Partners ready MAWS for commercialization

Four-year investment moves ahead

Since late 1990, the U.S. Department of Energy through its Office of Technology Development has supported a systems approach to environmental restoration known as Minimum Additive Waste Stabilization. The MAWS concept integrates various waste processing technologies into a system, achieving cost effectiveness through volume reduction and durability of the final waste form. In the MAWS systems approach, vitrification technology plays the primary role, fusing both primary and secondary waste streams into glass and crystalline composite waste forms. For more than four years, DOE's partners in developing the MAWS concept have been GTS Duratek, Lockheed Environmental Services Corp., Argonne National Laboratory, the Vitreous State Laboratory of Catholic University of America, and Pacific Northwest Laboratories.

In late June 1994, MAWS demonstrated its potential to cost effectively transform low-level mixed waste into safe, disposable glass. In a pilot-scale test at the Fernald Environmental Management Project in Ohio, MAWS was used to successfully treat waste in the process pits known as Operable Unit 1. This waste, typical of much of the waste found at other DOE sites, is a mixture of hazardous constituents and low-level radioactive components generated during the site's former life as a producer of pure uranium metal for DOE. Low concentrations of uranium and thorium were deposited with other inorganic process residues in the five on-site waste pits of OU-1. Studies have shown radionuclide contamination is present in both the pit sludges and in the surrounding soils and groundwater. Similar waste is found in tanks, sludge pits, evaporation ponds, injection wells, and rubble pits throughout the DOE complex.

The MAWS approach demonstrated at Fernald linked the technologies of vitrification, soil-washing, and ion-exchange. Soil-washing and ion-exchange processes were used as pre-treatments for waste, releasing clean soil and water to the environment and leaving contaminated silica-rich residuals as feed to the vitrification system. By removing clean soil and water, these pre-treatments helped accomplish waste volume reduction, a major MAWS benefit. At Fernald, a soil-washing process called TRUclean developed by Lockheed Environmental Services Corp. separated contaminated soils into clean soil and soil concentrates by passing the soil through a series of scrubbers and separators. The ion-exchange process treated contaminated wastewater generated from the soil-washing, a secondary waste, by stripping uranium and thorium from the water.

The next step in the MAWS approach is the blending of wastes, both primary and secondary, as feed to the vitrifier or melter. In addition to residues from the soil-washing and ion-exchange processes, other low-level mixed waste may include transite, asbestos, or incinerator ashes. The melter's electrodes heat the blended waste streams from 1,000¡ C to 1,300¡ C, a moderate temperature for mixed-waste vitrification. The current passing between the electrodes transforms the waste into a molten state. This process, known as joule-heating, decomposes and dissolves inorganics into the glass melt. The melter's off-gas system captures and treats any vapors from the melt, and cycles it back into the melter for processing. The molten material cools, producing a stable, leach-resistant glass. The MAWS system will dispense the waste glass product into drums as smooth, rounded glass pieces, called gems, of various sizes to reduce volume and facilitate packing.

MAWS is cost-effective, because the selected waste stream contains the optimal combination of basic chemical building blocks--formers, fluxes, and intermediates for making durable glass waste forms. MAWS promotes high waste loading, which means a large percentage of the glass is derived from the waste itself with addition of a minimal amount of expensive, non-waste ingredients such as sand, soda ash, borax, lime, or alumina. When waste loading is high, more time is spent actually processing waste rather than additives. Either the treatment facility can be smaller, reducing capital costs; or the processing period shortened, reducing operational costs.

The moderate temperature at which MAWS vitrification was conducted in the Fernald demonstration contributed to its cost effectiveness. Higher temperature melters cause more of the waste to escape as gases, necessitating a more elaborate off-gas system and contributing to reduced waste loading in the glass produced. Glass property characterization and modeling to achieve optimal blending of wastes which satisfy processing requirements will continue to expand the types of waste treatable by the MAWS approach. Development of efficient, higher-temperature melters is being studied and may prove to be feasible.

Another reason for MAW's cost effectiveness is vitrification minimizes waste volume through evaporation of water, combustion of organics to harmless gases, and consolidation of contaminants into non-porous solids. The smaller volume reduces disposal costs. In addition, the long-term durability and leach resistance of the waste glass could lead to the waste glass being delisted, or removed from the Environmental Protection Agency's list of hazardous wastes. Delisting would save costs for barriers to contain the glass and for monitoring the stored waste.

MAWS's cost effectiveness was borne out in the pilot-scale test of MAWS at Fernald Environmental Management Project. The melter ran for a total of 36 days during which there were four runs. More than 18,000 liters of feed--primary waste, secondary waste, and a small amount of additive were processed ranging in solid contents from 21 to 30 percent, producing approximately 4,800 kilograms of glass in both bulk and gem waste forms. Waste loadings of about 90 percent were achieved with volume reduction of about 89 percent.

The project also developed a life-cycle cost model to project the total life-cycle costs of MAWS to treat OU-1 wastes at FEMP. The model compares the total life-cycle costs of conventional cementation, which is encapsulation of waste in cement, to MAWS vitrification. Using Monte Carlo techniques and probability distributions to quantify uncertainties, the model estimates a life-cycle cost savings for MAWS of $100 million over the cementation method for treatment of FEMP's OU-1 wastes.

In an effort to broaden the range of DOE waste treatable by a MAWS system, the Vitreous State Laboratory of Catholic University of America, Argonne National Laboratory, and Pacific Northwest Laboratories have been conducting crucible melts to assess the acceptability of glass and glassy waste products produced by various waste streams. VSL, ANL, and PNL are characterizing wastes and identifying specific processing parameters required to produce durable glass and glassy slag products. VSL is concentrating on waste similar to that found at OU-1. These wastes vitrify at lower temperature to produce durable glass. ANL and PNL are assessing wastes that require higher temperatures and produce glassy slags. This research defines the trade-offs between final waste form quality, ease of processing, and waste loading.

VSL, ANL, and PNL are also evaluating the leachability and durability of their respective waste forms to understand how waste glass will weather during long-term storage. Testing provides a database of quality, consistency, homogeneity, leachability, and durability of vitrified products and links the products to the types of wastes producing them. The database along with appropriate models will predict the properties and performance of potential untested glass compositions.

Another major task is developing and adapting waste processing technologies for incorporation into a MAWS system. GTS Duratek, a medium-sized technology company based in Columbia, Maryland, collaborated with VSL to develop the low-temperature joule-heated melter, DuraMelter, used at the FEMP demonstration. MSE, Inc. in Butte, Montana in conjunction with Retech in Ukiah, California is evaluating a high temperature melter developed by Retech, the Plasma Centrifugal Furnace, to handle waste streams with high metal and organic contents and low fluxes.

Related developments include GTS Duratek's plans to use its vitrification technology to treat 700,000 gallons of M-Area mixed waste sludge at DOE's Savannah River Site in South Carolina by the end of 1996. In addition, GTS Duratek will provide a 1,000 kilograms per day joule-heated ceramic melter system to Fernald for high temperature vitrification of roughly 14,000 cubic yards of K-65 silo wastes. Plans are also afoot for GTS Duratek in partnership with Chem-Nuclear System, Inc. to build a system similar to the one demonstrated at FEMP at an existing Chem-Nuclear waste management facility. The DuraChem facility will be designed to convert low-level radioactive waste from industrial facilities, commercial nuclear power plants, hospitals, and laboratories into durable glass. It will be the first dedicated fixed facility in the United States to convert low-level radioactive waste into glass.

MAWS has been selected for future demonstration at a western site by the Western Governors' Association Committee to Develop Onsite Innovative Technologies.