An estimated 25 million tons of animal manure is produced globally every year, causing considerable impact to the environment. These impacts can be managed through the use of anaerobic digestion (AD) This process achieves waste degradation through enzymatic activity, the efficiency of the AD process is directly related to microorganisms that produce these enzymes. Biomethane potential (BMP) assays remain the standard theoretical framework to pre-determine biogas yield and have been used to determine the feasibility of substrates or their combination for biogas production. However, an integrated approach that combines substrate choice and co-digestion would provide an improvement to the current predictive models. PICRUSt (Phylogenetic Investigation of Communities by Reconstruction of Unobserved States) addresses the limitations of assays in this regard. In this paper, the biochemical functions of horse, cow, and pig manures are predicted. A total of 135 predicted KEGG Orthologies (KOs) showed amino acids, carbohydrate, energy, lipid, and xenobiotic metabolisms in all the samples. Linear discriminant analysis (LDA) combined with the effect size measurements (LEfSe), showed that fructose, mannose, amino acid and nucleotide sugar, phosphotransferase (PST) as well as starch and sucrose metabolisms were significantly higher in horse manure samples. 36 of the KOs were related to the acidogenesis and/or acetogenesis AD stages. Extended bar plots showed that 11 significant predictions were observed for horse-cow, while 5 were predicted for horse-pig and for cow-pig manures. Based on these predictions, the AD process can be enhanced through co-digestion strategies that takes into account the predicted metabolic contributions of the manure samples. The results supported the BMP calculations for the samples in this study. Biogas yields can be improved if this combined approach is employed in routine analysis before co-digesting different substrates.

Anaerobic digestion (AD) of organic waste is considered a sustainable solution to energy shortage and waste management challenges. The process is facilitated by complex communities of micro-organisms, yet most wastes do not have these and thus need microbial inoculation using animal manures to initiate the process. However, the degradation efficiency and methane yield achieved in using different inocula vary due to their different microbial diversities. This study used metagenomics tools to compare the autochthonous microbial composition of cow, pig, chicken, and horse manures commonly used for biogas production. Cows exhibited the highest carbon utilisation (>30%) and showed a carbon to nitrogen ratio (C/N) favourable for microbial growth. Pigs showed the least nitrogen utilisation (40%), which explains its high C/N above the optimal range of 20–30 for efficient AD. Manures from animals with similar gastrointestinal tract (GIT) physiologies were observed to largely harbour similar microbial communities. Conversely, some samples from animals with different GITs also shared common microbial communities plausibly because of similar diets and rearing conditions. Insights from this study will lay a foundation upon which in-depth studies of AD metabolic pathways and strategies to boost methane production through efficient catalysis can be derived.

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.