The process is being broadcast live by leading Japanese television companies.
TEPCO has collected more than 1 million tonnes of wastewater more than a decade after a massive tsunami triggered a meltdown at the Fukushima Daiichi plant in March 2011.
On Tuesday, Japanese Prime Minister Fumio Kishida said that the release of treated radioactive water from the Fukushima Nuclear Power Plant into the ocean would begin on August 24 if weather and marine conditions allow.