BIOREACTORS

GAYATHRI MOHAN

CONTENTS• Introduction to Bioreactors • Classification and types of Bioreactors• Operation • Applications of Bioreactors in waste water

treatment

BIOREACTORSBioreactor: A bioreactor may refer to any manufactured or engineered device or system that supports a biologically active environment.

A bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms.

This process can either be aerobic or anaerobic.

They vary in size and complexity from a 10 ml volume in a test tube to computer controlled fermenters with liquid volumes greater than 100 m3.

Applications of Biotechnology

Classification of BioreactorsBroadly classified as

Suspended Growth Bio-reactors: Biological catalyst is suspended in the growth medium. Eg: Batch reactors, CSTR’s, Plug Flow reactor

Biofilm Bio-reactors: Microorganism are kept attached to a surface and this type find major applicability in waste water treatment.

The different kinds of biofilm reactors include membrane,fluidized bed, packed bed, airlift, and upflow , anaerobic sludge blanket reactors.

Based on mode of operation Batch Continuous Fed-Batch

Batch Reactors• Reactants are charged in to the vessel at

the starting of operation and the products are withdrawn at the end.

• Complete mixing of the reactor volume• uniform composition everywhere in

reactor but changes with time

Eg: BOD digestion bottle

Continuously stirred tank reactors• In CSTR, reactants and products are continuously

added and withdrawn. Micro- organisms that grow within the reactor

continuously replace the micro organisms removed from the reaction in the effluent.

• The basic characteristic of the ideal CSTR is that the concentration of the substrate and microorganisms are the same everywhere through out the reactor.

• Mechanical or hydraulic agitation is required to achieve uniform composition and temperature

• Mainly used in industrial applications and in waste water treatment where no sterile conditions is needed

Applications of CSTR• Activated sludge reactors are one of the widely used CSTR s

in waste water treatment.• Ideal for growth associated products, ie, Primary

metabolites• Other application include the production of single cell

proteins, ethanol production, Lactic acid production.• One of the drawback is high power requirement for mixing.• High shear force may damage the cells.• Also less productive strain will be dominant in a continuous

culture.

Plug Flow Reactors (PFR)• Also referred to tubular reactor or piston

flow reactor. (PFR)• Flow of fluid through reactor with order

so that only lateral mixing is possible.• The liquid or slurry stream continuously

enters one end of the reactor and leaves at the other end.

• Flow moves through the reactor with no mixing with earlier or later entering flows.

• Concentrations of substrates are highest at the entrance of the reactor, which tends to make rates there quite high

Applications of PFR• Mainly used in small laboratory scale production and

pilot-plant studies.• A typical plug flow reactor could be a tube packed with

some solid material (frequently a catalyst) are called packed bed reactors or PBR's. Sometimes the tube will be a tube in a shell and tube heat exchanger.

• Plug flow reactors are used for the following applications: Fast reactions Homogeneous or heterogeneous reactions Continuous production High-temperature reactions

Disadvantages

Major disadvantages are Temperature is hard to controlHigh Maintenance cost

Packed Bed reactors• The medium to which the microorganisms are attached

is stationary (e.g plastic media or pea sized stones). • Commonly packed bed reactors are used for aerobic

treatment of waste waters and are known as tricking filters and or biological towers.

ADVANTAGES: There is improved contact between the waste stream

and the micro organisms .

Trickling filter

Fluidized Bed Reactor• The fluidized bed reactor depends upon the

attachment of particles that are maintained in suspension by a high upward flow rate of the fluid to be treated. The particles are often called biofilm carriers. The carriers may be sand grains, granular activated carbon, diatomaceous earth. ADVANTAGES:

1. Uniform particle mixing2.Uniform temperature gradients3. The ability to operate reactor in continuous state.

DISADVANTAGES:

1. Increased reactor vessel size2. pumping requirements and pressure drop3. Pressure loss scenario

Fluidized Bed Reactor

Environmental application of BioreactorsGASEOUS EFFLUENTS

Bioreactors provide better containment and superior environmental controls that allow faster, more complete and cost-effective treatment

• 1. BIOFILTERS: Biofilters are beds of soil or compost, about 1 m deep, with an underlying distribution system for the contaminated gas. As the contaminant laden gas moves up through the moist bed, the pollutants are removed by sorption and oxidized by the microbial population immobilized in the bed.

• 2. BIOSCRUBBERS: Conceptually similar to conventional gas scrubbers, Bioscrubbers are employed when heavier contaminant loadings, less soluble contaminants or contaminant toxicity make biofilters unsatisfactory. Activated sludge mixed in water is contacted with the gaseous effluent in a packed bed absorption tower. Contaminants transfer to the sludge-water slurry which is taken to holding or sedimentation tanks where most of the degradation takes place.Clarified liquor from the sedimentation tanks is recycled to the absorption column.

Bioscrubber

ARTIFICIAL WETLAND, OR 'REED BED'

• Bioreactor systems for reduction of biochemical oxygen demand (BOD 5 ) and total suspended solids (TSS) in municipal and industrial wastewaters.

• Aquatic plants such as bulrush, cattails, common reed, water hyacinth, swamp potato and duck potato rooted in rock and gravel media beds flooded with wastewater flowing though the bed and root zone, make-up the wetland filters.

Conclusion

Bioconversion of wastes to harmless substances or higher value products already has a significant role in environmental pollution control and improved resource utilization. Both in- situ and bioreactor based treatment processes are experiencing rapid development and increasing deployment in practical applications.

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