The mixing of potable water tanks has come to the forefront of consideration and attention in recent years. It has become apparent that there are issues of stagnant water in tanks, high levels of disinfectant by-products, and inconsistent contact time resulting in varying levels of disinfectant throughout the distribution system. To improve water quality, water systems are encouraged to assess overall water condition and take the necessary steps to improve the mixing of their tanks.
Mixing/Circulating of tanks may involve a passive system of a dedicated inlet line discharging water towards the top of the tank, with an outlet pipe located at the bottom of the tank. This ensures that new, colder water will circulate through the tank from top to bottom. A modification of this is a single line rising into the tank with a series of valves producing inflow at the top of the tank and outflow at the bottom.
Several options of commercial mixers are available involving an electric motor/pump mixing unit, which can be lowered into the tank and plugged into an outlet located at the top of the tank. These units circulate the water in the tank continuously ensuring turnover of the tank to eliminate stratification, provide uniform temperature, thorough contact time of disinfectants, and better removal of disinfectant by-products.
A side benefit of a mixing system is the reduction of ice buildup in the tank during the winter months, through the continual movement of the water in the tank. We all know that moving water does not freeze. Mixing systems do not remove all of the ice in a tank but can reduce it considerably depending on what part of the country you live in. Additionally, a mixer will not keep a riser from freezing shut if there is no water outflow.
For those operations in the extreme northern climates, an elevated tank may be better served with a circulating system. Circulators involve a dedicated 1-2 inch PVC circulating line located inside the existing inlet line, connected to a pump located on the ground at the bottom of the tank. The circulator pulls water from the bottom of the riser and pumps it to the top of the riser and into the tank. The circulating system provides all the benefits of the mixers, but also helps to ensure that the tanks riser will not freeze shut during extreme cold weather by continually moving water through the inlet pipe and the tank.
Maguire Iron has extensive experience in dealing with water tanks from the heat of the south to the cold of the extreme north. Maguire Iron can help your water system with design and installation of mixing and circulating systems for your water tanks. Contact your regional Maguire Iron Field Representative for a consultation.
Submitted by: Rich Kemmis, Maguire Iron Field Representative for SD, ND, MN, MT, WY, ID