Abstract
Shea butter extraction involves the generation of waste of environmental concern. To help find a solution to the management of this waste, and to search for an alternative source of energy, this study was carried out to investigate the potential of shea waste in generating methane gas through anaerobic digestion. At high concentrations, volatile fatty acids produce a low pH, which may inhibit hydrolysis, thereby affecting the stability of the anaerobic digestion process. The influence of pH changes on process stability (or otherwise) was also investigated. The study involved two fermentation processes: mono-fermentation and co-fermentation involving a mix of shea waste and cattle dung. The mono-fermentation investigations comprised six treatments - three organic dry matter concentrations of 7%, 5% and 3%, combined with two hydraulic retention times of 30 and 60 days. The co-fermentation investigations comprised three treatments of shea waste and cattle manure mix in proportions (by volume) of 50:50, 75:25 and 90:10. The results showed that changes in pH were a good parameter for indicating process instability. The results also showed that monofermentation of shea waste was not a viable option in anaerobic digestion for biogas production, whilst only the substrate with 50% cattle manure in the co-fermentation trials showed process stability, producing biogas with adequate methane content.
Highlights
Temperature and substrate changes (Razaviarani et al, 2013; Nagao et al, 2012)
Characteristics of raw shea waste and cattle dung: The characteristics of the basic raw materials as well as the various concentrations and types of input substrates used for the digestion process are summarized in Tables 1 and 2
The pH-values of the basic substrate, shea waste, under all odm concentrations, as well as those of the shea waste and cattle dung mix at 7% odm fell below the optimal pH range of 6.8-7.2 (Abdel-Hadi, 2003) for anaerobic digestion of organic materials (Table 2)
Summary
Even though fats (lipids) are fairly rapidly degraded, they may be the reason for problems associated with its own degradation Lipids and their hydrolysis products, the long chain fatty acids (LCFA), might adsorb to surfaces and as such hinder (physically) the attack of exo-enzymes, which hydrolyse the substrate, and the transport of substrates through bacteria membranes (Romero et al, 2016; Subramanian et al, 2015). It is estimated that for every metric ton of nuts processed, 450-600 kg of waste is produced. The objective of the study is to investigate the influence of substrate pH on the stability of anaerobic digestion in mono- and cofermentation of shea waste for biogas/methane production under continuous-flow system
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