The operator of the crippled Fukushima No. 1 nuclear plant said Wednesday it had finished filtering 620,000 tons of extremely toxic water stored in tanks on the premises of the complex to lower its radiation level.

Tokyo Electric Power Co. says the risk of radiation leakages from the water tanks is now much lower. However, around 400 tons of radioactive water is still being generated everyday as groundwater is seeping into the plant and mixing with tainted water more than four years after the nuclear crisis began.

According to Tepco, some 440,000 tons of the water has been treated through a water processing system that is said to be capable of removing 62 different types of radioactive material, with the exception of tritium. The remaining 180,000 tons has been processed through another facility capable of removing strontium, but still contains other types of radioactive substances and needs further treatment.

The highly radioactive water has been generated during the process of cooling the plant's reactors, which suffered meltdowns after the facility was struck by a magnitude-9 earthquake and tsunami in March 2011.

When Prime Minister Shinzo Abe visited the Fukushima plant in September 2013, Tepco President Naomi Hirose pledged that the company would filter all the water kept in tanks by March 31, 2015 to reduce the amount of radioactive material it contained.

But the process has been delayed due to a series of problems with key water treatment facilities.