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Treatment and disposal
Treatment, storage and disposal facilities (TSD) for HW are just as well liked as the HW itself. In their site evaluation, EPA also considers the risks associated with the "no-action" option, but in sites that are on the NPL, this is not an acceptable choice. Risks to public health and the environment are also associated with any given remedial action. However, the risks associated with remediation must be smaller than the risks associated with leaving the site as it is.
Since every HWS is a unique situation, each requires a unique solution in form of a remediation plan, also called remedial design/remedial action (RD/RA). In addition to the clean up standards that have to be met, the costs associated with the technologies have to be taken into account when selecting a "treatment train". There are many technologies that are used to treat, store and dispose of HW. Some are "tried and true", other remediation technologies are currently being developed, so-called "innovative technologies". I want to give you three examples for technologies that are currently in use: bioremediation, landfilling and incineration.
Bioremediation
Biorem - as it is called in the industry jargon - is still considered by many an innovative technology, although it has been used for decades. It is used to clean up contaminated waste water, sludge or soil, usually with microorganisms (algae, fungi, yeast, etc.). The University of Hawaii has a webpage summarizing the technologies associated with biorem. By metabolizing the organic contaminants, the microorganisms use carbon as an energy source. In some cases the indigenous microorganisms present in the soil are sufficient to degrade the contaminants, sometimes non-native micro-organisms are added to the waste stream (bio-augmentation). Sometimes other materials are added (water, oxygen, nutrients, etc.) to optimize the growing conditions. Biorem is often tried in-situ (meaning: in place, without excavating) which makes it usually one of the most economic option for remedial action of contaminated soil. It is very difficult to predict whether or not biorem will meet the clean up standards: sometimes it just does not work, and no one seems to be able to explain why. If you want to find out more about innovative remediation technologies, go to EPA-s clu-in.org site, or to DoE's GNET site.
Land-disposal / Landfilling
RCRA restricts the disposal of hazardous waste into landfills, unless they have been treated first (Best Demonstrated Available Technology - BDAT), but landfilling remains a common disposal technology for municipal and hazardous waste. The number of landfills in the US has fallen from ca. 8000 in 1988 to 2300 ten years later, in 1998. However, the landfill capacity actually increased, because now mostly big landfills are operating.
The following link is from Environmental Technology Council. ETC is a trade association of commercial environmental firms that are involved in recycling, treatment, disposal and clean up of HW. Please read the text and look at the drawing of a HW landfill.
A landfill can be looked at as temporary, controlled storage . It is storage, not treatment. Everybody agrees, that all landfills will leak - the question is when. A modern landfill is designed like a bath-tub: the Flexible Membrane Liners (FML) prevent liquids from reaching the surrounding soil. A hazardous waste site has at least two FMLs and in addition a leachate collection system is installed above each liner. In that way the leachate can be collected and pumped out.
Wastes are placed in "containment cells" into the landfill and covered daily with soil, to prevent water and pests from reaching the waste and to absorb any leaking material. During closure, another liner is installed on top and covered thickly with soil.
Landfill gas: A landfill is an anaerobic environment in which methane and H2S are formed, among other chemicals. This landfill gas can migrate underground Ð if there is a leak in the liner system Ð and pose a potential explosion hazard. Therefore additional wells are placed within a landfill that pump out this gas. After collection it is purified and burned for energy recovery. In the NJ Meadowlands, the methane gas from the waste piles is collected and sold to PSEG for $5 million per year in the early 1990s. Landfill gas serve can serve as a revenue source.
Incineration/thermal treatment
Incineration of HW is a very costly form of treatment (capital, operational, labor and maintenance costs). There are 21 commercial hazardous waste incinerators operating in the US and Canada. Thermal treatment reduces the volume of the waste greatly (about 90%) and reduces the weight of the waste by approximately 70%. Since the waste is destroyed there is no remaining liability for the previous owner of the waste. This feature is especially appreciated from the industry.
Here is the layout of a thermal treatment plant - in this case a municipal waste incinerator. This plant is located in Newark, NJ, and operated from American Ref-Fuel. It is called the Essex county Waste-to-Energy Facility.
Walk-through of MSW Incinerator
Once unloaded (1), waste is stored and mixed (2), predried and burned in a burning chamber (3). During burning the waste has to be moved and must be in contact with sufficient oxygen to allow complete combustion. The residence time in the burning chamber is about 30-120 min., at a temperature of at least 650°C. In hazardous waste incinerators the temperature must be at least 1100°C. Off gas values are monitored from the control room (4).
At this point we are generating our first waste stream: the bottom ash, from which ferrous metals are recovered. This waste stream must be tested according to the four criteria mentioned at the beginning of this section (ignitability, corrosivity, reactivity and toxicity). Depending on the outcome of the tests the waste goes to a municipal solid waste landfill or - at greatly increased costs - to the hazardous waste landfill.
The off gases are usually treated further in a the secondary burning chamber (afterburner) at a higher temperature, to reduce the amount of products of incomplete combustion (PIC) such as Dioxins, Furans, PAH. The hot process gas is then cooled in a heat exchanger, and the steam is used to drive the turbines for energy recovery (5). The electricity is sold to PSE&G: about 70 MW plus the energy to run the incinerator. The off gas is ãscrubbed" to remove particles and acid gases: this plant has 3 electrostatic precipitators and 3 dry scrubber systems. At that point the fly ash is collected, our second waste stream. A mercury emission control system was to be installed for this plant at the time of our visit. The off gas that leaves the stack is our third waste stream. It is controlled for several parameters, among them temperature and CO. According to the values measured one can adjust the air and auxiliary fuel input, to make sure that complete combustion is achieved.
In the European Community air emission standards that are more stringent than in the US. For example, there must be separate steps to take out
- nitric and nitrous oxides (de nox)
- three steps of particle removal (spray dry, electrostatic precipitator and bag house)
- dioxin/furan removal. The emission standard in Europe is 0.1 ng/Nm3 (in terms of technical equivalents of TCDD). In the US the limit is 300 times higher at 30 ng/Nm3, and that is for new MSW incinerators.
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