MERCER02-14
10
water, thereby asphyxiating the organisms. The treated areamust be sealed and ventedwith one-
way flow, but water is re-aerated quickly upon discharge. Given the tolerance of
G. franciscana
to low oxygen conditions, this approachmay have limited success andmaintaining deoxygenated
conditions can be costly.
4.2.3. Heat (experimental)
A heat system that increases water temperature to a point where organisms are killed is energy
intensive. Cysts and some bacteria can require very high temperatures, but organisms like
bivalves are killed after approximately one hour at 40°C. Water flow disruption is not
appropriate in this application, but typically is necessary to raise temperatures adequately.
Microwave heating is being tested, but is currently not cost effective.
4.2.4. Ultrasonic/Ultrasound (experimental)
Ultrasound systems disrupt an organism's cell wall by using high frequency vibrations that cause
microscopic bubbles. Vibration intensity and exposure length vary. Low frequencies appear to
be the most effective. Some indications are that free-field ultrasonic radiation is effective in
highly turbid environments, but costs to implement industrial scale applications are not known.
5. Coatings
The types of coatings on themarket are classified as either containing a biocide or biocide-free.
Biocide containing coatings include antifouling paints, epoxy based, and fluorinated powder
coatings. Biocide-free coatings are marketed as non-toxic because they rely on low surface
energy properties to reduce biofouling.
5.1. BiocideCoatings
Antifouling coatings prevent organisms from attaching by carefully controlling the release of
biocides. There are typically three components: a binder (polymeric compound to hold the paint
together), a resin (water-soluble compound to allow seawater access to toxin), and a
toxin/biocide (to confer antifouling property to the paint) (Nair 1999). The coatings are solid
metals or in powdered formwith the biocide incorporated into a coating matrix. Incorporation
methods include galvanization and thermal spray. When the coating erodes and sloughs off, the
embedded biocides are released (Wells andSytsma 2009).
The efficacy of different antifouling coatings varies with target species. For example, copper is
highly effective against macrofoulers, but not microfoulers (e.g., algae). For this reason, booster
biocides are often used alongside metals. On January 1, 2008, the International Maritime
Organization banned coatings containing tributyltin (TBT) because of toxicity concerns (Wells
andSytsma 2009).
Most antifouling paints have a lifespan of one to two years in flowing water. Performance is
approximately three years in staticwater (Skaja 2012). Copper, copper alloymetals, and thermal
sprays have an expected lifespan of greater than five years, unless the substrate is coatedwith a
biofilm; the effective lifespan is three years on a substrate coatedwith a biofilm.
