The importance of classifying biochemical operations according to bioreactor type follows from the fact that the completeness of a given biochemical transformation will be strongly influenced by the physical configuration of the bioreactor in which it is being carried out. Therefore, it is important to get a clear picture of the many bioreactor types available.
Wastewater treatment bioreactors fall into two major categories, depending on the way in which microorganisms grow in them: suspended in the liquid under treatment or attached to a solid support. When suspended growth cultures are used, mixing is required to keep the biomass in suspension, and some form of physical unit operation, such as sedimentation, is used to remove the biomass from the treated effluent prior to discharge. In contrast, attached growth cultures grow as a biofilm on a solid support and the liquid being treated flows past them. However, because organisms can slough from the support, a physical unit operation is usually required before the treated effluent may be discharged.
Suspended Growth Bioreactors. The simplest possible continuous flow suspended growth bioreactor is the continuous stirred tank reactor (CSTR), which consists of a well mixed vessel with a pollutant-rich influent stream and a treated effluent stream containing microorganisms. The liquid volume is constant and the mixing is sufficient to make the concentrations of all constituents uniform throughout the reactor and equal to the concentrations in the effluent. Consequently, these reactors are also called completely mixed reactors. The uniform conditions maintain the biomass in a constant average physiological state. Considerable operational flexibility may be gained by the addition of a physical unit operation, such as a sedimentation basin, which captures the biomass, as shown in Figure 1.1. As discussed previously, the overflow from the sedimentation basin is relatively free of biomass, while the underflow contains a concentrated slurry. Most of that concentrated slurry is recycled to the bioreactor, but a portion is wasted. Because the wasted biomass is organic, it must be treated in an appropriate process before release to the environment.
Connecting several CSTRs in series offers additional flexibility as feed may be added to any or all of them. Furthermore, biomass recycle may be employed about the entire chain or any portion of it. The behavior of such systems is complex because the physiological state of the biomass changes as it passes from bioreactor to bioreactor. Nevertheless, many common wastewater treatment systems use bioreactors with split influent and recycle streams. One advantage of multistage systems is that different environments may be imposed upon different stages, thereby allowing multiple objectives to be accomplished. This is very common in biological nutrient removal processes.
A batch reactor is a completely mixed reactor without continuous flow through it. Instead, a "batch" of material is placed into the vessel with the appropriate biomass and allowed to react to completion as the microorganisms grow on the pollutants present. As growth proceeds, reaction conditions change and consequently, so does the growth environment. Batch processes can be very flexible and are particularly well suited for situations with low or highly variable flows. Furthermore, by changing the nature of the electron acceptor temporally, it is also possible to accomplish nutrient removal in a single bioreactor. Because their operation follows a sequence of events, they are commonly called sequencing batch reactors (SBRs).
A perfect plug-flow reactor (PFR) is one in which fluid elements move through in the same order that they enter, without intermixing. Thus, the perfect PFR and the CSTR represent the two extreme ends of the continuum representing all possible degrees of mixing. Because of the lack of intermixing, perfect PFRs may be considered to contain an infinite number of moving batch cultures wherein changes occur spatially as well as temporally. Both, however, cause the biomass to go through cycles of physiological change that can have strong impacts on both community structure and activity. Because perfect PFRs are difficult to achieve in practice, plug-flow conditions are generally approximated with a number of CSTRs in series. In Chapter 4, we will examine ways of characterizing the mixing conditions in suspended growth bioreactors.
Attached Growth Bioreactors. There are three major types of attached growth bioreactors: (1) packed towers, (2) rotating discs, and (3) fluidized beds. The microorganisms in a packed tower grow as a film on an immobile support, such as plastic media. In aerobic bioreactors, the wastewater flows down the media in a thin film. If no recirculation of effluent is practiced, there is considerable change in reaction environment from top to bottom of the tower as the bacteria remove the pollutants. Recirculation of effluent tends to reduce the severity of that change, and the larger the recirculation flow, the more homogeneous the environment becomes. The performance of this bioreactor type is strongly influenced by the manner in which effluent is recirculated. Organisms are continually sloughed from the support surface as a result of fluid shear. If they are removed from the effluent prior to recirculation, pollutant removal is caused primarily by the activity of the attached biomass. On the other hand, if flow is recirculated prior to the removal of the sloughed-off microorganisms, the fluid stream will resemble that of a suspended growth bioreactor and pollutant removal will be by both attached and suspended biomass. In anaerobic packed towers, the media is submerged and flow may be either upward or downward.
Microorganisms in a rotating disc reactor (RDR) grow attached to plastic discs that are rotated in the liquid. In most situations, the horizontal shaft on which the discs are mounted is oriented perpendicularly to the direction of flow and several reactors in series are used to achieve the desired effluent quality. Consequently, environmental conditions are uniform within a given reactor, but change from reactor to reactor down the chain. This means that both the microbial community structure and the physiological state change from reactor to reactor.
In fluidized bed bioreactors (FBBRs), the microorganisms grow attached to small particles, such as sand grains, which are maintained in a fluidized state by the upward velocity of the wastewater undergoing treatment. The effluent from such bioreactors generally contains little suspended biomass, but particles must be continually removed and cleaned to maintain a constant mass of microorganisms in the system. The cleaned particles are continually returned to the bioreactor while the wasted biomass is sent to an appropriate treatment process. Recirculation of effluent around the bioreactor is usually needed to achieve the required fluidization velocity and thus the system tends to behave as if it were completely mixed.
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