NKK has commercialized a new ozone-contact water purification system that uses pure oxygen to generate ozone. In a joint research project NKK conducted with the Hanshin Water Supply Authority (HWSA) in Kobe City, NKK has verified cost advantages of the new system in comparison with conventional systems that rely on air for raw material. Major benefits include a reduced volume of harmful byproducts formed and a doubling of processing efficiency, which more than offset additional costs of the new system.

The HWSA recently placed a ¥1.7-billion order with NKK to supply an underground U-tube ozone reactor system, combined with an activated carbon filtering device, to the New Amagasaki Water Purification Plant being built in Amagasaki City. The space-saving vertical system will have a water treatment capacity of 190,000 tons per day.

The U-tube ozone-contact technology, which NKK licensed from DEGREMONT S.A. of France, involves running the raw water through a vertical U-tube, buried in the ground, where it is exposed to ozonized gas. The vertical design saves space and is cheaper to operate than the conventional horizontal design.

Artist's impression of the new water purification plant in Amagasaki City, which will use an oxygen-based ozone-contact system

The oxidizing effect of ozone decomposes organic compounds in raw water and kills bacteria. Using pure oxygen instead of air, 70% of which is nitrogen, for generating ozone through electric discharge, reduces nitrogen compounds that become nitric acid, etc. and yields 7 to 15 times the concentration of ozone.

The HWSA supplies about 70% of all clean water to four neighboring cities, including Kobe and Amagasaki. Construction of Amagasaki's new water purification plant (total water treatment capacity of 373,000 tons per day) began in fiscal 1997. The oxygen-based system has been chosen because the plant is located next door to Osaka Sanso Kogyo's oxygen plant, meaning that oxygen can be purchased at low cost and piped in directly.

One system at the new Amagasaki plant, capable of treating half of the projected daily total, is to be completed in March 2001. It will consist of two U-tubes each with a 0.75-meter inner diameter and 27-meter depth. The concentration of ozone generated by electric-discharge ozone generators will be more than 10% of the weight.