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Bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. They are commonly cylindrical, ranging in size from some liters to cubic meters, and are often made of stainless steel. Bioreactors use microorganisms in attached or suspended biological systems to degrade contaminants in water. It speeds up the production of biogas. It involves collecting the leachate, which is then reinjected into the body of waste. Contributing moisture and nutrients for the bacteria at work on the waste, it speeds up the degradation process thereby facilitating the recovery of methane used as a source of energy. Collecting and using the methane has ecological and economic advantages: it reduces its contribution to the greenhouse effect and is a significant energy resource. Under optimum conditions the microorganisms or cells will reproduce at an astounding rate. The vessel's environmental conditions like gas (i.e., air, oxygen, nitrogen, carbon dioxide) flow rates, temperature, pH and dissolved oxygen levels, and agitation speed need to be closely monitored and controlled. Heat exchange is needed to maintain the bioprocess at a constant temperature. Biological fermentation is a major source of heat; therefore in most cases bioreactors need water refrigeration. They can be refrigerated with an external jacket or, for very large vessels, with internal coils.

Types of Bioreactors:

1.Batch reactor: Free enzyme

  •  High viscosity or insoluble substrate can be used
  •  New enzyme required for each batch
  •  Substrate inhibition can be a problem

2.Continuous stirred tank reactor (CSTR): Free or Immobilized enzyme

  •  pH control simple
  •  Enzyme addition/replacement simple
  •  Colloidal or insoluble substrates can be used
  •  Less problem with substrate inhibition

3.Continuous-flow stirred tank with ultrafiltration: Free or Immobilized enzyme

  •  Colloidal or insoluble substrates can be used
  •  Colloidal or insoluble substrates can be used
  •  Poor enzyme stability over long term operation
  •  Enzyme denatured or adsorbed at membrane surface

4.Plug-flow: Immobilized enzyme

  •  High conversion efficiency
  •  Less problem with product inhibition
  •  Cannot be used with insoluble or high viscosity substrates

5.Fluidized-bed: Immobilized enzyme

  •  Better heat and mass transfer
  •  Insoluble and high viscosity substrates can be used
  •  Low pressure drop
  •  Low pressure drop


Classification of bioreactor:

1.On the basis of cell culture

  •  Those that are used for cultivation of anchorage dependent cells (e.g. primary cultures derived from normal tissues and diploid cell lines).
  •  Those that are used for the cultivation of suspended mammalian cells (e.g. cell lines derived from cancerous tissues and tumors, transformed diploid cell lines, hybridomas).

In some cases the bioreactor may be modified to grow both anchorage dependent and suspended cells. Ideally any cell culture bioreactor must maintain a sterile culture of cells in medium conditions which maximize cell growth and productivity.

2. On the basis of biodegradation:

(i) Aerobic- In an aerobic bioreactor landfill, leachate is removed from the bottom layer, piped to liquids storage tanks, and re-circulated into the landfill in a controlled manner. Air is injected into the waste mass, using vertical or horizontal wells, to promote aerobic activity and accelerate waste stabilization. It is further divided in two parts

  •  In suspended biological systems, such as aerated lagoon, activated sludge, fluidized beds, or sequencing batch reactors, contaminated water is circulated in an aeration basin where microbes aerobically degrade organic matter and produce carbon dioxide, water, and biomass. The biomass is settled out in a clarifier, then either recycled back to the aeration basin or disposed of as sludge.
  •  In attached growth systems, such as upflow fixed film bioreactors, rotating biological contactors (RBCs), and trickling filters, microorganisms are grown as a biofilm on a solid growth support matrix and water contaminants are degraded as they diffuse into the biofilm. Support media include solids that have a large surface area for bacterial attachment.

(ii) Anaerobic- In an anaerobic bioreactor landfill, moisture is added to the waste mass in the form of re-circulated leachate and other sources to obtain optimal moisture levels. Biodegradation occurs in the absence of oxygen (anaerobically) and produces landfill gas. Landfill gas, primarily methane, can be captured to minimize greenhouse gas emissions and for energy projects. It is further divided in two parts

  •  In suspended biological system uses biological agents in an oxygen-free environment to remove impurities from wastewater. After undergoing such a treatment, water can be safely released back into the environment. Ex. Complete mix suspended growth anaerobic digester, Anaerobic Contact Process, Anaerobic sequencing batch reactor.
  •   In attached growth systems, includes Upflow attached processes, Downflow attached processes.

(iii) Hybrid (Aerobic-Anaerobic)- The hybrid bioreactor landfill accelerates waste degradation by employing a sequential aerobic-anaerobic treatment to rapidly degrade organics in the upper sections of the landfill and collect gas from lower sections. Operation as a hybrid results in the earlier onset of methanogenesis compared to aerobic landfills. Problems in bio-reactor

  •  Optimal oxygen transfer is perhaps the most difficult task to accomplish. Oxygen is poorly soluble in water -and even less in fermentation broths- and is relatively scarce in air (20.8%). Oxygen transfer is usually helped by agitation that is also needed to mix nutrients and to keep the fermentation homogeneous. There is however limits to the speed of agitation, due both to high power consumption (that's proportional to the cube of the speed) and the damage to organisms due to excessive tip speed. 
  •  Fouling can harm the overall sterility and efficiency of the bio reactor.

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