CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
Supercritical Water Oxidation (SCWO) is a high temperature and pressure technology that uses the properties of supercritical water in the destruction of organic compounds and toxic wastes. The process was developed by Modell and co-workers based on investigations undertaken in 1975.
The SCWO technology has been applied to treat aqueous waste streams, sludges and contaminated soils. It is applicable to the treatment of a range of contaminants including acrylonitrile wastewater, cyanide wastewater, pesticide wastewater, PCBs, halogenated aliphatics and aromatics, aromatic hydrocarbons, MEK and organic nitrogen compounds. The SCWO process treats a range of wastes from municipal and industrial sources including biological sludges, industrial organic chemicals, plastics, synthetics, paints and allied products, industrial organics, agricultural chemicals, explosives, petroleum and coal products, rubber and plastic products.
For contaminated aqueous streams, SCWO is achieved by mixing an oxidant (either oxygen or hydrogen peroxide or a combination of both, or nitrate, nitrite or any other oxidants) with the waste stream and heating the mixture to the reaction temperature under supercritical pressure. For non-aqueous wastes, water can be added in a recycling system to provide a medium for the reaction. Under supercritical conditions, the reaction occurs in a homogeneous phase where carbon is converted to carbon dioxide, hydrogen to water, chlorine atoms derived from chlorinated organic compounds to chloride ions, nitro-compounds to nitrates, sulphur to sulphates, and phosphorus to phosphate. Since the reactions are exothermic, the process can become thermally selfsustaining at the appropriate concentration of the organic waste (Shanableh, 1995).
The process is implemented using totally enclosed treatment facilities in either an aboveground reactor, or an underground deep-well reactor (eg. comprising concentric tubes for separation of downflow and upflow, with the weight of liquid providing some or all of the pressure necessary to achieve the desired reaction conditions). The oxidant is injected as required based on heat transfer, thermal and kinetic considerations. The process results in the formation of a range of end products including disposable ash and releasable gases and a residual brine solution.
Figure 15.1 depicts a typical arrangement of a deep-well SCWO reactor.
The literature reports destruction and removal efficiencies (DREs) of greater than 99% for the treatment of numerous hazardous organic compounds using SCWO. For example, bench scale tests have shown DREs of 99.999% or higher for chlorinated solvents, PCBs and pesticides, and >99.99994% for dioxin contaminated MEK (US Congress, 1991). More recently, laboratory studies have shown that SCWO of chlorinated organics (eg. trichloroethylene) was essentially complete (>99.97%) at 450 0C with only 40 seconds residence time using hydrogen peroxide as the oxidiser (Foy et. al., 1996).
Deep-well vertical reactor technology was demonstrated to be an effective and cost-competitive sludge management option in Longmont, Colorado in the US. The Longmont reactor consisted of a 25 cm diameter stainless steel sub-surface tubular reactor reaching a depth of approximately 1600 metres. A metallurgical study after five years of operation showed insignificant corrosion, consistent with the 20 year design life.
The effectiveness of the technology for the destruction of hazardous organic wastes has been reported as having been demonstrated using bench scale and pilot scale, batch and continuous flow reactors. Treatment is achieved in totally enclosed treatment facilities. The high temperatures and pressures require special equipment, reactor materials and safety precautions. Less information is available on the oxidation mechanisms and the resulting by-products and emissions. However, the resulting ashes are generally found to contain contaminants in a non-leachable form, although the ash may still require disposal in a landfill. Generally the SCWO is limited to waste which is liquid or has a particle size of less than 200 m (US Congress, 1991). This may prove prohibitive for wastes which are predominantly solid in nature, such as soil. Although SCWO can treat wastes with a high organic content, it is usually limited to wastes with an organic content of less than 20% (US Congress, 1991).
A research effort is being established at the Queensland University of Technology in Australia by Dr. Abdullah Shanableh. Research work is also being carried out at the University of New South Wales (Foster, 1994). MODAR Inc. may be interested in licence opportunities in the region.
A portable pilot plant continuous SCWO system is in the process of being designed at the School of Civil Engineering at QUT (Jomaa, 1997). This system will represent a totally enclosed treatment facility which can be mounted on a semi-trailer for on-site testing and real life applications.
SCWO research and development has been active in the USA, at the Centre for Energy Studies at the University of Texas. The SCWO program was supported by The Gulf Coast Waste Disposal Authority, Gulf Coast Hazardous Substance Research Centre, Oxidyne Corporation, USEPA and Texas Energy Research and Application Program (Shanableh, 1994).
Process development activities overseas include:
(a) Proponents (in Australia)
Research into the use of this technology is being conducted at the Queensland University of Technology and the University of New South Wales.
(b) Wastes Applicable
Wastes listed in Section 15.1, especially wastes which are pumpable and have a solids size range less than 200 µm, and with an organic content of less than 20%.
(c) Contaminants Applicable
Acrylonitrile wastewater, cyanide wastewater, pesticide wastewater, PCBs, halogenated aliphatics and aromatics, aromatic hydrocarbons, MEK and organic nitrogen compounds.
The technology is in the research phase in Australia. Several units are being developed in the USA including a full-scale reactor which began operation in the USA in 1993.
(e) Timing for Commercialisation in Australia
Unknown. Depending on commercial viability and regulatory approvals, there is potential for a SCWO facility to be established within a relatively short period of time (say, 2 or 3 years) drawing on experience gained with commercial scale units in the US.
(f) Cost (example only)
The costs (US dollars) of SCWO can range from $120 to $140 / dry ton assuming some pretreatment (eg. thickening the feed sludge to 12%, operating at a lower pressure of 135 atm) (Hutchenson, et. al, 1994).
(g) Safety/Environmental Risk
The process operates in closed treatment facilities. End products include disposable ash, brine or salt solution and releasable gases. The ash is reported to generally contain non-leachable contaminants. However, it will probably require disposal to landfill unless it can be reused (eg concrete manufacturing).
The high temperatures and pressures used in the process require specialised equipment, reactor materials and safety precautions.
(h) Non-Technical Impediments
At this stage this is not known although deep wells in the ground may be of concern where groundwater has significant beneficial uses.
(i) Preferred Mode of Implementation
End products such as ash and brine require disposal. This technology is limited to the treatment of waste which is liquid or has a particle size less than 200 µm and is most applicable to wastes with an organic content of less than 20%.
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