CMPS&F - Environment Australia
Appropriate technologies for the treatment of scheduled wastes
Review Report Number 4 - November 1997
A wide range of technologies are available or are potentially available for the treatment of scheduled wastes, as discussed in the earlier chapters of this report. However, there are relatively few commercially available processes that destroy the contaminants and are able to treat a range of waste types.
Technologies such as PLASCON, Base Catalysed Dechlorination (BCD) and Eco Logic may be effective in treating waste streams in a liquid or gaseous phase. However, they require a separate extraction process, such as solvent extraction or thermal desorption to remove the contaminants from bulk solid media such as capacitors or contaminated soil. Similarly, technologies such as thermal desorption should be viewed as only part of the solution, in that while they remove the contaminant from a waste, they do not always destroy the contaminant and subsequent treatment of the off-gas may be required.
This chapter introduces some of the technologies which are largely pretreatment processes. These are mainly used to separate and concentrate the contaminants for further treatment. Pretreatment technologies can be divided into the following categories:
A short summary of these processes follows.
Thermal desorption is an ex situ process for physically separating volatile and semi-volatile contaminants from soil, sediment and sludge by heating to temperatures high enough to volatilise the organic contaminants.
Thermal desorption may use either indirect or direct heat exchange, and air or an inert gas to transfer vaporised contaminants from the contaminated medium.
The bed temperatures (usually between 170 to 550oC) and residence times used by thermal desorption systems will volatilise selected contaminants and drive off water, but typically will not oxidise or destroy organic compounds. Thermal desorption must then be linked with another destruction process (eg an afterburner, PLASCON, BCD or Eco Logic unit).
Thermal desorption has been widely applied to the treatment of tar contaminated soils; the off-gases are passed through an afterburner. Thermal desorption followed by direct combustion (eg in an after burner) can be likened to incineration, and it has the potential to have acceptability problems with local communities if used to treat scheduled wastes. Some thermal desorption systems, such as the PCS Technology (PCS) process (refer Section 16) incorporate other treatment processes such as pyrolysis, prior to a final combustion step.
There are a variety of thermal desorption units available, including:
Although thermal desorption units are commonly available, some systems may not be appropriate for treating chlorinated waste streams.
There are a number of Australian based clean-up contractors employing thermal desorption technology including Theiss Environmental Services and ADI Limited. ADI Limited has applied thermal desorption both independently (eg clean-up of explosive organic contamination at the St. Mary's site, Sydney, NSW) and in conjunction with the BCD process.
McLaren-Hart had established a thermal desorption capability in the region using a combined infra red/vacuum unit that is operated batchwise, in contrast to most other systems which are designed to operate continuously. McLaren-Hart have since ceased their Australian operations and hence their thermal desorption system is no longer available.
19.2.2 The Therm-O-Detox System
The Therm-O-Detox system is an example of a thermal desorption system linked with a further treatment process; in this case BCD.
The Therm-O-Detox system, developed by ETG Environmental Inc., uses an indirectly heated thermal desorber to separate organic compounds from contaminated media (Shieh, 1994). The unit is designed to achieve feed material temperatures of up to 510oC thereby allowing effective treatment of soils and sludges contaminated with a wide range of low and high boiling point compounds. Applications include oily sludges, pesticides, herbicides, PCBs, coal by-products, wood treating compounds, dioxins and furans.
The Therm-O-Detox system consists of a feed hopper, feed conveyer, and a low or medium temperature thermal desorption unit (LTTD and MTTD) heated by a hot oil heater. Off-gases are treated by a vapour recovery system which includes an oil venturi, an oil scrubber, a water scrubber, a condensing unit and vapour phase carbon adsorption for final polishing of gases exiting the condensing unit.
A cooling conveyor or mixer is used to cool and condition the processed waste in order to control dusting and to promote compactability for replacement on site or for off site disposal. The equipment design is claimed to offer good mixing, a low sweep nitrogen gas flow (to prevent air backflow and auto-ignition) and good contaminant recovery.
The system throughput rate depends on the waste type, moisture content, contaminant level and treatment standards. A typical system throughput can be expected to be 4 to 14 tonnes per hour depending on the variation in feed material characteristics and regulatory requirements.
Contaminants and moisture volatilised from the contaminated material (such as soil) are entrained in the off-gas and are condensed and recovered by the scrubbers/condensers. The condensed mixture is separated and the organic contaminant is collected for recycling via solvent recovery, fuel substitution or treatment using the BCD process.
Separated water can be treated by liquid phase carbon adsorption and sand filtration. Most of the treated water can be recycled back to the process for use in the scrubbers and cooling conveyor. Excess treated water must occasionally be bled from the process.
Off-gases exiting the vapour recovery system are discharged to the atmosphere after carbon adsorption polishing. A further discussion of this technology is in conjunction with the BCD process is provided in Chapter 5 (section 5.3.1) including details on the consideration of the application of the technology and experience and availability of the process.
Solvent extraction is a physico/chemical means of separating hazardous waste contaminants from equipment, soil or sediment, thereby concentrating the contaminants and reducing the volume of hazardous material that needs to be destroyed. This process produces relatively clean soil or sediment which can be returned to site or disposed of to landfill. Cleaned equipment may be suitable for reuse or disposal to landfill. Solvent extraction is generally applicable to organic wastes, using an organic chemical as a solvent to strip the contaminants of concern from the soil, sediment or other solid matrix (eg PCB contaminated capacitor internals). Solvent extraction has also been applied to the removal of heavy metals, generally with less success than achieved with organic chemicals.
Solvent extraction for soil treatment is an ex situ process and requires the contaminated soil to be excavated and mixed with the solvent, usually in a relatively intensive process. A wide range of liquid/solid contacting devices have been used for solvent extraction processes together with a variety of solvent types.
The selection of the solvent and contacting device is to some extent, determined by the nature of the contaminant and the matrix from which it is to be extracted.
Solvent extraction has been proposed for the treatment of PCB contaminated capacitor internals with limited success (refer Section 5.2.4). Solvent washing, however, has been used for the treatment of transformer internals and has been used for cleaning a range of non-porous articles.
19.3.2 Technology Description
Solvent extraction uses organic solvents such as liquefied gases and low boiling point solvents to extract organics from sludges, contaminated soils and wastewater. Propane, butane and hexane are some of the solvents used for sludges and contaminated soils, while carbon dioxide has been used for wastewater streams. Solvent extraction technology is available for continuous flow processes for wastes which can be pumped or as a batch process for soils and sludges.
In solvent extraction treatment, contaminated solids, slurries, or wastewaters are fed into the extraction system together with the solvent as shown in Figure 19.1. Following separation of the contaminated solvent from the treated feed, the solvent mixture passes to a solvent recovery system. Once in the recovery system, the solvent can be vaporised or otherwise regenerated before being recycled as fresh solvent. The extracted organics are drawn off and either reused or destroyed.
Different extraction system designs are used for contaminated wastewaters and for semisolids. A tray tower contactor is used for wastewaters and a series of extractor/decanters are used for separating solids and semi-solids from the liquid fraction.
Solvent extraction technology can be applied to soils and sludges containing volatile and semi-volatile organic compounds and other higher boiling complex organics, such as polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, and pentachlorophenol (PCP).
Solvent extraction treatment is similar in some respects to thermal desorption in that the treatment does not provide a total solution to the problem.
The extracted contaminants are not converted to less toxic substances which can be disposed of by ordinary methods and in estimating the total cost of treatment, the costs of subsequent treatment and disposal must be included.
The efficiency of solvent extraction depends on the solubility of the organic contaminants present; dioxins can strongly adsorb to particulate matter and may not be successfully desorbed, although reasonable efficiency with PCBs (which also tightly adsorb to particulates) suggests that a useful desorption would be achieved. Tests may be needed to establish the efficiency of extraction for each application.
In general, solvent extraction is likely to be less efficient at removing organic contaminants than thermal systems, although the technology may find more application where the contamination is present at low concentrations, or the site use is less sensitive.
Being predominantly a liquid phase process, the risk associated with release of contaminants to the atmosphere are minimal. However, because some solvents are gases under normal atmospheric pressure (eg butane) or are flammable (eg hexane) there is potential for fire or explosion if uncontrolled release occurs.
19.3.4 Experience and Availability in Australia
CMPS&F is not aware of solvent extraction technology being used commercially for site remediation or hazardous waste treatment in Australia with the exception of solvent washing of non-porous articles such as PCB contaminated transformer internals. Solvent extraction is widely used in the mining industry for uranium and copper extraction so, in principle, the technology should have an adequate level of local technical support.
An example of the application of solvent extraction to the remediation of contaminated soil is the system designed by CF Systems. Under the USEPA SITE Program, a mobile demonstration unit designed by CF Systems was tested with PCB contaminated sediments from the New Bedford (Massachusetts) Harbour Superfund site during September 1988 (USEPA, 1993). The technology was demonstrated concurrently with dredging studies managed by the US Army Corps of Engineers. Contaminated sediments were treated by the CF Systems Pit Clean-up Unit, using a liquefied propane and butane mixture as the extraction solvent.
Despite some operating problems, demonstration of the CF Systems unit at the New Bedford site yielded the following results:
CF Systems Corporation completed the first commercial on-site solvent extraction treatment operation in the USA at Star Enterprise, in Port Arthur, Texas. The propane based solvent extraction unit processed treated refinery wastes, producing treated solids that met EPA land-ban requirements. The unit operated continuously from March 1991 and March 1992 and was on-line more than 90% of the time. Following fixation for heavy metals, the treated solids were disposed of in a secure landfill.
A range of technologies are available for the adsorption or absorption of scheduled wastes, ranging from adsorbents using in the regeneration of transformer oils, to adsorbent materials used for the clean-up of spills and leaks. In general, adsorbent products are used on a one off basis and designated for separate treatment, along with other wastes contaminated by scheduled wastes. However, there are some processes that have been developed for specific applications that allow for regeneration of the adsorbent and production of a concentrated waste stream. The concentrated waste stream may then be more readily handled for separate treatment.
(a) Technology Description
The Stopped Counter Flow adsorption process has been developed to the pilot plant stage by Environmental Services International Pty Ltd for the treatment of PCB contaminated oil. Preliminary information on the process was provided by Starks (1995) of PEC Pty Ltd for inclusion in earlier reviews.
The process involves the use of a proprietary adsorbent to selectively remove PCBs from oil. The adsorbent is applied using a simple two step counter-current process. A first contact with partially exhausted adsorbent from a previous stage is then followed by a second contact with reactivated adsorbent. The adsorption process can be repeated if the residual concentration of PCBs does not meet the required level.
The adsorbent is reactivated for use and produces a small quantity of oil containing a high concentration of PCBs which must be treated in a separate process (such as Eco Logic, PLASCON or BCD) to destroy the PCBs.
The process of removing PCBs from the oil phase, and subsequent reactivation of the adsorbent, results in the majority of the oil being decontaminated and the remainder (3 to 5 %) having a high PCB content.
The low level of PCB oil would typically less than 10 ppm but can be reduced to less than 2 ppm to produce "PCB free" oil suitable for recycling or energy recovery.
Starks advises that the process is self contained and does not involve high temperatures or dangerous procedures and may be operated as either a fixed or mobile plant. The recycled oil can be passed through a final polishing stage, ie. filter/clarifier/moisture absorber and have additives replaced if required before return/recycle to the transformer. Other oil contaminants that are produced during transformer use are removed using this process. Such contaminants include oxidation and breakdown products, moisture, extracts from cellulose, rubber and timber of construction, etc. Oil colour, appearance and electrical properties can be improved to a degree that the oil conforms to new oil specifications. The method has been developed for transformers but could also be adapted to suit other low level scheduled waste decontamination requirements.
The cost of this treatment method will be substantially less than the cost of replacement with new oil depending on the initial concentration of PCBs in the oil, the number of operating cycles, plant capacity and whether a mobile or fixed treatment unit is required and the final destiny of the decontaminated oil (Starks, 1997).
Starks advises that laboratory work has achieved a level of decontamination for each stopped counter flow operation of approximately 75%, for oil contaminated initially at the 100 to 200 ppm level with Aroclor 1260. Triple processing is required to reduce oil with 100 ppm PCBs to less than 2 ppm. Decontamination efficiency falls off at higher levels and is about 50% for 10000 ppm PCBs contamination (Starks, 1995).
PCBs removed from the recovered oil remain in the oil associated with the adsorbent at the regeneration stage. This oil, containing high levels of PCBs, must be separately treated and disposed of, and fresh oil must be added to the system to replace the lost oil. Oil lost in this way is likely to be 5% of the total but will increase with the initial concentration of PCBs and the required level of decontamination efficiency.
Starks advises that oil contaminated with levels of Aroclor 1260 at the 10000 ppm level can be economically decontaminated to less than 10 ppm using this process.
In the process of decontaminating PCBs from transformer oil, aldehydes, ketones and terpenes are also extracted from the transformer structure.
(c) Experience and Availability in Australia
Both PEC and Environmental Services International Pty. Ltd. are Australian companies. To date, all work has been completed on a laboratory scale. A pilot plant with a capacity of 1.5 tonnes per day is currently under construction.
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