Effect of aerobic pre-treatment on production of hydrolases and volatile fatty acids during anaerobic digestion of solid sisal leaf decortications residues

Abstract

The effect of aerobic pretreatment on the production of hydrolases and volatile fatty acids during anaerobic 
digestion of solid sisal decortications leaf residue (SLDR) was investigated. Batch solid waste bioreactors 
with working volume of 2 litres were used in this study. Batch loads of aerobically treated or untreated sisal 
leaf residue inoculated with activated sludge mixed culture were packed into the bioreactors and operated 
anaerobically for 400 h. The fermentation products were mainly (mg/g total volatile fatty acids, VFAs): acetic 
acid (287), n-butyric acid (201), n-valeric acid (96) and caproic acid (62) as well as with low amounts of 
propionic acid and iso-butyric acid for aerobic pre-treated sisal leaf waste solids. Contrarily, for the untreated 
system, the fermentation products were chiefly (mg/g total volatile fatty acids): propionic acid (317), iso
butyric acid (276), n-butyric acid (96), acetic acid (84) and insignificant amounts of n-valeric acid, iso-valeric 
acid and caproic acid. Although the activities of hydrolytic enzymes found were similar for both treated and 
untreated SLDR, proportions of VFAs obtained with the former residues appeared to be better substrates for 
biomethanantion than those obtained from the latter substrates. These results indicated the potential of 
aerobic pre- treatment for enhanced bioconversion of SLDR. The present study, reports for the first time the 
types and levels of VFAs and hydrolases produced during anaerobic digestion of aerobic pre-treated SLDR 
and could be used as a basis for designing a pilot scale process.