CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
The Base Catalysed Dechlorination (BCD) 1 process, was developed to treat halogenated organic compounds. The process was developed from work by the USEPA on earlier forms of dechlorination (in particular the "KPEG" process). This work was undertaken at the Cincinnati Risk Reduction Research Laboratory. The proponents claim BCD is suitable for treatment of wastes which contain up to 100000 mg/kg of halogenated aliphatic or aromatic organic compounds such as PCBs. In practice, the formation of salt within the treated mixture can limit the concentration of halogenated material able to be treated. Reduction of chlorinated organics to less than 2 mg/kg is achievable (Rogers, 1991).
The BCD process can involve direct dehalogenation or decomposition of the waste material, or can be linked with a pretreatment step such as thermal desorption which yields a relatively small quantity of a condensed volatile phase for separate treatment by the BCD process.
5.2.1 Technology Description
The following description of the BCD process focuses on the BCD process applied either to the waste itself or to the separated volatiles from a preceding thermal desorption process. The description has been drawn from the BCD patent application (Rogers, 1991) and indicates some possible process variations. ADI Limited (ADI) in particular, has undertaken a considerable quantity of work aimed at refining elements of the process. In the following section, the application of thermal desorption for pretreatment of wastes is discussed.
The BCD process involves the addition of an alkali or alkaline earth metal carbonate, bicarbonate or hydroxide to the contaminated medium containing one or more halogenated or non-halogenated organic contaminant compounds. The BCD patent indicates that the alkaline chemical may be added to the contaminated medium in an aqueous solution, or in a high boiling point solvent. If the chemical is added in the form of a solid dispersion or suspension in water, the water assists in distributing the metal compound homogeneously throughout the contaminated medium. If the chemical is added with a high boiling point solvent, the solvent must have a boiling point of at least 200oC, and preferably be in the range from 200oC to about 500oC. Otherwise, it will distil from the mixture during treatment.
Alkali is added to the contaminated medium in proportions ranging from 1 to about 20 percent by weight. The amount of alkali required is dependent on the concentration of the halogenated or non-halogenated organic contaminant contained in the medium.
A hydrogen donor compound is added to the mixture to provide hydrogen ions for reaction with the halogenated and non-halogenated contaminants, if these ions are not already present in the contaminated material. The hydrogen donor compound may comprise the high boiling point solvent in which the alkali or alkaline earth metal compound is added, or it may include fatty acids, aliphatic alcohols or hydrocarbons, amines or other similar compounds. In order to activate these compounds to produce hydrogen ions a source of carbon must be added, either in solution or in suspension. An inexpensive carbon source which is water soluble and suitable for use, is a carbohydrate such as sucrose.
The mixture is heated at a temperature and for a time sufficient to totally dehydrate the medium. This may be performed at atmospheric or at reduced or elevated pressure. The water which is included in the aqueous solution allows homogeneous distribution of the alkali throughout the mixture and acts as a wetting agent and penetrant. When the water is removed from the medium during the dehydration step, the alkali is concentrated to a reactive state.
After dehydration, the medium is further heated at a temperature between 200oC and 400oC for a time sufficient to effect reductive decomposition of the halogenated and non-halogenated organic contaminant compounds, typically 0.5 to 2 hours. At this temperature the carbon source (eg the carbohydrate) acts as a catalyst for the formation of a reactive hydrogen ion from the hydrogen donor compound. This catalysed reaction is represented by the following reaction formula:
where R is the hydrogen donor compound, M is the metal compound, C refers to a source of carbon, for example a carbohydrate, and H is the hydride ion. The reactive hydride ion then reacts with the halogenated organic compounds contained in the contaminated medium according to the following reaction:
where R-X is the halogenated organic contaminant, X is the halogen atom and R-H is the reduced form of the contaminated compound.
Finally, the mixture is neutralised by the addition of an acid, preferably to a pH of 7 to 9. Depending on the nature of the feed material, the reagent additions and the site use, it may be possible for the treated material to be returned to the site if desired, although this may not be possible if the treated material is oily or has a high salt content.
Generally, oxygen will not adversely affect the BCD process and therefore air does not need to be excluded. When applied to the decontamination of hydrocarbon fluids, either aliphatic or aromatic, air needs to be excluded in order to prevent ignition of the hydrocarbon at the elevated temperature of the BCD reaction. This is achieved by passing nitrogen gas through the reaction vessel.
Given the process employs relatively small amounts of alkali and solvent (if used), recovery of excess reagents for reuse is not generally proposed. The treatment is usually carried out as a batch process with all steps completed within a single reactor.
Test results show that the BCD process is able to reduce PCB from 10000 mg/kg to below detectable limits in approximately 2 hours (Rogers, 1991). A sample of contaminated soil containing 2,200 mg/kg of Aroclor "1260", 1000 mg/kg of Aroclor "1242", 1000 mg/kg of PCP, 1000 mg/kg of dieldrin, 1000 mg/kg of lindane and 500 mg/kg of 2phenylnaphthalene, was treated by this process and the contaminants reduced to less than 1.0 mg/kg each. The 2phenylnaphthalene was also reduced to a cyclic hydrocarbon (Rogers, 1991).
The process mainly involves chlorine stripping. In treatment of chlorinated aromatic hydrocarbons the removal of chlorine atoms results in an increased concentration of lower chlorinated species (eg higher congeners are replaced by lower congeners). This is not a problem with contaminants such as PCBs. However, with constituents such as dioxins the lower congeners (eg TCDD) can have a higher toxicity than the more highly chlorinated congeners (eg OCDD). Therefore the process must be monitored to ensure that the reaction continues to completion.
In the case of treatment of PCBs and PCB contaminated oils, treatment will typically reduce the PCBs to less than detection (0.1 mg/kg total PCBs for the lower congeners, and 0.01 mg/kg for the higher congeners) if sufficient reaction time is allowed. Given that the process is a batch operation, it is possible to allow the reaction to proceed until the required level of destruction has been confirmed.
5.2.3 Considerations in the Application of the Technology
The BCD process is largely contained and the emission of gases is very small compared with other combustion systems. For example, air emissions associated with treating a contaminated soil containing 5000 mg/kg of PCP have been reported as follows (Carlisle, 1994a):
If volatile solvents are present (such as occurs with pesticides), then preferably these should be removed by distillation and the resulting sludge slurried in oil for treatment. The effect and therefore limitation of treating wastes containing volatile solvents is a reduction in boiling point of the BCD oil; high concentrations of solvents will reduce the boiling point and not allow the desired operating temperature to be achieved. As the system operates under reflux conditions, some solvent (including volatile chlorinated organic material) can be accepted for treatment.
Risk associated with process upset is considered to be low. The main concern would be with regard to air ingress which could result in auto ignition of the oil phase and an uncontrolled emission to air. The provision of a nitrogen atmosphere over the reactor is designed to ensure that this cannot occur. The occurrence of a fire in 1995 at the Victorian BCD facility operated by Technosafe was apparently the result of operation of a storage vessel without a nitrogen blanket (see below).
The BCD process is not favoured for treating large volumes of aqueous media (including wet sludges) because of the cost of evaporating the water. This restriction also applies when the waste material is pre-processed by a thermal desorption system as again energy is required to dry the waste. This is discussed further in Section 5.3.
5.2.4 Treatment of Capacitors
Direct treatment of capacitors containing PCBs by the BCD process is not appropriate because they contain aluminium and under the alkaline conditions of the BCD process hydrogen is evolved. Solvent extraction of shredded capacitors has been proposed. However, a large number of repeated extractions (eg 30 sequences) is required to obtain residual PCB concentrations which are suitable for landfill disposal (eg < 50 mg/kg) (Krynen, 1994b). On this basis, various proponents of the BCD technology have sought to develop alternative processes.
BCD Technologies have developed a pre-treatment step to avoid this problem (Krynen, 1994b,c). They shred the capacitor and treat the shredded material with sodium hydroxide at ambient temperature. This allows hydrogen to be generated and vented to the atmosphere at ambient temperature and avoids the higher temperature and increased explosion potential of the BCD process. The material is then treated in the normal BCD process.
BCD Technologies received an amendment to their license in September 1994 which allows them to treat capacitors containing PCBs in commercial quantities. As a result, a treatment plant was constructed and commissioned and is now in operation (Krynen, 1995).
5.2.5 Experience and Availability in Australia
Three proponents of the BCD technology are ADI Limited, BCD Technologies (Brisbane) and Technosafe (Melbourne). As originally established the licence status of each group was as follows:
The BCD licence for each group has recently been renegotiated such that each group is now able to directly apply the BCD technology to liquids and solids in Australia. While the licences now allow a number of groups to treat soils and other solids in Australia, in practice the facilities currently available are limited to the treatment of liquids and contaminated equipment.
The status of development and application of the BCD process by each of the proponents in Australia is outlined as follows:
Technosafe have re-established a BCD facility in Melbourne following a fire in 1995 which rendered the original unit inoperable (Carlisle, 1995). The fire damaged the treatment system and building. It is understood that the fire resulted from a combination of factors (Carlisle, 1995). The nitrogen blanket was in place over the reactor, however, on discharge of hot oil into a storage vessel without an adequate nitrogen blanket, the fire occurred in the storage vessel. The auto ignition point of the hot oil was lower than expected and was exceeded. The new unit has received approval from the Victorian Environmental Protection Authority (EPAV) (February, 1997) and Technosafe are again operating on a commercial basis, focussing on PCB contaminated oils, transformers and capacitors.
A licence was issued to Technosafe for a fixed PCB liquids treatment facility for treatment of liquids containing up to 2% PCBs and soils. However, at this stage, Technosafe are focussing on the treatment of PCB oils and equipment, rather than soil.
The BCD process is in operation in Brisbane (BCD Technologies) (Krynen, 1994a) for the treatment of liquids. Regulatory approvals for the Brisbane BCD facility extend to the treatment of liquid PCBs and a range of halogenated pesticides, and the use of the BCD plant on a portable basis (eg relocated and used on site) (Krynen, 1994b). A second plant for the treatment of organic liquids has been constructed in Brisbane in order to meet market demand. The new plant has a treatment capacity of 2500 tonnes per annum (Krynen, 1995).
While the BCD Technologies plant is capable of and licensed to treat organochlorine pesticide wastes, to date this has only been a limited component of the plant throughput. The treatment of pesticide wastes and derivatives of these contaminants require the fitting of additional odour control processes, which interrupt the treatment of PCB wastes (Krynen, 1996).
The new facility being established by BCD Technologies will have improved odour control and hence the capability of the unit to treat pesticide wastes will be enhanced. The BCD process should be capable of treating mixed pesticide wastes, eg. DDT and arsenic mixtures, however arsenic would remain in the process residue and amendments to licence conditions may be required to handle the arsenic waste generated. To date mixed pesticide wastes have not been treated in significant quantities in the Australian BCD facilities (Krynen, 1997).
BCD Technologies reports that it is currently treating capacitors containing PCB liquids at the rate of about 1 tonne per day but it hopes to significantly increase this rate. BCD are also in the process of developing an alternative capacitor treatment process. The excisting market demand for treatment of PCB contaminated materials is sufficient to ensure full utilisation of the current facilities.
Until recently, ADI Limited only held a BCD licence in Europe and had to operate in Australasia via a sub-licence. As part of renegotiation of the licence with the BCD Group, ADI has been granted an unrestricted licence, allowing for the treatment of soils and liquids in Australia. As part of the licence renegotiation, ADI and the BCD Group reached a commercial resolution with respect to the use of the Soil Thermal Treatment Process (STTP ) (the BCD variant developed by ADI) in Australia and New Zealand and will now jointly promote the BCD process.
While much of ADI's attention is focused on the treatment of soils and other solid wastes it also has a capability to treat liquid wastes. ADI has spent considerable effort developing in-house expertise in the BCD process and further modifying and refining the process. Through its work on the BCD process, ADI has developed the STTP which is a new process. STTP is outlined in detail in Section 5.4.
ADI, in conjunction with Institute of Environmental Science & Research Limited, NZ (ESR), has demonstrated the treatment of PCP and dioxin contaminated soil using the BCD/STTP process in New Zealand. The trial was conducted on behalf of the Ministry for the Environment and the Timber Industry Environment Council. A 60 kg/hour continuous thermal desorption unit was used to treat PCP and dioxin contaminated soil from a timber facility. The process achieved <20 ppb PCP and <1 ppb dioxin (TE) in the treated soil, confirming the ability to treat contaminated soils in a single stage process. The report covering this work is expected to be publicly available in September, 1997 (Truong, 1997).
ADI is also conducting treatability trials in New Zealand for chlorinated pesticides (eg. DDT and dieldrin from pesticide collections) using a liquid BCD treatment plant. The plant incorporates a 300 L batch reactor and the trials are expected to be completed by the end of August. The trials also address treatment of solid OCP wastes (eg powders).
ADI has submitted several proposals to apply the BCD process in Australia, however, to date no trials have been undertaken (Coniglio, 1997).
Further information regarding the application of the BCD process in conjunction with thermal desorption in the region is outlined in the following section. At this stage, the commercial use of the BCD process in Australia is limited to treatment of organic liquids and PCB contaminated equipment.
When contaminated soil is treated directly by the BCD process, the resulting soil is likely to be oily and disposal options may be limited. For example, the soil may require disposal in a secure landfill.
To avoid this problem, some suppliers of the BCD process now propose the use of a thermal desorption unit (TDU) to remove these contaminants, to concentrate them into a liquid phase for separate treatment by the BCD process (Shieh, 1994 and Tozer, 1994). As such, the soil is not treated directly by the BCD process and the BCD reagents (including alkali and hydrocarbons) are not added directly to the soil. This avoids the problem of residual hydrocarbons in the soil. However, it does rely on the thermal desorption unit to provide adequate removal of contaminants without the chemical reaction inherent in the BCD process. One such thermal desorption system is the "Therm-O-Detox" System which has been developed by ETG Environmental Inc (ETG). This system, and thermal desorption in general, is discussed further in Chapter 20. Thermal desorption has been used in conjunction with the BCD process on a commercial basis in the United States.
Thermal desorption can be applied to soil directly without addition of reagents. However, a patented variation involves the addition of sodium bicarbonate to the soil to enhance the efficiency of desorption and reduce the operating temperature of the desorber. The sodium bicarbonate will not necessarily increase the dechlorination of the chlorinated soil constituents. In the case of PCBs for example, BCD Technologies advises that with the addition of sodium bicarbonate some 95% of the PCBs are volatilised and 5% are dechlorinated (Krynen, 1994b). In the case of constituents such as pentachlorophenol, the proportion dechlorinated in the thermal desorber is likely to be higher (eg 50%). Thermal desorption variants are discussed further in Section 5.4.
As part of its further development of the BCD process, ADI has developed a variation on the BCD process, referred to as STTP (refer Section 5.4). This process can achieve dechlorination of contaminants in soil within the thermal desorption unit. Some recycling of the vapour stream may be required to achieve the necessary destruction efficiency, depending on the contaminant. The ability to effect treatment of contaminated soil in a single stage process is expected to result in significant cost savings (Coniglio, 1997). A brief overview of recent trials of this process is presented in Section 5.2.
A complete TDU-BCD process is shown in Figure 5.1.
As an example of the application of the thermal desorption system, in late 1992 ETG demonstrated the medium temperature thermal desorption (MTTD)/BCD technology using the Therm-O-Detox system at a Koppers site in Morrisville, North Carolina under the Superfund Innovation Technology Evaluation (SITE) program (USEPA, 1993).
The Koppers site in Morrisville was a former wood preserving operation utilising the Cellon process, which involved pressure treatment of wood with pentachlorophenol and subsequent steaming. A pentachlorophenol contaminated rinse water was generated in the process. The rinsate from this process was placed in unlined lagoons where leaching into the soil occurred. Concentrations of pentachlorophenol in excess of 8000 mg/kg and lesser concentration of dioxins and furans were present in the soil.
Following completion of bench-scale testing an MTTD/BCD system was mobilised, which was capable of handling 0.22 - 0.44 tonnes per hour of throughput. The equipment was placed within a portable containment pad having approximate dimensions 18 m x 24 m. Soil was excavated from the documented "hot spots" and hand screened to less than 12 mm. Contaminated soil was then placed in 210 litre drums for transport to the processing area.
Information from test runs at the Koppers site (Shieh, 1994) indicated that:
Detection limits varied from sample to sample due to dilution factors. Two other performance test runs achieved similar results.
A brief overview of recent trials by ADI of the single stage BCD soil treatment system was presented in Section 5.2. A report on the trials is expected to be released to the public in September, 1997, however we understand the trials have been successful in demonstrating the treatment of PCP and dioxin contaminated soils in a single stage process.
5.4.1 BCD Plus
BCD Technologies advises that it has developed a variation of the BCD process, which it calls "BCD Plus" (Krynen, 1994b,c). In this process standard desorption equipment is used to remove the contaminants from the soil. In addition, proprietary reagents are used to convert the dechlorinated hydrocarbons into carbon dioxide and water, apparently by a catalysed combustion process which operates at a relatively low temperature. Thus, the decontaminated soil is free of oil and the inorganic salts produced by the dechlorination process remain in the treated soil. The soil can be disposed of to any landfill, assuming that the soil does not contain heavy metals. A BCD Plus plant designed to treat 50 tonnes/day of contaminated soil has been constructed by BCD Technologies. An application for regulatory approval for a BCD soil treatment system has been lodged with the Queensland Department of Environment; however, this is not being actively pursued (Krynen, 1997).
The BCD Plus process was developed, in part, to circumvent licence restrictions associated with the conventional BCD soil treatment process. Given BCD Technologies is now licensed to apply the BCD soil treatment process, the implementation of a soil treatment system is likely to follow a conventional BCD process rather than the BCD Plus process. In any case, the demand for a BCD soil treatment system has not been great and therefore the development of facilities of this kind is still dependent on the market (Krynen, 1997).
ADI Limited has developed a modified BCD process for the treatment of contaminated soil called STTP (Soil Thermal Treatment Process). Where the BCD Technologies process is based on a hydrodechlorination reaction, the STTP process is said to be based on a carbonisation reaction. (Patents have been lodged by ADI for this process as an alternative treatment process to the BCD process.)
The STTP reaction can be carried out in a solid phase within a thermal desorption unit or in a liquid phase in a separate reactor. The primary focus in development of the STTP process has been the treatment of contaminated soils and other solid wastes and therefore use of the STTP process in conjunction with thermal desorption.
As with the "BCD Plus" process, STTP aims to achieve a significant proportion of the carbonisation/dechlorination/decomposition within the thermal desorption unit. For some wastes the STTP system is able to achieve effective carbonisation/dechlorination/ decomposition in a single stage, obviating the need for a separate reactor to treat condensate from the thermal desorber. The ability to complete the BCD reaction in a single stage rather than a two stage process, has an obvious cost advantage when treating soils. ADI advises that the single stage STTP process has been demonstrated on a pilot scale with PCP contaminated soil, with a high destruction efficiency of the PCP within the desorber. Dioxins if present, will be desorbed in this process and will require condensation and treatment in a separate reactor (Coniglio, 1996). Alternatively, recycling a portion of the vapour stream back through the thermal desorption process has been found to provide for adequate treatment of the dioxins (Coniglio, 1997). Treatment of dioxins in condensate from PCP contaminated soil has been demonstrated to a level in excess of 99.999% (DRE).
ADI is currently proposing to use an indirectly heated thermal desorption unit with the majority of contaminant destruction occurring within the thermal desorber, followed by a reactor for treatment of condensate (only if required). This system is known as the STTP Solids System.
While the main focus of ADI's STTP work is treatment of soils, the process is capable of treating liquids. Pilot plant trails using PCB contaminated oils have demonstrated reduction in PCB concentrations from 20% to PCB-Free as defined in the PCB Management Plan (1995) ie < 2 mg/kg.
5.4.3 Considerations in the Application of the Technology
Thermal desorption places constraints on the physical form of the waste to be treated, depending on the type of thermal desorber being used.
In the case of indirectly heated rotary kilns, a range of waste types can be treated. However, a typical feed size limitation of 25 mm usually applies (Carlisle, 1994a; Tozer, 1994; Krynen, 1994a). If this size is exceeded, then the desorption can be incomplete or the desorber mechanism may be blocked (this is dependent on the desorber system). In practice, desorption can be enhanced by increasing the temperature or by adding reagents. Higher boiling point waste materials such as PCBs and chlordane may not desorb effectively unless a reagent such as sodium bicarbonate is added to the mixture.
Materials handling problems can be expected to be significant for some waste materials, such as concrete (particularly if it includes steel reinforcing), rubbers and tars. Such materials are present for example, in the hexachlorobenzene (HCB) wastes held at Botany.
Thermal desorbers currently under development by the Australian BCD licensees need to be portable in nature (able to fit in one or two shipping containers) and should be easily relocatable from site to site (Krynen, 1994b; Tozer, 1994).
The treated soil from this process is sterile, but is expected to be in a form which will permit its return to a site (Carlisle, 1994a). This is in contrast to incineration, where the soil structure is permanently changed such that landfill disposal is required.
5.4.4 Experience and Availability in Australia
The use of thermal desorption for treatment of wastes in conjunction with a liquid BCD plant has not yet received approval by the relevant regulatory authorities in Queensland. Use of thermal desorption in conjunction with a liquid BCD Plant is not currently being pursued in Victoria.
BCD Technologies (Qld) advises it has constructed a thermal desorption (rotary kiln) system and is presently pursuing licence renewal for its trial unit. To date, the demand for a BCD soil treatment facility has not been sufficiently high to make the commissioning and approval of this unit a priority compared to the treatment of PCB contaminated oil and equipment.
BCD Technologies proposes to operate the unit as a normal desorber with condensation of the off gases for treatment in the liquid BCD plant. This desorber is expected to be portable (2 shipping containers) and it should be possible to relocate this unit to other States, if appropriate licence approvals can be obtained. The proposed thermal desorption system will extend the capability of the process for treatment of a wider range of waste materials than is presently possible.
Technosafe (Vic) has a licence to treat soils using the BCD process and has built a prototype thermal desorber for trial purposes. The process has not been re-established since the fire in 1995 and Technosafe advises that at this stage they are unlikely to pursue further development or approvals for this treatment process.
ADI advises that it has a thermal desorption unit under construction for use in Europe and has conducted extensive trials in Australia and New Zealand at laboratory and pilot scale. ADI has refined the BCD-thermal desorption process with the aim of applying the system to the treatment of contaminated soil both in Europe and Australasia. ADI also has experience in the field use of thermal desorption for contaminated site clean up. It has a direct fired thermal desorption unit rated at some 20 tonnes per hour available for use in Australia and New Zealand. However, because this unit is direct fired, it is not suitable for use in conjunction with the BCD process.
(a) Proponents (in Australia)
BCD Technologies (Brisbane).
Technosafe Waste Disposal (Melbourne).
(b) Wastes Applicable
Low volatility organic liquids and high volatility organic liquids (following evaporation of volatile solvents) (BCD reactor only).
Soils, sludges, irregular larger inert solids (following size reduction) and semi-solid materials (BCD in conjunction with thermal desorption or solvent extraction).
PCB contaminated transformers (BCD in conjunction with solvent extraction).
PCB contaminated capacitors (BCD in conjunction with size reduction and alkaline pretreatment, or solvent extraction (although a large number extractions is required)).
(c) Contaminants Applicable
All scheduled compounds.
Commercially available in Queensland for low volatility liquids and PCB contaminated transformers and capacitors. Victorian treatment facility is back in service following the 1995 fire, allowing treatment of PCB contaminated oils, transformers and capacitors. The technology is commercially available overseas for treatment of solids. ADI does not yet have a unit commercially available, although have successfully trialed treatment of contaminated soil.
(e) Timing for Commercialisation in Australia
Treatment capacity currently available for liquids. Potential to be commercially available for soils and other solids within 6 to 12 months, depending on regulatory approvals and market pressures. Development of a soil treatment capability in Australia is more likely to occur over a period of 1 to 5 years, although 6 to 12 months would be possible.
(f) Cost (example only)
$250 to $400/tonne for contaminated soil (pending).
$1000/tonne for lower concentration PCB contaminated oils, higher for high concentration oils.
$4.50 to $5/kg for larger capacitors, up to $12/kg for smaller lighting capacitors.
Waste holders state that costs have been increasing whereas the waste treatment groups state costs for treatment of liquids have been reduced, although the cost of treating capacitors has increased.
(g) Safety/Environmental Risk
Emissions associated with treating 5000 mg/kg PCP in soil by BCD have been reported as < 10 µg/m3 (organochlorine compounds eg PCP, PCB) and <100 ng/m3 (TCDD eq). Potential to form dioxins and furans is low as the system operates under an inert atmosphere and dioxins should be dechlorinated by the process. If dioxins are to be treated, there is potential for higher chlorinated congeners (OCDD) to be dechlorinated to form more toxic lesser chlorinated congeners (TCDD) and the reaction conditions should be selected to ensure the reaction goes to completion. System operates at only moderately elevated temperatures, therefore any accidental release should be able to be contained with appropriate precautions. Exclusion of air from the BCD process is important to avoid auto-ignition of hot oil used in the process.
Some associated processes such as the alkaline pretreatment of capacitors and solvent extraction carry with them a significant fire and explosion risk, and hence appropriate precautions must be taken.
(h) Non-technical Impediments
The BCD process is generally not regarded adversely by the community.
(i) Preferred Mode of Implementation
Currently established as small centralised, liquid only, BCD reactors in Brisbane and Melbourne. The Brisbane unit is portable. Any soils treatment system would most likely be a mobile unit.
Uneconomical to treat large volumes of aqueous wastes. While sufficient for most purposes, the destruction efficiencies achievable are low compared with incineration systems. Salt build-up when treating concentrated chlorinated wastes can halt the reaction prematurely, requiring the waste to be pre-diluted to attain the required destruction efficiencies. Treatment efficiency of soils is limited by the efficiency of the thermal desorption process.
The BCD process is not adversely affected by the presence of arsenic or other contaminants in mixed pesticide wastes, although treatment of such material has been limited by restrictions on the disposal of the arsenic containing residue. Energy costs for the treatment of pesticide wastes may be higher, given the solvents will need to be distilled from the mixture in order to reach the operational temperature.
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