To supply safe, good-quality tap water, Osaka Municipal Waterworks Bureau (OMWW) introduced an advanced water treatment system that incorporates ozonation and granular activated carbon (GAC) treatment into conventional treatment processes. By March 2000, all water purification plants in Osaka City were equipped with advanced water treatment facilities. This new treatment system has successfully removed odours and flavor and reduced trihalomethane. GAC treatment, the last stage of the water treatment, plays a vital role in removing organic matter in water, in conjunction with ozonation. However, GAC performance changes over time. To optimize the GAC effect, it is essential to conduct proper GAC management based on a thorough grasp of the change patterns of GAC performance. To determine GAC's water treatment capability, we analyzed data on 69 water quality items; obtained from past plant tests and chemical substance addition experiments using GAC actually used in the purification plants. Analysis revealed the deterioration patterns and time of GAC performance. By considering physical properties and economic factors as well as deterioration patterns of GAC performance, it is possible to establish an optimal GAC management system. This paper describes the findings obtained from the study on GAC water treatment performance conducted by OMWW.

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