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TECH-TIP: Distribution storage tank monitoring

Distribution Storage Tank Monitoring

Paul Handke – Water Program Specialist, Pennsylvania Department of Environmental Protection

Tanks - the most visible part of a water system, often welcoming visitors to the areas that they serve.  They are also key components to distribution system operations, providing adequate pressures and volumes of water to customers during normal use and during times of need, such as a fire.  Because of this, the operation and maintenance of storage facilities, such as tanks, is one of the performance improvement variables considered in the Partnership’s distribution system self-assessment process.  The self-assessment questions associated with storage facilities cover the areas of sampling, water use, inspection, and cleaning. This Tech Tip focuses on monitoring water quality in storage tanks. 

These highly-visible and much needed vessels of water can also be one of the biggest challenges to sustaining water quality within the distribution system.  To gain a better understanding of the impacts of tanks on water quality within your system, data must be collected to help support or validate operational changes or capital improvements that may be instituted to improve water quality.  Data collection can range from simple to complex, depending on the capabilities and needs of the system.

Grab samples at the inlet and outlet of the tank are a simple mechanism to determining the impact of a tank on water quality and can provide a very clear picture of the degradation that is occurring within a tank.  Although most systems sample at their tanks, little thought is typically put into tank operations at the time of the sampling (i.e. is the tank filling or drawing and where is it during that cycle?).  Optimally, sampling should take place at the end of a fill cycle, which typically represents the best water entering the tank, and at the end of the subsequent draw cycle, which typically represents the worst water leaving the tank.  A variety of parameters can also be measured, including but not limited to; free chlorine, total chlorine, monochloramine, free ammonia, nitrite, temperature, pH and disinfection by-products.  The parameters monitored will depend on the type of disinfection and the issues that are occurring within the distribution system.

Continuous monitoring at the inlet and outlet will provide a better and more consistent picture and trend.   Continuous monitoring is not only useful when developing an overall performance trend for the tank; it is very useful in detecting anomalies that may not be detected with grab sampling and that can indicate issues that may otherwise be hidden, such as severe stratification resulting in the loss of a chlorine residual that is only seen when a tank goes out of its normal operating range, as indicated in Figure 1.   Continuous monitoring will also provide better feedback when operational or design changes are made by providing real-time and trending data.

In-tank monitoring is the next step when determining the overall water quality within the tank itself, keeping in mind that at some point the water within the tank will be exiting the tank and reaching customers.  Some modern tanks are being designed with taps that are located at various levels within the tank, allowing for quick and easy in-tank monitoring.  The majority of older tanks do not have this feature and require accessing the tank from the top to collect samples.  Several types of in-tank monitoring tools are available to get a better picture of tank mixing and overall water quality.

Temperature is a good indicator of whether stratification exists within a tank.  Temperature dataloggers are relatively inexpensive and can be placed at various levels within a tank for an extended period of time.  If there is significant temperature separation thru the depth, it is a strong indicator the tank is not mixed and is likely negatively impacting water quality.  Figure 2 displays a graph of temperature readings at various depths within a storage tank, represented by the colored lines.  The differences in temperatures are indicative of stratification.

If temperatures are consistent from top to bottom throughout both fill and draw cycles, sufficient mixing is most likely occurring within the tank.  Figure 3 displays a graph of temperature readings at various depths within a storage tank.  In Figure 3, temperatures are consist throughout the water column, indicating a sufficiently mixed tank.

In addition to temperature monitoring, water quality grab sampling can also be conducted within the tank using a variety of depth sampling tools, including well sampling purge pumps and Kemmerer samplers.  Beyond temperature, this sampling can provide water quality data that may help pinpoint tank operational issues and the resultant impacts on distribution water quality.  Figure 4 demonstrates the impact of tank depth on disinfection by-product concentration. 

If performed in conjunction with temperature dataloggers, water quality sampling should be conducted at the same depths as the dataloggers so the data can be more easily compared and interpreted.

As the industry and regulatory focus continues to move towards distribution system water quality, including the Partnership for Safe Water Distribution System Optimization Program, the impacts of tanks must be taken into consideration.   As with other aspects of water treatment and distribution, comprehensive data collection and analyses must be the first step in determining what those impacts are and what types of changes may be made in order to remedy identified limiting factors.