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Figure 21.14 Effect of SOL per unit of residence time on the percent of BOD, applied that is removed by attached biomass growing on vertical plate media in CSAG systems '"

discussed in Section 18.2.1. This value is generally on the order of 35 m/hr for typical sand media. The THL is not a factor for CSAG processes.

21.2.4 Solids Retention Time

It is possible to measure the SRT for some attached growth bioreactors, such as FBBRs using sand media. In such instances, the SRT can be used to quantify and control process performance, just as with a suspended growth process, although it must be recognized that the thickness of the biofilm will determine the effectiveness of the biomass as discussed in Section 18.2.3. For an FBBR, samples of bioparticles can be collected and composited to produce a representative sample of the entire bioreactor. The volatile suspended solids (VSS) concentration in this sample can then be measured, and from it, the total biomass inventory determined. The biomass wasted from the system in both the effluent and by operation of the growth control system can also be measured, and the SRT calculated in the same fashion as for a suspended growth system. When this is done, bioreactor performance can be correlated with the SRT, as illustrated in Figure 21.15."" If data on the effect of SRT on effluent quality can be collected for a SAGB, kinetic parameters can be determined for the particular wastewater and bioreactor configuration. It must be borne in mind, however, that the parameters are not intrinsic, but implicitly incorporate the mass transfer characteristics for the particular system. Consequently, they cannot be extrapolated to other systems. Procedures for completing this analysis for FBBRs are

Figure 21.15 Effect of SRT on the total BOD, in the effluent from an FBBR treating automotive metal working wastewater. (From P. M. Sutton, Biological Fluidized Beds for Water and Wastewater Treatment: A User's Forum, Proceedings of the seminar held at the Sheraton University Inn, Ann Arbor, Michigan, P. M. Sutton & Associates, 1990. Reprinted with permission.)

Reactor Solids Retention Time, Days

Figure 21.15 Effect of SRT on the total BOD, in the effluent from an FBBR treating automotive metal working wastewater. (From P. M. Sutton, Biological Fluidized Beds for Water and Wastewater Treatment: A User's Forum, Proceedings of the seminar held at the Sheraton University Inn, Ann Arbor, Michigan, P. M. Sutton & Associates, 1990. Reprinted with permission.)

conceptually the same as those used for suspended growth bioreactors, discussed in Chapter 8.

The SRT can also be used to characterize the performance of CSAG processes. Two values of the SRT can be calculated, one based only on the suspended biomass (suspended growth SRT) and the other based on the suspended plus the attached biomass (total SRT). It must be recognized, however, that the nature of the suspended and attached biomass may be different and, consequently, they may not be easily equated. Experience with CSAG systems indicates that the presence of the attached biomass allows them to achieve effective nitrification with suspended growth SRTs equal to or less than the minimum SRT for the growth of nitrifying bacteria.'117:7 The suspended biomass is predominantly heterotrophic and maintains a low bulk liquid phase organic substrate concentration. Since the suspended growth SRT is low, nitrifiers cannot grow in suspension, but they can form effective biofilms because the low organic substrate concentration allows them to compete effectively for space in the biofilm. Consequently, the biofilm provides the nitrifiers required to achieve process stability and to allow the system to successfully process peak ammonia-N

loadings. Experience also indicates that significant denitrification can occur in the biofilm of such systems.

21.2.5 Hydraulic Residence Time

In general, the performance of an attached growth process is not affected by the HRT, as long as the TOL and THL are satisfactory. This is because the amount of biomass in the system is determined by the surface area of media provided, the TOL, and other operating factors, not the length of time that the flow remains in the bioreactor. However, if the HRT exceeds the minimum SRT for biomass growth on the substrate provided, biomass will grow in suspension, which is undesirable in packed beds and FBBRs because the suspended growth will pass into the effluent, increasing its organic matter content and reducing effluent quality. The growth of suspended biomass will also interfere with biofilm development because it reduces the amount of substrate available to the attached biomass. This can be particularly troublesome during startup. This is because low organic loading rates may be needed initially to avoid process overloading as the biofilm develops. If the low TOL is achieved by applying only a portion of the wastewater flow, the resulting HRT may allow significant suspended growth to develop, hampering biofilm development and slowing startup. In such cases, the wastewater should be diluted to allow operation at a reduced TOL while maintaining a short HRT to avoid the growth of suspended biomass.

21.2.6 Other Factors

Like other biological processes, the performance of SAGBs is influenced by a variety of environmental conditions, such as temperature, pH, and dissolved oxygen (DO) concentration. As with trickling filters and RBCs, the importance of mass transport into the biofilms means that the impact of temperature is less significant than for suspended growth systems, until it is sufficiently low (~15°C) to make the biological reaction rates controlling. Relatively little information is available in the literature on the effects of temperature on substrate removal rates, but they can generally be assumed to be similar to those observed for RBCs (see Section 20.2.4).

When considering other environmental factors, it is important to recognize that the conditions inside the biofilm will be quite different from those in the bulk fluid. The concentrations of electron donors and electron acceptors will change with depth, as illustrated in Figure 15.19. This means that denitrification can occur in the biofilm even though DO may be present in the bulk liquid. The only way to fully analyze these effects is through the use of generalized models, such as those discussed in Part IV, but such models are not yet commonly used in design. Consequent!), care must be exercised to ensure that the imposed operational conditions will achieve the desired result. This can be done most effectively with pilot studies.

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