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The Tokyo Metropolitan Government is upgrading the sewerage system to clean the water in the moats of the outer garden of the Imperial Palace as part of its bid to host the 2016 Olympic Games.

Its plan is aimed at improving the sewerage system to reduce the amount of polluted water flowing into the moats after heavy rainfall and to stop deterioration of the water quality by fiscal 2015.

The International Olympic Committee is scheduled to meet in Copenhagen on Oct. 2 to choose the games site. The candidates are Tokyo, Chicago, Madrid and Rio de Janeiro.

Covering about 370,000 sq. meters, the palace moats consist of 13 small and large moats surrounding the outer garden, which is the largest green zone in the capital.

According to the Environment Ministry, the now defunct purification plant in Shinjuku Ward used to supply the moats with clean water until its closure in 1965. Rainfall is the main source of water now.

Officials said trash and sludge are filling the bottom of the moats because of the lack of water circulation. Another problem is the flow of polluted water from sewers due to the use of the same pipes to carry dirty water and rainwater in many areas of Tokyo.

Normally, polluted water runs through the pipes but in the event of heavy rain, rainwater pours into the pipes to prevent homes from flooding.

Polluted water joins water released from the exit sites of four moats when the flow exceeds the capacity of sewer disposal facilities. Such water contains phosphorus and nitrogen and encourages substances low in nutrition to become rich in nutrients, generating a larger volume of plankton.

This develops into a serious problem in summer as the blue-green algae that have proliferated start rotting and smelling foul.

“Blue-green algae show up all over the surface of the water in some parts of the moats and it doesn’t look good,” said a 72-year-old man who frequently jogs around the palace.

The metro government plans to prevent polluted water from entering the moats by extending the main sewer pipe connected with the discharging exits of the moats by about 2.5 km into the Sumida River in the eastern section of the capital.

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