Figure 13.10 Typical media used in anaerobic filters: A. crosstlow; B. tubular; C. pall rings. (From J. C. Young. Factors affecting the design and performance of upflow anaerobic filters. Water Science and Technology- 24(8):133-156, 1991. Copyright © Elsevier Science Ltd.; reprinted with permission.)

vantages for the crossflow modular media because of its superior gas-liquid-solids separation capabilities.'1's"

Figure 13.11 provides a schematic of the overall AF process. Influent wastewater and recirculated effluent are distributed across the bioreactor cross-section and flow upward through the media. Treatment occurs as a result of the suspended and fixed biomass retained by the media. Effluent exits the top of the media section and is collected for discharge. Gas is collected under the bioreactor cover and is conveyed to subsequent use. Effluent is typically recirculated to maintain a reasonably uniform hydraulic loading on the bioreactor in spite of varying influent flow rates, thereby maintaining uniform bioreactor hydrodynamic conditions. Although performance is determined by the SRT maintained, accurate assessment of the bioreactor suspended solids inventory is not generally possible. Consequently, bioreactor designs are based on the HRTs and VOLs used successfully in other applications. Hydraulic retention times between 0.5 and 4 days are typical, along with VOLs in the 5 to 15 kg COD/ (m'-day) range. The biomass inventory is typically controlled by the hydrodynamic conditions that develop in the media as a result of the influent (wastewater plus recirculation) flow applied. Excess biomass is washed out of the system as it develops




Influent t _i

Figure 13.11 Anaerobic filter.

and becomes a part of the effluent. In some instances the capability to remove settled solids from the bioreactor bottom may be provided since heavy solids and precipitates can accumulate there. Solids removal from this location does not constitute an SRT control mechanism, however, because most of the active biomass is retained within the media section.

Hybrid Upflow Anaerobic Sludge Blanket/Anaerobic Filter. Hybrid UASB/AF systems combine aspects of the UASB process with aspects of the AF process."1 As illustrated in Figure 13.12, influent wastewater and recirculated effluent are distributed across the bioreactor cross-section and flow upward through granular and floc-culent sludge blankets where anaerobic treatment occurs. The effluent from the sludge blanket zone then enters a section of media identical to that used in AF systems where gas-liquid-solids separation occurs. Treated effluent then exits the media section and is collected for discharge from the bioreactor. Gas collects under the bioreactor cover and is transported to storage and/or use. The hybrid UASB/AF process primarily uses suspended biomass, and process loadings are similar to those used with the UASB process. The solids removal system is similar to that used with the UASB process.

Downflow Stationary Fixed Film. Downflow stationary fixed film systems use media just like an AF system, but flow is in the downward rather than the upward direction.2114 As a consequence, suspended biomass tends to be conveyed through the media rather than retained by it, and the process depends to a large extent on attached rather than suspended biomass. As illustrated in Figure 13.13, influent wastewater and recirculated effluent are distributed evenly across the bioreactor and flow downward through the media. Biomass growing on the media surface and that portion of the suspended biomass that is trapped within the media accomplish treatment of the applied wastewater. Solids removal systems are not generally provided because of the reduced importance of suspended biomass. However, the capability to remove heavy solids from the bottom of the DSFF bioreactor may be provided, just as with the AF process, but such provisions may be less useful as effluent is already being removed from this section of the bioreactor. Treated effluent exits the media zone and is collected for recirculation and discharge. Gas produced in the media flows upward and is collected under the bioreactor cover for transport to

Figure 13.12 Hybrid USAB/AF process.

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