Controlling invasive mussels16 May 2013
A new biological treatment offers new hope for controlling invasive mussels at hydroelectric projects, writes Sarahann M Rackl
Zebra and quagga mussels may be small in stature, but there's power in numbers and these tiny creatures are having a big ecological and economic impact throughout North America. The invasive mussel problem has been particularly pervasive and costly for the hydroelectric power industry, but recent scientific developments offer new hope for controlling these pesky invaders.
Originating in the Caspian and Black Seas, invasive zebra and quagga mussels were transported to North America in ballast water from a cargo ship. First discovered in the Great Lakes in the late 1980s, invasive mussels make themselves at home in freshwater lakes and rivers, where they spread rapidly and form thick, heavy colonies.
The average female zebra mussel, which is ready to reproduce in its first year of life, can release 30,000 to 40,000 eggs per year. After hatching, the planktonic larvae, or veliger, can not only move great distances in flowing water, but can also easily invade small places in both natural systems and industrial systems that draw from infested waters. And once mussels settle, their colonies build up very quickly. It has been reported that these mussels have been able to clog a three-foot-diameter pipe in less than three months (U.S. Department of Energy, National Energy Technology Laboratory [NETL] 2006). Added challenges are that these mussels can rapidly disperse to other water bodies, primarily by the larval movement and their inadvertent transport by barge and boat traffic, and can survive for many days out of water, factors that have caused the zebra and quagga mussel invasion to spread to many previously uninfested waters throughout the US and Canada.
Colonies of zebra and quagga?mussels can damage mechanical equipment and infrastructure; they can also clog pipes, filters and ?screens, obstructing or stopping the flow of critical cooling water. Increased corrosion is also common. Once infested, hydropower plants incur ongoing maintenance costs, and often expected and unexpected outages and a reduction in power generation.
Indeed, invasive mussels are a vexing problem. To minimize their impact, hydropower plants require regular, ongoing maintenance and control measures. Largely, these efforts have revolved around chemical control solutions that, while offering a high level of efficacy, come with significant drawbacks and risks.
A very common approach to mussel control is applications of chemicals such as chlorine. Chlorine-based methods using hypochlorite, chlorine gas and chlorine dioxide can pose significant risk to humans and non-target species. They also corrode equipment. Chemical treatments of adult mussels take days to complete and the efficacy of such methods may be impacted by water temperature and/or water quality.
Toxic chemicals are also subject to extensive regulatory restrictions and require special precautions for storage and handling, both of which increase costs. At a minimum, treatments may disrupt normal operations and workers are required to wear specialized personal protective equipment when working with them. Furthermore, in hydropower plants these chemicals are discharged within the tailrace into open water where they may negatively impact the environment if released in greater amounts than planned or prescribed by the regulator. As a result, facilities using chlorine and other chemical-based molluscicides may be required to deactivate or detoxify the treated? water before discharge to meet environmental requirements (NETL 2006). An additional disadvantage of using chlorine is that the mussels perceive the chlorinated water as a threat, causing them to shut their valves to stop siphoning. This behavioral response can significantly extend the needed applications time to achieve results.
As an alternative to aqueous solutions, microfiltration and UV light treatments have been tested in a few locations. These ?methods are not hazardous to employees or the environment, but they typically involve hefty capital investments as well as installation, operation and continual maintenance of specialized equipment. In addition, microfiltration and UV light treatments address only veliger control and they provide control only at the equipment location. Any temporary failure or shutdown of this equipment - even a nonfunctioning UV lamp or a micro tear in a filter - could allow veligers to move downstream in the system and settle. Further, seasonal cloudy, murky water or high solids from algae blooms can compromise these methods.
The need for a new control method drove extensive research that led to an industry-changing discovery. Faced with the threat of zebra mussels fouling electric power facilities within New York State, a research consortium of electric power generation companies contracted with New York State Museum Field Research Laboratory in 1991 for the screening of bacteria as potential biological control agents. The use of microbial, natural product compounds already had a clear record of commercial success and environmental safety in the control of invertebrate pests in North America, as well as globally.
Extensive laboratory screening trials of more than 700 bacterial strains identified a North American isolate, strain CL145A of Pseudomonas fluorescens, to be lethal to zebra and quagga mussels (Molloy 2002). Pseudomonas fluorescens is worldwide in distribution and is present in all North American water bodies. In nature, it is a harmless bacterial species that is found protecting the roots of plants from diseases.
In 2007 Marrone Bio Innovations (MBI) entered into a commercial partnership with the New York State Museum to bring this naturally occurring soil microorganism to market for the control?of zebra and quagga mussels. The result was Zequanox®, which was registered by the U.S. Environmental Protection Agency in early 2012.
Zequanox is the first biological solution for controlling invasive zebra and quagga mussels that offers efficacy comparable to chemical solutions but does not endanger employees or result in harmful impacts to other aquatic organisms. The product is composed of dead Pseudomonas fluorescens cells. The cells contain natural compounds that, when ingested, are lethal to zebra and quagga mussels during all life stages (veliger to adult). The mussels perceive Zequanox as a nonthreatening food source and readily consume the product along with their normal phytoplankton diet. This feeding mechanism contrasts with biocides such as chlorine, which the mussels sense as threatening, causing them to quickly shut their valves to guard themselves against the chemical.
Zequanox offers several advantages over chlorine and other chemical pesticides, including safety, flexibility and ease of use. First and foremost, Zequanox poses very limited to no risk to workers, non-target species and the environment. As a reduced-risk pesticide, Zequanox is safe to store, handle and apply; only minimal personal protective equipment is needed. In addition, the use of Zequanox does not require detoxification before discharge of the treated water.
Applications of Zequanox have been shown to be less labor intensive and less operationally disruptive than chemical methods as they can be done during normal facility operations and typically occur within a six- to eight-hour period. Zequanox also offers additional flexibility in that it is proven effective in a broader range of water conditions and temperatures than chlorine, thus expanding the "treatment season" during which Zequanox treatments can be effective. With Zequanox, plant operators benefit from the unmatched ability to tailor the treatment regimen to achieve the desired balance of mussel control, application frequency and shell debris management.
Compared with UV and microfiltration solutions, Zequanox also has several advantages. It can be applied using standard injection equipment, so facility operators can implement a Zequanox control program quickly and easily. Zequanox can be employed without having to undergo an arduous capital budgeting process and equipment installation, and without incurring the additional overhead of ongoing equipment maintenance. The aqueous formulation of Zequanox provides the added benefit of being able to reach and treat even the smallest of crevices in the water system, whereas mechanical solutions offer control only at a fixed location.
While still in its launch year, Zequanox is living up to its promise and is proving to be a highly effective alternative to chemicals. In one case study, a 1,000+ MW coal plant in the southwest used Zequanox to treat their static intake bays and achieved a greater than 95 percent success rate. Similar results were achieved from the treatment of a cooling water system of a hydropower plant in Ontario, Canada where 90+ percent mortality was observed.
As the battle against invasive and destructive mussels wages on, intensified by their unrelenting spread and complicated by increasing regulatory pressures, such as stricter discharge permits, Zequanox promises to offer the hydropower industry a highly effective alternative to traditional treatments. While initially targeted toward cooling water systems in industrial and power facilities, Zequanox also may soon be used to control mussels in aquaculture, fish hatcheries, as well as reservoirs and recreational waterways.
For more details on Zequanox, please visit www.marronebioinnovations.com