Beneficial biofilms are used in all types of bioreactor as they are simple, more productive, and surfaces that support biofilm growth are available at reasonable cost.
In industrial bioreactors two types of biofilms are commonly used
- Biofilms that grow onto supports such as charcoal, resin, bonechar, concrete, clay brick, or sand particles, and
- Biofilms that are formed as a result of flocs and aggregate formation.
On supports, biomass grows all around the particles and the size of the biofilm particles grows with time usually to several mm in diameter. The density of the support particles is usually higher than the fermentation broth and for this reason bioparticles tend to remain in the lower section of the reactor.
Where no support is used and cells form biomass granules and flocs that also grow in size with time. This type of biofilm is called granular biofilm and the reactor where this biofilm is used is called granular biofilm reactor. Granule formation may take from several weeks to several months. The cells produce sticky, extracellular polymeric substances (EPS) that binds the cells firmly in the form of flocs and aggregates.
Biofilm reactors can be assembled in a number of configurations including
- Batch: The immobilized cells have to be used in repeated batches. Productivity is reduced due to downtime necessary to fill and empty the reactor. If the reactor is packed with biofilm particles, some cells may die or become inactive due to lack of feed during emptying and filling of the reactor. As a result, it is viewed that batch reactors are not very practical for biofilms.
- Continuous stirred tank: Feed medium is fed to the reactor and product is withdrawn at the same rate as feed. They are stirred using a mechanical device such as impeller. This type of system can be used for the production of butanol and lactic acid in continuous operation with a constant feed and a constant effluent from the reactor.
- Packed bed: The reactor is packed with suitable support material followed by inoculation with the culture to form biofilm. The reactor is supplied with a feed that is not deficient in nutrients. Depending on the culture, nutrients, and support, biofilm formation may take a few to several days. Such reactors are usually fed at the bottom, thus getting product at the top of the reactor. However, these reactors are prone to blockade due to excessive cell growth.
- Trickling bed: Fed at the top of the reactor, thus obtaining product at the bottom. However, in such reactors some of the biofilms may not get sufficient feed which reduces reactor efficiency/productivity. Also, in gaseous fermentations gas may occupy significant space in the reactor and may form stagnant pockets. In anaerobic waste water treatment and acetic acid production, these reactors have been used at large scale successfully.
- Fluidized bed: Used in the degradation of toxic phenolic chemicals and butanol production. In these reactors, cell growth occurs around the adsorbent particles. Formation of active biofilms around the particles and accumulation of sufficient biomass in the reactor may take from 2 to 4 weeks. A major advantage in these reactors is that they can be operated for much longer periods than PBR or CSTRs (with fibrous bed). These reactors do not block due to excessive growth. In these reactors butanol production was increased by approximately 40–50 times that of the batch reactors. These reactors have been operated successfully for longer than 4 months in continuous operation.
- Airlift reactors: These reactors contain two concentric tubes, a riser (an inner tube) and a downcomer (an outer tube). In these reactors, mixing is achieved by circulating essentially air at the bottom of the reactor. As a result of force applied by the air (at the bottom of the inner tube), the liquid in the inner tube moves up which then overflows (the inner tube) downward thus creating eddies to mix the liquid.
- Upflow anaerobic sludge blanket: used for anaerobic treatment of wastewater/industrial effluents. As the name suggests, the flow in these reactors is in upward direction. At the top of the reactor provisions are made for gas/es to escape and sludge particles to settle to the bottom part of the reactor. Reactor effluent is removed from the top of the reactor.
Further reading on biofilms and chemical production
Beneficial biofilms: wastewater and other industrial applications https://www.sciencedirect.com/science/article/pii/B9781845694777500181
Qureshi, N., Annous, B.A., Ezeji, T.C. et al. Biofilm reactors for industrial bioconversion processes: employing potential of enhanced reaction rates. Microb Cell Fact 4, 24 (2005). https://doi.org/10.1186/1475-2859-4-24
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