Abstract:
Sweet potato (SP) is a tuber crop which is grown in most parts of the world; the
crop generates lots of waste throughout its supply chain. Currently the waste has
little commercial utilization; the unutilized sweet potato root waste (SPW) is highly
perishable and release methane as they decompose posing a serious problem of
environmental pollution. Biogas potential of sweet potato has been tested as a
mono substrate as well as co-substrate. Other studies have also reported that sweet
potato has a complex molecular structure which is resistant to enzymatic digestion;
however no studies have reported any pretreatment mechanism to alter the complex
structure to ease anaerobic degradation. Consequently, the main objective of this
research was to evaluate energy recovery through anaerobic digestion of
thermochemically pretreated SPW. The following specific objectives were
investigated: physicochemical characterization of SPW; thermochemical
pretreatment of sweet potato waste; and determination of pretreatment factors
combination which produces optimum biogas. The quality of biogas produced in
terms of methane content as well as the resultant digestate as a biofertilizer were
also analyzed. SPW was milled and then subjected to thermo-chemical
pretreatment where sodium hydroxide of concentration (0.6g/L-3.5g/L),
temperature (50 - 90 ) and pre-treatment time (30-120minutes) were varied.
The experimental setup was based on central composite design with all the three
factors at five levels and biogas yield as a response. The pretreated SPW was
anaerobically digested under mesophilic condition over an incubation period of 22
days. The results from the study revealed that, thermochemical pretreatment on
SPW improved both biogas yield and methane. The optimum conditions for biogas
production were obtained at: NaOH concentration 2.9g/L, heating temperature
82 and treatment time 102 minutes. The pretreated SPW had superior results than
the untreated SPW; the untreated SPW cumulatively yielded 28.23 ml/gSPW of
biogas, while the thermochemically treated SPW at optimal conditions produced
37.80ml/gSPW, therefore pretreatment improved biogas yield by 33.88%. The
untreated SPW produced 42% methane while the thermochemically treated SPW
produced 64%, hence pretreatment improved Methane by 22%. SPW in its natural
form had a pH value which ranged from 4.8 to 5.0, this was lower than the
optimum pH for anaerobic digestion, and hence neutralization step was necessary
prior to anaerobic digestion. Carbon nitrogen (C/N) ratio of SPW of 40.86% was
obtained in the study was which was higher than the recommended C/N ratio for
anaerobic digestion, therefore the use of thermochemically treated SPW as a co-
substrate with nitrogen rich substrate could help balance the nutrients. From the
analysis of digestate it was observed that, the digestate from the untreated SPW had
more macro nutrients (NPK) than the thermochemically treated SPW. It was
therefore concluded that, thermochemical pretreatment of SPW improves both
biogas yield and methane along with reduction in digester retention time though the
quality of digestate as a fertilizer was degraded. Therefore other pretreatment
methods on SPW should be tried to obtain a suitable pretreatment method to
improve biogas yield without compromising with the quality of fertilizer.