(4) Effluent Treatment by Chisso Minamata Plant

2.Mercury Poisoning and Control Responses

Table 3-3 shows the chronology of effluent treatment processes of acetaldehyde manufacturing section of Chisso Minamata Plant. As shown in the table, the process water had been discharged into Minamata bay from Hyakken Wastewater Outlet after collecting mercury in a scrap iron chamber reductor since the plant resumed operation after the war in February 1946. The above was abolished in September 1959, and neutralization precipitation was introduced. Effluent kept in the residue pit was sent to the Hachiman pool to separate solid component by sedimentation and discharged at the mouth of the Minamata River. This method, continued until September 1959, was not yet sufficiently effective.
In December 1959, encouraged by the instruction of the Ministry of International Trade and Industry, Chisso Minamata Plant established a circulation and sedimentation now system, which was the earliest attempt to reduce pollution among other Japanese chemical plants except paper mills. Even this system, which was one of the biggest in Japan employing the latest technology, could not realize the complete removal of methylmercury.
In August 1960, they introduced a drain circulation system for partial wastewater recycling in which mercury is collected as the effluent passes through a mercury collecting unit (scrap-iron-filled tower).
Since then, patients with acute symptoms of Minamata Disease have not been observed, and mercury concentration in the shellfish has decreased in Minamata Bay from 85ppm down to the level around l0ppm within 10 months (from January through October) according to the continuous follow-up surveys. The level of mercury in shellfish has not made a further increase proving the successful performance of the above system, In pursuit of a means of retaining all kinds of effluents associated with mercury handling, Chisso Minamata Plant established a complete circulation system for wastewater recycling in 1966, in which all effluent including drainage, pump ground leakage, washing water for periodical maintenance are collected in an underground tank and recycles them for acetaldehyde manufacturing. Discharge of effluent containing methylmercury compound was then completely checked after which.
In May 1968, Chisso Minamata Plant closed acetaldehyde manufacturing process.

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