by which suspended particles are brought together into
larger units called flocs.
Flocculation or floc formation depends on the physical
collision between suspended matter and their adhesion
(Alldredge and Jackson, 1995; Hansen et al., 1995).
Droppo (2001) described flocs as heterogeneous, composite structures composed of an active biological component, a nonviable biological component, inorganic
particles, and water held within or flowing through pores.
Because of the diverse origins of flocs, their characteristics
are highly variable (Alldredge and Silver, 1988). These
can also be viewed as individual microecosystems with
autonomous and interactive physical, chemical, and biological functions or behaviors operating within the floc
matrix (Droppo et al., 1997). Microflocs are dense,
quasi-spherical, resistant to turbulent mixing, and small
with sizes ranging from 100 to 160 mm (Verney et al.,
2009). Under favorable conditions, microflocs collide
with each other, flocculate, and form macroflocs (Simon
et al., 2002; Verney et al., 2009). Macroflocs are formed
from microflocs up to several millimeters and can rapidly
disintegrate back into microflocs.
Factors affecting flocculation
Flocs are formed within the water column or on the surface of a bed by a variety of complicated physical, physicochemical, and biological means (Droppo, 2001;
Simon et al., 2002). The physical mechanisms that bring
particles together in the ocean are Brownian motion, fluid
shear, and differential settlement (Mccave, 1984;
Alldredge and Silver, 1988; Simon et al., 2002).
Brownian motion dominates interactions of fine particles
(less than 8 mm). Differential settlement should dominate
coagulation between similarly sized particles between
1 and 100 mm in surface waters. It is important for sinking
of particles in the water column and in slack water in tidally affected shallow seas and estuaries (Simon et al.,
2002). McCave’s (1984) calculations indicate that collisions of small particles with larger ones resulting in
floc formation should be controlled primarily by shear.
Shear collides similarly sized particles and leads to scavenging of small particles by large ones more effectively
than differential settling. In agreement, a study
conducted by Chen et al., (2005) in Scheldt reported that
floc formation in the estuary was rather controlled by current velocity and the suspended matter concentration
than salinity.
Flocs are composed of biological components, inorganic particles, and water that carry with them negative
surface charge, hence, are affected by varying pH and
salinity. Salinity is used by oceanographers as a measure
of the total salt content of seawater. Clay particles which
are usually negatively charged have a high cationic
adsorption capacity. Interparticle forces then become
attractive at increased salinities, causing particles to
collide and flocculate (Day et al., 1989). pH is a measure
of acidity or alkalinity which can either increase (e.g., salt
influences) or decrease (organic matter load) due to the
ionic composition of the system. It has been reported in
some experiments that changes in surface charge due to
pH variations affect floc stability (Wilén et al., 2000).
Another factor called bridging occurs when the loops
and tails of a polymer adsorbed to one particle
become attached to one or more other particles
(Droppo et al., 2005). Divalent cations such as Ca
2+
and Mg
2+ can in some cases act as chemical bridging
agents between negative charges of the polymers and negative charges on the particle surface, enhancing the attraction and sticking properties of electronegatively charged
particles and polymers (Simon et al., 2002).
Flocculation can be biologically mediated by either particulate or dissolved organic matter. Organic matter is
adsorbed at the surface of sediment particles, giving it a
negative charge. Sticky organic compounds (transparent
exopolymer particles) produced in the water column by
phytoplankton, bacteria, and macrophytes also promote
aggregation and sedimentation of particles (Passow,
2002).
Summary and conclusion
Diverse studies on flocculation have been conducted
through the years. These studies lead to better understanding of the factors affecting flocculation at given environmental conditions. Moreover, applications of
flocculation studies can be valuable to society such as in
water and wastewater treatment, management of harmful
algal blooms, and siltation control.
Bibliography
Alldredge, A. L., and Jackson, G. A., 1995. Aggregation in marine
systems – preface. Deep-Sea Research Part II-Topical Studies in
Oceanography, 42(1), 1–7.
Alldredge, A. L., and Silver, M. W., 1988. Characteristics, dynamics, and significance of marine snow. Progress in Oceanography,
20(1), 41–82.
Chen, M. S., Wartel, S., Van Eck, B., and Van Maldegem, D., 2005.
Suspended matter in the Scheldt estuary. Hydrobiologia,
540(1–3), 79–104.
Day, J. W., Hall, C. A., Kemp, W. M., and Alejandro, Y., 1989. Estuarine Ecology. New York: Wiley-Interscience.
Droppo, I. G., 2001. Rethinking what constitutes suspended sediment. Hydrological Processes, 15(9), 1551–1564.
Droppo, I. G., Leppard, G. G., Flannigan, D. T., and Liss, S. N.,
1997. The freshwater floc: a functional relationship of water
and organic and inorganic floc constituents affecting suspended
sediment properties. Water Air and Soil Pollution, 99(1–4),
43–53.
Droppo, I. G., Leppard, G. G., Liss, S. N., and Milligan, T. G. (eds.),
2005. Flocculation in Natural and Engineered Environmental
Systems. Boca Raton, Florida: CRC Press.
Eisma, D., 1986. Flocculation and deflocculation of suspended matter in estuaries. Netherlands Journal of Sea Research, 20(2/3),
183–199.
328
FLOCCULATION
larger units called flocs.
Flocculation or floc formation depends on the physical
collision between suspended matter and their adhesion
(Alldredge and Jackson, 1995; Hansen et al., 1995).
Droppo (2001) described flocs as heterogeneous, composite structures composed of an active biological component, a nonviable biological component, inorganic
particles, and water held within or flowing through pores.
Because of the diverse origins of flocs, their characteristics
are highly variable (Alldredge and Silver, 1988). These
can also be viewed as individual microecosystems with
autonomous and interactive physical, chemical, and biological functions or behaviors operating within the floc
matrix (Droppo et al., 1997). Microflocs are dense,
quasi-spherical, resistant to turbulent mixing, and small
with sizes ranging from 100 to 160 mm (Verney et al.,
2009). Under favorable conditions, microflocs collide
with each other, flocculate, and form macroflocs (Simon
et al., 2002; Verney et al., 2009). Macroflocs are formed
from microflocs up to several millimeters and can rapidly
disintegrate back into microflocs.
Factors affecting flocculation
Flocs are formed within the water column or on the surface of a bed by a variety of complicated physical, physicochemical, and biological means (Droppo, 2001;
Simon et al., 2002). The physical mechanisms that bring
particles together in the ocean are Brownian motion, fluid
shear, and differential settlement (Mccave, 1984;
Alldredge and Silver, 1988; Simon et al., 2002).
Brownian motion dominates interactions of fine particles
(less than 8 mm). Differential settlement should dominate
coagulation between similarly sized particles between
1 and 100 mm in surface waters. It is important for sinking
of particles in the water column and in slack water in tidally affected shallow seas and estuaries (Simon et al.,
2002). McCave’s (1984) calculations indicate that collisions of small particles with larger ones resulting in
floc formation should be controlled primarily by shear.
Shear collides similarly sized particles and leads to scavenging of small particles by large ones more effectively
than differential settling. In agreement, a study
conducted by Chen et al., (2005) in Scheldt reported that
floc formation in the estuary was rather controlled by current velocity and the suspended matter concentration
than salinity.
Flocs are composed of biological components, inorganic particles, and water that carry with them negative
surface charge, hence, are affected by varying pH and
salinity. Salinity is used by oceanographers as a measure
of the total salt content of seawater. Clay particles which
are usually negatively charged have a high cationic
adsorption capacity. Interparticle forces then become
attractive at increased salinities, causing particles to
collide and flocculate (Day et al., 1989). pH is a measure
of acidity or alkalinity which can either increase (e.g., salt
influences) or decrease (organic matter load) due to the
ionic composition of the system. It has been reported in
some experiments that changes in surface charge due to
pH variations affect floc stability (Wilén et al., 2000).
Another factor called bridging occurs when the loops
and tails of a polymer adsorbed to one particle
become attached to one or more other particles
(Droppo et al., 2005). Divalent cations such as Ca
2+
and Mg
2+ can in some cases act as chemical bridging
agents between negative charges of the polymers and negative charges on the particle surface, enhancing the attraction and sticking properties of electronegatively charged
particles and polymers (Simon et al., 2002).
Flocculation can be biologically mediated by either particulate or dissolved organic matter. Organic matter is
adsorbed at the surface of sediment particles, giving it a
negative charge. Sticky organic compounds (transparent
exopolymer particles) produced in the water column by
phytoplankton, bacteria, and macrophytes also promote
aggregation and sedimentation of particles (Passow,
2002).
Summary and conclusion
Diverse studies on flocculation have been conducted
through the years. These studies lead to better understanding of the factors affecting flocculation at given environmental conditions. Moreover, applications of
flocculation studies can be valuable to society such as in
water and wastewater treatment, management of harmful
algal blooms, and siltation control.
Bibliography
Alldredge, A. L., and Jackson, G. A., 1995. Aggregation in marine
systems – preface. Deep-Sea Research Part II-Topical Studies in
Oceanography, 42(1), 1–7.
Alldredge, A. L., and Silver, M. W., 1988. Characteristics, dynamics, and significance of marine snow. Progress in Oceanography,
20(1), 41–82.
Chen, M. S., Wartel, S., Van Eck, B., and Van Maldegem, D., 2005.
Suspended matter in the Scheldt estuary. Hydrobiologia,
540(1–3), 79–104.
Day, J. W., Hall, C. A., Kemp, W. M., and Alejandro, Y., 1989. Estuarine Ecology. New York: Wiley-Interscience.
Droppo, I. G., 2001. Rethinking what constitutes suspended sediment. Hydrological Processes, 15(9), 1551–1564.
Droppo, I. G., Leppard, G. G., Flannigan, D. T., and Liss, S. N.,
1997. The freshwater floc: a functional relationship of water
and organic and inorganic floc constituents affecting suspended
sediment properties. Water Air and Soil Pollution, 99(1–4),
43–53.
Droppo, I. G., Leppard, G. G., Liss, S. N., and Milligan, T. G. (eds.),
2005. Flocculation in Natural and Engineered Environmental
Systems. Boca Raton, Florida: CRC Press.
Eisma, D., 1986. Flocculation and deflocculation of suspended matter in estuaries. Netherlands Journal of Sea Research, 20(2/3),
183–199.
328
FLOCCULATION