Anaerobic Digestion of Organic
Wastes for Biogas Production
Ally Mayer
Energy Systems Engineering Institute
ABSTRACT
INTRODUCTION
Anaerobic digestion of organic
wastes
and
wastewaters
provides
a
tremendous
opportunity
to
supply
a
sustainable source of energy
while reducing the amount of
waste that ends up in landfills.
This work discusses the up-todate literature on the anaerobic
digestion of cow manure and
the
organic
fraction
of
municipal solid wastes with a
focus on industrial food
wastewaters and food wastes.
Each waste was studied and
compared to determine the
optimum
feedstock
and
operating conditions for energy
production. From this study, it
was observed that food waste
and wastewaters represent the
most promising categories of
organic
feedstocks
for
anaerobic digestion for the
production
of
methane.
Additionally,
thermophilic
reactor conditions produced
higher quantities of biogas
than mesophilic reactors in
single stage systems. At higher
organic loading rates however,
two-phase digesters yielded
better results. Pre-treatments
were also studied in this work.
Anaerobic digestion is a series of biochemical processes
in which waste enters the digester and in the absence of
oxygen, microorganisms degrade the biodegradable
organic matter in the wastes through biological oxidation to
form biogas. The biogas is mainly composed of methane
and carbon dioxide. The methane can then be used as an
energy source while the solid-liquid slurry digestate may
be used as fertilizer that is free of pathogens and high in
nutrients. Anaerobic digestion can also reduce emissions
of landfill gases such as carbon dioxide and methane by
capturing the emissions within the digester. Thus, the
complete resource recovery from the anaerobic digestion
process reduces the cost of treatment while providing an
incentive for reducing pollution.
OBJECTIVE
To compile and review the
information available in the
scientific literature relative to
the anaerobic digestion of
organic wastes with a focus on
food wastes and wastewaters
for the production of biogas
and methane to be used as an
energy source. The operating
conditions, limitations, and
pre-treatments needed for a
successful digestion were also
researched. Finally, the results
of food waste digestion have
been
compared
to
the
digestions of cow manures
and the organic fraction of
municipal solid wastes.
CONTACT
Ally Mayer
Energy Systems
Engineering Institute
Email: [email protected]
Phone: (570) 498-6104
Poster Design & Printing by Genigraphics® 800.790.4001
DISCUSSION
Pre-treatments: Manual separation of biodegradable
components yields better results than mechanical
separation. Maceration increases methane production,
reaction rates, and biodegradability. pH should be adjusted
accordingly. Thermal pretreatments reduce digestion time.
Temperature: Thermophilic conditions yielded better
results than mesophilic conditions in single stage digesters.
Phase
configuration:
Two-phase
configurations
outperformed one-phase systems, especially at higher
organic loading rates. Temperature-phased systems show
high potential to control each process condition separately.
Feedstock: Food waste yielded higher methane contents in
the biogas than cow manure. Methane contents ranged
from 50-89.9% for food waste and 59-72.7% for manure
with corresponding biogas productions of 0.15-3.8 L/d and
0.0375 – 46.9 L/d, respectively. Food waste is also the best
performing feedstock of the organic fraction of municipal
solid wastes.
VS
destroyed
(%)
Manure
47
OFMSW
73
Food
78
Waste
Biogas
Methane Content
(L/d)
1.77
13.5
(%)
64
53
Methane
Production
(L/d)
1.14
7.16
19.9
65
12.9
PROBLEMS ENCOUNTERED
FEEDSTOCKS
Manure: The feces and urine of animals. In this case, dairy
cow manure was used as the focus point.
Organic fraction of municipal solid waste (OFMSW):
The umbrella term for the portion of municipal solid waste
that can be degraded. It includes food wastes, plants, yard
wastes, and paper. OFMSW makes up nearly 25% of urban
waste streams.
Food Waste: Uneaten food and food preparation leftovers
from residences, commercial establishments, and industrial
source. On average, Americans throw away about 44
million tons of food each year.
Food Wastewaters: Wastewater that is produced from
processing foods and beverages and includes: winery
wastewaters, olive mill wastewaters, milk processing
wastes, dairy wastes, spoiled fruit juices, etc.
After compiling the data into tables, it was seen that it is
very difficult to compare each experiment to one another.
• There is no standard unit or conventional method to
convert to a standard unit
• Generic formulas could be used to convert units, but
inaccurate results would be generated
• Due to varying hydraulic retention times, the biogas
productions should be normalized for comparisons
• Many factors are missing and assumptions would need to
be made
• Only significant comparisons come from varying
operating conditions within one experiment
• Experiments yield results specific to each operating
condition
making
comparisons
across
different
experiments difficult
CONCLUSIONS
Anaerobic digestion of organic wastes such as manure, the
organic fraction of municipal solid wastes, food wastes,
and food processing wastewaters has proven to be
successful under many different operating process
conditions. The solids contents have been reduced with a
production of an economic and sustainable fuel source in
the form of methane. It should be noted that the correct
operating conditions for each feedstock should be
determined prior to implementation of a full-scale system.
Anaerobic digestion can reduce pathogens and total solids
content while producing fertilizer and energy, making this
technology very well suited for organic wastes. Of all of the
feedstocks, food waste showed the most potential for
producing the highest amounts of methane.