Zebra and Quagga Mussels: The Inland Fouling Problem

By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.

Underwater Inspection | Commercial Diving Contractors Alabama Back in 2019, we pulled Unit 3's traveling screens and found a gravel parking lot encased in shells. The drive motor was tripping because the basket was too heavy to move. That was our first real break at the plant. Up until then, we'd gotten away with minor fouling on the condenser tubes — a few hundred mussels here and there. But that stretch of hot, low-flow weather let the veligers settle, and nobody caught it early. By the time we shut down for a weekend, we had a four-foot thick mat on the intake screens. If you're running a power plant, a refinery, or a drinking water facility with a raw water intake, you're going to deal with this. There's no 'if' — just 'when.' This is how we approach removal, from the mechanical to the chemical, and what I'd tell my own crew if we were doing it again.

Know Your Invader

Zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels aren't the same animal, and it matters. Zebras attach to hard surfaces, so they'll cover your trash racks, screens, and pipe walls. Quaggas can live on mud and tolerate colder water, so they'll colonize the intake tunnel floor just as fast. They both spawn veligers — microscopic larvae that drift through your system and settle wherever the flow slows. Your intake screens are paradise. Plenty of food, warm water in the summer, and hard mesh to grab. We've seen densities of 100,000 per square meter on our racks. That's not unusual. What's worse, they build on each other — dead shells make the substrate for the next generation. You can remove them one day, and within six weeks you'll see a fresh set the size of peppercorns. The key is to tell them apart early, because quaggas are more tolerant of cold water, so your treatment window starts earlier in the spring. Call it a 5°F difference, but that's enough to throw off a schedule that's based on zebra mussel behavior.

The Mechanical Route

Mechanical removal is the first thing you try, because it's straightforward and doesn't require chemical permits. For the screens themselves, shut off the pump and pull the racks. A high-pressure washer with a 40° fan tip at 3,000 psi will strip most of the shells off the mesh. You'll want a surface cleaner attachment for the bigger racks — the kind you see used on driveways, only with industrial fittings. We rented a 4,000 psi unit from Grainger and it did the job in a weekend. The key is to hit the shell at a sharp angle, not straight on, so you don't drive the debris deeper. For the pipes, you're into pigging. A foam pig with a steel core will scrape the walls, but you have to know your pipe layout — if you've got a thermocouple or a flow meter protruding into the bore, that pig will tear itself apart. We run a swab pig first to flush out loose stuff, then a hard bristle pig for the shell. You can hire a company to pump the pig through and capture the debris at the outfall. It's expensive — call it $20,000 to $40,000 for a big line — but it beats a week of manual chipping. Here's a digression that makes the point. Back in '03, we had a 16-inch raw water line to the backup fire pump that we'd ignored for years. When we finally pumped it dry and cut a spool piece, we could stand on the mussel bed inside the pipe. Took two days with sledgehammers to get it clean. That's why you stay on top of mechanical work.

Chemical Treatment

Chemical treatment is the only way you're going to reach the inside of a condenser bundle or a long intake tunnel. Chlorine is the old standby — sodium hypochlorite or bleach, injected at the pump bay. You're targeting the veligers, not the adults, so the key is chronic treatment during spawning season, not a one-time shock. We run a continuous 0.5 mg/L residual for the months when water temperature is above 50°F. That's a common approach. It works, but you're feeding a chlorine-containing discharge, so you've got to monitor the residual and possibly dechlorinate at the outfall. That's a whole NPDES permit dance. If you want something more targeted, there are proprietary molluscicides like EarthTec and Zequanox. EarthTec is a copper-based product labeled for zebra mussel control. We've tested it on a side stream and it kills veligers at low concentrations. But it's not a magic bullet — it's expensive, and you still need to know your flow rate and dosage. And you'll need to work with your state environmental agency before you discharge anything. Cost? Depends on the plant. Chlorine is the cheapest, but the safety equipment — containment, gas scrubbers if you use chlorine gas, or day tanks for sodium hypochlorite — can run into six figures. The proprietary products cost more per gallon, but you use far less. For our plant (300 MW, once-through cooling), a chemical program runs somewhere between $10,000 and $50,000 a year. That's a rough range. The real number depends on your intake flow, your discharge limits, and how bad the infestation is.

Ongoing Control and Prevention

You don't get rid of zebra mussels. You manage them. After a cleanout, the clock starts on the next colonization. So you need a plan. Inspect your intake screens weekly during spawning season. We use a small inspection camera from RIDGID attached to a pole, and we look for mussel growth on the screen wire. If you see any, treat it immediately — you don't want to wait until the mats form. Consider coating your screens. A non-toxic silicone or urethane coating makes it harder for byssal threads to grip. We've seen a few products that extend the cleanout interval by a couple of months. But no coating is 100% — the mussels will still find a way in through the crevices. Track water temperature and flow. Veligers settle in the 68-77°F range, give or take. We keep a log and set our treatment schedule accordingly. And run a biocide during an outage, whether chemical or thermal. Heat treatment works: you can bring the intake tunnel up to 90°F for a day and most of the mussels die. But that's disruptive, so save it for a scheduled shutdown. Finally, document everything. You'll need it for your NPDES permit and to show your insurer you're taking action. We keep a binder and a spreadsheet. It's not glamorous, but it's what keeps the plant from tripping off at 2 a.m. On a Sunday.

Frequently Asked Questions

How often do I need to treat my intake for zebra mussels? During spawning season, typically late spring through early fall, you should be running a continuous low-dose biocide if your system allows it. Plus, do a shutdown treatment once a year with a higher dose or heat. How exactly that looks depends on your plant's flow, temperature, and discharge permit. Can I really use a pressure washer to remove zebra mussels? Yes, on screen baskets and hard surfaces. Use a 4,000 psi unit with a 40° or 25° tip and hold it at a sharp angle, not perpendicular. Collect the debris so it doesn't re-settle. It's the cheapest way to handle a light infestation. What's the difference between zebra and quagga mussels for my intake? Quaggas are hardier — they handle colder water and softer bottoms. That means your treatment season starts earlier, and you might need to clean your intake tunnel floor, not just the screens. Zebra mussels stick to hard surfaces, so mechanical removal is more straightforward. Is chlorine the best chemical for mussel control? Chlorine is the most common and cheapest, but it's not always the best. You've got to deal with discharge residuals and safety. Copper-based molluscicides like EarthTec work well and are EPA-labeled, but they cost more. Monochloramine is an option if you're worried about corrosion. It depends on your water chemistry and regulatory limits.