demanded that Chisso install a complete treatment system for plant effluent and suspend operation until the installation of such a system. Meanwhile, patients of Minamata Disease organized sit-ins in front of the main gate of the Chisso Minamata Plant, demanding compensation.
The MITI gave a guidance to Chisso in October 1959 to install a plant effluent treatment system, and Chisso completed the installation of a coagulation sedimentation system on December 19, 1959. With the reporting of the completion of the plant effluent treatment system by the mass media, there was rising local expectation of the treatment of the effluent by this system. However, it was later discovered that the system was not designed to remove mercury and was useless for the removal of methylmercury compounds in the effluent.
Some movement was made in December 1959 regarding the issue of compensation. A compensation agreement for fishermen was reached on December 25, 1959 between Chisso and the Federation of Fishermen's Cooperatives in Kumamoto Prefecture with the help of the Arbitration Board for Fisheries Disputes in the Shiranui Sea, members of which included the Governor of Kumamoto Prefecture and the Mayor of Minamata City. The same Arbitration Board also helped with the signing of the so-called consolation payment agreement between Chisso and the Mutual Help Group of Households with Minamata Disease Patients. One clause of this agreement stated that recipients of this payment would not demand further compensation even if Minamata Disease was found to be caused by effluent from the Chisso Plant in the coming years.
The installation of the coagulation sedimentation system, fisheries compensation payment and consolation payment to patients eased the intensity of the local dispute on Minamata Disease by December 1959, and the problem of Minamata Disease ceased to be a major topic of social concern without clarification of its cause. Although the research by the University of Kumamoto to investigate the cause continued, hardly any progress was made regarding control or prevention measures by the governments until the outbreak of the same disease in Niigata Prefecture in 1965.
Outbreak of Niigata-Minamata Disease to Consensus Opinion of the Government
On May 31, 1965, Professor Tsubaki and other staffs of the University of Niigata reported to the Health Department of the Niigata Prefectural Government that patients suspected to be organomercury poisoning appeared in Niigata. In June 1965, the Niigata Prefectural Government established the Research Headquarters for Mercury Poisoning in Niigata Prefecture and conducted a health survey on people living in the Agano River basin with the cooperation of the University of Niigata and others. The Special Investigation Team on Mercury Poisoning in Niigata set up by the MHW in September of the same year submitted its report to the Ministry in April 1967. This report claimed that effluent from a plant of Showa Denko was responsible for the disease based on epidemiological study results and other data even though the company blamed agrochemicals for the cause.
On September 26, 1968, the MHW and the Science and Technology Agency announced the consensus opinion of the government that Minamata Disease in Kumamoto Prefecture was caused by "a methylmercury compound generated by the acetaldehyde and acetic acid manufacturing facilities" at the Chisso Minamata Plant and that the same disease observed in Niigata Prefecture was caused by "a methylmercury compound generated as a by-product of the acetaldehyde manufacturing process" at the local Showa Denko Plant.