Zebra Mussels Have Reached Places They Should Not Be Able To
By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Amanda McCallister, editor.
Environmental Diving | Commercial Diving Contractors Alabama June 11, 2018. I was doing a five-year inspection on a circulating water pump at the Waukegan Generating Station. The pump house smells like dead alewives and grease. When we unbolted the discharge bonnet, a stream of lake water poured out, and behind it came a cascade of zebra mussel shells the size of coffee beans. They'd built up a reef on the backside of the impeller vanes—right where the water speed should have scoured anything loose. That wasn't the surprise. The surprise was the one we found six months later, back inside the plant, in a condensate pump seal flush line that ran at 140°F. That line is supposed to be self-cleaning. It's a small-bore pipe off a closed loop, filtered to 100 microns, and we run it hot enough to kill mussels. Yet there it was, a live juvenile not more than a quarter inch long, attached just past a tee where no water flow ever goes. It shouldn't have been able to get there. But zebra mussels don't care about what should be. They survive in bilge water, on a tarp, inside a fire hose, in the mud on a boot. They can sit out of water for days, and they can go through a pump and come out alive. If you've got raw water coming into your plant, they will find the one dead leg that never gets inspected.
How They Actually Get In
Zebra mussels don't need an open pipe to a lake. They appear inside firewater tanks fed by trucks, inside condensers after an outage, inside heat exchangers that were cleaned with a pressure washer last year. The veligers are microscopic and can pass through screens. Once they're spawning in a raw water intake, every line that carries raw water—even a line that's closed for nine months—already has larvae in it. The adults then hitch rides on mowers, trailers, dive gear, and maintenance tools. In one plant I worked at, we traced an infestation to a gauge used by a contractor who had previously worked on a boat lift. The gauge sat on a cart in our pump house for a week. Inside its fitting was a cluster of juvenile mussels.
Inspect the Spaces You'd Rather Not
You'll find them in dead legs, on the backside of orifice plates, inside pump casings, in the cooling jacket of a bearing, in the bend of a small-bore impulse line. They like darkness, low flow, and something rough to grip. When we do a summer inspection, we pull a sample line and a bonnet every month. We also cut a boroscope into the dead legs at the ends of our distribution manifolds. The strange thing is they rarely appear in the high-velocity areas. You'd expect the water that's moving fast to carry them, but they get swept through. They settle where the velocity drops below about 1.5 feet per second. So focus your inspection on the back half of every heat exchanger, the outlet channel, and any line that dead-ends. A routine inspection should include a flashlight and a mirror. The juveniles are translucent—you can miss them.
What Kills Them Without Killing Your System
You have three options: chemical, thermal, mechanical. Chemical: chlorine or a stabilized oxidizer. But intermittent dosing is often useless—mussels sense the oxidizer and close their shells for hours. You need continuous low-dose or a strong periodic shock, and you need to monitor the residuals at the far end of the line. Thermal: recirculating hot water above 104°F will kill them, but you need a system that can tolerate the heat and you have to get the temperature up to the last drop. Mechanical: scraping, freezing with dry ice, or flushing with a rubber pig. We've found the most effective approach is a combination: a continuous low-dose oxidizer plus an annual mechanical cleaning. Don't try to fight a mature colony with a single shock; you'll end up with thousands of dead mussels that decay and produce ammonia and sulfides. That's a contamination problem in itself.
The Habit Beats the Machine
The best 'how to' I can give you is not a purchase order. It's a schedule. Keep a log of where you've found mussels, and put a checklist on the wall. Add sample ports to every dead leg longer than a few feet. Once a month, open the petcock and look for shells. Once a year, drain and inspect the intake screens and the condenser inlet. The moment you let a summer go by without checking, you will lose a season to a fouled tube sheet. I also mark the calendar for late August—that's when new larvae settle. If you catch them young, they're just a coating. Wait until the spring, and you're lifting a 100 lb slimy reef off the tube bundle.
Frequently Asked Questions
I've heard zebra mussels can survive out of water. Is that really true? Yes. Like many freshwater bivalves, they can close their shells and hold moisture. On a damp tarp or inside a fitting they can last for days. We've found live mussels in a valve that had been drained for two weeks. That's why equipment transfers and contractor gear are a risk. If I add chlorine, doesn't that just create more problems for discharge? It can. Chlorine reacts with organic material to form regulated compounds. Many plants now switch to chlorine dioxide or run lower, continuous doses. You have to balance the treatment against your discharge permit. That's why an integrated strategy—mechanical plus chemical—is worth the engineering time. How do I know if my plant is at risk? If you draw raw or finished water from an infested lake, river, or reservoir, you're at risk. If you're on municipal water, the risk is lower but not zero, because municipalities don't always treat to kill mussels in the pipe. The best way is to check your condensate and cooling water inlet screens during a scheduled outage. If you see one shell, you have a colony.