Bubbles Beat Back Biofouling
Biofouling is the accumulation of plants and animals on submerged, hard surfaces in the marine environment. Biofouling on a ship’s hull is a major problem for the marine industry, with significant economic and ecological consequences. Hull fouling increases drag, which increases fuel consumption; increases labor costs associated with hull cleaning; and plays a role in the transport and introduction of non-indigenous species to new areas. An efficient antifouling system is necessary to reduce fuel consumption and maintenance costs for military, commercial and recreational fleets, as well as to prevent the transfer of non-indigenous marine species.
A range of fouling control methods exists, but many are costly, time-consuming in their application, or have a negative impact the environment. Until recently, successful antifouling methods included coatings that incorporate copper, tri-butyl-tin (TBT), and biocides. The widespread use of TBT and heavy metal-based paints has been linked to marine pollution because of their toxic effects to marine communities in areas adjacent to ports, marinas and busy shipping lanes. Concern over the impacts of TBT has lead to an international ban on its use. Copper-based paints are currently banned in the state of Washington, as well as several other countries for similar reasons. Following bans on TBT and other heavy metal-based paints, novel antifouling coatings and technologies have been recently proposed within the marine industry.
One of the novel methods entails introducing “bubbles” to the hull of ships to prevent biofouling organisms from settling or to dislodge newly settled organisms. The use of aeration as a means to prevent biofouling has been studied by various researchers since the 1940s. In more recent years, groups from the United States and Australia have revisited the idea. Aeration as an antifouling technique employs a continuous stream of air bubbles directed at the surface of the hull. The advantages of employing this technique are two-fold: it is relatively cost-friendly and it is also environmentally friendly as aeration produces no toxins and consumes minimal amounts of energy.
In 2011, warfare center sponsored marine scientists to evaluate a range of existing antifouling strategies (i.e., commercially available products including copper bottom paint, SeaGuard, and Intersleek) and aeration. The objective of their 2011 research was to determine which method maximizes efficacy against biofouling, while minimizing cost and environmental risk. To evaluate the efficacy of paints and aeration, fiberglass panels were deployed in Rhode Island’s Narragansett Bay from May through September 2011. Control panels and those treated with paints were mounted on four box-kite assemblies. Panels were mounted on a small model hull to which aeration was applied. Test panels were monitored on a monthly basis to determine which panels had the greatest percent cover of biofouling.
Results indicate that not all antifoulants perform in the same manner. Visual observations revealed that only a biofilm (a thin layer of microorganisms and bacteria) developed on those panels treated with SeaGuard and copper paint. Biofouling accumulated on the control panels and those panels treated with Intersleek and aeration. SeaGuard appeared to be the most effective antifoulant, followed by copper paint and Intersleek. Panels treated with aeration had virtually no accumulation during the first month of the experiment, but very quickly accumulated biofouling later in the deployment. A look at the challenges the researchers faced during this study may explain why.
As with many experiments, technical difficulties were encountered that hindered the ability to regulate air pressure in the beginning of the study. Aeration is not meant to have the power-washing effect, but rather supply a steady, continuous stream of bubbles to a surface. Higher, unregulated pressure early in the experiment was more efficient at preventing fouling accumulation than later in the experiment, when pressure was regulated to mimic conditions used by prior researchers. This finding only stressed the importance of understanding the mechanisms behind aeration (i.e., air pressures) to successfully deter biofouling organisms. Determining the optimum air flow may be the key to aeration’s success.
The next challenge was posed by Mother Nature. Hurricane Irene hit the New England Coast in August 2011 (in the middle of the study) and required the panels to be removed from the water for three days. Interestingly, aerating panels following Hurricane Irene revealed the technique is effective at removing growth that has died-back due to environmental conditions. This has implications for aeration as a mitigation technique, especially when timed with seasonal mortality of biofouling communities. Applying aeration to vessels that have experienced a brief vessel haul-out may maximize efficiency.
The scientists at the Naval Undersea Warfare Center in Newport, R. I., concluded that future research is necessary to determine what factors make aeration effective. These include answering questions such as: At what flow rate does aeration prevent biofouling organisms from settling on a surface? Does it matter at what angle the air is delivered to the hull? A follow-on study is currently underway and a series of laboratory experiments will be conducted during the summer. Information gathered during these experiments could be incorporated into the design of an aeration system prototype with the ultimate goal of deploying around vessels of all types.
Aeration may provide secondary benefits, including a reduction in hull husbandry costs associated with maintaining the integrity of the hull coating. Scrubbing and scraping fouling from the hull is a standard practice that results in scratching the hull coating, which can lead to increased corrosion and patches of bare hull susceptible to colonization by additional biofouling organisms. Aeration is gentle in comparison and would unlikely penetrate the surface of the coating. It is hypothesized that when used in conjunction with fouling release coatings, the dynamic and turbulent nature of bubbles might maximize fouling prevention. Cost-effective prevention and remediation measures could result in significant cost savings to the marine industry from reduced fuel consumption and maintenance (e.g., dry-docking, antifoulant application). As with most developing technologies, more research is required to further this concept, however aeration is showing promise as a “green” alternative for controlling biofouling in the marine environment.
Natasha Pinckard and Kari Heinonen are marine scientists at the Naval Undersea Warfare Center