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Article

Harnessing Horsepower from Horse Manure at the EARTH Centre in South Africa: Biogas Initiative Improve the Facility’s Operational Sustainability

by
Charles Rashama
1,2,
Tonderayi Matambo
1,*,
Asheal Mutungwazi
3,
Christian Riann
2 and
Godwell Nhamo
4
1
Centre for Competency in Environmental Biotechnology, College of Agriculture and Environmental Science, University of South Africa, Cnr Pioneer and Christian De Wet Roads, Private Bag X6, Florida 1710, South Africa
2
Department of Life and Consumer Sciences, College of Agriculture and Environmental Studies, University of South Africa, Cnr Pioneer and Christian De Wet Roads, Private Bag X6, Florida 1710, South Africa
3
Infrastructure Services Department: Major Projects, Mount Isa City Council, 23 West Street, Mount Isa, QLD 4825, Australia
4
Institute for Corporate Citizenship, College of Economic and Management Sciences, University of South Africa, Preller Street, Muckleneuk, Pretoria 0002, South Africa
*
Author to whom correspondence should be addressed.
Energies 2025, 18(7), 1808; https://doi.org/10.3390/en18071808
Submission received: 27 February 2025 / Revised: 24 March 2025 / Accepted: 1 April 2025 / Published: 3 April 2025
(This article belongs to the Special Issue New Challenges in Waste-to-Energy and Bioenergy Systems)
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Abstract

:
This study investigated the sustainability aspects of implementing a small-scale biogas digester project at the EARTH Centre, a horse-riding facility for the disabled, in South Africa. Firstly, an energy audit of the facility was conducted. From this exercise, energy-saving opportunities through anaerobic digestion of horse manure were identified. Biomethane potential tests (BMPs) were then performed using the Automatic Methane potential test system II (AMPTS II) of BioProcess Control (Lund, Sweden). The horse manure BMP result was 106 L/kg.VS with the biogas averaging a methane content of 40%. This BMP was lower than that of common substrates such as cow manure which can range from 150–210 L/kg.VS. The gas production rate was almost constant in the first 13 days indicating a long hydrolysis period for horse manure. The microbial species in the digester did not change much during the incubation period although small changes were visible in the proportions of each species as the reaction progressed from start to finish. The energy audit showed that 47% of the EARTH Centre’s energy requirements, which equated to 14,372 kWh/year, could be secured from biogas or solar instead of obtaining it from the national grid which is powered mainly by unsustainable coal-fired systems. As a starting point, a 10 cubic meter biogas digester was installed to produce 5512 kWh of energy per year in the form of biogas. To boost biogas production and continue running the system smoothly, it was evident that the horse manure-fed digester would require regular spiking with cow manure as a bioaugmentation strategy. The digester also produced bio-fertiliser and several sustainable development goals were fulfilled by this project. Current efforts are focused on process optimization of this technology at the Earth Centre to further improve the sustainability of the whole business.

1. Introduction

Sustainability is a concept that focuses on issues around nature’s regenerative capacity that will enable current human activities to proceed without endangering future generations’ potential to also derive benefits from the same nature [1]. The United Nations (UN) member states set sustainable development goals (SDGs) in 2015 which target to address global sustainability concerns [2]. Although sustainability is premised on three (3) main pillars (or 3Ps), which are economics (profit), social (people) and the environment (planet), breaking down these three into actionable items produced seventeen (17) SDGs [3]. Each SDG specifically addresses unique aspects such as pollution and climate change, poverty and jobs, clean and affordable energy, water and sanitation, just to mention a few. To operationalise these SDGs and yield the best results, every person has to participate. Programs at the national level, projects at the institutional level and even initiatives at the personal level in every sector of the economy can be designed to fulfill these SDGs.
Biogas is an energy carrier gas produced from the microbiological degradation of organic materials under anaerobic conditions. Matambo et al. [4] reviewed anaerobic digestion (AD) technology’s contribution to sustainability and pointed out that the implementation of an AD system could potentially impact thirteen (13) SDGs. A case study conducted in Nepal concluded that household-scale digesters make a modest contribution to sustainability if these investments are supported by the necessary policies, especially in the space of clean cooking [5]. Additional studies by Lohani et al. [6] in Nepal household-level AD systems showed that replacing firewood with biogas resulted in carbon monoxide emission avoidance of 4 g/MJ and PM2.5 emission avoidance equal to 400 mg/MJ. Replacing fossil-derived electricity in cooking with biogas cooking may potentially be a sustainable option. The sustainability of this technological switch can be evaluated in South Africa where 93% of the country’s electricity is generated from coal and the country hosts a vibrant agro-industry that generates substantial amounts of organic waste [7]. The 30.5 million tonnes of organic waste generated per year in South Africa can be sustainably used through biogas production if the appropriate policies and incentives to promote this industry are put in place [8].
Horse manure is one of the common animal manures that present disposal challenges hence sustainability concerns. Horse rearing is broadly practised in South Africa. However, horses are not native to South Africa. Owing to this, they suffer several illnesses in this country [9]. To manage these diseases, horses are vaccinated, maintained on certain drugs, and given special nutrition to adapt to the Southern African environment [10]. These interventions affect the horse manure’s chemical and nutritional profiling which further impacts the biogas yields and kinetics of using horse manure as a biogas digester substrate [11]. Horse manure has been previously used in a few biogas productions elsewhere and co-digestion schemes [12]. Studies involving horse manure co-digestion schemes gave higher biogas yields being 142 L/kg.COD with pig manure [13]. Most of these studies were, however, performed with horse manure sourced from those specific sites and not the South African environment.
Each biogas project has its own unique measurable contribution to sustainability. Site-specific variables such as the type of substrate used, national energy mix and costs, site energy demand, applicable environmental constraints and many other factors result in different extents of addressing the three sustainability pillars. No significant studies linking sustainability to horse manure anaerobic digestion have been reported in South Africa in public literature to date. The University of South Africa, through its Institute for the Development of Energy for African Sustainability (IDEAS), engages in research and dissemination of small-scale sustainable energy technologies to industries and local communities. An opportunity to produce biogas from horse manure was seized at the EARTH Centre, a horse-riding facility for the disabled in 2017. Evaluating the sustainability of this specific biogas initiative with a vision to replicate it countrywide was the main objective of this study.

2. Materials and Methods

2.1. Baseline Energy Audit

An energy audit was conducted at the EARTH Centre, Johannesburg, South Africa. The estimated energy consumption at the site was established from this exercise. This energy consumption focused on small household and office equipment only. The equipment excluded the electric fence, flood lights, lawn mowers and big four-wheeled vehicles. Such an audit aimed to identify opportunities for grid power replacement by a small to medium-scale renewable energy alternative source. The South African grid power is predominantly sourced from coal combustion and, therefore is an unfavorable choice for fulfilling sustainability requirements. Energy-saving opportunities that could be achieved through different interventions were identified and their suitability for implementation at the EARTH Centre was evaluated in consultation with the Operations team of that facility. The results of this exercise were used to design and justify the implementation of this horse manure conversion to biogas project.

2.2. Elemental Analysis Methods

Determination of the organic elements (CHNSO) in the horse manure was performed following the procedures outlined by Krotz and Giazzi [14]. The horse manure was first macerated to 2 mm size particles in a coffee grinder model SCG-250. The fine manure was oven-dried at 55 °C until its mass stopped changing when measured 3 consecutive times. Finally, the dry samples were loaded and analyzed in the Thermofischer FLASH 2000 organic elemental analyser to complete the analysis. Data from the analyser were processed through Thermo Scientific TM Eager Xperience software Version 1.4.

2.3. Biomethane Potential Analysis

Horses at the EARTH Centre are fed hay, a grass containing cellulose material, that has been dried to feed the animal’s feed. Fresh horse manure biogas potential testing was conducted in an automatic methane potential testing system (AMPTS II). Fresh cow manure was used as the AMPTS’s digester inoculum source for microbial consortia. The AMPTS II is less labour-intensive than conventional BMP assays and allows for biogas composition monitoring without GCs. Gas production (both biogas and methane) is accounted for throughout the experimental run. This provides useful information for the understanding of waste degradation dynamics. A blank with 400 mL (64 g cow manure in water) of only the inoculum slurry and a positive control with 400 mL (64 g of cellulose solution) were also prepared and incubated. The volatile solids (VS) content, inoculum to substrate ratio (I:S) and mixing revolutions per minute (rpm) were specified for all 5 reactors. Nitrogen gas was then bubbled for 30 s into the loaded vials to create anaerobic conditions before starting the digestion. The caustic solution for carbon dioxide scrubbing and the rest of the system were set up and operated according to instructions provided by the equipment manufacturer as shown in Figure 1 [15]. The reacting mixtures in digestors were agitated for 60 min at 60 rev/min after every 2 h throughout the 30-day incubation period.

2.4. Microbial Analysis

i.
Horse manure
Microbial DNA was isolated from fresh horse manure using the DNAeasy PowerSoil kit (UNISA, Johannesburg, South Africa). This analysis was executed in triplicate and was also extended to digester slurry samples which were taken at 3-day intervals. Amplification of the isolated DNA was achieved through a polymerase chain reaction that employed two bacterial primers F (5′-AGAGTTTGATCMTGGC-3′) and 518 R (5′-GTATTACCGCGGCTGCTGG-3′). Procedures followed to perform this amplification are detailed by Selvarajan et al. [16]. After amplification, extracted DNA was purified using the Clean and Concentrator Kit (ZYMO RESEARCH, Irvin, CA, USA) to ensure that the DNA was of high quality. Quality control was checked by running samples in triplicate and performing an ANOVA on the results obtained before accepting the results as valid. Further processing of these samples involved pooling, followed by library preparation and sequencing. The detailed operating conditions and equipment for subsequent stages that included cleaning the PCR products, validation of the fragments, buffering of the DNA, denaturing and sequencing on the Illumina Miseq sequencer were completed at Inqaba Biotechnical Industries (Pty) Ltd. (Pretoria, South Africa). The output from the Sequencer was trimmed for adapters and primers, checked for quality and channeled for bioinformatics according to detailed steps reported by [17]. The clustering of both forward and reverse reads culminated into operational taxonomic units (OTUs) with a sequence similarity of 97% being adopted for species-level identification. A similarity of 97% offers a compromise between the potential inflation of the number of OTUs due to sequencing errors and the cut-off used for taxonomic classification [18]. The OTUs were statistically analysed using different R-studio functionalities.

2.5. Metabolomic Analysis

During the anaerobic digestion run, 2 mL samples were drawn from the reacting slurry as demonstrated in Figure 2. This exercise was repeated at 3-day intervals and the collected samples were used for DNA extractions and slurry metabolites profile analysis. The equipment and detailed protocols on how digester intermediate products were analysed using High-Performance Liquid Chromatography are reported by Asheal et al. [19].
Line graphs derived from the quantified compounds given on the chromatograms were then correlated with pH and AD results.

2.6. Digester Designs, Installation and Commissioning

A survey was conducted on household-sized digesters readily available in the South African market, highlighting their advantages and disadvantages. Fixed dome digesters were disqualified in this project because of their main weakness of failure by wall cracking. Jojo tanks and steel tanks were also disqualified due to their poor temperature control capabilities. The red mud (balloon-type) digester was chosen for its simplicity and moderate capital outlay. The balloon-type digester consisted of a horizontally placed cylindrical PVC bag measuring 7 m in length and 1.4 m in diameter allowing it to hold a slurry volume of 6 m3 and provide a gasholder headspace volume of 4 m3. Each of the domed ends of the cylinder had an opening to provide one inlet and one outlet for the slurry. At the centre along the cylinder bag length, there was one gas outlet provision. The digester was manually fed with 40 L/day of a well-mixed manure-water slurry of 12% weight solids content. This feed rate translates into a slurry hydraulic retention time of 60 days. This digester sizing was based on manure’s availability at the site, manure moisture content, manure BMP and site energy needs of the facility. The digester was installed and commissioned following the manufacturer’s instructions. A level ground was chosen and a pit was dug. A supporting brick wall was constructed on the walls of the pit and the digester bag was placed into this pit and commissioned by adding horse manure with cow manure inoculation.

2.7. Troubleshooting and Optimization

After the digester installation, substrate feeding and process monitoring were initiated. The intention of all this was to test the digester’s performance and establish optimal operating parameters. During this period, the EARTH Centre operational team was also trained on aspects of digester troubleshooting and management. Several challenges were identified and resolved over a 3 month period. A final operational manual was developed from lessons learned during the project implementation and the project was handed over to the EARTH Centre management. As part of the long-term agreement between the partners in the project, other ancillary activities including applied and academic research work using these digesters will continue to be carried out by the University of South Africa’s IDEAS team throughout the lifespan of the installation. This intervention is progressing well at the EARTH Centre.

3. Results

3.1. Energy Audit

The energy consumption patterns at the EARTH Centre for low to medium-consuming equipment as described in the methods section showed that the facility’s total energy demand is 16,516 kWh/year. Refrigeration consumed the highest energy amounting to 53% of the total energy consumed by small equipment at the EARTH Centre. The remaining 47% of energy consumed in this facility is shared across different equipment. Space heating and hot water generation (for cooking and coffee/tea making) consume the largest portion of this shared energy component. We established that energy requirements for generating warm water needed in cooking, coffee making, bathing, laundry, dishwashing and handwashing could be easily replaced with heating from biogas or solar power. Replacing these energy needs with biogas makes more technical and economic sense for the EARTH Centre than the solar option because of the horse manure substrate that is freely available at the site and the lower capital outlay required for digester installation. A total potential electrical energy replacement valued at 14,372 kWh/year can be achieved if biogas fridges and biogas space heaters are used instead of electrical ones. The energy picture for potential electrical savings and what was actually saved by installing a small digester at the EARTH Centre is portrayed in Figure 3. Biogas replaceable energy component (A) covers energy required in hot water needs, refrigeration as well as space heating while component (B) covers only hot water needs that are addressed by the current digester installation. Only 2756 kWh/year of grid power was relieved by the 10 m3 biogas digester indicating that a bigger digester probably in the scale of 70–100 m3 would allow for full replacement of grid electricity supply to small equipment and household appliances at this site. This expansion shall be explored after accruing lessons from the ongoing optimization exercises with the current small digester. Currently, refrigeration and space heating are off the biogas system and are still running on grid electricity. Direct application of biogas in heat generation is generally a more efficient energy conversion route than biogas conversion to electricity [20]. Therefore, electrical energy replacement for ironing, computers and lighting which equates to 2107 kWh/year could not be foreseen as an energy-saving opportunity in the case of the EARTH Centre operations.

3.2. Elemental Analysis Results

The horse manure’s elemental characterisation is reported in Table 1. Horse manure presents a balanced organic elemental profile that is typical of most organic materials used in digesters. For example, cow and buffalo manure’s elemental profiles in a study by Tsai and Liu [21] revealed CHNO levels ranging from 43.67–46.20, 5.45–6.08, 0.59–0.84% and 50.07–47.13%, respectively. The C/N ratio (25.26) was found to be within the recommended range for stable biodigester operations. The sulphur level was insignificant, indicating minimal potential hydrogen sulphide production during anaerobic digestion.

3.3. Gas Production, Microbial and Metabolomic Analysis

The daily biogas and biomethane production trends during the horse manure batch incubation are displayed in Figure 4, Figure 5 and Figure 6. These Figures portray three distinct anaerobic digestion phases, namely, (i) hydrolysis (first 13 days), (ii) the intermediate phase (day 14-day 24) and the late phase (day 25-day 30). The total gas production during the three phases amounted to 1870.2 NmL of biogas and 670.8 NmL of methane. During the hydrolysis stage, the gas production rate was low and near constant and it started to rise sharply in the intermediate stage (Figure 4). This intermediate phase in the batch AD comprises three key microbial processes namely acidogenesis, acetogenesis and methanogenesis. The late stage is predominantly acetogenesis and methanogenesis and in this phase, the metabolites are depleted in the digester leading to a decrease in gas production rates until the process comes to a halt.
  • Phase 1—Hydrolysis
The rate-limiting effect of the hydrolysis phase (13 days) is evident from the results shown in Figure 4. Though hydrolytic microbial processes are generally known to be slow especially when lignocellulosic are involved, the case for horse manure is extreme. The AD of cow manure where hydrolytic times ranging 5.5 days have been reported while that of water hyacinth has been around 7 days [22]. This slow kinetics with horse manure hydrolysis can be attributed to the horse diet which is rich in lignocellulosic components and the low microbial diversity and abundance in the horse gastro-intestinal gut (GIT) to facilitate pretreatment of this substrate as is the case in cow GIT. The horse stomach is also smaller, being 100–140 L, while that of the cow has a volume of between 150 and 180 L [23,24,25]. This hydrolysis time can be reduced by pre-treatment of the manure before digestion. Recent excellent reviews on the subject of manure pretreatments have been given by Orlando et al. [26] and Preeti et al. [27]. However, the best pre-treatment choice for horse manure and each specific case can only be ascertained by conducting trials. A reduction in the lag phase time (hydrolysis time) from 1.3 days to 0.8 days was reported for horse manure AD after a milling pre-treatment [28].
Microbial analysis of horse manure revealed a few lignocellulosic degrading microbiota capable of unlocking the substrate’s nutrients [29]. The microorganisms present and dominating at each stage of horse manure AD are shown in Figure 5. Ranking the microbial dominance by abundance during hydrolysis we note that Fibrobacter > Weissella > Escherichia > Leuconostoc > Lactobacillus > Streptococcus > Enterococcus > Acinetobacter.
Fibrobacter which appears in high abundance in horse manure is a cellulose gedrader [30]. Weissella is well known for its participation in fermentation and carbohydrate degradation capacity where it produces mainly lactic and acetic acids [31]. Starch degradation is the main target for Enterococcus genera. Lactobacillus produces hydrogen, carbon dioxide, lactate, acetates and ethanol from a wide range of sugars [32]. The monosaccharide degrading capacity of Escherichia and Leuconostoc generated acetates, carbon dioxide, hydrogen and ethanol [33]. These metabolites from hydrolysis and acidogenesis, though little in quantity during hydrolysis, were converted by the small population of methanogens in inoculum to give the little biogas reported during this time [34]. Acinetobacter genera are aerobic and non-fermentative, hence they became extinct in this first phase due to the anaerobic conditions of the digester. Phase 1 also witnessed a gradual drop in pH from 7.5 to 6.8 as volatile acids increased concentration in the digester while acetogens and methanogens populations were still to grow and later convert these acids into biogas.
  • Phase 2—Intermediate phase
This phase in the batch incubation of horse manure was characterised by both microbial (Figure 5) and metabolomics shifts (Figure 6). The increasing volatile acids assays consisting of lactic, formic, propionic, butyric, acetic and ethanol and the accompanying microbial shifts resulted in heightened acetogenic and methanogenic activities. These activities are manifested through increased biogas production rates from 5.9 NmL/day to 41.4 NmL/day during this period. The volatile acids had increased from the hydrolysis stage further bringing down the pH in Phase 2 from 6.8 to 5.8 during the period from day 12 to day 18. However, the pH drop halted and started to go up as acetogenesis and methanogenesis increased after day 18. The observed trends in volatile acids, pH, biogas production and microbial shifts confirm that all three processes of acidogenesis, acetogenesis and methanogenesis were taking place simultaneously during this period
The five genera responsible for the observed intermediate product trends were Fibrobacter (26.797–30.803%) Weissella (19.351–26.855%), Escherichia (13.797–17.814%), Leuconostoc (12.532–20.513%), and Lactobacillus (8.403–12.43%). Fibrobacter and Weissella were observed to have given way to Escherichia, Leuconostoc, and Lactobacillus. The mixture of acids i.e., acetate, formate, lactate and CO2, H2 and ethanol produced by the highly versatile Escherichia, Leuconostoc and Lactobacillus were converted to methane by acetoclastic and hydrogenotrophic means accordingly.
  • Phase 3—Late phase
During the third phase, methane, acetic acid and all the intermediate products’ production rates decreased; lactic (0.57 g/L/day), formic (0.21 g/L/day), propionic (0.18 g/L/day), butyric (0.11 g/L/day) acids and ethanol (0.09 g/L/day).
  • Summary of AD process dynamics observed for horse manure
A review by Menzel et al. [35] reported that animal manure AD suffers challenges of ammonia inhibition, early acidification, and low biodegradability due to high fibre content. These challenges are similar to the findings observed in this current study, whereby cumulative biogas production was 1870.2 ± 31.5 mL at 39.5% methane content. This translated to a BMP of 106 LCH4/kg.VS. This relatively low BMP value is in the range of other highly lignocellulosic substrates such as water hyacinth which gave BMP assays of 185 LCH4/kg.VS [22]. The major challenges with horse manure digestion included low rates of hydrolysis and acidogenesis which can be attributed to poor feed maceration by horses which keeps the recalcitrant lignocellulosic material intact and locks up cellulose for digestion. Another challenge noticed in the acetogenesis and methanogenesis stage pertains to the low diversity and abundance of methane-producing micro-organisms. Menzel et al. [35] went further to propose that the highlighted challenges could be solved by co-digestion of high nitrogen substrates with carbon-rich substrates, thermophilic hydrolysis and stage separation of the AD processes to allow for the enrichment of specific microorganisms in each stage. The C/N ratio has traditionally been used to select feedstocks for co-digestion and the ratios to use in performing the co-digestion runs. In the present work, although we had expected that horse manure would require no co-digestion since its C/N ratio (25.45) was well within the recommended range, it surprisingly exhibited poor hydrolysis and methanogenesis. This confirms that focusing on C/N or other physico-chemical operational parameters alone for co-digestion and other AD process control decisions is not sufficient.
The current study findings help in determining the best-fit solutions and do not allow for poorly informed decisions in biodigester optimization and/or troubleshooting. The results confirm that horse manure AD is rate-limited at the hydrolysis stage and experiences poor methanogenesis due to this shortcoming. Horse manure co-digestion with cow manure could bioaugment the digester with the necessary hydrolytic and methanogenic microorganisms to alleviate these challenges and improve biomethane yields. Operating at thermophilic temperatures as a strategy to improve hydrolysis would, however, not work for horse manure since most of these hydrolytic microorganisms which are commonly found in cow manure, all operate optimally within a mesophilic temperature range of 35–40 °C and a pH of 6.7–7.2 [36,37,38]. This would also mean that stage separation of hydrolysis and methanogenesis based on using different operating temperatures and pH would not be logical since the acetoclastic methanogens (Methanosarcina) identified in cow manure also operate optimally at similar conditions as the hydrolytic bacteria [39,40]. Other bases for stage separation such as nutrient supplementation would have to be investigated and possibly applied if there are notable variations between the needs of these two sets of microorganisms. With the application of this co-digestion strategy, horse manure is highly likely to produce a higher yield of bio-methane at a faster rate. A closer look at the differences in microbial profiles of horse manure and cow manure slurries (Table 2) gives more insights into the AD performance of these two substrates.

3.4. Designs

From the energy audit, it was clear that 14,372 kWh/year electrical grid energy could be saved by using biogas at the EARTH Centre. In the first phase of the project, energy replacements for the fridge and space heating were excluded until the biogas potential of the specific feedstock was ascertained. This leaves a requirement of only 2756 kWh/year of energy requirement from biogas. Biogas at 60% methane content has an energy potential of 6 kWh/m3 and conversion efficiency to thermal energy ranges 16–84% depending on the type of conversion technology, gas composition and other factors [20,42]. Using 50% as an average value, the EARTH Centre would need to produce approximately 5512 kWh/year in the form of biogas (15 kWh/day) to meet the electrical energy substitution proposed by the above energy audit. A sustainable design for such small-sized digesters would be to buy off-the-shelf units which are easy and cheap to ship as well as assemble. A balloon-type red mud digester with a total volume of 10 m3 and a liquid operating volume of 6 m3 was chosen (Figure 7). This leaves a 4 m3 gas holding capacity which is 63% over design to the EARTH Centre needed to cater for bad days, especially when ambient temperatures drop, causing a reduction in microbial activities. These microbial activity fluctuations affect the reliability of biogas generation from the digester [43]. Biogas generated from this balloon-type digester using source provided horse manure as substrate was pumped by a small battery-powered pump which can be solar charged to improve the eco-friendliness of the system by avoiding as much as possible the use of grid electricity.
At the point of use (the kitchen and offices), the biogas was used to boil water for cooking and laundry on a biogas stove (Figure 7). Water traps were installed along the gas conveying pipes to reduce biogas humidity, as it impacts the gas’s calorific value. To improve safety and gas handling, a pressure relief valve was also incorporated on the gas outlet lines to open and vent out gases during times when gas production exceeds gas usage. An operating manual to ensure that consistent organic loading rates are adhered to and possible digester fouling is avoided was also developed and provided to the client. However, after commissioning the biodigester, it could not perform to the expected targets and a troubleshooting exercise was embarked on to debottleneck the biogas system. The next step in this project is to ramp up production through optimisation studies so that a biogas geyser, biogas office and room heater as well as the biogas fridge can also be connected to the system.

4. Discussion

The energy audit showed that the top three (3) energy-consuming activities at the EARTH Centre can be powered with biogas. Grid electricity savings can also be achieved by installing solar systems but the relative benefits of biogas towards addressing SDGs are higher than that of solar, therefore, digester installation was the preferred alternative energy route chosen for the EARTH Centre [44]. The biogas for this site could be produced from horse manure despite the challenges of low biogas generation associated with this substrate as compared to most manures [45]. The presence of horse manure at the site is another factor that favoured biogas instead of solar installation at the EARTH Centre because by producing biogas from horse manure, other sustainable development goals in addition to that of energy will also be addressed simultaneously, a situation not so achievable in the solar systems option [44,46]. Some of the sustainability contributions of installing a biodigester to replace grid (coal-based) power at the EARTH Centre are reported in Table 3. Biogas-powered fridges, boilers and space heaters are now commercially available on the market making these recommended energy use changes possible [47]. It is also known that biogas chemical energy conversion to thermal energy is more efficient than conversion to electricity through biogas-powered generators as previously stated in this paper. So, equipment that requires biogas conversion to electricity first (e.g., lights, ironing, computers) was not shortlisted for the energy-saving opportunities presented by biogas at the EARTH Centre.
Biogas peak production in horses was reported on day 19 after incubation as opposed to day 9 in cow manure digesters. Cumulatively, cow manure produced more biogas than horse manure. The daily gas trends in our study differed from those observed by Alfa et al. [48]; however, both studies agreed that cow manure produced a higher cumulative gas yield than horse manure. In the research of Alfa et al. [48], the daily biogas production patterns of cow and horse manure closely mirrored each other. This variance in patterns of gas production may be attributable to differences in antibiotic programs that may affect the final microbial patterns in the digesters fed on different manures [49]. A probable reason for different cumulative gas observations by different researchers on similar substrates arising from different sites could also be linked to variations in specific animal diets for every farmer, although the validity of this reason is debatable among scholars [50,51,52]. Another interesting observation of horse manure biodegradation to biogas is the relatively low methane composition in the gas as compared to that of cow which is the most popular substrate for biogas. This trend of low methane fraction in horse manure-based biogas compared to that of cow manure-based gas was also observed by Weide et al. [53]. A possible explanation for this is the highly carbonaceous and lean lipid substrate nature of horse manure as opposed to cow manure which has a little bit of protein and fats. Lipids and proteins produce higher methane content when they degrade as compared to carbohydrates [54,55]. Fortunately, towards the end of the incubation period, the methane composition at 50% is within the average composition typically achieved across common anaerobic digestion substrates [56]. The BMP achieved in this study for horse manure was 176 LCH4/kg.VS which is comparable to that of cow manure, a well-known biogas substrate whose BMP ranges 148–208 LCH4/kg.VS [57]. Thus, horse manure can be used to produce combustible biogas. If this biogas is generated in the right quantities, it can therefore be used to replace part of grid power at the EARTH Centre.
An off-the-shelf design solution proved cost-effective compared to constructing the digester from scratch in the case of fixed dome brick and mortar structures, which are slowly losing popularity because of leakage and short life span challenges [58]. Other viable designs [59] were evaluated but the balloon-type digester had more positive attributes than others such as being economical in terms of space usage, natural temperature regulation when partly buried underground, etc. After the digester commissioning at the EARTH Centre, troubleshooting exercises were undertaken to ensure that the digester performed as expected. Low quantities of biogas were generated, especially in winter. It was suspected that the low temperatures likely slowed methanogenic population growth rates. To address this, a 20 kg dose of cow manure was added once every fortnight to sustain microbial diversity and population, effectively resolving the issue [48]. Stones and other indigestible components in the feed blocked the digester inlet and a screen was installed at the digester inlet to allow only fine manure into the digester chamber. Another problem that was identified was the floating of grass in the feedstock used. Inconsistent feeding resulted in digester fouling. This was resolved by training of operators and increased supervision from management in the early days after commissioning.
Table 3. The EARTH Centre biogas project’s contribution to various SDGs.
Table 3. The EARTH Centre biogas project’s contribution to various SDGs.
SDG Name (Number) 1Project FindingsComparison to Other Research and Significance
Affordable and clean energy (7)Biogas use replaced 5512 kWh/year of coal-fired grid electricity. Biogas combustion is cleaner than coal in terms of particulate emissions.Biogas is obtained at no additional operational cost. Daily maintenance of the digester takes only 20 min from the employee’s daily routines.
Burning coal in power plants generates 81 mg/MJ of particulates versus a near-zero emission from biogas combustion [60]
Climate action (13)Approximately 1000 m3 of methane gas emissions from open manure heaps were diverted to energy use, thus reducing the global warming potential (GWP) effect of open dumped manures.Instead of methane going into atmosphere and damaging the ozone, it is captured and then used in a controlled complete combustion technology where the only GHG generated is carbon dioxide. Methane GWP effect is 25 times stronger than that of carbon dioxide [61].
Zero hunger (2)At least 3 employees have 3 meals per day with vegetables grown onsite using digestate, saving almost US$5 on relish expenses per day.Liquid digestate was evaluated and it was demonstrated that 58% of the different samples met the minimum N, Zn and Cu requirements for agriculture [62].
Good health (3), Clean water and sanitation (6)Reducing coal usage in power plants by using biogas reduces the associated pollution from power plants. Flies around the Sables were reduced. Manures are no longer washed away into open environments.Coal power plants emit 1360 mg/MJ of SOx and 583 mg/MJ of NOx of power produced [60]. These emissions have a negative impact on the health of humankind.
Quality education (4)The EARTH Centre is used for promoting STEM education in the Gauteng province of South Africa. Students visit the EARTH Centre for science demonstrations and University postgraduates formulate research problems and acquire field data from the installation.
Life (14 and 15)Plant life (lawn and flowers) growth improved at the EARTH Centre after the use of digestate on the lawns. This was witnessed by an increase in frequency of lawn mowing and flower tree pruning from 1 to 2 times per month before and after the digestate use periods.Jurgutis et al. [63] applied digestate on grass at a rate of 170 kgN/ha and the grass biomass grew 3 times compared to the grass that had no digestate application.
Partnerships for the goals (17)Government, Academia and Not-for-Profit partners were involvedUniversity of South Africa (Academia), South African Energy Institute (Government) then EARTH Centre (Not-for-Profit)
1 SDGs reported in order of perceived extent of contribution from the project.

5. Future Actions for a Sustainable EARTH Centre

Currently, the first phase of the EARTH Centre biogas project which involved replacing the thermal power requirements has been accomplished. A number of initiatives are now being incorporated into the project in the second phase of implementation to improve its sustainability. Knowing very well that digesters consume a lot of water [64] and water use and/or contamination is a critical topic in the SDG matrix, it is paramount that rainwater harvesting be incorporated into the EARTH Centre biogas project. This project component has been adopted and is at an advanced stage of implementation. The bioslurry (digestate) arising from the anaerobic digestion process is being used in a small horticultural garden to provide fresh vegetables for the staff at the EARTH Centre. Future use of this popular biofertiliser in the lawns and flower management at the site will also be explored in an effort to lower or eradicate the use of synthetic fertilizer which has been well known to contribute to negative climate change effects [65]. It is also anticipated to introduce a greenhouse above the digester to improve temperatures in the digesters hence boosting microbial activities which in turn will improve biogas production [66]. This is particularly important during the winter and cold nights of Southern Africa.

6. Conclusions

Horse manure is a viable biogas digester substrate although not as good as cow manure and other popular substrates due to its highly fibrous lignocellulosic carbohydrate-rich components that tend to lower the BMP. The microbial species autochtonous to horse manure lack the desired level of diversity and population of methanogens as opposed to the situation in cow manure. This suppressed biogas production with horses only as a substrate, giving a BMP of 108 L/kg.VS, although this could go up to almost 176 L/kg.VS with cow manure supplementation. Bioaugmentation through regular cow manure doses or co-digestion would be an inevitable requirement to improve the horse manure’s biodegradation. Anaerobically treating horse manure for biogas production offers significant sustainability benefits, especially in a setup where the horse manure is available onsite (zero cost) and the site has the capacity to use the generated gas onsite to replace grid electricity. With proper integration of additional processes at each client’s site, such as digester use as biofertilizer, providing entertainment and education to locals, etc, multiple SDGs can be addressed simultaneously through horse manure–cow manure co-digestion. The significance of this research is that it unpacks the possibilities, limitations and solutions for realising sustainable goals at small-scale institutional level facilities by considering local conditions rather than relying on generalised global data which may miss benefits that come with understanding local constraints and opportunities. The results and procedures followed can be replicated in other small institutions around the country and the overall effect will be to harness bigger sustainability benefits for humankind.

Author Contributions

Conceptualization, C.R. (Charles Rashama) and T.M.; methodology, C.R. (Charles Rashama) and A.M.; formal analysis, A.M.; investigation, C.R. (Charles Rashama) and A.M.; resources, T.M. and G.N.; data curation, A.M.; writing—original draft preparation, C.R. (Charles Rashama) and A.M.; writing—review and editing, T.M. and C.R. (Christian Riann); visualization, A.M.; supervision, T.M.; project administration, T.M.; funding acquisition, G.N. and T.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research and APC were funded by National Research Foundation, South Africa (NRF Grant Number: 138093, Awarded to Prof TS Matambo), and the Department of Science and Innovation (South Africa) and the Technology Innovation Agency (TIA) sponsored the funding for developing the Centre of Competence in Environmental Biotechnology.

Data Availability Statement

Data collected during the research has been reported in the article.

Acknowledgments

We would like to thank the EARTH Centre for affording us the opportunity to implement this project at their site. We also acknowledge the South African National Energy Institute (SANEDI) for participating in the digester siting negotiations with the EARTH Centre as well as providing government oversight on the project. We are also grateful to the Institute for the Development of Energy for African Sustainability (IDEAS) students and staff members including Trevor S Malambo, Dolly Mazwi and Lucia S. Vuiswa who volunteered to provide labour during the digester installation. We are also grateful to Rosina Nkuna who assisted us with the bioinformatics.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
AMPTSAutomatic Methane Potential Testing System
SDGSustainable Development Goals
ADAnaerobic Digestion
SANEDISouth African National Energy Development Institute
UNISAUniversity of South Africa
IDEASUNISA’s Institute for the Development of Energy for African Sustainability
CAESUNISA’s College of Agriculture and Environmental Sciences
GHGGreenhouse Gas

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Figure 1. The AMPTS II experimental setup for the BMP testing.
Figure 1. The AMPTS II experimental setup for the BMP testing.
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Figure 2. Liquid sampling from a reactor on the AMPTS® II system.
Figure 2. Liquid sampling from a reactor on the AMPTS® II system.
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Figure 3. Electrical energy savings (kWh/yr) through biogas use at the Earth Centre.
Figure 3. Electrical energy savings (kWh/yr) through biogas use at the Earth Centre.
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Figure 4. Biogas and methane daily production from horse manure.
Figure 4. Biogas and methane daily production from horse manure.
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Figure 5. Microbial profile during horse manure digestion.
Figure 5. Microbial profile during horse manure digestion.
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Figure 6. Intermediate products formed during horse manure digestion.
Figure 6. Intermediate products formed during horse manure digestion.
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Figure 7. Biogas digester at the EARTH Centre and the biogas stove under operation.
Figure 7. Biogas digester at the EARTH Centre and the biogas stove under operation.
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Table 1. Horse manure ultimate analysis results.
Table 1. Horse manure ultimate analysis results.
% C% H% N% S% OC/N
48.0 5.61.90.355.825.26
Table 2. Comparison of microbial profiles in cow and horse manure AD at different stages.
Table 2. Comparison of microbial profiles in cow and horse manure AD at different stages.
AD Stage.
(Batch Experiments)		Dominant Microorganisms (Abundance)Remarks
Cow Manure [41]Horse Manure [This Study]
Early stage (first 1.5 weeks)Ruminiclostridium 1 (39%)
Butyrivibrio 2 (20%)
Acinetobacter (16%)
Aquabacterium (8%)
Macellibacteroides (5%)
Lactobacillus (3%)
Ruminococcaceae UCG-009 (2%)		Fibrobacter (21–27%)
Weissella (9–19%)
Escherichia (11–14%)
Leuconostoc (10–13%)
Lactobacillus (7–8%)
Streptococcus (9–7%)
Enterococcus (6–3%)
Acinetobacter (8%).		There is a completely different set of microorganisms in the two digester manure slurries, except for the Acinetobacter and Lactobacillus which are found in both. The Acinetobacter is much more densely populated in the cow manure than it is in the horse manure where it was observed to be decreasing as the reaction progressed to the end. The Lactobacillus abundance is low in both manures. The difference in microbial species and abundance between these two explains the faster hydrolysis in cow manure digesters than in horse manure ones.
Mid-stage (2–3 weeks)Acinetobacter (27%)
Ruminiclostridium 1 (16%)
Ruminococcaceae UCG-002 (7%)
Ruminococcaceae UCG-009 (4%)
Saccharofermentans (4%)
Desulfovibrio (4%)
Macellibacteroides (2%)		Fibrobacter (27–31%)
Weissella (19–27%)
Escherichia (14–18%)
Leuconostoc (13–21%)
Lactobacillus (8–12%)		Again, the speciation of microorganisms in the two manures is very different. However, there is more diversity and evolvement of the microbial dynamics in the cow manure than the horse manure. The horse manure microbial population profiles did not change much and this was also reflected in the biogas production rate which was almost stagnant during this period and was only accompanied by a few increases in metabolites.
Late stage (4–5 weeks)Ruminiclostridium 5 (24%)
Acinetobacter (14%)
Lactobacillus (9%)
Bifidobacterium (6%)
Haliangium (5%)
Lachnospiraceae UCG-004 (4%)
Butyrivibrio 2 (4%)
Aquabacterium (4%)		Lactobacillus (33%)
Leuconostoc (27%)
Escherichia (24%)
Weissella (10%)
Fibrobacter (4%)		In both manures, there were visible microbial shifts with Lactobacillus featuring in both manures. Acinetobacter which was consistently high in the cow manure slurries from the beginning continued to exist in high proportions although this was not the case in horse manure where the Acinetobacter population quickly decreased from the onset of the digester to around day 13 when these genera was never found in horse manure again.
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Rashama, C.; Matambo, T.; Mutungwazi, A.; Riann, C.; Nhamo, G. Harnessing Horsepower from Horse Manure at the EARTH Centre in South Africa: Biogas Initiative Improve the Facility’s Operational Sustainability. Energies 2025, 18, 1808. https://doi.org/10.3390/en18071808

AMA Style

Rashama C, Matambo T, Mutungwazi A, Riann C, Nhamo G. Harnessing Horsepower from Horse Manure at the EARTH Centre in South Africa: Biogas Initiative Improve the Facility’s Operational Sustainability. Energies. 2025; 18(7):1808. https://doi.org/10.3390/en18071808

Chicago/Turabian Style

Rashama, Charles, Tonderayi Matambo, Asheal Mutungwazi, Christian Riann, and Godwell Nhamo. 2025. "Harnessing Horsepower from Horse Manure at the EARTH Centre in South Africa: Biogas Initiative Improve the Facility’s Operational Sustainability" Energies 18, no. 7: 1808. https://doi.org/10.3390/en18071808

APA Style

Rashama, C., Matambo, T., Mutungwazi, A., Riann, C., & Nhamo, G. (2025). Harnessing Horsepower from Horse Manure at the EARTH Centre in South Africa: Biogas Initiative Improve the Facility’s Operational Sustainability. Energies, 18(7), 1808. https://doi.org/10.3390/en18071808

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