Inside the reactors of Fukushima Daiichi Nuclear Power Station Units 1~3, there is fuel that melted and solidified during the accident (fuel debris). This fuel debris is being kept cool by continuously spraying it with water, but when this water comes in contact with the fuel debris it is exposed to the highly concentrated radioactive substances and becomes “contaminated water.” This “contaminated water” that contains highly concentrated radioactive substances accumulates inside the reactor buildings and when it mixes with groundwater or rainwater flowing into the buildings even more “contaminated water” is produced.
This “contaminated water” is being treated/purified to reduce the concentrations of radioactive substances using multiple types of equipment. After risk has been sufficiently reduced in this manner , it is stored in tanks on site as “treated water*”.

The ALPS is one of the pieces of equipment used to purify contaminated water at the Fukushima Daiichi Nuclear Power Station. Radioactive substances are removed by passing the contaminated water through adsorption towers that are filled with adsorbents thereby enabling this equipment to remove the most of radionuclides with the exception of tritium.
The government has stipulated two different regulatory standards for contaminated water. The first are regulations for contaminated water if it is to be stored in tanks. And, the second are regulations for contaminated water if it is to be discharged into the environment. Preventing impact on the surrounding environment has been our first priority, so contaminated water has been purified/treated using the ALPS etc. while adhering to the regulations for storing contaminated water in tanks. Therefore, at current time, all of the water that has been processed with the ALPS (treated water*) meets the regulations for tank storage, however more than 80% of it does not yet meet the regulations for environmental discharge.
If TEPCO decides to dispose of the water treated with multi-nuclide removal equipment, etc. (for which the sum of concentration ratios required by law equals or exceeds 1) by discharging it into the environment the water will first be purified again (secondary treatment) in order to reduce the amount of radioactive substances in the water as much as possible (with the exception of tritium) and ensure that it meets the regulations for environmental discharge.

Tritium is hydrogen with two extra neutrons (hydrogen-3) and it has the same characteristics as hydrogen.
Tritium is a radionuclide that emits beta radiation but the amount of energy emitted is weak and can be blocked with a sheet of paper.
Tritium is generated when cosmic radiation from space mixes with the Earth’s atmosphere. In the atmosphere the tritium bonds with oxygen instead of hydrogen which is why it exists in the form of water in the water vapor and rain in the atmosphere, as well as in the oceans, etc. We ingest tritium through the water we drink in our daily lives but our metabolism prevents it from accumulating/concentrating in our bodies and it is immediately excreted.
Tritium is generated through the process of nuclear fission in nuclear power facilities both in Japan and overseas (nuclear power stations and reprocessing facilities), and each country has regulations for discharging tritium into the oceans and atmosphere in a controlled manner.

Treated water* being stored in tanks contains other radioactive substances in addition to tritium. The concentration of these radioactive substances is analyzed using two methods. “Gross beta measurement” is used to measure the total amount of beta rays being emitted by various radioactive substances over a short period of time. And, “nuclide analysis” is used to assess the total of each concentration of the primary seven radioactive substances (※) that have relatively high detection concentrations when compared to legally required concentration levels.
In October 2018, a discrepancy (deviation) between gross beta measurement values and nuclide analysis totals was found, so an investigation was conducted and it was discovered that carbon-14 and technetium-99, which are not included in the seven radioactive substances mentioned above, were the cause of this discrepancy (announced on January 17, 2019).
Going forward, carbon-14 and technetium-99 will be analyzed in addition to the primary seven radioactive substances to confirm that there is no discrepancy with gross beta values. An investigation shall also be conducted to look for undiscovered radioactive substances other than carbon-14 and technetium-99.
Furthermore, if treated water* is to be discharged into the environment, any treated water* for which the sum of legally required nuclide concentrations, with the exception of tritium, equals or exceeds 1 shall be subject to secondary treatment to reduce the sum of legally required nuclide concentrations to less than 1.

(※) Primary seven radioactive substances (primary seven nuclides)
Cesium-134, cesium-137, cobalt-60, antimony-125, ruthenium-106 (+rhodium-106), iodine-129, strontium-90 (+yttrium-90)

There are Japanese regulations that stipulate the concentrations required by law for approximately 1,000 radioactive materials if they are to be discharged into the environment from a nuclear facility.
Of all the radioactive materials contained in contaminated water, 62 have been targeted for removal due to the risks that they pose to humans and the environment. Multi-nuclide removal equipment has been designed to have the ability to reduce the concentrations of these 62 types of radioactive materials (excluding tritium) to below legally-accepted concentrations (concentrations required by law).
When the equipment was first put into operation, there were periods when removal performance was unstable. There were also times when the “standards for water that is to be stored in tanks” were followed because priority was given to protecting the surrounding environment. According to our assessment, with the exception of the water that was treated during these periods, the sum of the ratios of concentrations required by law for the aforementioned 62 radioactive materials is less than 1* for the contaminated water that has been treated.
The graph below shows some of these 62 radioactive materials that are present in contaminated water for which concentrations are relatively high when you compare them to the concentrations required by law. The concentrations of these radioactive materials prior to purification and treatment with multi-nuclide removal equipment (at the inlet) and after purification and treatment (at the outlet) have been indicated.

If you look at the post-treatment numbers, you will see that the concentrations have been reduced to below the standards for discharging water into the environment (concentrations required by law).
And, we have confirmed that the concentrations of radioactive materials other than tritium and the 62 nuclides mentioned earlier, such as carbon-14, are sufficiently below government regulations.

 
* Assessment based on analysis of water sampled from the outlet of multi-nuclide removal equipment (post-treatment)