844.51 ± 2.575.56 ± 0.24
TVFA (mgAcH/L)8.59 ± 1.23159.02 ± 22.7286.52 ± 12.3689.12 ± 12.73
C/N48.79 ± 0.0530.43 ± 0.07 5.06 ± 0.10
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Candel, M.; Ballesteros, L.; Fernandez-Rodriguez, J.; Perez, M.; Solera, R. Study of the Effect of Temperature to Optimize the Anaerobic Digestion of Slaughterhouse Sludge by Co-Digestion with Slaughterhouse Wastewater. Recycling 2025, 10, 47. https://doi.org/10.3390/recycling10020047

AMA Style

Candel M, Ballesteros L, Fernandez-Rodriguez J, Perez M, Solera R. Study of the Effect of Temperature to Optimize the Anaerobic Digestion of Slaughterhouse Sludge by Co-Digestion with Slaughterhouse Wastewater. Recycling. 2025; 10(2):47. https://doi.org/10.3390/recycling10020047

Chicago/Turabian Style

Candel, Maria, Laura Ballesteros, Juana Fernandez-Rodriguez, Montserrat Perez, and Rosario Solera. 2025. "Study of the Effect of Temperature to Optimize the Anaerobic Digestion of Slaughterhouse Sludge by Co-Digestion with Slaughterhouse Wastewater" Recycling 10, no. 2: 47. https://doi.org/10.3390/recycling10020047

APA Style

Candel, M., Ballesteros, L., Fernandez-Rodriguez, J., Perez, M., & Solera, R. (2025). Study of the Effect of Temperature to Optimize the Anaerobic Digestion of Slaughterhouse Sludge by Co-Digestion with Slaughterhouse Wastewater. Recycling, 10(2), 47. https://doi.org/10.3390/recycling10020047

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