Section 11. FOOD WASTES
HYBRID BIOLIQUEFACTION AND BIOGASIFICATION FOR PAPAYA
PROCESSING WASTES IN THE TROPICS
P. Y. Yang, Professor
C. Y. Chou, Graduate Research Assistant
Agricultural Engineering Department
University of Hawaii at Manoa
Honolulu, Hawaii 96822
INTRODUCTION
Anaerobic fermentation of fruit processing wastes holds potential for producing alternative energy
in addition to reducing the pollution problems. The biogas (50-70% of methane) produced from the
anaerobic fermentation of fruit processing wastes could have been used in equipment designed for
natural gas fuel with no or only minor purification. In order to utilize this potential, the anaerobic
fermentation of papaya processing wastes was preliminarily studied and reported in Hawaii [1,2]. It
has been indicated that the biogasification of papaya processing wastes for pollution control and
energy utilization is feasible. Also, the biogasification process with sludge recycling permits smaller
reactor volume without any deterioration of methane production rate and methane content. However,
the water addition (about 10 times that of raw papaya wastes) for preparation of slurry prior to
methane fermentation is considered as the main weakness. This is not appropriate because of more
problems of possible post-treatment or handling of diluted-digested papaya processing wastewater.
Although solid state fermentation has been studied for methane production from organic wastes with
high solid content [3,4], it requires very long retention time in order to stabilize the organic wastes.
Koster [5] investigated the liquefaction and acidogenesis of tomato solid wastes and suggested that
recirculation of the effluent from the methane reactor and addition of water should be considered
in order to stimulate liquefaction. Combined liquefaction and gasification treatment of organic residues were investigated with various solid agricultural and industrial organic residues including straw,
grass, cabbage leaves, waste-onions, sugar-beet pulp, cannery wastes, tomato-plant wastes, municipal
solid wastes, etc. [5-10]. This combined system offers a significant advantage by eliminating the post-
handling and treatment of the digested effluent and is free of the drawbacks of the dry anaerobic
fermentation (i.e., requirement of a long reaction time to stabilize the organic residues). However,
they are complicated in both structure and operation. Additionally, the detailed design and operational criteria for the system have not been established yet. Further study is needed to develop a
system which is efficient and simple in construction and operation with detailed design and operational criteria.
MATERIALS AND METHODS
Feedstock Preparation
The papaya processing wastes used in this study were collected from a fruit processing plant in
Honolulu, the Aloha Produce Corporation. The waste was transported to the Agricultural Engineering Department of the University of Hawaii and was stored in a freezer unit before needed for
experiments. It was withdrawn to defrost for two hours before use.
Experimental Set-up
The hybrid bioliquefaction and biogasification system consisted of an 8-liter bioliquefaction unit
and a 20-liter biogasification unit (Figure 1). This system was tested in a walk-in incubation room
at 30 ± 1 C. The bioliquefaction unit (Reactor 1), shown in Figure 2, was an acrylic plastic column
with a grease-sealed cover and a sieve plate on the bottom. The biogasification unit (Reactor 2),
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