Environmental Engineering Reference
In-Depth Information
1. In hydrolysis, facultative and anaerobic hydrolytic bacteria transform polysaccharides
into simple sugars (pentoses and hexoses), proteins into amino acids, fats into free fatty
acids and glycerol, and nucleic acids into purines and pyrimidines.
2. During acidogenesis, sugar compounds formed during the first phase are transformed into
organic acids, such as acetic, propionic and butyric, and low amounts of ethanol, carbon
dioxide, and hydrogen.
3. Acids produced during acidogenesis are transformed into acetate, hydrogen, and carbon
dioxide.
4. During methanogenesis strictly anaerobic methane-forming bacteria produce methane
according to two different routes: in one by combination of carbon dioxide and hydrogen
and in a second by cleaving acetic acid into methane and carbon dioxide (Henze et al.,
2008; Tchobanoglous et al., 2004).
Anaerobic wastewater treatment systems
Anaerobic wastewater treatment systems can be open to the air or closed. Anaerobic lagoons,
generally used to treat wastewater from animal facilities, are examples of open systems.
Some food processing plants also use anaerobic or facultative lagoons to treat their waste-
water. The disadvantage of these lagoons is that the methane gas is vented without the pos-
sibility of capture and the production of intense offensive odors. A relatively inexpensive
solution is creating lagoons lined and covered with an impermeable membrane (geomem-
brane) to grant anaerobic conditions, contain odors, and maximize methane generation. In
addition the biogas produced, which is a mixture of methane and carbon dioxide, can be
captured and used. Anaerobic lagoons are not heated and therefore not suitable for cold cli-
mates. Anaerobic mesophilic bacteria need at least 20
°
C to develop and higher temperatures
to function optimally.
Instead of lagoons, a controlled option is the use of anaerobic reactors. These are closed
insulated containers that built in different configurations. They generally have a heating
device, an inlet for the wastewater, an outlet for the effluent, a gas outlet, and in some cases
stirrers. The anaerobic fermentation process works at its optimum when the temperature is
maintained constant.
Most digesters work between 30 and 38
C, which promotes the development of mesophilic
bacteria, but they can also be set at a range that encourages the development of themophilic
microorganisms that thrive between 50 and 70
°
C. In the thermophilic range, formation of
biogas takes place more rapidly, but the digestion is more sensitive to disturbances. In colder
areas, such as in the northern hemisphere, digesters need to be kept at a constant temperature
with some type of heating mechanism. The best option is to use part of the biogas produced to
generate heat for the reactor (Soares, 2007).
In cases in which the biogas is used to run an engine, or a microturbine, gases from the
exhaust can be used to heat the reactor or the engine coolant can be circulated through a heat
exchanger mounted inside or surrounding the reactor (a typical example of combined heat and
power generation). The use of biogas to produce energy or heat is considered a renewable
source of energy with net-zero carbon dioxide emissions because it is “biogenic” carbon that
is part of the natural carbon balance that does not add additional carbon dioxide to the atmos-
phere (Intergovernmental Panel on Climate Change [IPCC], 2007).
In terms of design, anaerobic systems are classified as follows:
°
Anaerobic suspended growth processes.
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Anaerobic sludge blanket processes.
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