The technology does not generate toxic byproducts, such as dioxins or furans, it
operates at pressure and temperatures close to the ambient, it does not require fuel
(it minimizes secondary residues), and it can simultaneously remove dangerous
organics and emissions like SO x /NO x . It does not require catalysts. However, this
technology has not reached yet sufficient level of development to be used in practice,
but some work on the design of the reactors has been recently attempted (Stratton
et al. 2015).
7.2.8 Oxidation in Sub- and Supercritical Water
These technologies allow the oxidation of the pollutants in a mixture of water with
oxygen, or air at high pressures and temperatures (Domènech et al. 2004; Kronholm
and Riekkola 1999; Zhang and Chuang 1999; Martino and Savage 1999; Tungler
et al. 2015).The operating process under subcritical conditions is called wet air
oxidation (WAO); it works at pressures between 1000–22,000 kPa and temperatures
between 150–370
C. The mechanism involves the primary carbonization of the
organic compounds and the further reaction with HO
• produced in the catalytic
transformation of dissolved O 2 (DO) on the surface of the carbon center, Eqs. 7.43
and 7.44. Nitrogen, halogens, and sulfur moieties are also mineralized, Eqs. 7.45,
7.46 and 7.47. Equation 7.48 shows the origin of the main byproducts.
O 2 ! H 2 O 2 ! O 2 and HO
•
ð7:43Þ
HO
•
þ C ! CO 2 þ H 2 O
ð7:44Þ
N ! NH 3 , NO 3
À or elemental N
ð7:45Þ
X ! X À
ð7:46Þ
S ! SO 4
2À
ð7:47Þ
C þ HO
•
! low molecular weight organic acids
ð7:48Þ
Figure 7.1 shows a typical flow diagram of a WAO process (Mishra et al. 1995).
Since the oxidation reactions are exothermic, sufficient energy may be released in
the reactor to allow the WAO system to operate without any additional heat input at
or above a chemical oxygen demand (COD > ~10,000 mg/L), and the process can be
self-supported; in comparison, incineration requires COD between 300,000 and
400,000 mg/L. The addition of an oxidant such as O 2 , H 2 O 2 , or potassium persulfate
(Kronholm and Riekkola 1999) improves the efficiency. Any kind of wastes can be
treated this way, including sludge, water wastes of high COD, biorecalcitrant wastes,
municipal sewage sludge, distillery wastes, paper pulp, black liquors of textile
waters, wastewaters with CN
– and nitriles, wastes adsorbed on the carbon used in
treatment of effluents (for regeneration), etc. Polyphenols, perchlorophenols,
134
M. I. Litter
operates at pressure and temperatures close to the ambient, it does not require fuel
(it minimizes secondary residues), and it can simultaneously remove dangerous
organics and emissions like SO x /NO x . It does not require catalysts. However, this
technology has not reached yet sufficient level of development to be used in practice,
but some work on the design of the reactors has been recently attempted (Stratton
et al. 2015).
7.2.8 Oxidation in Sub- and Supercritical Water
These technologies allow the oxidation of the pollutants in a mixture of water with
oxygen, or air at high pressures and temperatures (Domènech et al. 2004; Kronholm
and Riekkola 1999; Zhang and Chuang 1999; Martino and Savage 1999; Tungler
et al. 2015).The operating process under subcritical conditions is called wet air
oxidation (WAO); it works at pressures between 1000–22,000 kPa and temperatures
between 150–370
C. The mechanism involves the primary carbonization of the
organic compounds and the further reaction with HO
• produced in the catalytic
transformation of dissolved O 2 (DO) on the surface of the carbon center, Eqs. 7.43
and 7.44. Nitrogen, halogens, and sulfur moieties are also mineralized, Eqs. 7.45,
7.46 and 7.47. Equation 7.48 shows the origin of the main byproducts.
O 2 ! H 2 O 2 ! O 2 and HO
•
ð7:43Þ
HO
•
þ C ! CO 2 þ H 2 O
ð7:44Þ
N ! NH 3 , NO 3
À or elemental N
ð7:45Þ
X ! X À
ð7:46Þ
S ! SO 4
2À
ð7:47Þ
C þ HO
•
! low molecular weight organic acids
ð7:48Þ
Figure 7.1 shows a typical flow diagram of a WAO process (Mishra et al. 1995).
Since the oxidation reactions are exothermic, sufficient energy may be released in
the reactor to allow the WAO system to operate without any additional heat input at
or above a chemical oxygen demand (COD > ~10,000 mg/L), and the process can be
self-supported; in comparison, incineration requires COD between 300,000 and
400,000 mg/L. The addition of an oxidant such as O 2 , H 2 O 2 , or potassium persulfate
(Kronholm and Riekkola 1999) improves the efficiency. Any kind of wastes can be
treated this way, including sludge, water wastes of high COD, biorecalcitrant wastes,
municipal sewage sludge, distillery wastes, paper pulp, black liquors of textile
waters, wastewaters with CN
– and nitriles, wastes adsorbed on the carbon used in
treatment of effluents (for regeneration), etc. Polyphenols, perchlorophenols,
134
M. I. Litter