In the latter half of the 1960s, Lake Biwa, Japan's largest freshwater lake known as the "water reservoir of the Kinki region,"^* experienced an abnormal growth of phytoplankton due to eutrophication. In the 1980s, metabolites from this phytoplankton caused a musty smell in tap water. The water quality of the Yodo River, which is fed mainly by Lake Biwa, also deteriorated due to population growth and industrial development associated with rapid economic growth. Trihalomethane, which is produced during the water treatment process, became a major social problem because of its potential to cause cancer. At that time, tap water was not safe to drink, and society longed for safe and delicious water that could be drunk straight from the tap.

The Kunijima water purification facility in Osaka City was one such water treatment plant that used water from the Yodo River as its source. Its water purification treatment method was the rapid filtration system, which was common at the time. While it was excellent at removing water turbidity, it was not good at removing musty odors and reducing trihalomethanes dissolved in raw water. Furthermore, the presence of environmental hormones due to trace organic matter such as pesticides also became apparent. Although trihalomethane concentration levels were below the set water quality standards, Osaka City nevertheless had to take immediate action to ensure the safety of its tap water, the drinking water for its citizens.

In order to solve this problem, Osaka Municipal Waterworks Bureau and Kubota introduced a highly advanced water purification treatment pilot plant to improve the overall quality of tap water, including the removal of strange odors and the reduction of trihalomethane. This involved the addition of ozone and granular activated carbon treatment to the conventional water purification system.

Ozone, with its strong oxidizing power, is effective in decomposing and removing organic substances that cause musty odors, as well as in disinfecting water. Granular activated carbon adsorbs organic substances and substances that cause trihalomethane in its small pores, and the microorganisms attached to the surface of the activated carbon decompose them.

By combining both treatment processes, the musty odor was completely eliminated, and approximately 60-70% of the trihalomethane that remained in the water purified by the conventional system was removed. These efforts resulted in delicious tap water that can be drunk with peace of mind. However, this was only the start of a much bigger challenge, and Kubota's efforts to improve the quality of tap water continued.