Process Design

21.3.1 General Design Procedures

The design of SAGBs is largely empirical in nature. Loading versus performance correlations, such as those illustrated in Figures 21.9 through 21.14 are used to select a volumetric loading (TOL, Eq. 19.1; TNL, Eq. 21.1; or TAL, Eq. 21.2) or a surface loading (SOL, SNL, or SAL), thereby providing the required bioreactor media volume. The basic relationship between volumetric loading and surface loading is given by Eq. 19.2. The bioreactor configuration is then selected based on minimum and/ or maximum hydraulic loading rates and typical media bed depths. This procedure is similar to that used in the design of trickling filters (see Section 19.3). For packed bed bioreactors one constraint is the maximum THL since excessive headloss will occur if the bioreactor is operated at higher THLs. This sets the minimum cross-sectional area and the maximum media depth. For FBBRs a minimum THL must be maintained to fluidize the bed while a maximum THL must not be exceeded to prevent media loss. When coupled with recirculation requirements, these set the maximum and minimum cross-sectional areas, respectively. Procedures for determining the minimum and maximum THLs for FBBRs are presented in Section 18.2. For CSAG systems, the relative amounts of suspended and attached growth must be determined. This determination is generally based on experience with the specific system and application. Volume requirements are then determined in the same manner as for an activated sludge system, as discussed in Section 10.3.

Excess biomass production rates and heterotrophic oxygen requirements are calculated based on net process yields, Y,„ and process oxygen stoichiometric coefficients, Y()j, just as is done in Eqs. 9.3 and 9.4 for the preliminary design of activated sludge systems. However, in some cases significant organic matter will be in the process effluent, requiring those equations to be corrected for its presence. In addition, if nitrification is occurring, the oxygen requirement for nitrification, ROA, is calculated with a modified form of Eq. 10.16. Since the observed yield of nitrifiers is quite low, the right term within the brackets of that equation is typically ignored, allowing it to be simplified to:

The concentration of nitrogen available to the nitrifiers, SNa, is calculated with Eq. 10.17. Use of that equation requires knowledge of the heterotrophic nitrogen requirement, NR, which for activated sludge systems is calculated with Eq. 5.36. However, in this case, because only the net process yield, Y„, is known, NR can be estimated with:

Some denitrification applications require the addition of an electron donor, such as, methanol. For these systems, the concentration of methanol, S^.on, that must be added to remove the total amount of electron acceptor present is calculated from basic stoichiometry:

SMt()H = 2.47SN(K) + 1.53Sn„2„ + 0.87Soo (21.5)

where SNO(, is the influent nitrate-N concentration in mg/L, SN,,:() is the influent nitrite-N concentration in mg/L, and S,K, is the influent DO concentration in mg/L. The stoichiometric coefficients in Eq. 21.5 are based upon the net biomass yield for growth on methanol as the electron donor and carbon source. They can be calculated using the procedures presented in Chapters 3, 5, and 6. When exact information on the nature of the influent is not available, a methanol dose of 3 mg MeOH/mg NO^-N is typically used, where NO,-N represents the sum of the influent nitrate-N

and nitrite-N concentrations. The design of DFPB bioreactors must also accommodate frequent air purge cycles to release nitrogen gas that accumulates in the bio-reactor. Procedures for accomplishing these computations are presented elsewhere.'

Bioreactor geometry is an important factor for several of the SAGBs and the resulting physical constraints must be considered during design. For example, packed and fluidized bed bioreactors require even flow distribution over the entire cross-sectional area, which is accomplished by nozzles located in either the influent or effluent regions. The physical constraints in accomplishing this distribution are very much like those involved in the design of a granular media filter. One result is a maximum allowable cross-sectional area corresponding to the area over which adequate flow distribution can be achieved. The depth of an aerobic packed bed bioreactor must be adequate to allow the aeration system to meet the oxygen requirements. Furthermore, the depths of both packed and fluidized bed bioreactors must also be sufficient to produce the required effluent quality, which they should do if the loading was appropriately chosen. Ongoing developments by system developers and suppliers are refining these restrictions.1" Bioreactor geometry is generally less restrictive for the CSAG systems.

0 0

Post a comment