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Home > Outline of Japan's Industrial Pollution Abatement > Approaches to Water Pollution Control (Case Study-2) Minamata City, Kumamoto Prefecture > (1) Bottom Sludge Treatment Program in Minamata Bay and Other Water Areas

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Update:April 1, 2010

(1) Bottom Sludge Treatment Program in Minamata Bay and Other Water Areas

4.Approach to Environmental Regeneration

The amount of mercury discharged and sedimented in Minamata Bay was estimated to be 70-150t. Fish and shellfish from the bay sometimes contains abnormal levels of mercury exceeding the provisional control standards established by the national government.
Aiming to treating such poisonous sludge containing mercury and protecting the residents' health, Kumamoto Prefecture took the initiative in making a Pollution Control program that includes reclamation and dredging of the bottom sludge containing mercury more than the prefectural standard (25ppm) based on the Provisional Standard for Bottom Sludge Containing Mercury as decided by the Environment Agency from October 1977.
For the operation, the reclamation site and the dredging site were first separated with a watertight embankment made of steel sheet piles. Then, the bottom sludge on the sea bed of the dredging site was scooped and fedinto the reclamation site using a pump specially designed to prevent turbidity in the dredging site. After this, the reclamation site was covered with fine quality mountain soil after surface processing (See Figure 3-8, and 3-9). In the course of the operation, the water quality and aquatic life were strictly monitored to prevent secondary problems.
In March 1990, the above program was successfully completed without any fear of secondary pollution after 14 years and 48.5billion yen of expenses. Because of the program, the water quality in the bay was dramatically improved; mercury concentration of the sludge decreased to 4.69ppm. In addition, a reclamation site of 58ha was acquired.
Chisso paid approximately 60% of the total operation expenses and therest was borne by the national government.
Aside from with the above, sludge treatment programs were conducted at Hyakken Wastewater Outlet by Minamata City and at Marujima Port by Kumamoto prefecture.