Inspection and cleaning of potable water storage tanks can pay dividends for public health as well as the health of the structures
It's not only bridges that come crashing down when deterioration and age cause structural failure. Drinking water storage structures can be overtaken by interior corrosion and fail as well.
In addition, over time, sediment can accumulate on the floors of potable water storage tanks, providing a breeding ground for bacteria.
Today, cutting-edge technology can deliver high-quality tank inspections, allowing utility managers to assess tank conditions and make sound decisions on repair, rehabilitation and cleaning.
By using remotely controlled underwater cameras, or by engaging specially trained tank divers, inspection contractors can get a good look at tank interiors without interrupting service and without wasting water. These inspections can be completed at a fraction of what the utility would spend if it used its own employees to drain and inspect the tanks.
What will you find?
Without a thorough inspection, there is no way to know how much corrosion or sediment is inside a water storage tank. It is common to find two to three inches of sediment on the floor of water storage tanks and towers. If the facility has gone 10 or 20 years without cleaning, the accumulation is sometimes measured in feet.
This buildup of sediment creates a habitat for bacteria, protozoa and even viruses to grow, as they are sheltered from exposure to treatment chemicals. Keeping tanks clean is one of the best ways to ensure the health of a public water system.
Texas is one of few states that require tank inspections and has done so for about 20 years. Since 2003, Florida has required potable water tanks to be cleaned at least annually. Also, according to Section 62-555.350 (12)(c) of the Florida Administrative Code, "All suppliers of water shall keep records documenting that their finished-water-storage tanks ... have been cleaned and inspected during the past five years."
A closer look
A qualified inspection and cleaning contractor can save water utilities man-hours and water. Traditionally, tank inspections have been costly and disruptive. In a typical process, utility workers remove the tank from service and drain it. Then a worker enters the tank to inspect it.
When the tanks are drained, typically only about two-thirds of the water can be put back into the system; the rest empties onto the ground. Before the tank is returned to service, it must be decontaminated, wasting even more water. Fortunately, this method is becoming obsolete.
Now, utilities can hire an inspection contractor with an underwater camera, saving tens or hundreds of thousands of gallons of water on each tank inspected. By having interior conditions professionally photographed and by obtaining video footage, managers can see precisely what is going on inside their storage tanks and towers just as the inspector witnessed it. These inspections take one to four hours to complete, depending on the method and condition of the facility.
Most often, the video camera used in an inspection is a forward-facing, wide-angle, color camera in a fixed position and focus, with 420 lines of resolution and 0.3 lux. When an inspection is completed with such a camera, it is considered a remote video inspection. Operated by one inspector, the equipment is designed for use in potable water applications and is sanitized with a 200 ppm chlorine solution before it enters the tank.
When using the camera, the operator scans the exterior and interior roof first. Then the inspector manually lowers the unit into the water tank, where it films the interior in great detail. Once it is submerged, the inspector rotates the camera for a view of the center of the tank. The inspector then gets footage of the walls as the camera makes the descent to the tank floor.
Sometimes, the tank's height and structure dictate that a remote operated vehicle (ROV) camera be used for an inspection. ROV cameras are neutrally buoyant but require a two-man crew. Propellers move the camera about the tank. The cameras are equipped with food-grade silicone seals for safe use in potable water. Two high-efficiency halogen lights attached to the camera illuminate the inside of the tank.
With both remote video inspection and ROV inspection, video footage is recorded on a portable HD recorder. Inspectors under the charge of a professional engineer can then review the footage and determine what maintenance work is necessary. Inspec-tion reports are delivered with the video footage on DVD, ena-bling the tank owner to ob-serve how much sediment is on the floor and how much cleaning is necessary.
Occasionally, it is in the owner's best interest to obtain an inspection performed by a professional diver using a handheld underwater video camera. Divers qualified for this type of inspection wear a dry suit and gear purchased for and used only in potable water. Their suits completely seal them from the water supply.
Before entering the tank, they are sanitized with a chlorine solution to meet American Water Works Association (AWWA) standards. Divers enter the tank and document the interior condition in detail.
Getting it clean
Potable water dive crews are also used to clean water storage tanks. With this innovative method, the diver enters the structure and removes sediment from the floor with minimal water loss or disruption in service.
Typically, a three-diver team is needed for a tank cleaning. The sediment is removed with a vacuum system and is properly disposed. Additionally, dive teams can repair any corroded spots that are noticed during the inspection.
Tank owners should assign this work only to specialists in potable water diving. Equipment must be designed for and used only in potable water. Firms focused on diving offshore or in lakes and rivers may dive in potable water tanks only as a sideline. The AWWA has had standards for potable water diving for more than 20 years.
Inspection and diving contractors need special capabilities and attributes. For example, climbing to heights of 150 to 250 feet to enter a tank or tower is not for the faint of heart. Qualified contractors have the training, equipment and experience to get the job done efficiently and safely.
Using specialized inspection technicians and divers, water utility managers can keep their systems inspected and cleaned with minimal water loss and little if any disruption in service. Advances in technology allow tank inspection and cleaning to be quick and cost-effective. Maintaining healthy water storage tanks is an attainable goal for every utility, and the practice can go a long way toward keeping aging infrastructure safe.
About the Author
Ron Perrin owns Ron Perrin Water Technologies, based in Fort Worth, Texas, and is author of the book, Inspecting & Cleaning Potable Water Storage. He can be reached at 888/481-1768 or firstname.lastname@example.org.