with mesophilic digestion, and reduced benefits of pre-treatment
can be expected.
Some commercial processes were developed based on thermal
pre-treatments. The Norwegian company Cambi developed a
system based on thermal hydrolysis [144]. A solids solubilisation
of approximately 30% was reported (dependant on the type of
sludge being processes) for a 30 min treatment at 180 1C. An
associated increase of biogas production by 150% is reported by
the company. A similar thermal treatment is sold as BioThelyss by
Kru¨ ger Inc., a subsidiary of Veolia Water.
Evidently, the thermal pre-treatment requires the input of a
considerable amount of heat, since the sludge feedstock needs to
be preheated to the operating temperature (700 kJ/m3) at the
expense of using some of the biogas produced.
5.3. Mechanical pre-treatment
Mechanical treatment employs several strategies for physically
disintegrating the cells and partly solubilising their content.
The use of a colloid mill (with stationary and rotating disc) for
disrupting microbial cells was first reported by Harrison [145].
The heating of the suspension by energy dissipation can moreover
enhance the disintegration. The same paper also describes the use
of a high-speed shaker ball mill for sludge disintegration. In the
treatment reactor, moving impellers transfer kinetic energy to
grinding glass beads thereby creating high shear stresses that
break the cell walls. Alternative ball mills using ceramic or steel
beads were also reported. The use on an agitator ball mill was
studied by Kunz et al. [146]. Sludge was pressed through a
cylindrical or conical space by an agitator inducing shear stresses
of sufficient magnitude to break the bacterial cell walls.
One of the most frequently used methods for large-scale
operation is high-pressure homogenisation, compressing the
sludge to 60MPa [145,147]. The compressed suspension is then
depressurised through a valve and projected at high speed against
an impaction ring. The cells are hereby subjected to turbulence,
cavitation and shear stresses, resulting in cell disintegration.
Some studies reporting the effects of these mechanical
methods on AD are summarised in Table 10.
Although less results are available than for the other pretreatment
methods, it is seen that their efficiency of improving AD
of sewage sludge is rather low, compared to the other methods.
Although most techniques consume a lot of power [130], they do
not require the addition of chemicals or heat.
5.4. Chemical pre-treatment
Chemical pre-treatment to enhance the AD treats the sludge to
hydrolyse the cell wall and membrane and thus increase the
solubility of the organic matter contained within the cells. Various
chemical methods have been developed, based on different
operating principles. The major groups are (i) acid and alkaline
(thermal) hydrolysis, (ii) ozonation, and (iii) advanced oxidation
methods. These methods are described hereafter.
5.4.1. Acid and alkaline (thermal) hydrolysis
In (thermo)chemical hydrolysis methods, an acid or base is
added to solubilise the sludge. The addition of acid or base avoids
the necessity of high temperatures and these methods are thus
mostly carried out at ambient or moderate temperatures. An
overview of these methods is presented in Neyens and Baeyens
[153]. Some experimental results are given in Table 11. The
methods are shown to be an effective albeit cumbersome method
for sludge solubilisation since required pH levels are extreme, and
sludge needs subsequently to be re-neutralised. Their use as a pretreatment
for AD is hence rather limited.
5.4.2. Oxidative sludge pre-treatment
Oxidative waste sludge destruction was first practised in the
aerobic Zimpro process originally designed as a wet oxidation
method in the USA (1954). This process uses oxygen or air at high
temperatures (260 1C) and pressures (10MPa) [159]. An effective
solubilisation of a large part of the sludge was achieved. Problems
with mesophilic digestion, and reduced benefits of pre-treatment
can be expected.
Some commercial processes were developed based on thermal
pre-treatments. The Norwegian company Cambi developed a
system based on thermal hydrolysis [144]. A solids solubilisation
of approximately 30% was reported (dependant on the type of
sludge being processes) for a 30 min treatment at 180 1C. An
associated increase of biogas production by 150% is reported by
the company. A similar thermal treatment is sold as BioThelyss by
Kru¨ ger Inc., a subsidiary of Veolia Water.
Evidently, the thermal pre-treatment requires the input of a
considerable amount of heat, since the sludge feedstock needs to
be preheated to the operating temperature (700 kJ/m3) at the
expense of using some of the biogas produced.
5.3. Mechanical pre-treatment
Mechanical treatment employs several strategies for physically
disintegrating the cells and partly solubilising their content.
The use of a colloid mill (with stationary and rotating disc) for
disrupting microbial cells was first reported by Harrison [145].
The heating of the suspension by energy dissipation can moreover
enhance the disintegration. The same paper also describes the use
of a high-speed shaker ball mill for sludge disintegration. In the
treatment reactor, moving impellers transfer kinetic energy to
grinding glass beads thereby creating high shear stresses that
break the cell walls. Alternative ball mills using ceramic or steel
beads were also reported. The use on an agitator ball mill was
studied by Kunz et al. [146]. Sludge was pressed through a
cylindrical or conical space by an agitator inducing shear stresses
of sufficient magnitude to break the bacterial cell walls.
One of the most frequently used methods for large-scale
operation is high-pressure homogenisation, compressing the
sludge to 60MPa [145,147]. The compressed suspension is then
depressurised through a valve and projected at high speed against
an impaction ring. The cells are hereby subjected to turbulence,
cavitation and shear stresses, resulting in cell disintegration.
Some studies reporting the effects of these mechanical
methods on AD are summarised in Table 10.
Although less results are available than for the other pretreatment
methods, it is seen that their efficiency of improving AD
of sewage sludge is rather low, compared to the other methods.
Although most techniques consume a lot of power [130], they do
not require the addition of chemicals or heat.
5.4. Chemical pre-treatment
Chemical pre-treatment to enhance the AD treats the sludge to
hydrolyse the cell wall and membrane and thus increase the
solubility of the organic matter contained within the cells. Various
chemical methods have been developed, based on different
operating principles. The major groups are (i) acid and alkaline
(thermal) hydrolysis, (ii) ozonation, and (iii) advanced oxidation
methods. These methods are described hereafter.
5.4.1. Acid and alkaline (thermal) hydrolysis
In (thermo)chemical hydrolysis methods, an acid or base is
added to solubilise the sludge. The addition of acid or base avoids
the necessity of high temperatures and these methods are thus
mostly carried out at ambient or moderate temperatures. An
overview of these methods is presented in Neyens and Baeyens
[153]. Some experimental results are given in Table 11. The
methods are shown to be an effective albeit cumbersome method
for sludge solubilisation since required pH levels are extreme, and
sludge needs subsequently to be re-neutralised. Their use as a pretreatment
for AD is hence rather limited.
5.4.2. Oxidative sludge pre-treatment
Oxidative waste sludge destruction was first practised in the
aerobic Zimpro process originally designed as a wet oxidation
method in the USA (1954). This process uses oxygen or air at high
temperatures (260 1C) and pressures (10MPa) [159]. An effective
solubilisation of a large part of the sludge was achieved. Problems
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