chemical formula for organic waste is C6H 10
O4 (Shefali & Themelis 2002) Hydrolysis
reaction of organic fraction is represented by
following reaction:
C6H10O4 + 2H2O → C 6 H 12 O6 + 2H 2
(Ostrem & Themelis 2004)
Acitogenesis
This stage is facilitated by microorganisms
known as acid formers that transform the
products of the hydrolysis into simple organic
acids such as acetic, propionic and butyricacid
as well as ethanol, carbon dioxide and
hydrogen. . Acid forming stage comprises two
reactions, fermentation and the acetogenesis
reactions. During the fermentation the soluble
organic products of the hydrolysis are
transformed into simple organic compounds,
mostly volatile fatty acids such as propionic,
formic, butyric, valeric etc, ketones and
alcohols. Typical reactions occurring at this
stage are the following - Conversion of the
glucose to ethanol: - Conversion of the
glucose to propionate: (Ostrem & Themelis
2004) The acetogenesis is completed
through carbohydrate fermentation and results
in acetate, CO₂ and H₂, compounds that can
be utilized by the methanogens. The presence
of hydrogen is critical importance in
acetogenesis of compounds such as propionic
& butyric acid. These reactions can only
proceed if the concentration of H₂ is very low
(Ralph & Dong 2010). Thus the presence of
hydrogen scavenging bacteria is essential to
ensure the thermodynamic feasibility of this
reaction (Ostrem & Themelis 2004).
Important reactions during the acetogenesis
stage are as follow (Ostrem & Themelis 2004)
- Conversion of glucose to acetate:
- Conversion of ethanol to acetate.
- Conversion of propionate to acetate.
- Conversion of bicarbonate to acetate.
Methanogenesis.
Methanogenesis is a reaction facilitated by the
methanogenic microorganisms that convert
soluble mater into methane . Two thirds of the
total methane produced is derived converting
the acetic acid or by fermentation of alcohol
formed in the second stage such as methanol.
The other one third of the produced methane
is a result of the reduction of the carbon
dioxide by hydrogen. Considering that the
methane has high climate change potential the
goal is to find an alternative in order to lower
the environmental foot print of the organic
waste treatment. Therefore this stage is
avoided and instead of methane the
production of volatile fatty acids is targeted.
The reactions that occur during this stage are
as follows (Ostrem & Themelis 2004).
- Acetate conversion
2CH₃CH₂OH + CO₂ ↔ 2CH₃COOH + CH4
Followed by: CH₃COOH ↔ CH₄ + CO2
- Methanol conversion
CH₃OH + H₂ ↔ CH₄ + H₂O
- Carbon dioxide reduction by hydrogen
CO₂ + 4H₂ ↔ CH₄ + H₂O
PARAMETERS AFFECTING THE
ANAEROBIC DIGESTION OF KITCHEN
WASTES
1- pH value
The pH value of the reacting material is a
pivotal factor in the AD of kitchen waste. The
importance of the pH is due to the fact that
methanogenic bacteria are very sensitive to
acidic conditions and their growth and
methane production are inhibited in acidic
environment. In batch reactors pH value is
closer dependent of the retention time and
loading rate. Different stages of the AD
process have different optimal pH values.
Also the pH value changes in response to the
biological transformations during different
stages of AD process. Production of organic
acids during the acetogenesis can lower the
pH below 5 what is lethal for methanogens
and cause decrease in the methanogens
population. Consequently this would lead to
acid accumulation, since the methanogens
are responsible for the consumption of the
formed acids, and digester failure. Constant
pH is crucial in the start-up phase because
fresh waste has to go first thru the stage of
hydrolysis and acidogenesis before any
methane can be formed, which will lower the
pH. In order to keep the value of pH on the
equilibrium buffer has to be added into the
system, such as calcium carbonate or lime.
2 -Composition of the kitchen waste
It is important to know the composition of the
kitchen waste in order to be able to predict
both the bio-methanization potential and most
efficient AD facility design. The biomethanization
potential of the waste depends
on the concentration of four main components:
proteins, lipids, carbohydrates, and cellulose.
This is due to different bio-chemical
characteristics of these components (Nerves
et al. 2007)
The highest methane yields have systems with
excess of lipids but with longest retention time.
The methanization of the reactors with excess
of cellulose and carbohydrates respectively.
The lowest rates of the hydrolysis are with an
excess of lipids and cellulose, indicating that
when these components are in excess, a