WATER STORAGE TANKS THE SCIENCE OF MIXING AND IMPROVING WATER QUALITY Michael J Duer, P.E., Chief Engineer, Tideflex Technologies
Water storage tanks and reservoirs are a critical component of distribution systems, yet they can pose a significant challenge for water utilities as they often have a negative impact on water quality, due to low turnover or inadequate mixing resulting in short-circuiting.
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n water distribution networks, reservoirs are required for flow equalisation, to sustain pressure, to hold several days of storage for redundancy, and provide fire and emergency storage. In order to achieve these goals, reservoirs need to have adequate storage volume based on worst-case hydraulic scenarios and also must be designed to supply the system allowing for future community growth. Some of these design goals are contrasted by what is generally required to maintain safe drinking water. Common problems in storage tanks and reservoirs are the loss of residual disinfectant, bacteria regrowth, spikes in disinfection by-products (DBPs) and nitrification (chloramines), resulting from hydraulic short-circuiting, poor mixing and circulation, poor turnover and excessive retention time. Many of these water quality problems can be specifically attributed to the location and orientation of the inlet and outlet piping. In order to minimise water age, tanks must be turned over — that is, water volume must be exchanged to and from the tank by fluctuating tank levels. The required amount of turnover varies depending on the system, but a fairly common turnover goal is 3–5 days, or 20–33% daily fluctuation. However, tanks can have a significant localised increase in water age when they short-circuit and are not completely mixed, even if they are fluctuated 20–33%. Often the increased water age and all associated water quality problems are specifically attributed to the inlet and outlet pipes.
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There are three primary design goals to preserve storage tank water quality: 1. Design the piping or mixing system to separate the inlet and outlet to eliminate short-circuiting. 2. Design the mixing system to achieve complete mixing during fill cycles. 3. Fluctuate tank levels to exchange water volume, or turnover the tank, to minimise water age. The first two are the responsibility of the mixing system designer but they are hydraulically linked with the third. So, the designer must not only understand circulation patterns and mixing characteristics, but must also know how to design based on the tank turnover.
Short-circuiting The simplistic description of short-circuiting is the last water that entered the tank is the first water drawn from the tank (last in, first out). Water quality problems develop for two reasons: 1. The entire tank volume is not completely mixed. 2. The oldest water cannot be drawn from the tank due to the location of the outlet pipe. Short-circuiting is often not problematic over one or several days, but it is the consecutive daily fill and draw cycles with persistent short-circuiting that result in a localised increase in water age — resulting in the development of water quality problems such
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