Biofouling

Anti-fouling is the ability of specifically designed materials (such as toxic biocide paints, or non-toxic paints)^[1] to remove or prevent biofouling.^[2]

The buildup of biofouling on marine vessels poses a significant problem. In some instances, the hull structure and propulsion systems can be damaged.^[3] The accumulation of biofoulers on hulls can increase both the hydrodynamic volume of a vessel and the hydrodynamic friction, leading to increased drag of up to 60%.^[4] The drag increase has been seen to decrease speeds by up to 10%, which can require up to a 40% increase in fuel to compensate.^[5] With fuel typically comprising up to half of marine transport costs, antifouling methods save the shipping industry a considerable amount of money. Further, increased fuel use due to biofouling contributes to adverse environmental effects and is predicted to increase emissions of carbon dioxide and sulfur dioxide between 38% and 72% by 2020, respectively.^[6]

Biology

Biofouling organisms are highly diverse, and extend far beyond the attachment of barnacles and seaweeds. According to some estimates, over 1,700 species comprising over 4,000 organisms are responsible for biofouling.

Marine fouling is typically described as following four stages of ecosystem development. Within the first minute the

Governments and industry spend more than US$5.7 billion annually to prevent and control marine biofouling.^[9] Biofouling occurs everywhere but is most significant economically to the

Medical devices often include fan-cooled heat sinks, to cool their electronic components. While these systems sometimes include HEPA filters to collect microbes, some pathogens do pass through these filters, collect inside the device and are eventually blown out and infect other patients.^[12] Devices used in operating rooms rarely include fans, so as to minimize the chance of transmission. Also, medical equipment, HVAC units, high-end computers, swimming pools, drinking-water systems and other products that utilize liquid lines run the risk of biofouling as biological growth occurs inside them.^[13]

Historically, the focus of attention has been the severe impact due to biofouling on the speed of marine vessels. In some instances the hull structure and propulsion systems can become damaged.^[3] Over time, the accumulation of biofoulers on hulls increases both the hydrodynamic volume of a vessel and the frictional effects leading to increased drag of up to 60%^[5] The additional drag can decrease speeds up to 10%, which can require up to a 40% increase in fuel to compensate.^[5] With fuel typically comprising up to half of marine transport costs, biofouling is estimated to cost the US Navy alone around $1 billion per year in increased fuel usage, maintenance and biofouling control measures.^[5] Increased fuel use due to biofouling contributes to adverse environmental effects and is predicted to increase emissions of carbon dioxide and sulfur dioxide between 38 and 72 percent by 2020.^[6]

Biofouling also impacts aquaculture, increasing production and management costs, while decreasing product value.^[14] Fouling communities may compete with shellfish directly for food resources,^[15] impede the procurement of food and oxygen by reducing water flow around shellfish, or interfere with the operational opening of their valves.^[16] Consequently, stock affected by biofouling can experience reduced growth, condition and survival, with subsequent negative impacts on farm productivity.^[17] Although many methods of removal exist, they often impact the cultured species, sometimes more so than the fouling organisms themselves.^[18]

Detection

Shipping companies have historically relied on scheduled biofouler removal to keep such accretions to a manageable level. However, the rate of accretion can vary widely between vessels and operating conditions, so predicting acceptable intervals between cleanings is difficult.

LED manufacturers have developed a range of UVC (250–280 nm) equipment that can detect biofouling buildup, and can even prevent it.

Fouling detection relies on the biomass' property of fluorescence. All microorganisms contain natural intracellular fluorophores, which radiate in the UV range when excited. At UV-range wavelengths, such fluorescence arises from three aromatic amino acids—tyrosine, phenylalanine, and tryptophan. The easiest to detect is tryptophan, which radiates at 350 nm when irradiated at 280 nm.^[19]

Methods

Antifouling

Antifouling is the process of preventing accumulations from forming. In

Non-toxic anti-sticking coatings prevent attachment of microorganisms thus negating the use of biocides. These coatings are usually based on organic polymers.[20]

There are two classes of non-toxic anti-fouling coatings. The most common class relies on low

Biocides are chemical substances that kill or deter microorganisms responsible for biofouling. The biocide is typically applied as a paint, i.e. through

Ultrasonic transducers may be mounted in or around the hull of small to medium-sized boats. Research has shown these systems can help reduce fouling, by initiating bursts of ultrasonic waves through the hull medium to the surrounding water, killing or denaturing the algae and other microorganisms that form the beginning of the fouling sequence. The systems cannot work on wooden-hulled boats, or boats with a soft-cored composite material, such as wood or foam. The systems have been loosely based on technology proven to control algae blooms.^[26]

Energy methods

Pulsed laser irradiation is commonly used against

Similarly, another method shown to be effective against algae buildups bounces brief high-energy acoustic pulses down pipes.[27]

Other methods

Regimens to periodically use heat to treat exchanger equipment and pipes have been successfully used to remove mussels from power plant cooling systems using water at 105 °F (40 °C) for 30 minutes.^[28]

The medical industry utilizes a variety of energy methods to address bioburden issues associated with biofouling. Autoclaving typically involves heating a medical device to 121 °C (249 °F) for 15–20 minutes. Ultrasonic cleaning, UV light, and chemical wipe-down or immersion can also be used for different types of devices.

Medical devices used in operating rooms, ICUs, isolation rooms, biological analysis labs, and other high-contamination-risk areas have negative pressure (constant exhaust) in the rooms, maintain strict cleaning protocols, require equipment with no fans, and often drape equipment in protective plastic.^[29]

UVC irradiation is a noncontact, nonchemical solution that can be used across a range of instruments. Radiation in the UVC range prevents biofilm formation by deactivating the DNA in bacteria, viruses, and other microbes. Preventing biofilm formation prevents larger organisms from attaching themselves to the instrument and eventually rendering it inoperable.^[30]

History

Biofouling, especially of ships, has been a problem for as long as humans have been sailing the oceans.^[31]

The earliest attestations of attempts to counter fouling, and thus also the earliest attestation of knowledge if it, is the use of pitch and copper plating as anti-fouling solutions that were attributed to ancient seafaring nations, such as the

A recorded explanation by Plutarch of the impact fouling had on ship speed goes as follows: "when weeds, ooze, and filth stick upon its sides, the stroke of the ship is more obtuse and weak; and the water, coming upon this clammy matter, doth not so easily part from it; and this is the reason why they usually calk their ships."^[34]

Before the 18th century, various anti-fouling techniques were used, with three main substances employed: "White stuff", a mixture of

In 1708, Charles Perry suggested copper sheathing explicitly as an anti-fouling device but the first experiments were not made until 1761 with the sheathing of HMS Alarm, after which the bottoms and sides of several ships' keels and false keels were sheathed with copper plates.^[31]

The copper performed well in protecting the hull from invasion by worm, and in preventing the growth of weed, for when in contact with water, the copper produced a poisonous film, composed mainly of

With the rise of iron hulls in the 19th century, copper sheathing could no longer be used due to its galvanic corrosive interaction with iron. Anti-fouling paints were tried, and in 1860, the first practical paint to gain widespread use was introduced in Liverpool and was referred to as "McIness" hot plastic paint.^[31] These treatments had a short service life, were expensive, and relatively ineffective by modern standards.^[1]

By the mid-twentieth century, copper oxide-based paints could keep a ship out of drydock for as much as 18 months, or as little as 12 in tropical waters.^[31] The shorter service life was due to rapid leaching of the toxicant, and chemical conversion into less toxic salts, which accumulated as a crust that would inhibit further leaching of active cuprous oxide from the layer under the crust.^[36]

The 1960s brought a breakthrough, with self-polishing paints that slowly hydrolyze, slowly releasing toxins. These paints employed organotin chemistry ("tin-based") biotoxins such as tributyltin oxide (TBT) and were effective for up to four years. These biotoxins were subsequently banned by the International Maritime Organization when they were found to be very toxic to diverse organisms.^[37][38] TBT in particular has been described as the most toxic pollutant ever deliberately released in the ocean.^[24]

As an alternative to organotin toxins, there has been renewed interest in copper as the active agent in ablative or self polishing paints, with reported service lives up to 5 years; yet also other methods that do not involve coatings. Modern adhesives permit application of copper alloys to steel hulls without creating galvanic corrosion. However, copper alone is not impervious to diatom and algae fouling. Some studies indicate that copper may also present an unacceptable environmental impact.^[39]

Study of biofouling began in the early 19th century with

One trend of research is the study of the relationship between wettability and anti-fouling effectiveness. Another trend is the study of living organisms as the inspiration for new functional materials. For example, the mechanisms used by marine animals to inhibit biofouling on their skin.[42]

Materials research into superior antifouling surfaces for

A study of the biotoxins used by organisms has revealed several effective compounds, some of which are more powerful than synthetic compounds. Bufalin, a bufotoxin, was found to be over 100 times as potent as TBT, and over 6,000 times more effective in anti-settlement activity against barnacles.^[44]

One approach to antifouling entails coating surfaces with polyethylene glycol (PEG).^[45] Growing chains of PEG on surfaces is challenging. The resolution to this problem may come from understanding the mechanisms by which mussels adhere to solid surfaces in marine environments. Mussels utilize adhesive proteins, or MAPs.^[46] The service life of PEG coatings is also doubtful.

See also

• Fouling
• Biomimetic antifouling coatings
• Tributyltin
• Bottom paint
• Corrosion engineering

References