Thermal stratification is often an invisible problem inside water storage tanks. Without taking water temperature measurements at different depths, a municipality may be unaware that warm water is trapped and aging for days or even weeks in the top
of their tank. Typically, the only warning signs of thermal stratification are sudden changes in disinfectant residual levels and taste and odor complaints, usually during warm summer months.
The City of Covina, Calif., faces water quality challenges that are typical for many urban and suburban water systems across the southern United States. Nestled in the heart of the greater Los Angeles area, Covina receives water from several sources and distributes finished water through multiple pressure zones. With large variations in seasonal demand, operators must balance maintaining supply and reducing water age with requirements for emergency preparedness.
Pinellas County, Fla., has a distribution system that is typical of many major metropolitan water systems, with over 700,000 customers, 2,000 miles of piping and several large water storage facilities. The Pinellas County Department of Environment and Infrastructure (DEI) has seen a decline in water usage over the last decade, due to both active water conservation programs and downturns in the regional economy. This decrease in water usage, combined with warm southern temperatures, has increased water age and incidences of nitrification in parts of its chloraminated system.
The City of Rockville, Md., located just outside of Washington, has taken a proactive approach to meeting water quality regulatory compliance for its drinking water system. However, an unexpected notice of violation for exceeding the maximum contaminant level (MCL) of total trihalomethanes (TTHMs) in 2008 caused the city to critically examine water quality in its water distribution system. TTHMs are chemicals that form when chlorine reacts with naturally occurring organic matter found in drinking water, such as Rockville’s drinking water source—the Potomac River.
January 2014 brought a major weather event to the northern United States and Canada, and many challenges to water utilities in the region. An ultracold mass of air descended out of Canada into the United States, and temperatures fell to record levels. Water utilities struggled with dangerous work conditions, water main breaks and ice formation in water storage tanks. The City of Atwater, Minn.—situated 100 miles west of St. Paul—has seen its share of cold winter weather, but January 2014 set new records.
South of Laramie Water and Sewer District (SLWSD) is a consecutive system that purchases water from the city of Laramie, Wyo. Since SLWSD was formed in 1996, managing icy conditions and maintaining water quality in their only tank—a 300,000 gal pedisphere—has been a concern.
Large underground water storage tanks are a common feature of many water distribution systems in the United States and Europe. These basins are typically rectangular, shallow and concrete and feature numerous columns that support a thick roof. Unlike aboveground storage tanks, which are often visible for miles, underground storage tanks are hidden from view. In many cases, municipalities utilize the land above the storage reservoir for recreation or parking.
The Spanaway Water Co. in Washington discovered thermal stratification in its standpipe after noticing heavy condensation on the outside wall of the steel tank. The operator expected that the cold water inside the tank was causing the condensation, but was alarmed that the condensation was only visible 20 ft up the side of the 127-ft standpipe. To test his theory that the tank was thermally stratified, the operator installed a series of submersible temperature probes at 20-ft intervals inside the tank.
While most people enjoy a little ice in their water glass, a large ice plug inside a water storage tank can spell disaster. Unfortunately, ice damage is difficult to avoid in northern climates. Depending on the air temperature, inlet water temperature, amount of turnover, and presence of tank insulation and heaters, ice formation can range from a thin skin to a several-ton ice cap.
Redwood City, Calif., is one of 24 municipalities in the San Francisco Bay Area that receives its water from the San Francisco Public Utility Commission (SFPUC).