Your Drinking Water Sits in a Tank Somebody Has to Swim In

By Marisol Vega, Municipal water operations manager. Reviewed by Dana Whitcombe, technical reviewer.

Potable Water Tank Inspection & Cleaning | Wastewater & Sewer Diving On a Tuesday in August, the air was 98 degrees and the water inside the 1.5-million-gallon tank was 58. I pulled on a dry suit over my coveralls, strapped a gas detector to my harness, and swung a leg over the hatch. Ten feet below me, the surface of your drinking water looked like a black mirror. It wasn't. It was a layer of silt, rust flakes, and algae that had settled over the winter. I'm Marisol. I manage 40 finished-water storage tanks for a municipal water system. This one was due for its five-year inspection. That means somebody has to go in. Not a robot. A person. Me or one of my crew. We swim in it, scrape it, vacuum it, and then disinfect it before it goes back online. This is how that happens.

Why We Get Wet

Your water sits in a tank for hours or days. Time does two things: sediment sinks, and chlorine fades. The sediment is mostly fine sand, silt, and corrosion flakes from the pipes on the way in. It builds up on the floor like a muddy coffee ground. On the walls, a biofilm forms—a translucent slime of bacteria that chlorine can't fully kill once it's anchored. Left alone, it becomes a place where pathogens can hide. So every few years, we take the tank out of service, drain it, and go in.

The Gear That Keeps You Alive

Walking into an empty tank is not like walking into a basement. It's a confined space, and the OSHA rules are there because people die. We bolt a tripod over the hatch, and it holds a winch with a rescue harness. Before anyone drops in, we lower a gas detector to check oxygen, hydrogen sulfide, and flammable vapors. The tank has been vented for days, but we still test. We also bring a blower to keep fresh air moving. The swimmer wears a dry suit, rubber boots, gloves, and a hard hat with a light. Nobody goes in alone. The attendant outside has a radio and can haul you out in under a minute.

The Swim

Once you're inside, it's cold water up to your thighs, and it's dark. The beam of your headlamp hits a wall of gray-brown slush. You start at the top and work down. A scraper is attached to a long pole, and you drag it along the walls, peel the biofilm like dead skin. It's slow work. The floor is worse. We use a hydrovac—a truck-mounted vacuum with a hose long enough to reach into the tank. You walk the nozzle across the floor, feeling the lighter sediment lift and hearing it gurgle through the line. It's messy. It's physical. You're hunched over, moving in half-steps, and after an hour your back is screaming. But you get every inch.

Inspection: What We're Looking For

With the walls bare and the floor swept, you can finally see the tank's real condition. We look for cracks in the concrete, blisters in the coating, and red-brown stains that mean rebar is rusting. If the old paint is peeling, that's a problem—flakes will cling to shells and end up in someone's glass. We also check the seams around the hatches, the level sensors, and the ladders. These tanks are like teeth: if you brush them regularly, you catch a cavity early. If you skip, you get a root canal. I once found a hairline crack that would have become a leak within a year. We patched it then, instead of draining the tank again in an emergency.

Kill It With Chlorine

After the last scrape, the final step is disinfection. We don't just spray bleach. We fill the tank with three or four feet of water, then add a dose of chlorine so strong that it's almost a shock. The target is a residual of at least 10 milligrams per liter—several times stronger than normal drinking water. We let it sit for a few hours, recirculating it through the sump, so every surface gets a slosh. Then we drain it and fill it completely with fresh, treated water. We don't test one sample. We test after 24 hours, then two days later, and again a week after that, to make sure the chlorine level has settled back to normal and no bacteria are growing.

The Schedule

There's no universal rule for how often a tank needs this. The American Water Works Association recommends inspecting every three to five years, cleaned if they look dirty. But the real schedule depends on your source water, how old your pipes are, and what the sediment says. If the raw water is muddy, you'll need more frequent tanks. If it's from a protected groundwater source, you might stretch it. I keep a spreadsheet color-coded for all 40 tanks. Green is fine. Yellow is due. Red is a problem. Right now I have three yellows. That means three dives this summer. And I'm already dreading the first one, because it's always cold.

Frequently Asked Questions

Is the water still safe to drink while a tank is being cleaned? No. The tank is completely taken out of service, and the water is routed around it. You might see lower pressure or a notice about hydrant flushing, but the water you get is never mixed with the cleaning tank. Why can't they clean the tank without someone going inside? Robots and cameras can't scrape a wall or feel a crack. They also can't replace a loose bolt or test the coating's adhesion. A human eye and hands are still the best tools for a one-million-gallon vessel. What happens if the tank isn't cleaned for too long? Sediment eats the chlorine faster, so the water has less protection. Biofilm can slough off, and you'll get taste, odor, or cloudiness. In the worst case, bacteria like coliform can regrow, and the tank is then a source of contamination, not just a holding cell.