Selection of Bioreactors

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The selection of a reactor is often determined by economics, reliability, or availability of a proven system.

Selecting an appropriate reactor-type or configuration for an immobilized cell system must be based on critical issues such as supply and removal of gases and solutes in the liquid phase and removal of excess biomass formed. The cell aggregates can only be fully active if the external supply or removal rates match the internal transport, utilization and production rates. The high cell densities in the reactors place higher demands on nutrient supply and transport rates and this is especially problematic for the sparingly soluble oxygen, requiring high circulation rates which may be in conflict with other design criteria. Correct selection of the reactor type can alleviate many of the aforementioned problems, but many reactor types can also be modified to adapt to the specific demands imposed by wastewater organic content, selected microorganisms or specific operational conditions.

Sterility and stringent cleaning procedures are usually not required in the case of wastewater treatment plants. Cost is obviously the main issue as no added value products are produced in a waste water treatment plant. A major motivation in using immobilized systems is the possibility to run in continuous operation. In this fashion, the unproductive time in batch and fed-batch, associated to filling, emptying and start-up of the reactor is eliminated. However, in many cases (e.g. biofilms or porous preformed supports) the biomass needs to be generated from the mixed population present in the non-sterile feed such as in the wastewater treatment units. This may take a significant amount of time (up to several weeks) and the associated cost can only be justified if the production time will be much longer. After long-term operation, the excessive growth of biocatalyst (films or within particles) may clog the reactor and then periodic cleaning may be necessary. Backwashing at high velocity of submerged fixed beds with vigorous injection of air, sometimes resulting in fluidization, or flooding of trickling filters followed by air injection, all lead to biofilm detachment and restoration of adequate flow conditions.

Some reactor systems are self-cleaning and excessive biomass or other solids can be eliminated continuously. In fluidized beds, thicker films on particles are removed by collision of the particles and the detached flocs are easily separated by sedimentation. Some rotating surface reactors also contain scrapers to remove excessive films, but as films grow thicker, the substrate and oxygen depletion in the interior will usually be sufficient for biofilm degradation and spontaneous detachment. In non-sterile mixed population operations such as wastewater treatment or biofilters, the pretreatment of the feed is not critical. Some removal of solids to avoid clogging may be useful, but in the case of fluidized bed operation, the denser solids may act as fluidization support and only require periodic purging to limit the solids content.

An important issue in immobilized cell reactors is the mixing pattern resulting from the conflicting demands on substrate supply and hydrodynamic requirements. In many instances, high flow rates are necessary for operation such as in fluidized beds, where the minimum fluidization velocity should obviously be exceeded but even higher velocities may be required to obtain sufficient bed expansion and mixing. The velocity is on the other hand limited by the entrainment velocity of the particles, which may only give a narrow window for good operation. As biomass accumulates on the particles, their apparent density decreases and hence their settling velocity. This may be influenced by careful selection of the carrier material (density) but even then the control is limited. A similar problem results when large quantities of gas are generated in packed beds requiring large flow rates to entrain this gas out of the reactor. Consequently, only in rare cases will the resulting flow rate match the substrate flow rate and recycling of the liquid through the reactor will be necessary for sufficient COD utilization. This results in completely mixed liquid operation and all biomass is exposed virtually the same conditions.

Engineers designing biological reactors need to take into account all the above considerations in order to achieve optimum performance of a system.

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