Collector Wells and Raw Water Intakes: The Inspection Nobody Schedules
By Marisol Vega, Municipal water operations manager. Reviewed by Amanda McCallister, editor.
Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska Last month I spent a morning in a boat twenty feet from a highway bridge, watching a diver in a neoprene hood feel along the slats of our raw water intake screen. He came up holding a tangle of zebra mussels and one plastic bag, and said "you've got a good foot of growth on the inside." The pump station had been pulling about fifteen percent less than the design capacity, and the pressure differential was screaming. That inspection should've happened six months earlier. Nobody schedules raw water intake inspections because nobody sees them. A finished water tank is a visible asset; you can walk around it, chip paint, climb the ladder. A raw water intake is buried in the bottom of a river, in a lake, or under a concrete cover in a floodplain. It gets ignored until the low-level alarm trips or a pump starts cavitating. The truth is, a collector well and its intake are the most critical pieces of infrastructure you don't look at. You can treat water all you want, but if the raw water doesn't get in, the plant doesn't run. I've been managing water operations for nineteen years, and I've learned to plan inspections like a mechanic plans a transmission service: before the breakdown, not after.
Why they never make the calendar
We schedule tank inspections religiously. We test backup generators every Tuesday. But the intake? For most utilities, the intake is a forgotten steel pipe with a screen on the end that was installed in 1967 and hasn't been touched since. Ask an operator when they last looked at their raw water intake and you'll get 'the dive team was out during the flood, I think.' The problem is out of sight is out of mind. But the raw water intake is doing the same job as a furnace filter, and it's clogging constantly. Leaves, silt, zebra mussels, plastic, old fishing line. In the Upper Midwest, invasive mussel growth can cut an intake screen's open area by half in a single summer. That's not a guess, that's what we see on our own intake. The river bottom slumps, a sandbar shifts, and a lateral in a collector well collapses under a bank and the plant starts sucking air. We tie the intake's health into pump performance, so we get fooled. A pump drawing more current than last year might be a worn impeller, or it might be twenty percent of the screen blocked with mussels. You have to separate the two, or you'll replace a motor that never needed replacing.
What an inspection actually looks like
Start with data, not water. Shut down one pump and run the others at a steady state. Read the raw water flow with a portable ultrasonic flow meter, say a Greyline TTFM 2.0. Clamp it to the raw water line and compare its reading to the station's magnetic flow meter. If they're more than 3% different, you've got a measurement problem, and you'll be pumping blind. Fix that first. Then check the pressure across the screen. You need a gauge upstream and downstream of the intake screen or a differential pressure transmitter tied into SCADA. If you don't have that, it's an easy retrofit. On our 24-inch intakes, normal differential is 1.5 to 2.5 feet of water. Above 4 feet, we're losing efficiency. Above 7 feet, we're courting structural failure. We've seen a screen collapse when the differential hit 11 feet during an ice jam. Now comes the underwater inspection. Either hire a dive team certified for swift water, or use an ROV. A VideoRay with a bright light and 4K camera is much safer in cold water, but a diver can feel for sand, torque on bolts, and take samples. I usually do an ROV annually and a diver every three years. The diver also checks the concrete encasement for the collector well, which an ROV can't easily assess. One thing I learned the hard way: the inspection site matters. We had a stretch of river shift after a spring flood, and the dive team was scheduled for the old intake while the actual flow had moved a hundred feet upstream. They spent two hours looking at the wrong screen. Now we survey the riverbed with a side-scan sonar the month before any dive, just to confirm where the intake actually is.
When to schedule, and why most plants won't
I tell my team to put the intake inspection on the same calendar week as the annual tank cleaning, and on the same work order as a raw water pump teardown. That way it's part of a bigger outage, and it can't be canceled by someone saying 'we're too busy today.' Frequency depends on the plant. On a clean, stable river with low sediment, annual visual might be enough. For a lake with quagga mussels, I'd want one every six months. For a collector well in a sandy aquifer, I'd do a step test every two years to assess the laterals' specific capacity. Call it: 12 months is the ceiling, 6 months is the floor if you've ever had a fouling event. After any major storm, flood, or ice jam, check the intake within 30 days. Not just the SCADA trend. You need a camera in the water. We had a 3-inch-diameter branch smash a grating after a lightning strike knocked out a tree upstream. The operator noticed the water level in the suction well drop, and that was enough to trigger a dive. Also watch the trend lines. If your screen differential rises more than a foot per month, the biofouling is exponential. Don't plan a dive for next quarter. Plan one for next week. I've seen a utility ignore a rising differential all summer, then a cold snap killed the mussel colonies, and the whole screen was sucked into the pump. They lost a pump and a bag filter, not to mention a day of production.
What you actually do with the findings
So the camera shows mussel growth on a screen, or a collapsed lateral in a collector well. Don't grab a hammer and a crowbar. That's how you damage the screen beyond repair. For soft biofouling, a diver with a power washer or hydrolaser at 3,000 psi can strip much of it off, but you'll need an airlift or big vacuum to carry the debris away. Alternatively, feed a low concentration of chlorine or a potable-water-approved antifoulant through the intake, but that must be coordinated with water quality and can affect downstream treatment. It's a drug, not a cleaner. For hard scale or zebra mussel shells, you often need mechanical reaming or a brush sled. That's a specialty job, and quotes are priced per foot of screen plus mobilization for the boat and dive supervisor. I've seen quotes vary by four grand based on river traffic alone. It's not cheap, but a collapsed intake grate is far worse. For a collector well, you might need to surge the laterals by alternately raising and lowering water level, sometimes with an air-lift pump. Hydrogeologist supervision is non-negotiable. And if a lateral is broken, you have to plug it or replace it; that's a capital project, not a maintenance task. We had a failed lateral on the east side. Plugging it lost about 15% of the well's capacity, but replacing it would have required a coffer dam, and that was not in the budget.
Frequently Asked Questions
How often should a raw water intake be inspected? At least once a year, and more often if you're in a biofouling zone or after a major storm. Watch the differential pressure trend; if it's rising unexpectedly, schedule a dive. Can I inspect an intake without sending a diver down? Yes, an ROV with a camera is a good first pass, but you'll still want a diver for sampling and physical checks. In shallow water, a drop camera on a pole from a boat can work for a quick check. What's the difference between a collector well and a surface water intake? A collector well pulls water from an aquifer through horizontal laterals, while a surface water intake has a screen in a lake or river. Collector wells need step testing and surging to keep laterals open; surface intakes need physical cleaning. How do I know if my screen is clogging? The first sign is usually a rising suction pressure or a higher differential pressure across the screen. Pump flow drops even at the same speed and water level. Don't ignore those clues.