Abstract
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.
Highlights
Agricultural processes as well as other industrial activities are synonymous with the generation of significant waste
In the broad categorization of anaerobic successional processes towards the production of methane biogas, the following sets of biochemical reactions occur, commencing with hydrolysis, which is the degradation of complex macromolecules, through the breaking of chemical bonds present to produce monomeric units that are degradable by microorganisms[18, 19]
If the ratios of these enzymes are reduced within the microbial consortia owing to various exogenous factors, such as, feed nutrient composition, environmental temperature and pH changes, organic load derived toxicity within the digester, etc., such changes will affect the outcome of the anaerobic digestion process and may reduce the final methane y ield[26]
Summary
Agricultural processes as well as other industrial activities are synonymous with the generation of significant waste. The anaerobic digestion (AD) of animal manure to produce biogas has the added advantage of reducing organic load but it decreases microbial load and infectious aerobic organisms as well as associated odors[16, 17] These outcomes are achieved through the sequential enzymatic degradation facilitated by rumen derived microorganisms autochthonous to animal manure. Current theoretical methods for predicting biogas yield do not generally consider stability, degradability, and kinetics, despite the strong influence of microbial activity as well as the associated inhibitory or stimulatory effects to the process. These methods tend not to consider the presence of recalcitrant organics (lignin)[35]
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