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Keeping water systems safe

Keeping water systems safe

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    The ability to quickly clean the inside of a potable water tank is important to the security of a municipality's water system. Photos: BIF LLC

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    Tanks, such as this one built by Crom, can hold two or three days of a city's water supply. This excess water supply is imperative when a disaster strikes, such as a terrorist attack or another catastrophe. Photo: Crom

One of the most problematic areas of our infrastructure security is the drinking water system. Though the United States' water supply is perceived to be adequate and well-protected, many points of vulnerability exist. Although water treatment facilities have increased levels of security and protection, a major area of vulnerability is the distribution network. Distribution systems are often spread over large geographic areas and through residential areas of cities, suburbs, and even rural areas, leaving water managers and operators looking for new and more effective methods to ensure security. These systems remain at significant risk.

According to the U.S. EPA, “Over the past several years, various Presidential Decision Directives and other orders have assigned the EPA responsibility for some very important aspects of the [infrastructure] security. These explicit responsibilities include being the lead federal agency charged with helping to protect the nation's water infrastructure from attack, being the lead agency responsible for the cleanup of any biological or chemical attacks, and having significant responsibilities in certain radiological attacks.”

“Since Sept. 11, 2001, the EPA has taken a number of steps to ensure its abilities to meet its homeland security responsibilities,” stated a report on the EPAWeb site. “The Agency is adding 75 response staff personnel to strengthen its ability to respond simultaneously to multiple incidents. In addition, the Agency is providing advanced training and state-of-the-art equipment to those who will respond to any chemical, biological, or radiological incident and is establishing a new Environmental Response Team West in Las Vegas to provide a quicker response time to any incidents that may happen in the western United States.

“The Agency has already awarded nearly $50 million in grants to the nation's largest drinking water facilities to assess their vulnerabilities and make security improvements and has upgraded its Cincinnati facility to handle level three contaminants. [In October 2002, EPA administrator Christie] Whitman announced a Homeland Security Research Center in the Agency's Cincinnati labs to coordinate research in areas such as building decontamination, rapid risk assessment, and drinking water protection.”

GATHERING DATA

Recently, significant work in a major water utility has shown that steps can be taken in advance to enhance the speed at which a system can recover from an attack. Through some coincidental and fortuitous events, important data were gathered related to the ability of a water utility to immediately recover from a catastrophic situation.

During the electricity blackout of 2003, a major Midwestern water utility was forced to drain the water out of all their distribution system storage tanks. When the power outage occurred that August, demand was high and pumping ability was immediately suspended. Continuous water usage by customers resulted in the tanks' water level dropping to the point of causing generalized depressurization in the distribution system.

After power was recovered, the utility began immediately pumping water into the storage tanks at the highest rate possible. Interesting comparisons became available when data on this tank recovery process were evaluated. Water-quality managers and engineers were able to look at tanks of different sizes and types to see how well they recovered with respect to each other. This information became particularly insightful in light of the EPA's interest in system recovery after a catastrophe.

While it is well-known that the recovery of these systems can take place over an extended period of time, it was interesting to see how the systems could recover with the addition of adequate mixing and blending equipment in the individual tanks.

BLENDING WATER

In this situation, two redundant ground storage tanks are located in the same service area of the distribution system and are supplied from the same water production facility. One of the tanks has a capacity of 3 million gallons and is fitted with an integral mixing system. The other does not have mixing capability.

Quality-control experts for the utility examined the differences in the water drawn from these tanks as the system began to recover. Initial thoughts were that the tanks would exhibit similar chlorine retention characteristics as they were filled and drafted. The chlorine residuals in the tanks were measured over time to determine how quickly the tanks could become fully operational.

In the tank with the mixing system, the chlorine residual continued to increase over time as the tank was filled and drafted. This is because the inflow of high disinfectant-residual water that enters the tank mixes with the water already in the tank, thus better sustaining a stable chlorine-residual level in the tank over time.

In the tank that did not include a tank mixing system, the tank chlorine residual cycled, as would be expected from the relatively high distribution-system flows that accentuated the short-circuiting. Short-circuiting is a common trait of tanks with no adequate mixing pattern. When water is allowed to enter the tank at the same time water is drawn out of the tank, a flow pattern develops that bypasses the majority of the tank's volume. In essence, water moves directly from the inlet to the outlet without mixing with the older water in the tank. Disinfectant residual in tanks diminishes with tank residence time. The rate of reduction is dependent on many factors. These factors, such as the amount of biological or chemical reactivity inside tanks, may be aggravated during emergency conditions.

When short-circuiting was taking place, high chlorine-residual water moved directly from the inlet to the outlet without interaction with resident tank water. Whenever the supply of water to the tank stopped, the older (low chlorine-residual) water in the tank was removed and the measured chlorine residual at the tank's outlet dropped. The chlorine residuals were almost exactly the same when the chlorinated water was originally introduced into the tanks, but over time, the tank without the mixing system exhibited wild fluctuations in quality due to short-circuting and water residence time. Customers served by the tank with the mixing system had consistently better water quality much sooner than customers on the tank without mixing system,

What does all of this mean with respect to security in water systems? Unfortunately we must be prepared for future terrorist attacks on water utility systems. And, we must prepared for recovery and remediation of storage tanks in the distribution system, Without question, the addition of the mixing system in potable water storage tanks significantly improves the quality of water and the ability of the water and the utility to quickly recover from a catastrophic event.