Semiconductor production consists of a series of between 200 and 300 processes, and since at every stage impurities are washed away from the product (silicon wafers), vast quantities of water are required. At the Ricoh Group’s Yashiro Plant, which handles the production of our semiconductors, we have been using a highly efficient closed water recycling system since the plant came online in 1989, with the objective of thoroughness in the effective use of water.
The processing of the wafers takes place in a clean room, at which stage ultra pure water is employed. The water from the water pipes is flushed through a pure water production system, in which after initial and then secondary treatment it is turned into ultra pure water and supplied to the production line. The water used on the production line (principally for cleansing wafers) is recovered in the drainage line after being divided into three grades according to the level of impurities it contains. After passing through the pure water collection and processing system the Level 1 drainage water with a comparatively low amount of impurities in it is returned to the pure water production system, and is used again as ultra pure water. By passing the Level 2 drainage water through the pure water collection and processing system it once again becomes pure water (of a lower grade than ultra pure water), and this is used in boilers and as cooling water for freezing units. The Level 3 drainage water undergoes neutralization treatment in the effluent treatment system, and is then flushed down the sewage pipes.
Furthermore, water other than the water used to cleanse wafers that is processed by the closed water recycling system is also subject to recycling activities. For example, the sampling water used to measure dissolved oxygen in once-through boilers is generally flushed away after measuring, but it is now possible for us to reuse it in boilers. When doing so, by altering the position of each device the water is fed to a hot well tank using the difference in elevation, making it possible to reuse the water without any pumps or energy.
At the Yashiro plant these sorts of activities have led to an overall water recycling rate of 63.9%, which means we have reduced the amount of water used on an annual basis by 245,000m3 (actual performance in FY 2012).