Flocculation is a process where particles in solution come out of solution and combine together. Finer particles remain separated due to electrostatic charge, but this charge can be neutralized using coagulants which causes the particles to collide and flocculate. Flocculating agents are added to fermentation media and wastewater treatment to promote aggregation of microbial cells and other particles to increase settling rate for easier separation. The mechanisms of flocculation involve neutralizing the negative surface charges on microbes through charge neutralization or reducing hydrophilicity to overcome repulsive forces between particles. Effective flocculation depends on the choice and dosage of flocculating agent and conditions for floc formation.

1. FLOCCULATION
DR. N. BANU, VELS UNIVERSITY

2. FLOCCULATION
• process where a solute comes out of solution
in the form of flocs or flakes.
• Particles finer than 0.1 µm in water remain
continuously in motion due to electrostatic
charge which causes them to repel each
other
• Once their electrostatic charge is neutralized
(use of coagulant) the finer particles start to
collide and combine together .
• These larger and heavier particles are called
flocs.

3. CELL AGGREGATION AND FLOCCULATION
 Flocculating agents are added in the fermentation
media for de-watering.
 Flocculation – effluent treatment plant for the removal
of microbial cells and suspended colloidal matter.
 Aggregates of microbial cells will sediment faster
because of increased diameter of the particles.
 Microbes in solution occurs as discrete units because
of:
 Surface of microbes are –ively charged and they
repulse each other.
 Cells have hydrophilic cell wall, so shell of bound
water is associated with the cell, which act as
thermodynamic barrier for cell aggregation.
 Due to irregular shape of the cell, it forms steric
hindrance.

4. MECHANISM FOR CELL FLOCCULATION
 Neutralization of anionic charges, carboxyl
and phosphate groups present in surface of
the cell, thus causes aggregation.
 Reduction in hydrophilicity.
 Use of high mol. Wt. polymer bridges.
(anionic, cationic, non-ionic polymers)

5. COAGULATION
 Process fluid + flocculating agent (high
shear in a stirred tank) – coagulation,
followed by gentle agitation – floc
formation and it grow in size.
 Flocculating agents for bacteria, yeast and
algae:
 Alum, calcium salts, ferric salts.
 Tannic acid, titanium tetrachloride and
cationic agents such as quaternary
ammonium compounds, alkyl amines and
alkyl pyridinium salts.

6. FLOCCULATION
 Depends on: choice of additive, dosage, and
conditions for floc formation.
 Effective agents: Mineral colloids and polyelectrolytes.
 Phosphoric acid – flocculating agent in scp process.
 Polyelectrolytes: act by charge neutralization and
hydrophobic interaction to link cells to each other.
 Coagulation of microbial protein released from the
cells by heating for short period.
 Bioflocculants: Rhodococcus erythropolis – alternative
to conventional flocculants.
 Genetic manipulation to alter the cell surface
properties to aid aggregation.
 Polymeric flocculation: cross- linked cationic polymers
may also be used in the processing of cell lysates and
extracts prior to downstream processing. E.g. Borax
used for yeast cell debris prior to decanter centrifuge.