Section 6 will focus on the possible techniques to upgrade the
biogas formed by removing CO2, H2S and excess moisture. A
special attention will be paid to the problems associated with
siloxanes (SX), including their origin and behaviour in sludge, and
the techniques to either reduce their concentration in sludge
by preventive actions such as peroxidation, or to eliminate the
SX from the biogas by adsorption or other techniques.
Section 7 will guide the reader to extensive publications
concerning the operation, control, maintenance and troubleshooting
of AD plants.
2. Basic principles and parameters of AD
2.1. Principles
The AD of organic material basically follows; hydrolysis, acidogenesis,
acetogenesis and methanogenesis as shown in Fig. 2. The
biological aspects of AD are dealt with in specialised literature [8–11].
AD is a complex process which requires strict anaerobic
conditions (oxidation reduction potential (ORP)o200 mV) to
proceed, and depends on the coordinated activity of a complex
microbial association to transform organic material into mostly
CO2 and methane (CH4). Despite the successive steps, hydrolysis is
generally considered as rate limiting [7,12–16].
The hydrolysis step degrades both insoluble organic material
and high molecular weight compounds such as lipids, polysaccharides,
proteins and nucleic acids, into soluble organic substances
(e.g. amino acids and fatty acids). The components formed
during hydrolysis are further split during acidogenesis, the second
step. VFA are produced by acidogenic (or fermentative) bacteria
along with ammonia (NH3), CO2, H2S and other by-products.
The third stage in AD is acetogenesis, where the higher organic
acids and alcohols produced by acidogenesis are further digested
by acetogens to produce mainly acetic acid as well as CO2 and H2.
This conversion is controlled to a large extent by the partial
pressure of H2 in the mixture.
Section 6 will focus on the possible techniques to upgrade the
biogas formed by removing CO2, H2S and excess moisture. A
special attention will be paid to the problems associated with
siloxanes (SX), including their origin and behaviour in sludge, and
the techniques to either reduce their concentration in sludge
by preventive actions such as peroxidation, or to eliminate the
SX from the biogas by adsorption or other techniques.
Section 7 will guide the reader to extensive publications
concerning the operation, control, maintenance and troubleshooting
of AD plants.
2. Basic principles and parameters of AD
2.1. Principles
The AD of organic material basically follows; hydrolysis, acidogenesis,
acetogenesis and methanogenesis as shown in Fig. 2. The
biological aspects of AD are dealt with in specialised literature [8–11].
AD is a complex process which requires strict anaerobic
conditions (oxidation reduction potential (ORP)o200 mV) to
proceed, and depends on the coordinated activity of a complex
microbial association to transform organic material into mostly
CO2 and methane (CH4). Despite the successive steps, hydrolysis is
generally considered as rate limiting [7,12–16].
The hydrolysis step degrades both insoluble organic material
and high molecular weight compounds such as lipids, polysaccharides,
proteins and nucleic acids, into soluble organic substances
(e.g. amino acids and fatty acids). The components formed
during hydrolysis are further split during acidogenesis, the second
step. VFA are produced by acidogenic (or fermentative) bacteria
along with ammonia (NH3), CO2, H2S and other by-products.
The third stage in AD is acetogenesis, where the higher organic
acids and alcohols produced by acidogenesis are further digested
by acetogens to produce mainly acetic acid as well as CO2 and H2.
This conversion is controlled to a large extent by the partial
pressure of H2 in the mixture.
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