By Jonny Lupsha, Wondrium Staff Writer
Drinking water at an elementary school near Pearl Harbor tested positive for petroleum. The tainted water supply, linked to a Navy storage facility, affects a total of 93,000 people. Water treatment usually cleans raw water.
Hundreds of residents living in Navy housing near Pearl Harbor have complained of a fuel-like odor coming from their tap water, while many have also complained of headaches and abdominal pain. No one using Honolulu’s municipal water supply has been affected. Preliminary results from a lab at the University of Hawaii showed that the Navy’s water supply contained petroleum, though further details and definitive results had yet to be released.
The contamination may be linked to an aquifer 100 feet underneath the Navy’s fuel storage tanks at Red Hill.
When operating properly, water treatment cleans and purifies raw water regardless of its origins. In his video series Everyday Engineering: Understanding the Marvels of Daily Life, Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point, explains how it all works.
Let’s Be Perfectly Clear
According to Dr. Ressler, water treatment plants perform three main functions: clarification, filtration, and disinfection.
“As raw water is pumped from a lake, river, or receiving reservoir into the treatment plant, it passes through a series of progressively finer gratings and screens to remove trash, leaves, aquatic weeds, and critters—like fish, tadpoles, and zebra mussels—which have no business swimming around in our drinking water,” Dr. Ressler said. “The water then enters a mixing basin, where a chemical coagulant is added and vigorously mixed.”
The coagulant causes particles that are suspended in water to cling together, forming larger clumps of materials called flocs in a process called flocculation. Eventually, the flocs settle to the bottom of the basin. The water is gently mixed so more clumps can form without disturbing the ones that already have. Then, after the water is stilled, the flocs at the bottom are raked out, dried, and disposed of in a landfill. The flocculation process takes 30 to 60 minutes.
Filtered and Disinfected
“The clarified water is now pumped through a filter, typically composed of horizontal layers of gravel, sand, and anthracite coal,” Dr. Ressler said. “As the water passes through this filter, any particulate matter that didn’t settle out during clarification is captured in the tiny voids between the sand grains. The coal serves as a source of carbon, which is particularly effective at capturing volatile organic chemicals and removing them from the water.”
The disinfection process usually begins with mixing a small quantity of dissolved chlorine into the water. If any bacteria or viruses managed to survive the previous two steps in the process, chlorine kills them. Alternatively, the water can be exposed to ultraviolet light or ozone instead of chlorine.
“Finally, the purified water passes into a holding tank, which gives the chlorine more time for disinfection, while also providing a reservoir for smoothing out fluctuations in demand,” Dr. Ressler said. “At this point, the water treatment process is complete, and the finished water enters the local distribution system.”