Standpipes are among the most problematic tank geometries to mix. Inlet velocities are typically small in magnitude and horizontal in direction. The substantial majority of water in the standpipe must remain in the tank to produce and maintain pressure in the distribution system, so there is often a hard limit (typically 70% to 90% of capacity) below which the operators cannot draw. This takes away the default (yet energy- and labor-intensive) method of mixing—the forced drawdown and refilling of tanks.
Redwood City, Calif., operates a 500,000-gal ground-level steel tank with a single inlet/outlet. Located in the Bay Area of Northern California, Redwood City purchases chloraminated water from a regional wholesale supplier. During the summer, the total chlorine residual of samples taken near the bottom of the tank typically remained at the target level of 2.0 mg/L, but residual levels in samples taken from the top few feet of the water inside the tank would drop significantly, sometimes to undetectable levels.
Stanly County, in central North Carolina, is typical of many smaller rural water utilities in the U.S. Due to excess capacity in the city water system that supplies the surrounding county, Stanly County receives finished water that is often at or above the maximum contaminant level (MCL) for total trihalomethanes (TTHMs). Without a treatment plant of its own, Stanly County is limited to few options for bringing its water into compliance.
A rectangular, in-ground concrete tank in Ontario, Calif., had poor circulation and low turnover, and it suffered from excessive water age. During the winter, this tank showed good water quality. But because of thermal loading on the roof in the summer, stratification between top and bottom would build to almost 16°F, which would isolate the top water from the rest of the reservoir, leading to complete residual loss and acceleration of bacterial and biofilm growth.
Monterey, Calif., is a seaside town that enjoys cool weather, picturesque beaches and, for the most part, excellent water quality. However, over the last few years, trihalomethane (THM) levels in the Ryan Ranch part of its system have risen dramatically. Despite aggressively flushing this part of the system and periodically boosting chlorine at the tank to improve residual levels, Monterey was on track to breach its total trihalomethane (TTHM) levels in the summer of 2011.
The City of Rifle, Colo., knows painfully well that during summertime in the Colorado desert, the daytime heat loading on drinking water tanks can be intense. When exposed to long hours of sunlight, the water inside these tanks is heated and rises to the top of the tank, forming a layer of isolated, high-temperature water that can be up to 10°C (22°F) warmer than the fresh, cool water delivered during the tank’s daily fill cycles. This warmer layer floats up and down like a piston on top of the cooler bottom water and is never refreshed.
Maintaining disinfectant residual levels in drinking water reticulation systems is a challenge, even under normal conditions. Water must travel through several kilometers of pipes and is often stored in water tanks and basins before reaching customers. However, when the distance between the treatment plant and customer is extensive, maintaining adequate disinfectant levels becomes even harder. Further challenges become apparent during times of low water usage as the age of water within the water reticulation system increases.
San Jose Water Co., located in the heart of Silicon Valley in California, is used to being on the forefront of innovation. As one of the largest and most technologically sophisticated investor-owned utilities in the U.S., it has taken a proactive approach to improving drinking water quality and lowering operating costs.
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).