Biofilm reactors

2. BIOFILM REACTORS
FOR PRODUCTION OF VALUE-ADDED
PRODUCTS
By Aref Farokhi-Fard
aarreeff@ymail.com

3. Microbial biofilms

4. A biofilm is any group of microorganisms in which
 Cells stick to each other
 Often these cells adhere to a surface
 These adherent cells are frequently embedded within a self-produced
matrix of extracellular polymeric substance (EPS).
 Cells growing in a biofilm are physiologically distinct from planktonic
cells of the same organism
 More than 90% of wet biofilm mass is water
 Extracellular polymeric substances (EPS) correspond to ≥70% of the
dry biofilm mass

5.  Microbial films can cause many detrimental effects
on human health
 But on the other hand, biofilms can be very useful
in many applications:
 Wastewater treatment
 Bioremediation: degradation of toxic pollutants
 Production

6.  Biofilm thickness can vary from a few microns to
even a few centimeters depending on factors such
as:
 Microbial species
 Biofilm age
 Available nutrients
 Liquid shear stresses

7. 1-The conditioning of the substratum by:
 Macromolecules in bulk liquid or
 Intentionally coated material
2-The microorganisms suspended in the liquid are then transported to the surface by:
 Diffusion
 Convection
 Self-motility
3,4-Weak reversible adhesions with the solid surface
5- Irreversible adhesions by:
Formations of polymer bridges between the conditioning layer (an adsorbed layer of
macromolecules on the solid surface) and the EPS excreted by the microbe.

9. 6- Growth of microorganisms
 More important role than transport
7-Detachment processes (erosion or disruption or sloughing off)
occur simultaneously and in response to
 Fluid shear
 Forces
 Weak internal cohesion
 Depletion of nutrients or oxygen in the biofilm

10.  During colonization, the cells are able to communicate via
quorum sensing (QS) using products such as N-acyl
homoserine lactone (AHL).
 coadhesion : secondary colonizers coadhere with organisms
already adhering to the surface. Coaggregates of organisms
may also form in bulk liquid and then adhere to the biofilm
surface in a process called coaggregation.
 When growth balances with detachment, the maximum
average thickness of biofilm is reached and the system is
considered as pseudo-steady state.

11.  Heterogeneous model:
Microorganisms form a dense, planar, homogeneous biofilm
exposed to the flowing liquid
 Heterogeneous mosaic model or pseudo-
homogeneous model:
Stacks consisting of cells hold together by EPS and appeared as
columns separated by water channels over a layer of cells about
5 μm.
 Mushroom or tulip model

12.  Is the most recent model revealed using:
 Confocal laser scanning microscopy
 Molecular probe like fluorescent markers
 In this model, the biofilm was formed in a mushroom-shaped
column surrounded by water channels through which oxygen
and nutrients were carried with the liquid flow.

14. 1. Shear forces
2. Nutrient compositions
3. Nutrient concentrations
4. Nutrient depletion

15. Biofilm reactors

16.  Increasing the biomass using:
 Cell-recycle reactors
 Hollow-fiber reactors
 Cell immobilization
 Immobilized-cell :
 Excellent examples of high-biomass density systems
 Lesser tendencies to develop membrane fouling
 Lower required capital costs
 high capital and operation cost
 potential for membrane fouling

17.  Active or artificial
Covalent bonding to surfaces
 Various coupling agents
 Entrapments in polymer matrix
 Passive or natural
Natural adsorptions of films or flocculants around or within the
solid support materials
 Adsorptions: mainly based on electrostatic interactions
 Colonization: based on a technique using porous biomass support
particles (BSPs)

18.  Toxicity of coupling/cross-linking agents on cell viability and activity
 Instability of the polymer matrix (e.g., calcium alginate gel) with various
anions including phosphate, citrate, EDTA, and lactate
 Cell leakage from the gel matrix
 Limited mass transfer across the beads
 Poor operational stability
 High cost of the carrier

19.  Their potential for development into a continuous culture
 Their exceptional stability
 Their lower nutrient requirements

20. 1. Higher biomass density
2. Higher operation stability
3. To retain 5 or 10 times more biomass per unit volume of reactor
4. Increasing production rates
5. Reducing the risk of washing out
when operating at high dilution rates during continuous fermentation
1. Eliminating need for re-inoculation during repeated-batch fermentation
2. Decreased viscosity and enhanced nutrient and oxygen transfer:
In the case of filamentous microorganisms, such as Aspergillus Niger
1. High resistance to extreme conditions of pH and temperature,
contaminations, hydraulic shocks, antibiotics, and toxic substances
2. Products can be easily recovered

21.  Fixed-bed reactors
Include all processes in which the biofilms develop on static media.
 Expanded-bed reactors
Include all biofilm processes with continuously moving media
maintained by high air or liquid velocity or by mechanical stirring.

22. (1) Submerged beds
The biofilm particles are completely immersed in the liquid
(2) Trickling filters
The liquid flows downward through the biofilm bed, while the gas flows upward
(3) Rotating disk reactors
The biofilm develops on the surface of a vertical disk that is partially submerged and
rotates within the liquid
(4) Membrane biofilm reactors
The microbial layer is attached to a porous gas-permeable membrane

23. (1) Fluidized beds
Particles move up and down within the expanded
bed in the well-defined zone of the reactor
(2) Moving beds
The whole expanded bed circulates throughout the reactors such
as airlift reactor and circulating-bed reactors.

25.  The support must:
 Favor microorganism adhesion
 Have a high mechanical resistance to liquid shear forces and particle collision
 Be inexpensive and widely available
 Some of the properties of solid support, dramatically affect the
adhesion of microorganisms:
 Surface charge
 Hydrophobicity
 Porosity
 Roughness
 Particle diameter
 Density

26.  Balance between the van der Waals forces of attraction and repulsive forces
 Generally, the bacterial cell surfaces and most of the existing solid materials
display a net negative charge when immersed in aqueous solution with pH near
neutrality
 The higher degree of hydrophobicity of solid surfaces strongly enhances
adhesions of microorganisms

27.  If the particles are porous, the film is formed not only on the surface, but also
within the pores.
 A porous matrix of materials provides niches sheltered from hydraulic shear forces
 Deficient nutrient diffusion to the inner area and accumulation of gaseous
metabolites inside porous carriers can be overcome by using materials with
adequately large pores and internal porous volume

28.  Developed at Iowa State University
(U.S. Patent Number: 5,595,893)
 Ideal physical structure
 Slow-released nutrients
 Is an extrusion product of
polypropylene and several
agricultural products
 Can be custom-made for specific
microorganism.

29. The PCS rings and
tubes were used as
solid supports for
biofilm formation
in packed-bed
reactors or as PCS
tubes attached to
the bioreactor
agitator shaft

31.  Before biofilm reactors can be applied in industrial-scale
production, many additional scale-up studies on several
parameters are necessary:
 Culture conditions
 Mass and heat transfer constraints
 studies on Kinetics

33. Thank you for your attention