Evaluation of Energetic Potential of Slaughterhouse Waste and Its Press Water Obtained by Pressure-Induced Separation via Anaerobic Digestion

Abstract

Anaerobic digestion has the potential to convert organic waste materials into valuable energy. At the same time, using press water from biomass materials for energy generation while taking advantage of the resulting cake for other purposes is an emerging approach. Therefore, this study aimed to investigate the residual potential expected from a typical biogas feedstock after it has been mechanically separated into liquid and solid phases. Hence, in this study, the rumen contents of ruminants (cow, goat, and sheep) and their proportionate ratios were obtained from an abattoir in Ghana. Resource characterization of the waste samples was carried out in the central laboratory of the HFR, Germany. Anaerobic batch tests for biogas (biomethane) yield determination were set up using the Hohenheim Biogas Yield Test (HBT). The inoculum used was obtained from an inoculum production unit at the Hohenheim University biogas laboratory. The trial involved two different forms of the sample: mixture of rumen contents, press water, and inoculum, each in four (4) replicates. The trial was carried out at a mesophilic temperature of 37 °C. Results obtained over a seventy (70) day period were transformed into biogas yields. Overall, the results show that the current contents are suitable for biogas generation as an option as opposed to the current form of disposal at a refuse dump. However, using these mixtures in their original forms is more technically viable than using press water without further treatment.

1. Introduction

Economic and social development depends largely on clean energy transition, thus influencing the drive for energy development goals. Essentially, there is the need for affordable and clean energy for all, especially Africans [1]. The African continent could hold the potential for the world to achieve net zero energy transition. In 2021, biofuel and waste accounted for 45.4% of the total energy supply in Africa. It is projected that by 2025, the world will generate 6 million tons of waste [2]. Zohaib Ur Rehman et al. (2023) cited livestock residues as one of four key feedstock for biogas generation [3]. Presently, 64% of Africans use agricultural and animal waste and wood for cooking, possibly resulting in deforestation [1]. Biogas generation is one option to ensure a sustainable energy supply and to offer an alternative for clean energy transition, especially through simple cooking applications and deriving energy from animal and agricultural waste [2,4,5,6]. Biogas, a renewable energy resource, is produced from the microbial breakdown of organic materials (biomass: manure, food waste, agricultural waste, waste water, green waste, sewage, municipal waste, etc.) under anaerobic conditions [4,7,8,9]. Bejor (2020) defined anaerobic as the absence of free oxygen [10]. This process involves relevant microorganisms of four separate generation processes: hydrolytic bacteria for hydrolysis, acidogenic bacteria for acidogenesis, acetogenic bacteria for acetogenesis, and methanogens for methanogenesis [11]. At the hydrolysis and acidogenic stages of biogas production, lipids, proteins, and carbohydrates are broken into complex long-chain fatty acids, glycerol, amino acids, and sugars. These are eventually converted into short-chain fatty acids, alcohols, hydrogen, carbon dioxide, and acetate. Through methanogenic actions, biogas is then produced from the acetates, carbon dioxide, and hydrogen [12,13], thus producing methane (CH₄:55–65%), carbon dioxide (CO₂: 35–40%), hydrogen sulfide (H₂S), moisture, and siloxanes in small amounts [14]. Biogas composition can generally summarized as 50–75% of CH₄, 25–45% of CO₂, and 2–7% of H₂ [3]. An equation for biogas generation was cited by Bejor (2020) as organic matter + Combined Oxygen +Anaerobic microbes→ CH₄ + CO₂ + Other end-products [10]. The energy content of biogas primarily depends on the fraction of methane [15]. The semi-solid or solid effluent produced is a source or useful raw material for biofertilizer applications [14]. While biogas, technically the CH₄ component, is useful for cooking, electricity generation, vehicle fueling, and bio-methanation, its production and use cycle is continuous, with no net carbon dioxide emissions being produced. That is, carbon absorbed during biomass growth offsets the carbon associated with its energy conversion if transport and processing emission are not taken into account [16]. Despite the positives of anaerobic digestion for biogas generation, there are still technological and microbiological considerations necessary to ensure economic feasibility [12].

Náthia-Neves (2018) classified biogas production processes as wet digestion, dry digestion and semi-dry digestion, based on the dry matter or total solid content of the initial substrate. A typical wet digestion should have an initial dry matter content that is below 10% and more than 20% for a dry digestion. Wet digestion has been used and suited for sewage sludge treatment and liquid waste with high moisture contents. Dry digestion, on the other hand, is suited for substrates with high dry matter contents [14]. A wet digestion, semi-dry, or dry digestion process could be transformed to another by transforming the substrate in order to affect the hydrolysis process. Hydrolysis in biogas production has two challenges: It is the most-time intensive of all four generation stages and has the limitation of speed in the use of substrates, which are in the forms of particles [17]. Intensifying the hydrolysis process therefore results in an increased performance [18]. Mechanical pre-treatment is one way of doing this (with biological, chemical, or a combination of any of the three as the others) [19]. Pressing out fluid from biomass, which is difficult to process by anaerobic digestion, e.g., due to its texture and high fiber content, is an emerging mechanical pre-treatment approach [20] and one way in which the hydrolysis stage can be modified.

Mechanical pre-treatment by pressing has been driven by several forces [17,21,22,23,24]. For instance, Corton et al. (2014) cited the use of mechanical pressing to obtain press water for biogas while densifying the resulting solid cake, which is fibrous in nature, for the purposes of solid fueling [20]. The approach was seen as an innovation to maximize the conversion of energy from low-input high-diversity biomass. Other research related to this integrated approach has been conducted by other authors [25,26]. Nayono et al. (2010) co-digested food waste with press water from organic municipal solid waste with the aim of improving the production of biogas [27]. The authors observed that adding more of either food waste or press water to biowaste co-digestion resulted in a stronger buffer medium. The biomethane potential obtained over a 11 day period was 500 mL/g_oTS [27]. Nayono et al. (2010) concluded that using press water from the organic part of municipal solid waste is a good resource for biogas generation [28]. In a mesophilic digestion at 37 °C, a BMP of 540 mL/g_oTS was obtained in a batch test for press water from the organic fraction of municipal solid waste. This feedstock was obtained from a composting plant, thus allowing the remaining cake to continue in the composting chain. This positive result is the basis for their conclusion. The focus of the study was to characterize and assess press water’s suitability for anaerobic digestion: to evaluate the potential of its energy recovery and for mitigating for problems with its handling [28].

Hensgen et al. (2014) used a screw press to obtain press waters from twelve (12) material types sourced from semi-natural habitats in Germany, Estonia, and Wales [25]. Biogas production determinations were carried out for press waters from the silages using VDI standards, at a mesophilic temperature of 37 °C. A fermentation time of 14 days was used in this trial. In Hensgen et al. (2014), the different sources of the materials used and their different compositions did not influence the biomethane potentials of their press waters significantly. Biomethane potential ranged from 312.1 to 405 mL/g_oTS. In the long run, the press cake ended up with a high fiber content [25].

Richter et al. (2011) studied biomass materials with the aim of using press water from the material (herbage from a low land hay meadow) to generate biogas while using the resulting cake for combustion [26]. The authors work was underscored by the challenges associated with the combustion of low-input high-diversity materials. Hydrothermal carbonization treatments were conducted for three (3) out of six (6) herbage biomass materials obtained on six different dates. Levels of elements that are detrimental to combustion were reduced in the press cake but were higher in the press waters [26]. The biomethane potential for the press waters without hydrothermal treatment ranged from 6.35 to 13.15%_TS. Between 0.09 and 0.23% of total solids (TS) contained in the silage were transferred into the press waters.

Sailer et al. (2022) highlighted dewatering biogenic wood fuels in the form of wood chips through mechanical pressing to reduce moisture content: Press waters were generated as by-products. As its oxygen demand is very high, biogas generation was examined as an option to utilize these waters [29]. They researched the anaerobic digestion potential of spruce-based and poplar-based press waters. Obtaining a biomethane potential (BMP) of 160 ± 12 mL/g_oTS for both inoculum and press water as compared to 95 ± 26 mL/g_oTS for only inoculum, the authors recommended that further research could explore the potential of press water as a co-substrate [29]. However, the challenge with press water is that its characteristics and utilization potentials are not truly known [29].

Like most biological wastes, managing slaughter waste presents a challenge in Africa, especially due to increasing urbanization and population growth. It is critical to transition the waste management landscape to a sustainable circular economy. There is potential in generating valuable energy from slaughter waste and additionally remedying huge costs and associated environmental challenges with disposal systems [3,22,23,24,30,31]. Wang et al. (2018) stressed that AD has been used as a viable technology for slaughterhouse waste to generate biogas (energy) and reduce negative environmental impacts [32]. This is evident in several studies highlighting biogas’ potential from slaughter waste: basic research, performance improvement, optimization, application, process techniques advancement, and a blend of any or all of these [30,32,33,34,35,36,37,38,39,40,41]. With a focus on anaerobic digestion, the energy potential of a cattle slaughterhouse was studied in Ireland. The study found a methane potential of 49.5 to 650.9 mL∙CH₄/g_oTS [39]. Omondi et al. (2023) co-digested water hyacinth with ruminant waste from a slaughterhouse. They observed that the slaughterhouse waste enhanced the production and yield of biogas from water hyacinth by 113% [42].

Salehin et al. (2021) studied the potential of biogas generation from slaughter wastes in Dhaka. They found that 7915 tons of slaughter waste is generated per year in the city, with a biogas potential of 2.15 million m³ [33]. Samadi et al. (2021) studied the potential of biogas generation from slaughter waste of poultry, co-digested with vegetables and fruits, in order to produce optimal conditions for a biogas generation. In this study, the highest biogas yield occurred for a C/N ratio of 30. The study stressed that generating biogas from slaughter waste (with co-digestion) is more advantageous than the use of depositing or burning as a disposal method [34]. Ware et al. (2016) obtained a BMP of 465–650 mL/g_oTS for offal of cattle, pig, and poultry from slaughterhouses, presenting an empirical pointer to the good biogas potential from slaughterhouse waste [43]. Aklaku et al. (2006) assessed the performance of a small-scale biogas digester for a slaughterhouse in Ghana [44]. This slaughterhouse produces waste from cattle, sheep, and goats. The study found that the digester is able to deliver energy in a form of biogas to replace wood as fuel while delivering by-products useful for fertilizing land [44].

These studies established that slaughter waste has a huge biogas potential for the energy landscape in Africa, with different studies targeting improvement in yield. The high moisture content of such feedstock (slaughterhouse waste) could therefore be subjected to mechanical pressing as a pre-treatment method that aims at an overall performance improvement of a biogas digestion system, including protecting digester–pumping and piping systems.

The current study aimed at assessing the potential of generating biogas from waste deposits generated at a typical abattoir (slaughterhouse) in Africa and evaluating the technical comparative performance of using absolute waste mixtures at slaughterhouses and pressing-out liquid (water with solved and dissolved organics). The main objective of the study was to assess the technical potential (biogas yield) of rumen waste mixtures from cow, sheep, and goat produced at the Sunyani Abattoir in Ghana. Additionally, the research sought to determine if it is technically prudent to press out water from these mixtures and use digester in the biogas generation process, keeping in mind that the remnant (cake) will be considered in practical scenarios for other conversion processes or uses, such as composting.

2. Materials and Methods

The substrate for anaerobic digestion (AD) trials was collected from the Sunyani Abattoir (Asuakwaa slaughterhouse) in Ghana, located on latitude 7.34376° and longitude −2.29457° [45]. The substrate was obtained as the key waste generated at the facility, as all types of offal are utilized, except partially digested rumen contents. They were obtained as cow rumen content in “pasty” form and a mixture of goat and sheep rumen in medium-liquid form. The current form of disposal is an irregular but mostly weekly collection cycle with storage in a waste dump. This not only entails the challenge of prolonged and exposed storage in a trough container along with associated costs and irregular pickups by the waste disposal company but also involves the release of emissions from methanogenic bacteria present in the waste.

After collection in Sunyani, the samples were weighed and dried at 105 °C until constant weight. They were further sealed and kept airtight for further transportation to Germany for analysis. The samples were kept in an oven at 105 °C for a day before the experimental trial started in Germany.

Figure 1 shows the current form of disposal in an open container, while the background shows the heaps of dried rumen content that are dumped there when the container limits are reached. Figure 2 shows the weighing of rumen content at the Sunyani abattoir.

A subsequent resource assessment was carried out to evaluate the specific waste generation and composition at the facility. This involved counting each cow, goat, and sheep slaughtered per day. This was repeated for three (3) days. For all three days, the masses of rumen content per cow, goat, and sheep slaughtered were measured, each in duplicate. This was used for estimating the average cow, goat, and sheep’s rumen content production. The quantities of each animal slaughtered and the resulting rumen masses were used to obtain a relationship between the mixtures produced by the slaughterhouse. The obtained ratios were subsequently used to mix the received samples.

Table 1. Summary of masses in kg of rumen content for different animals produced at Sunyani abattoir.

Animal Type	Cow	Sheep	Goat
Average quantity of each type of ruminant slaughtered
Per day	16	6	10
Per week	96	36	60
Average mass of rumen content per animal slaughtered (kg)
Per day	25	2.5	2.1
Total mass of rumen content disposed per animal
Per day	400	15	21
Per week	2400	90	125
Total	2615
Ratio of weekly waste	92%	3%	5%

summarizes the masses obtained for the rumen contents; the quantities of cows, goats, and sheep slaughtered at the facility; and the expected annual outputs.

The Hohenheim University Biogas Laboratory has a 400 L laboratory reactor designed and designated for standardized inoculum production. This facility cultivates bacteria continuously under controlled and favorable conditions. This reactor is supplied with rapeseed oil, soybean meal, shredded wheat, maize sludge, and digestate from different biogas plants in Baden-Württemberg, southwest Germany. The inoculum production plant had an organic loading rate (OLR) of 0.3 kg_oTS/m³d on a dry organic matter basis and a temperature of 37 °C. It had a hydraulic retention time (HRT) of 200 + 25 d. The inoculum was collected and transported in sealed and airtight containers over a period of one (1) hour. There was constant degassing of the inoculum while in transport. Its use was preceded with sieving using a mesh with a size of 0.5 mm, the same as in the case of Hülsemann et al. (2020) [46]. The characteristics of the inoculum were described by previous authors [29,46].

The analytical methods in this research were undertaken at the Central Laboratory of the Rottenburg University of Applied Forest Sciences. These methods followed the VDI 4630 [47] and associated standards.

In determining the total solids, approximately 50 g of each sample was placed into evaporating dishes and oven-dried at 105 °C for >12 h. The masses were checked after the drying period and further dried until consistent masses were achieved. The determination was carried out in four (4) replicates to ensure accuracy.

After transportation to Germany and repeated drying to ensure an anhydrous state, the oTS was determined according to ISO 21656 [48] and VDI guidelines [47]. The samples were milled with a sieve size of 1 mm, and uniform particle size was achieved according to ISO 3310-1 [47,49]. Empty crucibles were weighed, and 1 g of each sample was measured into the empty crucibles. The masses of the empty and filled crucibles were determined and recorded. They were kept in a muffle furnace set to operate with defined heating ramps (defined by standard DIN EN ISO 18122:2016-03) [50,51] up to a temperature of 550 °C for about 24 h. For each sample, measurements were taken in four (4) replicates. The crucibles were weighed after cooling to room temperature in a desiccator. The ash contents were determined and the oTS and calculated as the difference between the pre-ashing and post-ashing masses. The ash contents were expressed as a percentage of the total mass of dried samples used. These processes were repeated for the inoculum obtained from HHU biogas laboratory.

Next, 5 g of TS of each sample (cow solid, cow liquid, goat–sheep mixture, and press cake) was measured and milled using a 0.25 mm mesh. Approximately 80 mg of each sample was measured for elemental determination in an elemental analyzer (LECO CHN 828, LECO Instrumente GmbH, Mönchengladbach, Germany). The CHN proportions of these samples were determined according to VDI 4630 [47]. The oxygen fractions were determined as the difference between the total composition of all elements (100%) and the sum of the measured CHN organic fractions as well as the ash content, neglecting the fraction of sulfur.

Inductively coupled plasma–optical emission spectroscopy (ICP-OES) was used to evaluate the study samples’ concentration of trace and minor elements. The Aqua Regia treatment in accordance with ISO 11885 [52] and DIN 22022-2 [53] was followed in this procedure [29]. Each sample was analyzed with n = 4 or n = 3 repetitions, except for the press cake, which was analyzed with n = 2 due to the limitation of sample quantity. In the case of press water, the sample was diluted in a ratio of 1:10 using double-distilled water. Then, 1 mL of nitric acid (HNO₃) was added to acidify each sample. A Spectro Blue, ASX-260 auto sampler (SPECTRO Analytical Instruments GmbH, Kleve, Germany) was used to measure the trace element concentration of these samples.

Sample preparation for calorimetry was carried out in accordance with ISO 14780 [54,55]. About 1 g of each dry sample was pressed to a tablet size. Those tablets were burned in a bomb calorimeter (with IKA C6000 ISOPERIBOL) (IKA-Werke GmbH & Co. KG, Staufen, Germany) to measure the higher heating values (HHVs) of the samples in accordance with DIN EN ISO 18125 [50,56]. The setup had a bath, ensuring a fixed heat transfer between the bomb and the water. Double-distilled water was used to collect the liquid resulting from the calorimetry and to rinse the bomb. Chloride and sulfate anions were determined using ion chromatography (with Metrohm 883 Basic IC plus) (Metrohm Deutschland GmbH & Co. KG, Filderstadt, Germany).

The Hohenheim Biogas Yield Test (D-HBT) was used for the biomethane potential trials. This setup consisted of sixteen (16) 100 mL syringes. In the D-HBT setup, gas volume measurements were carried out manually. Due to the absence of a rotating drum, the substrates were regularly stirred by turning syringes in clockwise and anti-clockwise directions. The tests were performed according to the guidelines and procedures specified by VDI 4630 [47].

The inoculum obtained from the Hohenheim University Biogas Laboratory was used together with two different forms of waste portions prepared from effluent/feedstock obtained from the Sunyani Abattoir. The resulting forms were a dried mixture of cow rumen content, goat rumen content, and sheep rumen content (CGS). The mixture was based on a scientifically determined ratio, and the press water (PW) was obtained from a solution of the aforementioned mixture.

RC-CGS-M (mixture of rumen contents of cow, goat, and sheep from the Sunyani Abattoir) on a dry basis was formed using the deduced production ratio, applied on a dry basis to two different dried samples: rumen content of cow—liquid form (RCCL) and rumen content of goat–sheep mixture—liquid form.

Table 2. Description of variants of biomass feedstock considered in this study.

Variant	Sample ID	Description
1	RCCL	Rumen content of cow—liquid form
2	RC-GSL	Rumen content of goat–sheep mixture—liquid form
3	RCGL	Rumen content of goat—liquid form
4	RCSL	Rumen content of sheep—liquid form
Variant I	RC-CGS-M	A dried mixture of cow rumen content and goat–sheep rumen content
Variant II	PW-CGS-M	The liquid extract obtained from Variant I after mechanical pressing
Variant III	PC-CGS-M	The solid residue obtained from Variant I after mechanical pressing

describes different samples and their identities, including the actual obtained slaughter waste materials on the ground and the variants obtained from combinations of these variants used in the experimental study.

The wet equivalences of dry masses of RCCL and RC-GSL (RCGL + RCSL) available for the experimental trial were measured and mixed in the established ratio. The mixing was effectively performed with the use of a magnetic stirrer.

The preparation of PW-CGS involved two key intermediate steps: step I and step II. The activities of step I were repeated, except for relevant masses, which were used on a dry matter basis. The equivalent relevant water was calculated and added to the mixture. With the help of a magnetic stirrer, it was stirred to mimic the initial forms. The rehumidified samples were then stored overnight. Step II involved the design of a mechanical press to squeeze out the liquid available in the substrate. An appropriate mechanical press was designed and fabricated at the HFR’s mechanical laboratory. An appropriate weight was derived from an easily commercially available press. Thus, a pressure of 24.1 kg/cm² should act on the press surface, reinforced with a fine sieve, of 52.81 cm², induced by a pressing weight of 200 kg. Figure 3 shows the design of the mechanical press on the right, while the left side, the fabricated mechanical press can be seen.

After preparing the wet mixture, sample RC-CGS-M was pressed. The pressed-out water (liquid) was collected, weighed, and labelled as press water (PW-CGS-M).

After pressing in step II, the remnant cake was weighed and labelled as press cake (PC-CGS-M). The structure and set-up of the HBT is available in the literature [29,47]. The experimental procedure was carried out according to standards in VDI 4630 [2,3]. Figure 4 is a picture of the HBT digester used used. All determinations were carried out in four replicates. The four (4) replications for each substrate were in accordance with VDI 4630 [2], ensuring measuring accuracy. Each digester contained 30 mL of inoculum and X g of TS of sample in a manner ensuring that it achieved an oTS ratio of less than 0.5. For PW-CGS-M, X g of 2.21 g FM was used, and for RC-CGS-M, X g of 0.49 g TS was used. One setup, however, contained only inoculum to serve as control. For all three setups (inoculum, RC-CGS-M, and PW-CGS-M), their starting volumes were manually read and recorded. The setup was left for a 70-day period at a mesophilic temperature of 37 ± 0.5 °C. The volume levels of each substrate and/or inoculum were read over the period. The differences in volume between two measurements was estimated as the gas generation in mL over that period (under the specified operating conditions). The experimental systems were degassed in turns at appropriate times (when volumes of content of syringes were reasonably high, to prevent uncontrolled leakage). In the initial stages of the trial (about 8 days), readings were taken on average three (3) to four (4) times a day. Measurements were restricted to once a day in the last weeks of the trial. Considering the 0.5% criteria (with an increase in gas production of less than 0.5% per day for three days), the digestion process was terminated.

The biogas and specific methane yields were evaluated for both biogas yield and the estimated methane fractions (determined according to VDI 4630). Using first principle, the biogas (methane) potentials of the different variants were determined from the mass of waste generation and the biomethane potentials determined in this study [57]. The total waste generation per week was first transformed to the effective mass usable in biogas generation for each variant, that is, the mass of PW-CGS-M obtained after pressing and that obtained without pressing for RC-CGS-M variant.

Table 3. Fraction of waste generation extractible for biogas generation using different variants.

Variant	Fraction of Waste Extractible for Biogas Generation (%)
RC-CGS-M	100
PW-CGS-M	53

RC-CGS-M: a dried mixture of cow rumen content and goat–sheep rumen content; PW-CGS-M: the liquid extract obtained from RC-CGS-M after mechanical pressing.

presents the factors used to transform the waste generation for each variant. The potentials in terms of cubic meters of methane were converted to kWh of energy using Equations (1) and (2).

Theoretical models were adopted to determine the theoretical maximum biomethane potentials (TMBMP) of the studied samples in order to validate the obtained results. Boyle’s model, dependent on elemental composition, was used in the case of RC-CGS-M, and Boyle’s model as dependent on chemical oxygen demand (COD) measurement was used in the case of PW-CGS-M [58]. The equations used in the estimation are available in Rodrigues et al. (2016) [58].

A dilute solution of the press water was obtained by applying a ratio of 1:30 using distilled water. First, 2 mL of the dilute solution was added into a test tube containing Test 0–29 Nanocolor CSB 1500 (COD/DCO/DQ0) (ISO 15705) [59]. The mixture was shaken thoroughly and placed in a Nanocolor Vario C2 device to heat for two (2) hours. Afterwards, the COD was measured photometrically. The measured COD is presented in Table 4.

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{c}\mathrm{u}\mathrm{b}\mathrm{i}\mathrm{c} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{s} \mathrm{t}\mathrm{o} \mathrm{M}\mathrm{J}$$

(1)

$X\to$ 1 m³ of methane = 34 MJ of energy [60].

Conversion factor for MJ to kWh

(2)

$Y\to$ 1 MJ = 0.2778 kWh [61].

Table 4. AC, TS, oTS, and CHNO fractions; S, Cl, and HHV of raw and experimental samples.

Sample	TS [%]	oTS [% TS]	C [% TS]	COD (mg/L)	H [% TS]	N [% TS]	Cl [%]	S [%]	O [% TS]	AC [% TS]	C:N Ratio	HHV [kJ/kg]
Inoculum	4.06 ± 0.00	62.24 ± 0.00	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d
PW-CGS-M	1.58 ± 0.05	63.23 ± 0.02	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d
RCCL	10.85 ± 0.28	79 ± 0.01	41.88 ± 0.40	n.d	5.25 ± 0.16	2.55 ± 0.05	0.21 ± 003	0.13 ± 0.01	29.40	20.79 ± 0.01	19.16	17.48
RC-GSL	16.7 ± 0.09	82 ± 0.01	41.30 ± 0.59	n.d	5.46 ± 0.07	2.43 ± 0.05	0.22 ± 0.01	0.15 ± 0.02	32.94	17.72 ± 0.00	19.83	17.14
RC-CGS-M	11.34 ± 0.19	81 ± 0.00	40.65 ± 0.93	n.d	5.37 ± 0.18	2.45 ± 0.11	0.52 ± 0.02	0.18 ± 0.02	32.13	19.23 ± 0.00	19.40	17.46
PC-CGS-M	26.88 ± 0.34	82.25 ± 0.00	41.43 ± 0.20	26,380	5.49 ± 0.01	2.38 ± 0.02	0.19 ± 0.01	0.16 ± 0.02	32.79	17.75 ± 00	20.31	17.40

n.d: not determined; TS: total solid; oTS: organic total solids; C: carbon; COD: chemical oxygen demand; H: hydrogen; N: nitrogen; Cl: chlorine; S: sulfur; O: oxygen; AC: ash content; C:N: carbon–nitrogen; HHV: higher heating value. PW-CGS-M: the liquid extract obtained from variant RC-CGS-M after mechanical pressing; RCCL: rumen content of cow—liquid form; RC-GSL: rumen content of goat–sheep mixture—liquid form; RC-CGS-M: a dried mixture of cow rumen content and goat–sheep rumen content; PC-CGS-M: the solid residue obtained from RC-CGS-M after mechanical pressing.

Table 4. AC, TS, oTS, and CHNO fractions; S, Cl, and HHV of raw and experimental samples.

Sample	TS [%]	oTS [% TS]	C [% TS]	COD (mg/L)	H [% TS]	N [% TS]	Cl [%]	S [%]	O [% TS]	AC [% TS]	C:N Ratio	HHV [kJ/kg]
Inoculum	4.06 ± 0.00	62.24 ± 0.00	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d
PW-CGS-M	1.58 ± 0.05	63.23 ± 0.02	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d	n.d
RCCL	10.85 ± 0.28	79 ± 0.01	41.88 ± 0.40	n.d	5.25 ± 0.16	2.55 ± 0.05	0.21 ± 003	0.13 ± 0.01	29.40	20.79 ± 0.01	19.16	17.48
RC-GSL	16.7 ± 0.09	82 ± 0.01	41.30 ± 0.59	n.d	5.46 ± 0.07	2.43 ± 0.05	0.22 ± 0.01	0.15 ± 0.02	32.94	17.72 ± 0.00	19.83	17.14
RC-CGS-M	11.34 ± 0.19	81 ± 0.00	40.65 ± 0.93	n.d	5.37 ± 0.18	2.45 ± 0.11	0.52 ± 0.02	0.18 ± 0.02	32.13	19.23 ± 0.00	19.40	17.46
PC-CGS-M	26.88 ± 0.34	82.25 ± 0.00	41.43 ± 0.20	26,380	5.49 ± 0.01	2.38 ± 0.02	0.19 ± 0.01	0.16 ± 0.02	32.79	17.75 ± 00	20.31	17.40

n.d: not determined; TS: total solid; oTS: organic total solids; C: carbon; COD: chemical oxygen demand; H: hydrogen; N: nitrogen; Cl: chlorine; S: sulfur; O: oxygen; AC: ash content; C:N: carbon–nitrogen; HHV: higher heating value. PW-CGS-M: the liquid extract obtained from variant RC-CGS-M after mechanical pressing; RCCL: rumen content of cow—liquid form; RC-GSL: rumen content of goat–sheep mixture—liquid form; RC-CGS-M: a dried mixture of cow rumen content and goat–sheep rumen content; PC-CGS-M: the solid residue obtained from RC-CGS-M after mechanical pressing.

3. Results

3.1. Sample Characterisation

3.1.1. Waste Resource Assessment

The quantities (numbers and masses) of each of cow, goat, and sheep slaughtered at the Sunyani abattoir are presented in Table 1. The table captures the quantities and their averages per ruminant type, per day, and per week. Cow slaughtering is higher both per day and per week, while goat slaughtering is the second highest, followed by sheep. The figures for sheep are about one-third of those for cattle, while that of sheep are secondary to those for goat.

It summarizes the average mass of rumen content per each type of ruminant and the average masses of rumen contents per day corresponding to the different types of ruminants. The assessment indicates that an aggregate ruminant waste generated at the Sunyani abattoir will overwhelmingly be characterized by rumen content of cow waste. It Table 1 projects this ratio of waste generation by type of ruminant. The Sunyani abattoir, however, has a waste generation potential of 2615 kg per week. This presents a weekly potential of 2615 kg for biogas generation from rumen waste.

3.1.2. AC, TS, oTS, and CHNO Fractions; S, Cl, and HHV of Raw and Experimental Samples

The mean values ± standard deviations of TS and oTS of the test samples and their originally existing forms are presented in Table 4. The TS of RCCL is one-half of what Wijaya et al. (2020) obtained for cattle manure, chicken manure, rice straw, and hornwort in mesophilic mono-digestion; that of RCCS’L is about one-quarter of that obtained by Wijaya et al. (2020). The TS of PC-CGS-M is higher than that obtained by Wijaya et al. (2020) [62]. this confirms the general expectation that TS or, in return, the liquid, will be drawn out by the pressing process. The TS of PW-CGS-M is very low compared to that of the already-sieved inoculum used, indicating that the net generation of biogas from PW-CGS-M will be lower than that of the inoculum. It falls in the range (0.39 ± 0.01–3.17 ± 0.01%) obtained by Sailer et al. (2022) [29] for three different press waters (spruce-based and poplar-based). The TSs for RCCL, RC-GSL, and RC-CGS-M are comparable to the 13.1 ± 0.2% obtained for dairy manure by Achi et al. (2022) [63]. The TS of inoculum is higher than the 2.95 ± 0.01% obtained for inoculum by Achi et al. (2022) [63]. However, the sources are geographically different, and farm practices may differ from one area to the other. This will be a comparatively negative inference for the test samples if fermentation results are similar to that obtained by Achi et al. (2022) [63]. Hülsemann et al. (2020) reported similar TS and oTS of inoculum that was obtained from the same inoculum production source [46].

The oTS contents of RCCL, RCCGS’L, PC-CGS-M, and RC-CGS-M are similar to that obtained for cow dung by Marañón et al. (2011) [64] and comparable to the 87.3 ± 0.6% TS obtained for dairy manure by Achi et al. (2022) [63], although a little lower. This is positive because the different fractions of cow waste do not affect the organic fractions of the test materials. Another positive observation is that the press cake (PC-CGS-M) showed a similar oTS to that of the raw mixtures and the composite non-treated test material (RC-CGS-M). As expected, the oTS of the PW-CGS-M is lower than the literature values for rumen contents, which was confirmed by this study. The oTS measured, unlike the TS, deviates from the 86% to 89% obtained by Sailer et al. (2022) for three different press waters (spruce-based and poplar-based) [29]. As only the mass of oTS is convertible to biogas in anaerobic digestion, biogas generation per unit from the PW-CGS-G is estimated to be significantly lower than the per unit generation of biogas in the case of Sailer et al. (2022) [29]. The oTS of PW-CGS-M is similar to that of the inoculum. Under similar conditions, a similar generation pattern should be expected. The oTS of inoculum used is equally lower than the 73.6 ± 7.0% obtained by Achi et al. (2022) for dairy manure [63]. Different preparation methods, components, and conditions can explain this. With a lower oTS of inoculum, a good fermentation test result will indicate higher practical potential of the test materials in biogas generation. Generally, the TS and oTS characteristics of the inoculum conformed to the standard [29,47,65].

The C/N ratio is an important factor that affects the production of biogas. A C/N ratio of 25 to 30 has been cited as the optimal for biogas generation [63,66]. Table 4 summarizes the C/N ratios used in this study. For RCCL (16.45), RC-GSL (17.01), and PC-CGS-M (17.44), the C/N ratios do not fall within these categories but are comparable to the 15.2 obtained for dairy manure in one study [63]. A lower C/N ratio signifies that there is a possibility of excess nitrogen in forms such as NH₃, thereby inhibiting the biogas-production bacteria. Co-digesting the feedstock with more fibrous materials such as cassava waste could enhance the C/N ratio by increasing the carbon content [15,42,63].

3.1.3. Elemental Analysis, IC, and AC

The elemental composition (EA) of RCCL, RC-GSL, RC-CGS-M and PC-CGS-M is presented in Table 4. The EA of PW-CGS-M was not experimentally determined in this study due to limited quantity of these materials. Biomass materials’ suitability for energy generation and as a substitute for fossil fuel largely depends on CHNOS [67]. The obtained carbon values (41.43–41.88%) are comparable to those (39.98–43.08%) reported by Fajobi et al. (2022) for cow dung and selected lignocellulosic materials [67]. For hydrogen, the 5.52–5.49% measured in this study is lower than the 6.74–9.86% reported by Fajobi et al. (2022) [67]. Nitrogen and sulfur are known to release harmful and toxic gases. However, they are not relevant to biogas production itself. High levels of sulfur produce high levels of H₂S, which reduces the overall energy content of the biogas and is also harmful to piping and subsequent consumers [68]. Table 3 presents the results obtained for sulfur and nitrogen contents. The values obtained (0.13–0.18%) for sulfur for the studied samples are lower than that obtained (0.46%) by Fajobi et al. (2022) and are within acceptable levels [67]. The values obtained for nitrogen for the studied samples (2.38–2.55%) are higher than what (1.145–1.858%) Singh et al. (2017) obtained [69]. This signals that biogas production from studied feedstock (rumen content of cow goat and sheep mixture and press cake of this feedstock) will be associated with non-negligible levels of toxic substance (NH₄-N) release from nitrogen [69]. The measured oxygen contents, i.e., 50.30–50.71%, are comparable to the 46.69–51.82% reported by Fajobi et al. (2022). For carbon, hydrogen, nitrogen, sulfur, and oxygen, the measured values for all studied samples are similar, validating them for use as energy materials, except for PW-CGS-M, which could not be ascertained and cannot be directly related to any of the related studies [67,69].

The cake had a relatively high ash content, as should be expected, due to having the highest total solid, while oTS was not significantly higher than that of the other samples. This figure (4.77%) is about twice that obtained for raw cow dung by Fajobi et al. (2022) [67]. Due to fouling, the ash content primarily reduces the fuel quality of the cake, which is not the main purpose of the cake. The ash content of RC-CGS-M (assumed from the determined values of RCCL) is about half that of PC-CGS-M. The fouling effect will be lower and produce a better fuel quality in comparison to that of PC-CGS-M. This figure also is not very different from that obtained by Fajobi et al. (2022) [67].

3.1.4. HHV

The calorific values presented in this work are high heating values (HHV), which were experimentally determined. Table 4 contains these values. The figures measured (17.1–17.5 MJ/kg) are higher than the 14.7 MJ/kg obtained for cow dung by Fajobi et al. (2022) [67]. This may be because rumen contents might have lost methane (energy carrier) in the event of digestion before being released as fecal waste (cow dung), which is the case in the latter. It also implies that the use of rumen content for biogas generation will be more valuable per TS amount than for animal feces. The calorific values for all the studied samples are within reasonable ranges.

Figure 5 is a Van Krevelen diagram that shows the atomic ratios of O/C plotted against H/C for the studied samples. The diagram shows similar ratios of O/C plots against H/C, as evident in all the data points nucleating around the same point, except in the case of RCCCL, which falls to the extreme lower part of the plot. While the diagram confirms that all that all study samples have similar calorific values, the RCCL, unlike the others exhibit lower biomass properties (in terms of energy content) than the others. Thus, lower proportion of RCCL in slaughterhouse waste is likely to provide better heating values than a higher proportion of it. Heating value (fuel efficacy) is a factor of the atomic ratios of H/C and O/C. A low H/C ratio will correspond to a high-energy content and vice versa. With a high O/C ratio, there is a high energy content due to a high energy density [69]. Singh et al. (2017) explained it as resulting from the possession of more chemical energy in C-C bonds as opposed to C-O bonds [69].

3.1.5. TE Concentrations

Table 5. Concentration of trace elements of studied samples compared with inhibitory and optimum values.

TE	RCCL (mg/kg TS)	PW-CGS-M (mg/kg TS)	PC-CGS-M (mg/kg TS)	RC-CGS-M (mg/kg TS)	Inhibitory Levels (mg/L)	Optimum Ranges (mg/kg TS)
Fe	1879.82 ± 245.20	38.07 ± 5.74	884.48 ± 88.57	3188.03 ± 943.11	-	1500–3000
Mn	543.37 ± 13.78	5.50 ± 0.90	226.45 ± 20.51	449.94 ± 8.55	-	100–1500
Zn	66.204 ± 1.52	3.474 ± 2.44	42.775 ± 2.05	64.995 ± 1.62	150	30–150
Na	6223.86 ± 14.10	1235.02 ± 41.56	3077.15 ± 379.84	6218.10 ± 31.41	5000–15,000	-
K	6620.51 ± 491.65	916.06 ± 109.43	2347.70 ± 246.53	8259.85 ± 598.75	4800 (I (50))	-
Ca	8978.04 ± 291.32	125.18 ± 14.19	4054.04 ± 350.41	8952.164 ± 405.40	4800 (I (50))	-
Mg	1233.53 ± 5.82	52.46 ± 7.67	779.83 ± 82.64	1237.37 ± 5.28	1900 (I (50))	-
S	2020.49 ± 101.23	84.59 ± 9.98	851.01 ± 94.85	1993.68 ± 42.08	30 (H2S)	-

presents trace element concentrations of test samples and the optimum ranges and inhibitory levels defined by the literature [70]. Trace elements are significant to the potential of anaerobic digestion [29,71]. On one hand, they have the potential to improve anaerobic digestion, while on another hand, they have the possibility to offer toxicity. The former is possible due to the ability to increase energy yields. The latter depends on their concentrations in the digestion system. Fe, Co, Mn, Mo, W, Ni, and Zn are very important trace elements for anaerobic digestion [29,71]. Demirel and Scherer (2010) defined optimum ranges for the concentrations of trace elements that increase energy yields and inhibition levels for trace element concentrations that could offer toxicity [70]. A direct comparison can be made between the measured values and the defined values for optimum ranges. For inhibitory values, the study of Lima and Victor (2022) was first to make the assumption that if a TS of 20–27% has a density of 1009 to 1030 kg/m³, then an average TS of 11% in the case of the studied samples has a density ≈ 1000 kg/m³; therefore, the studied materials have a density of 1000 kg/m³ [72]. As a result, the concentrations of mg/kg determined that the defined levels of mg/L are equivalent.

For the studied samples, only Fe, Mn, and Zn, within the list of studied trace elements, have optimum ranges as defined by Demirel and Scherer (2010), whereas Zn, Na, K, Ca, Mg, and S have inhibitory levels also defined by [70]. The relevant levels of Fe found in the RCCL and RC-CGS-M are an indication that the ordinary rumen mixture should be preferred primarily for anaerobic digestion if non-biogas output significances are ignored. For both RCCL and RC-CGS-M, the measured levels conform to the optimum ranges defined in the literature, while those of PW-CGS-M and PC-CGS-M are below the defined lower limit. PW-CGS-M has a marginal value amongst all the materials. This certainly amounts to the lower (very low) biogas levels found in the PC-CGS-M and (PW-CGS-M). In the case of Mn, RCCL, PC-CGS-M, and RC-CGS-M have concentrations within the optimum levels. While the PW-CGS-M had the lowest and outside-of-range value, RC-CGS-M had the highest. This is in line with the observation of a comparatively high biogas potential. For all studied samples, only PW-CGS-M had Zn concentrations outside the optimum range. Again, RC-CGS-M had the highest concentration, affirming the highest biogas output in comparison with all other variants.

RCCL has a very high S content compared to the acceptable inhibition level. Although PW-CGS-M has a higher concentration in reference to the defined inhibition values, and the concentration is more reasonable than that of RCCL. As the trend has been for PW-CGS-M, the measured values for Zn, Na, K, Ca, and Mg are all outside the defined inhibitory levels. This is a positive confirmation for the use of press waters in AD, with limited process challenges, particularly inhibition, in this case. In the same line, PC-CGS-M is a better-placed variant in terms of inhibition potential. Zn, Na, K, Ca, and Mg are all outside the inhibitory range, although they are less attractive than the figures for PW-CGS-M. For both RCCL and RC-CGS-M, which were designed to not differ from each other due to composition and physicochemical properties, Zn and Mg have concentrations outside the inhibition ranges, despite being less attractive that the PW-CGS-M and the RC-CGS-M. In the same vein, both RC-CGS-M and RCCL have concentrations of Na, K, and Ca within the inhibitory levels. Thus, inhibition resulting from these TEs will be of concern in their raw use for AD. Additionally, all studied variants have S concentrations higher than the minimum for inhibition to occur. RC-CGS-M and PC-CGS-M, however, have very high concentrations of S, with RC-CGS-M recording the highest for all studied substrates. The S levels will also have implications for H₂S generation, whose impact was already discussed in this paper (Section 3.1.3).

The levels of Ni, Cu, Cr, Co, Pb, and Cd, which are relevant and concerning for anaerobic digestion, were within ranges not defined by the method used in the trace elements evaluations in this study. A future study with other estimation experimental methods may be necessary, for instance, for metals like Ni, which has been defined by the literature as very critical to methane-forming bacteria [70]. Other authors have cited the relevance of Mo and Se, which were not studied under the Aqua Regia treatment [29].

3.2. Biomethane Potential

Figure 6 presents the biogas yield of RC-CGS-M in mL/g_oTS. RC-CGS-M proves to be a potential biogas and renewable energy resource with a biogas potential of 292 ± 28.78 mL/g_oTS. This is lower than the 291 mL of CH₄/g_oTS obtained for cattle slurry by Thomas et al. 2018) [73] and higher than the 281 mL of biogas/g_oTS obtained by Wijaya et al. (2020) for mono-digestion of cattle manure, also for mesophilic conditions [62]. In the case of Thomas et al. (2018) [73], the higher results can be explained by the enhanced digestion resulting from continuous stirring and the use of a feeding pump and monitoring and control systems. Figure 6 shows a very fast generation from day 1 up to day 30 of anaerobic digestion, which is comparable to Wijaya et al. (2020) [62], whose results showed consistent generation until day 35 of anaerobic digestion. For the RC-CGS-M, the corresponding biogas potential is about 93% of the entire generation for the 30 days of digestion time. There is no evidence from the trial that the practical co-digestion of rumen contents of the cow and goat–sheep mixture has an advantage over single-use digestion, as shown in the literature [74,75,76]. As it presents a need to implement a comparison between the anaerobic co-digestion in this study and the anaerobic digestion of the individual samples, the key conclusion is that the resource available for use at the Sunyani abattoir occurs ordinarily in the mixture form used in this trial. At a mesophilic temperature of 37 °C, the anaerobic system planned to implement this result for the climate in Ghana, which is fundamentally at 27 °C daily (22 °C–32 °C) [77], is likely to produce a smaller biogas output or achieve the same biogas yield with a longer digestion time. This presents another need for research that will seek to evaluate the practicality of such a system with the climate in Ghana, without the use of temperature enhancers. At present, a cue can be taken from Chae et al. (2008), who obtained a 17.4% reverse-biomethane potential difference for an operating temperature of 30 °C and 25 °C [78]. Figure 7 presents the biogas yields in mL/g_FM of both RC-CGS-M (rumen content of cow and goat–sheep mixture) and PW-CGS-M (press water of cow and goat–sheep mixture). In both cases, the net production over the production from the inoculum is reported. RC-CGS-M shows a higher potential than PW-CGS-M, with about 70% more yield. This translates into yields of 26.70 ± 28.78 mL/g_FM and 7.47 ± 1.12∙mL/g_FM for RC-CGM and PW-CGS-M, respectively. RC-CGS-M proves the more efficient variant, as was generally expected. As cited by Sailer et al. (2022) [29], the PW-CGS-M only contains a very small amount of fiber, thus reducing the surface area.

The biogas potential obtained for PW-CGS-M is about 28% of the total potential obtained for RC-CGS- M on an FM basis. The rate of generation in this case was steady until about day 35, where it was relatively not significant. This is more closely similar to the observation for RC-CGS-M. This could be explained as having resulted from a lower TS: about 1.5 times lower than that of RC-CGS-M. This corroborates the findings of Sailer et al. (2022) highlighting that press waters have low biogas potential and, as such, are a low-potential renewable energy resource [29], suggesting low TS as one likely factor and pressing efficiency issues. It will therefore be prudent to look at the possibilities of improving both pressing efficiencies and TS, as suggested by Sailer et al. (2022) [29]. Additionally, Sailer et al. (2022) suggested that press waters provide a good alternative for co-digestion or as a supplement to AD [29]. In the event where press waters result as a by-product, then the potential biogas production may be worth it without any modifications. Nuchtang and Phalakornkule (2012), Morsink-Georgali (2022), and Smith et al. (2014) suggested the use of resulting cakes for composting purposes in implementing circular economy systems [79,80,81]. Nonetheless, the oTS value obtained for the press water signals a valuable quantity of organic materials that are available for biogas generation while offering inorganic materials that aid in anaerobic digestion [29].

Abubakr and Ismail (2012) and Nielsen and Angelidaki (2008) suggested that animal materials such as cow dung (in this case, the rumen contents) are highly lignocellulosic and thereby do not present optimal anaerobic processes [82]. With this limitation in mind, the biogas potentials evident from this research are significantly positive. Notwithstanding, pre-treatment methods such as hydrolysis may offer better potential for lignocellulosic materials [83]. Table 5 summarizes the oTS ratios used in the various fermentation setups. All of the ratios in the instance of the digestion trial are within the reasonable limits (below 0.5) suggested by Sailer et al. (2022) and VDI 4630 [29,47]. This indicates that there was stable and efficient anaerobic digestion [29,47].

This study did not practically assess or evaluate the methane fractions of the biogas generated. However, the BMP of the two samples was determined by adopting the method of Kasinath et al. (2021), who summarized the average methane fraction of biogas obtained by different studies as 55–75% for cattle manure [83]. The lower limit, 55%, was adapted for evaluating our study sample for safe approximation and conservativeness [82]. The oTS ratios of the substrates used is reported in

Table 6. oTS ratios of substrates used in the AD (oTS of experimental substrate/oTS of inoculum).

Trial Sample	oTS Ratio
RC-CGS-M	0.02
PW-CGS-M	0.07

RC-CGS-M: a dried mixture of cow rumen content and goat–sheep rumen content; PW-CGS-M: the liquid extract obtained from Variant I after mechanical pressing.

while

Table 7. BMP of RC-CGS-M and PW-CGS-M.

Substrate	BMP (mL/goTS)	BMP (mL/gTS)	BMP (mL/gFM)	SD	TMBMP (mL/goTS)
RC-CGS-M	160.4	129.9	14.7	28.78	495.93
PW-CGS-M	-	-	4.11	23	20.39

BMP: biomethane potential determined experimentally; SD: standard deviation; TBMMP: theoretical maximum biomethane potential.

summarizes the studied samples’ obtained BMPs in mL/g_oTS, mL/g_TS, and mL/g_FM. The BMP determined for RC-CGS-M is extremely low, as compared to the TMBMP of 495.93 mL/g_oTS. This presents a very promising potential of about 68% more yield, which could be explored using the optimization of generation conditions and pre-treatment methods. While the TMBMP of PW-CGS-M, comparable with reported values in the literature [84], is not practically achievable, it presents an opportunity for more reliable biogas yield estimations when used together with the experimental biogas yield in mL/g_FM. In addition, it provides a fair performance of the PW-CGS-M against the RC-CGS-M in terms of mL/g_oTS of BMP, which is achievable. From this study, it can be deduced that the highest BMP extractible from PW-CGS-M is lower than 13% of the minimum BMP extractible from RC-CGS-M. It is therefore recommended that for loss of organic acids and dissolved oTS, which negatively impacts biogas potentials, the evaluation of biogas yields of waters is preferable on a COD-removal basis, as is paramount in the literature [85].

The biomethane potential based on the total rumen content generation per week at the Sunyani abattoir and on assumption that the ordinary mixture is used in its occurring form is 38,675 liters (38.7 m³/week). This corresponds to 364 kWh of energy. Figure 8 compares the different methane potentials extractible from the waste generated based on each variant and the extractible waste based on mechanical pressing. Figure 8 summarizes the maximum extractible potential of energy in kWh from the RC-CGS-M. Clearly, almost all the waste generated per week can be used in biogas generation, making more total solids available for methane generation in relation to variant RC-CGS-M.

4. Conclusions

The study evaluated the comparative performance of biogas generation from the raw mixture of slaughterhouse waste and the use of its mechanically separated press water that could potentially enable additional use cases in the sense of bioeconomy. The study found that the facility has a waste generation potential of 2615 kg per week, which is sourced from rumen contents of cattle, goats, and sheep slaughtered at the facility for 6 out of 7 days a week. The biogas yield from this slaughter waste in an ordinarily existing mixture is 291.60 ± 28.78 mL/g_oTS or 26.70 ± 28.78 mL/g_FM. The biomethane potential (BMP) of this material is 160.4 ± 28.78 mL/g_oTS for its ordinary mixture. For the press water, the biogas yield is 7.47 ± 1.12 mL/g_FM. It has a TMBMP of 20.39 mL/g_oTS. In terms of nominal biogas volumes, the potential of the waste generated per week is 38.7 m³, which is equivalent to 364 kWh in the event of using the resource in its semi-solid form without pressing. The study concludes that although the use of pressing is technically viable, it is comparatively not as resourceful as the use of its ordinary mixture.

In any case, it is necessary to assess for each process chain why only press water is available for biogas generation. The high oxygen demand necessitates posttreatment of the PW, wherein anaerobic conversion can play a significant role. Furthermore, co-digestion with other materials that are richer in surface area and carbon can deliver substantially better results and thus energy output. However, the general energy output loss due to the separation of the two phases should always be compared with the desired added benefits of this additional process chain, for instance, alternative use of fiber in the context of bioeconomy or facilitated pumpability of the biogas substrate through addition of liquid/press water, as it is considerable in the presented setting without co-digestion.

Notwithstanding, with additional benefits such as reduced deterioration of pump and hydraulic systems, the approach is technically viable. It is recommended that the feedstock (slaughterhouse waste) is co-digested with more fibrous materials (in West African context, e.g., cassava, yam, or plantain waste), as this could enhance the C/N ratio and effectively increase the biogas generation output. HHVs of 17.1–17.5 MJ/kg were obtained for the studied samples. These are higher than the 14.7 MJ/kg value reported in the literature for cow dung, probably since rumen contents may have lost methane (energy carrier) in the event of digestion before being released as fecal waste (cow dung). It can be inferred that the use of rumen content for biogas generation will be more valuable per TS amount than for animal feces. Further research, however, is recommended for exploring the enhancement of anaerobic digestion using press waters and looking into pre-treatment options and optimization techniques. An experimental study to assess the quality of the resulting cake for use in composting is recommended.