As shown in Table 1.2, one type of attached growth bioreactor is the rotating disc reactor (RDR). In it, closely spaced discs are mounted on a common horizontal shaft placed very near to or touching the liquid surface in a long narrow tank. The shaft is rotated at constant speed, thereby allowing any point on a disc to be alternately submerged and exposed to the atmosphere. When water containing organic matter, nitrogen, and other nutrients flows through the bioreactor, microorganisms consume the substrates and grow attached to the disc as a biofilm. The rotating action imparts a shear force to the biofilm, keeping its thickness relatively constant by removing the cells generated by consumption of the substrate. The turbulence generated by the rotation transfers oxygen to the bulk liquid and keeps the sloughed microorganisms in suspension so they can be carried out in the effluent. The most common arrangement of the discs is with the shaft perpendicular to the direction of liquid flow, as shown in Figure 17.1. Under those circumstances, the turbulence is sufficient to make the substrate concentration uniform throughout the tank. In other words, for all practical purposes the tank can be considered to be completely mixed and can be modeled as such. We saw in Chapter 7 that bioreactors arranged in series perform better than a single bioreactor of similar total volume. Because of this and because of the modular nature of RDRs, most applications use a series of bioreactors. Consequently, the performance of an RDR system can be modeled as a series of continuous stirred tank reactors (CSTRs) containing biofilms.
Just as in a packed tower, when the concentration of organic matter is high, nitrifying bacteria are unable to compete with the heterotrophic bacteria for space in the biofilm and thus the main reaction is carbon oxidation. However, once the concentration of organic matter has been reduced, nitrifiers can compete effectively for space and nitrification becomes significant. As a consequence, in systems containing a series of RDRs, carbon oxidation is the predominant reaction in the first few bioreactors while nitrification is more important in the later stages. In some cases, wastewater from which the organic matter has been removed in an upstream biochemical operation is applied to an RDR specifically for the oxidation of ammonia-N to nitrate-N, in which case the biofilm is composed almost entirely of nitrifying bacteria. Regardless of the nature of the reactions, however, the configuration of the RDR gives it characteristics that differ from that of a packed tower. In order to gain an appreciation for the differences and similarities of the two attached growth bioreactor types we will consider only growth of heterotrophic bacteria with a single limiting organic substrate using the model of Grady and Lim/ The reader should recognize, however, that nitrifying biofilms will behave in a similar manner.
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