CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
In Europe, Israel, Japan and the US research is being conducted into the use of solar energy to replace or supplement the energy requirements for treatment of hazardous wastes. This has been the subject of a collaborative research program referred to as SolarPACES. In this context, the following discussion refers to the outcomes of a range of research projects focussing on the use of solar energy in hazardous waste treatment.
Solar energy can be utilised for the destruction of hazardous organic chemicals in soil, water and air, either by direct thermal decomposition or by photochemical reaction. The benefits include savings in fuel use, improved thermal destruction of contaminants, and a reduction in exhaust gas volumes, including products of incomplete combustion, such as soot and NOx.
Solar detoxification processes can either use thermal energy or take advantage of a range of photochemical reactions.
In thermal treatment organic chemicals in soil and water, including dioxins, polychlorinated biphenyls (PCBs), and furans can be destroyed. Thermal treatment can also be used for inorganic wastes by melting eg filter dusts, and forming a glassy, inert material.
For efficient use of solar energy in thermal treatment, concentration of the solar radiation is necessary to achieve the high temperatures required for decomposition or destruction of the contaminants being treated. Solar radiation is reflected by mirrors (heliostats) and absorbed by a receiver reaching temperatures of up to 2300 K depending on the absorber.
Solar enhanced thermal treatment has been developed for solar detoxification of soils by Science Applications International Corporation (SAIC) and Energy and Environmental Research Corporation (EER) in the USA. The system is used for detoxification of organic contaminants from both soils and liquid wastes. No auxiliary fuel is required and it has been demonstrated to show an improvement in the destruction and removal efficiency (DRE) of organics including, PCBs, by a factor of 100 or more over conventional thermal technologies. High destruction efficiencies can be achieved at a temperature of 7500C which is lower than the temperature required for thermal incineration. Field demonstrations of the system for the US Army Environmental Center are scheduled for mid-1997. These trials are designed to characterise the system's performance and identify optimal operating conditions (Doty, 1997).
The main photochemical processes that aid thermal treatment in solar detoxification include: photocatalytic oxidation using, for example, titanium dioxide (TiO2) as a catalyst; sensitised formation of singlet oxygen or hydroxyl radical using a dye-sensitiser; direct photochemistry where light is absorbed by a compound; and photo-Fenton chemistry with hydroxoiron (III) complexes.
Photocatalytic solar detoxification processes utilise solar radiation from various parts of the spectrum. Ultraviolet radiation is used to promote an oxidation reaction in photocatalytic reactions using a catalyst such as TiO2 in the presence of oxygen. The reactivity of singlet oxygen, irradiated with visible light in the presence of dissolved oxygen, is utilised in the dye-sensitiser processes. The reactive species formed can then react with contaminant molecules in the waste, effecting the treatment.
Research projects into photochemical processes have been undertaken in Europe for the detoxification of water using dye-sensitised and catalytic photoreactions. Oxidative degradation of pesticides, including Lindane in contaminated water has been tested with direct sunlight in a solar furnace. Three prototype reactors were constructed and the photocatalysts used were TiO2 for hydroxyl radical generation, dyes such as methylene blue and rose bengal for singlet oxygen generation, and ferric chloride/hydrogen peroxide in Photo-Fenton OH radical generation.
Singlet oxygen was effective against some of the pesticides but reacted slowly or not at all with others. All pesticides were degraded by OH radical generating agents (such as methylene blue), however, the cost of treatment with methylene blue is less than the Fenton system which in turn costs less than titanium dioxide catalysts. Each system has different capabilities which needs to be taken into consideration when making comparisons (Funken, 1997).
Field demonstrations using the solar detoxification soil system were due to commence in June 1997 in the USA, with final test results available in July.
No information was provided regarding excisting applications of this technology in Australia. Laboratory scale and some pilot scale experiments have been performed in Spain, Germany, Israel, Japan and the USA.
(a) Proponents (in Australia)
Nil at this stage.
(b) Wastes Applicable
Primary focus has been on contaminated soils and water, and gas streams.
(c) Contaminants Applicable
All scheduled compounds.
(d) Timing for Commercialisation in Australia
Uncertain. Still largely at a research and demonstration phase internationally.
(e) Costs (example only)
No information provided.
(f) Safety/Environmental Risk
Limited information provided. The solar detoxification processes should compare favourably with alternatives due to the use of a renewable energy source, limited use of other chemicals and lower offgas generation rates than for other thermal technologies.
(g) Non-technical Impediments
None envisaged at this stage.
(h) Preferred Mode of Implementation
Not stated. Both mobile and fixed facilities have been proposed.
No information at this stage.
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