Japan to use In-situ Soil-remediation Technology

A Japanese consortium shortly expects to commercially apply an imported U.S. technology developed in the early 80s. The application could prove the viability of a process to provide the country with a long-term solution to its contaminated soil sites.

The process works by melting soil in-place using electricity applied to four graphite electrodes inserted about a half meter into the ground in a square or rectangular pattern 8 m apart. A highly conductive mixture of graphite and glass-forming compounds is placed between the electrodes during startup.

A 3- to 4-MW potential is applied to the electrodes, creating a current flow through the starter path, which heats up and causes the surrounding soil to melt. Once the soil melts, it too becomes electrically conductive. Continued application of electricity results in joule heating within the molten soil between the electrodes.

With the melt fully established, the melt zone grows steadily downward and outward through the contaminated media. The process can treat 90 to 140 metric tons each 24 hours. Typical soils undergo a 25% to 50% volume reduction due to densification.

The process destroys most organic and some inorganic compounds by thermal decomposition (pyrolysis) in the oxygen-depleted environment in and around the melt zone. Compounds when pyrolyzed typically break down to their elemental components (carbon, hydrogen, chlorine.). Volatile components travel to the surface of the melt where nearly all oxidize. Any remaining volatile components are treated by an off-gas treatment system (see illustration).

Where the temperature has been above 1400ºC (2550ºF), the soil turns to glass, encasing such pollutants as heavy metals. Since the glass is structurally stable, the pollutants can be confined almost indefinitely.

The method as used in Japan costs 10,000 yen (U.S. $90) per metric ton of soil. This cost is considerably more than that for Japan's conventional method for dealing with contaminated soil, but reportedly provides near permanent security. (Japan's conventional method is to encase the soil in concrete for disposal elsewhere. However, the encased waste can leak as the concrete breaks down with time.)

The melt technology comes from Geosafe (a Richland, WA-based division of Battelle Memorial Institute) which became a partner in ISV Japan Ltd. when the consortium was established in 1995. Other members include the Japan Research Institute (JRI), Ube Industries Ltd., Konoike Construction Co., Hazama Corp., and Mitsubishi Corp.

To adapt the technology to Japanese sites, in 1994 JRI, an economic think tank, launched a development project with the government's New Energy and Industrial Technology Development Organization. The institute fined-tuned the method, experimenting with chemical agents to promote the process, based on data from an pilot facility at a Ube Industries' factory.

ISV Japan has conducted 10 trials since 1995, treating 1000 to 10,000 metric tons of polluted soil. The consortium said the recent economic slowdown in Asia has discouraged remediation companies from attacking soil-pollution problems with the new technology; they prefer the concrete process for its lower cost. Soon, Japan's first commercial application of the vitrification technology, it is thought, will occur at the country's equivalent of a U.S. Superfund site.

Japan is said to have an estimated 200 large sites contaminated with illegally deposited wastes

The previous case study was initiated by a story carried by the Nikkei English News service on Jan.11.