Environmental Engineering Reference
In-Depth Information
process is the formation of a conditioning ilm (
Figure 2.1
). When a surface is immersed
in the sea, it quickly absorbs organic material from the marine environment. Bacteria
attach, settle and grow on this ilm and start to secrete exopolysaccharide (EPS) with
which they attach themselves irmly to the surface. EPS changes the surface energy and
the electrostatic properties of the immersed surface, thus helping with the attachment
of the microbes. These events are followed by the attachment of algae and protozoa
to form the bioilm. The microbes that initiate biofouling are called microfoulers.
Thereafter, the surfaces are colonised by seaweeds and the marine invertebrates,
which are known as macrofoulers [4]. These attachments are a function of the type
of microfouling or the early colonisers that are present on the surface.
Reversible
attachment
of bacteria
EPS and
biofilm
formation
(hours-days)
Irreversible
attachment
of bacteria
Growth and
division
Conditioning
film
(Seconds-minutes) (Seconds-minutes) (Minutes-hours) (Hours-days)
Figure 2.1
Various stages in the formation of bioilm bacteria on an immersed
surface
A number of environmental factors affect the settlement of marine organisms on
immersed surfaces and these include light, ocean current speed, temperature, salinity,
dissolved oxygen, depth, weather conditions, geographic location, presence of other
inorganic salts and gravity. Marine invertebrates preferentially attach on surfaces in
the dark, whereas algae prefer light. A water speed of over 103 cm/s will prevent
settlement of invertebrates. Since algal spores are small enough to enter the laminar
sub-layer they can settle at higher current speeds [5]. It is observed that invertebrates
have a preference to settle on the bottom of surfaces while algae settle on the upper
surface to avoid the shear forces [5]. Salinity of the water also inluences the growth
rate and maximum organism size. A low salinity environment is unfavorable to a
few common fouling species. High water temperature around the year, leads to heavy
