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Figure 21.12 Effect of ammonia-N loading (ANL) on the volumetric ammonia-N oxidation rate in packed bed bioreactors. The data are from ten full-scale nitrification plants. (Erom R. Pujol, M. Hamon, X. Kandel, and H. Lemmel, Biofilters: flexible, reliable biological reactors. Water Science and Technology 29(10/1 l):33-38, 1994. Copyright © Elsevier Science Ltd.; reprinted with permission.)

observed as the TOL was increased, or by the volumetric removal rate expressed in kg/(nr - day). In the last case, a linear increase in the volumetric removal rate would initially be observed as the TOL was increased, much as in Figure 20.4. With further increases in TOL, however, the rate of increase in the volumetric removal rate would decrease until a relatively constant, plateau value was reached.

The total nitrogen loading (TNL) is calculated in the same manner as the TOL. except that the influent total Kjeldahl nitrogen (TKN) concentration is used instead of the influent COD or BOD, concentration:

where AN is the TNL and the sum of SNHO, SNSo. and XNS() is the TKN. Figure 21.10 illustrates the relationship between the TNL and the TKN removal efficiency for the same twelve full-scale facilities considered in Figure 21.9." In this instance, data from periods when the facilities were accomplishing combined carbon oxidation and nitrification are presented. Notice that the TKN removal efficiency is high at low values of the TNL, but begins to deteriorate as the TNL increases. This is exactly as expected when one considers that the TOL was high when the TNL was high. Figure 21.II"1 presents nitrification performance data for another facility accomplishing combined carbon oxidation and nitrification, but in this case ammonia-N removal is presented as a function of the TOL expressed on a COD basis. Both the ammonia-N removal efficiency and the ammonia-N volumetric oxidation (removal) rate are presented. Initially, the volumetric removal rate increases with increasing TOL because as the TOL is increased, so is the TNL. Thus, as long as the TOL is low enough to allow nitrifiers to compete for space in the biofilms, more nitrification will occur as more nitrogen is added. Ultimately, however, as the TOL is increased further the ammonia-N oxidation rate plateaus and declines as the nitrifiers have more dif ficulty existing in the system. The ammonia-N removal efficiency parallels the trends in volumetric removal rate. Note that, for a combined carbon oxidation and nitrification application using a particular wastewater, the TOL and TNL are directly related by the COD/TKN ratio of the influent wastewater. Consequently, only one of the two loading rates needs to be specified to define the performance of the bioreactor.

For tertiary nitrification applications, in which the TOL is low enough to allow full growth of nitrifiers in the biofilm and most nitrogen is in the form of ammonia-N, nitrification performance can be correlated with the ammonia-N loading, TAL:

where .\M, is the TAL. Figure 21.12 " illustrates the relationship between the TAL and the volumetric ammonia-N oxidation rate for ten full-scale packed bed nitrification applications. Most were tertiary nitrification applications. The range of TAL values applied in this instance was relatively limited. As a consequence, a linear increase in the volumetric oxidation rate with an increase in the TAL was observed. If a broader range of TALs had been used, the volumetric oxidation rate would have reached a plateau, just as the rate of nitrification did in RBCs (see Figure 20.5)

The relationship between loading rate and process performance will vary with both the application and the bioreactor type. For example, Pujol, et al. " indicate the following volumetric loading limitations for UFPB bioreactors using expanded clay media: organic matter removal, 10 kg COD/(m'- day); nitrification, 1.5 kg NH,-N/ (m'day); and denitrification using methanol: >4 kg NO,-N/(m'■ day). These are maximum loading rates, and lower values may be required depending on eflluent requirements. By comparison, a loading rate of 6.4 kg NO,-N/(m' • day) or greater can be achieved for denitrification by FBBRs using sand media.' 4:

The TOL concept can also be applied to CSAG bioreactors, but it is less useful than for bioreactors containing only attached biogrowth. This is because some means is required to equate the attached and suspended biomass within a CSAG bioreactor. Side-by-side comparisons between bioreactors with and without attached growth and/ or with different types of media can be made based on the relative TOLs that can be achieved. However, the actual performance is a function of the relative amounts of attached and suspended biomass. When the total biomass concentration and its activity can be quantified, approaches typically applied to suspended growth bioreactors, such as the SRT, may be more useful process loading measures.

21.2.2 Substrate Flux and Surface Loading

Substrate flux and surface loading also influence the performance of a SAGB. Their relevance for characterizing the performance of attached growth bioreactors is discussed in Section 19.2.1 and the relationship between SOL and TOL is presented by Eq. 19.2. Similar arguments and expressions apply for the fluxes and surface loadings of total nitrogen and ammonia-N in nitrifying SAGBs. One of the principal values of fluxes is that they allow comparisons to be made between different types of attached growth bioreactors and media. An example of the use of the ammonia-N flux for this purpose is provided by Figure 21.13 where fluxes for three packed bed

Ammonia-N Cone., mgN/L

Figure 21.13 Effect of ammonia-N concentration on the surface nitrification rate (ammonia-N biofilm flux) in three types of SAGBs accomplishing tertiary nitrification. The range shown is for aeration rates of 15, 20. and 25 m/hr. (From M. Boiler, W. Gujer, and M. Tschui. Parameters affecting nitrifying biofilm reactors. Water Science and Technology 29(10/ 11): 1 — 1 1, 1994. Copyright © Elsevier Science Ltd.; reprinted with permission.)

Ammonia-N Cone., mgN/L

Figure 21.13 Effect of ammonia-N concentration on the surface nitrification rate (ammonia-N biofilm flux) in three types of SAGBs accomplishing tertiary nitrification. The range shown is for aeration rates of 15, 20. and 25 m/hr. (From M. Boiler, W. Gujer, and M. Tschui. Parameters affecting nitrifying biofilm reactors. Water Science and Technology 29(10/ 11): 1 — 1 1, 1994. Copyright © Elsevier Science Ltd.; reprinted with permission.)

bioreactors using three types of media are compared as a function of the residual ammonia-N concentration.' Similar fluxes were obtained in two bioreactors but substantially lower fluxes were observed with the other. Figure 21.14 illustrates the use of a variation on the SOL to correlate the performance of attached biogrowth in CSAG systems."' The curves show the removal of BOD, by vertical plates in an activated sludge bioreactor as a function of the SOL divided by the HRT. In other words, the abscissa is the SOL per unit of residence time. Since no biomass recycle was used in the system from which these curves were obtained, they can be taken as being representative of the amount of the applied loading to a CSAG system that would be removed by the attached biomass. The empirical nature of this relationship is indicative of the fact that the development of these systems is in its infancy.

21.2.3 Total Hydraulic Loading

The THL for an SAGB, also referred to as the superficial velocity, is calculated in exactly the same manner as the THL for a trickling filter, as given by Eqs. 16.8 and 18.24. However, the THL affects the operation and performance of SAGBs in different ways, depending on the bioreactor type and application. For packed bed bioreactors it represents a constraint since an increase in the THL causes increased headloss, more frequent backwashing, and a larger total volume of backwash water. The specific upper limit depends on the media type, the influent suspended solids concentration, and the desired degree of treatment. For example, Pujol, ct al."" have suggested the following maximum THLs for UFPB bioreactors using expanded clay media: organic matter removal, 6 m/hr; tertiary nitrification, 10 m/hr: and denitrifi-cation using methanol, 14 m/hr. In contrast, for FBBRs a minimum upflow superficial velocity must be maintained to achieve the necessary fluidization of the media, as o c a> o it LU

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