Sediment in Water Storage Tanks Can Be a Haven for Pathogens. Here's What to Do About It.

After major storms such as hurricanes, water storage tanks should be inspected for breaches and for accumulated sediment on the tank floor.

Sediment in Water Storage Tanks Can Be a Haven for Pathogens. Here's What to Do About It.

The best approach to reducing the risk of contamination after a breach is an inspection, followed by cleaning if necessary.

While hurricanes like Harvey and Irma can cause multiple breaches in a water system, sediment already accumulated in water tanks can add to the problem by hosting a wide variety of new contaminants. In this situation, chlorine reserves may quickly be overwhelmed, leaving the system at risk.

One common fix to a breach is a chlorine burn. Increasing chemical disinfectant to clean water mains may be the first choice, but water storage tanks and towers have two significant dimensions missing from the piping: the large volume of water in tanks and sediment buildup on the interior tank floor. Chemicals alone are not enough after a breach — it may be necessary to clean the tanks.

Hazards in sediment

While a chlorine burn may yield immediate results, the question is whether its effects will last. The burn effectively removes contaminants from the underwater surfaces of pipes, lines, and water mains, but it is difficult if not impossible to raise the chlorine level in a storage tank high enough to do the job.

At best, the chlorine kills the microbes on the surface of the sediment on the tank floor. Under that sediment is where the problems lie, and they continue to grow each day. The sediment gives safe refuge to the exact pathogens the chlorine was designed to kill.

When the tank resumes normal operations, the microbes can once again grow under the protection of the sediment, depleting normal chlorine reserves and potentially causing a violation or, far worse, a community health crisis.

After any system breach, it is a good idea to have the tanks cleaned. This includes many types of breaches that commonly happen during and after a hurricane, tornado, or other extreme weather event. Common breaches include damaged or missing vent screens, open or missing hatch covers, holes and any other opening in a tank and, of course, flooding.

Dealing with breaches

Sediment builds up in almost all water storage tanks and towers over time. This soft accumulation can be a breeding ground for multiple pathogens. After a storm, a comprehensive inspection of the facilities should be performed as soon as it is safe to do so. Any breaches should be repaired. If the tank has a breach, it is more likely to also have some sort of microbial contamination. 

In fact, a few years ago, at a five-state meeting on drinking water at U.S. EPA Region 6 headquarters in Dallas, an EPA official referred to sediment on the interior floor of water storage tanks as a breach and said that sediment should be treated the same way as any other breach in the system.

To understand why this is so, one only needs to look at the kinds of organisms that can use sediment as a habitat. The list is long, but according to the Centers for Disease Control and Prevention, the most common villains include Giardia, Legionella, Norovirus, Shigella, Campylobacter, Salmonella, hepatitis A, Cryptosporidium, and E. coli.

Cleaning methods

The EPA recommends that potable water storage tanks be removed from service and steam-cleaned. While that is the most desired method, especially after a flood, it is extremely expensive and time-consuming and usually is not an option for smaller systems. Most systems cannot afford to have a storage tank offline for that long. Often, the best fit is to hire a diving contractor to remove all loose sediment. 

Removing sediment removes the habitat that microbes can use to get a foothold in your system. Afterward, chemical treatment becomes more effective and, more often than not, less chemical is needed to meet standards.

The best approach is to be proactive, keep the system safe, and reduce the risk of widespread contamination or Revised Total Coliform Rule violations by inspecting tanks and, if needed, cleaning them. If an RTCR violation does occur, an assessment is required, and an underwater tank inspection can be an important part of that process. If sediment is found in the tank, cleaning would be necessary.

The bottom line: After severe storms, water storage facilities need a comprehensive inspection, inside and out. If there are breaches, repairs need to be made. If sediment has accumulated, it should be treated as a breach and removed from the system. 

About the author

Ron Perrin ( is owner of Ron Perrin Water Technologies, a water tank inspection and cleaning company based in Fort Worth, Texas. 


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