Keeping the Tank in Service: Why Utilities Choose Divers Over Draining
By Marisol Vega, Municipal water operations manager. Reviewed by Dana Whitcombe, technical reviewer.
Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska Back in 2019, we had a water quality call on the north side. A 1.5-million-gallon elevated tank was sending brown sediment through the mains. My first instinct was to schedule a drain-and-clean outage. Then I looked at the calendar—and the weather—and the community's demand. That tank is the high service zone's buffer. So we called a dive team instead. They were in the water by 7 a.m. We never isolated the tank. That day taught me something. Draining is not the only way to clean a finished-water storage tank. In fact, for larger tanks, it's often the worst way.
Draining: The Old Default
Draining used to be the default. You isolate the tank, pump the water down, neutralize the residual chlorine, and discharge to a sewer or storm drain. Then you put a crew in a confined space with air monitors, a harness, and a bucket brigade. That's a minimum of two days for a small tank, three to five for a big clearwell. For a city with 40 tanks, that's a lot of lost reserve. Not just inconvenience. Fire flow. During a draining project in 2016, we had a structure fire a half-mile from the isolated tank. The pumper trucks had to rely on the smaller tank in the neighbor zone. Call it a coincidence, but it changed how I evaluate risk. Water loss is the hidden cost. A 2-million-gallon tank drained means 2 million gallons gone. We pay about $3 per 1,000 gallons for treatment and pumping, so that's $6,000. Then we pay to dispose of the dirty water—sometimes $2,000 to $4,000 at the wastewater plant. Then we refill and rechlorinate. The total bill is easily $12,000 to $15,000 before we even hire a cleaning crew. And the tank is still empty for the inspection.
What a Dive Team Actually Does
Divers aren't just swimming around with a brush. The crews we use are commercial divers with potable water experience. They wear dry suits and a tether, and every piece of gear gets wiped down with a chlorine solution before entering the tank. The tank stays full, though we often lower the level to a 12-foot working depth. That keeps the bottom accessible. And here's the key: the tank remains online. We may have it in a reduced-pressure status, but it's not isolated. That means we still have reserve volume for peak morning and evening demand. The dive contractor brings a command trailer, a surface-supplied air system, and underwater cameras. Our engineer sits at a monitor and watches the diver sweep the walls. They use submersible pumps with a suction hose to pull sediment out, and they carry rotary brushes for the floor and lower walls. For hard buildup, they use hydro-lancing equipment that shoots water at 3,000 psi. They also take sediment samples from the floor and send them to a lab for metals and bacteria. That's a huge advantage over draining: we get data about what's actually in the bottom.
Cost and Schedule: The Crossover Point
So how does the cost compare? For a small tank, say a 100,000-gallon standpipe, draining is usually cheaper. You can empty it in an hour, clean it in half a day, and refill it overnight. The water loss is only $300. But for anything over 500,000 gallons, the numbers flip. A 2-MG ground storage tank might take 8 hours to drain, but the lost water and disposal costs eat up your cleaning budget. A dive project for that tank runs $15,000 to $25,000, give or take, depending on the debris and the contractor. That's often about the same as the drain-and-dispose cost, but you eliminate the outage. Time is where divers really win. For a 1-MG tank, draining and cleaning takes three days. A dive team can do it in one day. That's not just a schedule win—it's a risk win. Every day a tank is off-line is a day we're relying on pumps and older tanks. During summer peaking, that's a real problem. So the decision is often driven by the calendar, not just the invoice.
Safety, Water Quality, and Confined Space
There's a common misconception that divers ruin the water quality. The opposite is true. We maintain a free chlorine residual of 0.5 to 1.0 mg/L in the tank during the dive. The divers' exposure is monitored, and they're trained to work in these conditions. The gear is disinfected, and we run a coliform test on the water after the dive to confirm nothing was disturbed. That's a standard practice, not a special event. Diving in potable water is an established industry, and OSHA's confined space standard (1910.146) applies just like any tank entry. But we still have to plan for a rescue. If a diver gets tangled or a pump kicks in, we need a secondary air source and a retrieval line. The dive team has their own rescue procedures, but we also have a trained standby crew on site. And we have to coordinate with our operations staff to ensure no pump starts during the dive. It's a controlled operation, but it requires the same vigilance as any confined space entry. You don't just drop a diver in the tank and walk away.
When We Still Drain
I'm not saying we never drain. We do. Any tank that needs structural repairs, sandblasting, or coating work has to be drained. There's no way to weld a roof vent underwater. So for major rehabilitation, we close the valve and plan the outage. Divers can inspect, clean, and even spot-repair, but they can't replace a tank floor. So here's my rule of thumb: if the tank is over 500,000 gallons and the work is routine cleaning and inspection, call the divers. If it's a coating failure, a structural concern, or a full rehabilitation, drain it. And if you're unsure, do a dive inspection first. It's cheaper than an outage, and you'll have the data to make the call.
Frequently Asked Questions
Is it safe to have divers in a potable water tank? Yes, when the crew is experienced and the tank is properly monitored. We keep a chlorine residual in the water and disinfect all equipment. The divers are confined-space trained, and we have a rescue plan. It's not something you wing. How often should a municipal storage tank be cleaned? AWWA recommends inspection every 3 to 5 years, but cleaning frequency depends on water quality and tank location. In our system, we put larger tanks on a 2-year dive cycle and smaller ones on a 4-year drain-and-clean cycle. Do divers really get the tank as clean as draining? They get the floor and walls clean, but draining lets you physically inspect every square foot. For sediment and biofilm, divers do just as well. For detailed coating condition, an empty tank is still better. That's why we use divers for routine cleaning and draining for capital projects. Can divers save money on a small tank? Usually not. For tanks under 500,000 gallons, the water loss cost is low, and a drain can be done in a day. Divers start to make sense above that, especially when downtime is a concern.