pressure and temperature, higher than those of the critical point of water,
1 between
450 and 700
C and pressures close to 25 MPa. In the supercritical state, the water
behaves as a fluid of relatively low viscosity, density, and dielectric constant. Under
supercritical conditions, the number of hydrogen bridges decreases, the solubility of
organic compounds and gases increases, and the solubility of the electrolytes
decreases. These reasons make SCWO an exceptional treatment system, with an
oxidation efficiency higher than 99.99% in very short contact times (5–60 s), not
requiring any additional treatment of the gaseous products. Due to the high solubility
of O 2 in supercritical water, there are no mass transfer problems. In addition, as the
surface tension is null, O 2 can penetrate into the smallest pores and can oxidize any
organic substance. On the other hand, it is possible to remove inorganic compounds
by precipitation. The process can be energetically self-supported if organics in
concentrations higher than 5% are treated, and the combustion heat can be recovered
as process heat at high temperature or power. However, the process requires severe
operative conditions; moreover, it is not attractive for diluted waters, and dibenzofurans and dioxins can be produced. Similarly to WAO, SCWO needs expensive
especial construction materials for high temperatures and pressures.
SCWO can be applied to the treatment of aqueous conventional organic wastes,
sludges, chlorinated and nitrogenated solvents, chlorinated, phosphorous and
nitrogen pesticides and herbicides, PCBs, dyes, plasticizers, plastics, byproducts
of the chemical and pharmaceutical industry, military products such as explosives,
propellants or war gases, etc. It is highly promising for treating hazardous wastes
as well. However, SCWO treatments at the pilot plant scale of real wastewaters are
scarce, and the application of this technology to industrial wastewaters has two
main drawbacks: corrosion and salt deposition; some other problems should be
also solved related to management of biphasic wastes, presence of suspended
solids, high costs, etc. Therefore, currently, the industrial scale-up and commercialization of the process is still a critical issue. Improvements, especially in the
design of the proper reactors, are imperative. The design of SCWO plants has to
include an energy recovery system in order to achieve economic feasibility of the
process. Although some commercial plants were built in the past, only two of them
are still in operation. Technical solutions to decreasing the capital and the operating costs in order to achieve full commercial development of this technology are
vital (Vadillo et al. 2013).
7.2.9 Electrohydraulic Discharge – Ultrasound
This technology uses high power ultrasound (from 15 kHz up to 1 MHz), taking
advantage of the electrohydraulic cavitation, i.e., the growing and cyclic collapse of
gas bubbles under these conditions. The gas implodes and very high local
1 Water critical points: T c ¼ 374
C and P c ¼ 22.1 MPa.
136
M. I. Litter
1 between
450 and 700
C and pressures close to 25 MPa. In the supercritical state, the water
behaves as a fluid of relatively low viscosity, density, and dielectric constant. Under
supercritical conditions, the number of hydrogen bridges decreases, the solubility of
organic compounds and gases increases, and the solubility of the electrolytes
decreases. These reasons make SCWO an exceptional treatment system, with an
oxidation efficiency higher than 99.99% in very short contact times (5–60 s), not
requiring any additional treatment of the gaseous products. Due to the high solubility
of O 2 in supercritical water, there are no mass transfer problems. In addition, as the
surface tension is null, O 2 can penetrate into the smallest pores and can oxidize any
organic substance. On the other hand, it is possible to remove inorganic compounds
by precipitation. The process can be energetically self-supported if organics in
concentrations higher than 5% are treated, and the combustion heat can be recovered
as process heat at high temperature or power. However, the process requires severe
operative conditions; moreover, it is not attractive for diluted waters, and dibenzofurans and dioxins can be produced. Similarly to WAO, SCWO needs expensive
especial construction materials for high temperatures and pressures.
SCWO can be applied to the treatment of aqueous conventional organic wastes,
sludges, chlorinated and nitrogenated solvents, chlorinated, phosphorous and
nitrogen pesticides and herbicides, PCBs, dyes, plasticizers, plastics, byproducts
of the chemical and pharmaceutical industry, military products such as explosives,
propellants or war gases, etc. It is highly promising for treating hazardous wastes
as well. However, SCWO treatments at the pilot plant scale of real wastewaters are
scarce, and the application of this technology to industrial wastewaters has two
main drawbacks: corrosion and salt deposition; some other problems should be
also solved related to management of biphasic wastes, presence of suspended
solids, high costs, etc. Therefore, currently, the industrial scale-up and commercialization of the process is still a critical issue. Improvements, especially in the
design of the proper reactors, are imperative. The design of SCWO plants has to
include an energy recovery system in order to achieve economic feasibility of the
process. Although some commercial plants were built in the past, only two of them
are still in operation. Technical solutions to decreasing the capital and the operating costs in order to achieve full commercial development of this technology are
vital (Vadillo et al. 2013).
7.2.9 Electrohydraulic Discharge – Ultrasound
This technology uses high power ultrasound (from 15 kHz up to 1 MHz), taking
advantage of the electrohydraulic cavitation, i.e., the growing and cyclic collapse of
gas bubbles under these conditions. The gas implodes and very high local
1 Water critical points: T c ¼ 374
C and P c ¼ 22.1 MPa.
136
M. I. Litter