Environmental Engineering Reference
In-Depth Information
4.6 Controlling Biofouling and Macrofouling
Use of antifouling paint is the most common and best approach for protecting the
parts of a ship that are under water. They contain water-soluble resins, pigments,
metal salts and inert illers. Copper oxide coatings are widely used. The basic
insoluble-matrix, rosin-copper(I)oxide coatings (using vinyl, chlorinated rubber,
and polyisobutyl resins) developed in the 1950s remain the same. These coatings are
capable of a static barnacle resistance of over 90% for almost four years. Organotin
coatings were introduced in early 1960s. There are two basic types of organotin
coatings: coatings that incorporate organotin compounds and coatings based on ilm
forming resins that contain a chemically bound organotin. Antifouling elastomers
with a slow release toxic reservoir are reported in reference [7]. Current research
towards identifying new marine-based products has led to the use of polyhydroxy
sterols isolated from octocorals.
Chlorination is generally preferred to prevent the settlement of fouling organisms in
the cooling water inlet lines of power stations. Macrofouling takes place when there
is only intermittent chlorination or when chlorine levels are below the threshold
value. Mussels and barnacles, once settled (during a period of no chlorination) are
able to resist subsequent chlorination cycles. During the breaks in chlorination they
feed and carry on their normal life [7].
Quaternary amine compounds and surfactants have been reported to act slowly on
molluscs. These compounds do not trigger the chemoreceptors of these molluscs.
The molluscs continue to ingest the antimicrobials throughout the exposure period,
but they experience slow mortality. Pressure washing, water jetting, sonic devices,
robotics, magnetic ields, reproductive control and cathodic protection are the other
techniques that have been found to be effective to some extent and are used in
some industries. Marine organisms are more sensitive to the ambient temperature
than their terrestrial counterparts. Warm waters in the temperature range between
50−70 ºC can kill nearly all organisms and it is found to be successful for localised
fouling control [29].
Controlling biofouling by biological means is gaining attention because it is potentially
cost effective, long lasting, less polluting, and a sustainable and green alternative.
There are several examples of such an approach but they have still not been made
available commercially for large areas. The common map turtle (
Graptemys
geographica
) is known to feed on zebra mussels (
D. polymorpha
), one of the most
proliic macrofoulers in American waters. The bull chub ish (
Nocomis raneyi
) can
crush and ingest hard-shelled molluscan foulers. Blue crabs, hermit crabs and stone
crabs prey on oysters (molluscs). Hydroids are eaten by some species of sea slugs,
ileishes, puffer ish and moorish idol (
Zanclus cornutus
) [12].
