BIOFILM REACTORSFOR PRODUCTION OF VALUE-ADDED

PRODUCTS

By Aref Farokhi-Fard

aarreeff@ymail.com

Microbial biofilms

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

What is the biofilm?

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

Benefits and harms

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

Biofilm thickness can be variable

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.

A formation of biofilm takes several steps

A formation of biofilm takes several steps

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

Growth and Detachment

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.

Quorum sensing, co-aggregation and pseudo-steady state

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

The structure of biofilm is not entirely understood, but 3 different models have been proposed

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.

Mushroom or tulip model

1. Shear forces2. Nutrient compositions3. Nutrient concentrations4. Nutrient depletion

Biofilm structure is affected by several factors

Biofilm reactors

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

One way to increase the productivity of fermentation is to increase biomass

high capital and operation cost potential for membrane fouling

Active or artificial Covalent bonding to surfaces

Various coupling agents Entrapments in polymer matrix

Passive or naturalNatural 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)

Cell immobilization methods can be divided into two major categories

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

Disadvantages of active/artificial immobilization

Their potential for development into a continuous culture

Their exceptional stability

Their lower nutrient requirements

Biofilm reactors have great potential for large-scale production

1. Higher biomass density2. Higher operation stability3. To retain 5 or 10 times more biomass per unit volume of reactor4. Increasing production rates5. Reducing the risk of washing out when operating at high dilution rates during continuous fermentation6. Eliminating need for re-inoculation during repeated-batch fermentation7. Decreased viscosity and enhanced nutrient and oxygen transfer:In the case of filamentous microorganisms, such as Aspergillus Niger8. High resistance to extreme conditions of pH and temperature,

contaminations, hydraulic shocks, antibiotics, and toxic substances 9. Products can be easily recovered

Biofilm reactors show many advantages over suspended cell reactors

Fixed-bed reactorsInclude all processes in which the biofilms develop on static media.

Expanded-bed reactorsInclude all biofilm processes with continuously moving media maintained by high air or liquid velocity or by mechanical stirring.

Biofilm reactors can be categorized into two categories

(1) Submerged bedsThe biofilm particles are completely immersed in the liquid (2) Trickling filtersThe liquid flows downward through the biofilm bed, while the gas flows upward

(3) Rotating disk reactorsThe biofilm develops on the surface of a vertical disk that is partially submerged and rotates within the liquid

(4) Membrane biofilm reactorsThe microbial layer is attached to a porous gas-permeable membrane

Fixed-bed reactors

(1) Fluidized bedsParticles move up and down within the expandedbed in the well-defined zone of the reactor

(2) Moving bedsThe whole expanded bed circulates throughout the reactors such as airlift reactor and circulating-bed reactors.

Expanded-bed reactors

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

Selection of solid supports for biofilm reactors

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

Adhesions of microorganisms to solid surfaces/support

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

Carrier particles and rough and/or porous surface materials for larger surface area

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.

Plastic composite support (PCS), used in many studies

PCS biofilm reactors

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

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

For scale-up

Reference

Thank you for your attention