Mixing and aging of water in distribution system storage facilities

Mixing and aging are two related phenomena that affect the water quality changes that can occur within distribution system storage tanks and reservoirs. Long residence times depress disinfectant residuals and can promote bacterial regrowth, and uneven mixing can result in zones of older water. As a result, minimizing detention time and avoiding zones of water that remain in the storage facility for long periods of time should be implicit objectives in the design and operation of distribution system storage facilities. In this article, the authors present information and tools to assist water utilities in the design, retrofit, or operation of distribution system storage facilities so as to minimize the probability of seriously diminished water quality resulting from poor mixing and excessive aging.