B

Figure 13.8 Systems for degassifying anaerobic mixed liquor before sedimentation: A cascade; B. (low-through tank; C. thin-film/trickle-film.

the range of HRTs associated with this range of SRTs is dependent on the strength of the wastewater and the concentration of active biosolids that can be attained in the bioreactor. Bioreactor suspended solids concentrations may range from 4 to 6 g/L as VSS to as high as 25 to 30 g/L as VSS, depending on the settleability of the solids that develop. The lower range of concentrations represents typical operation. Clarifier hydraulic loading rates on the order of 5 to 6 m/day are often used, with a solids recycle rate equal to the influent flow rate. Volumetric organic loading (VOL) rates often range between 0.5 and 10 kg COD/(m'-day).

Upjlow Anaerobic Sludge Blanket. The UASB process uses suspended growth biomass, but the gas-liquid-solids separation system is integral with the bioreactor. More importantly, the environmental conditions created in the bioreactor can result in the development of large, dense, readily settleable particles called granules, which allow very high concentrations of suspended solids, on the order of 20 to 30 g/L as VSS, to be accumulated. '' " These high suspended solids concentrations allow significant separation between the SRT and HRT, and operation at relatively short HRTs, often on the order of two days or less. Figure 13.9 provides a schematic of the process.

Influent wastewater enters the bottom of the bioreactor through a distribution system that is designed to provide relatively uniform flow across its cross section. A dense slurry of granules forms in the lower portion of the bioreactor, and the combined effects of the influent wastewater distribution and gas production result in mixing of the influent wastewater with the granules. Treatment occurs within the dense blanket of granules. For some wastewaters, a much less dense flocculent sludge also develops, and this accumulates on top of the blanket of granules. Other wastewaters contain suspended solids that are not trapped in the granular sludge, and these solids also accumulate as a flocculent sludge blanket overlying the granules. Treated effluent exits the granular and flocculent sludge zones and flows upward into the gas-liquid-solids separator. A variety of configurations can be used for this device, and the one illustrated in Figure 13.9 is only meant to represent the basic concepts used by several manufacturers. The device often consists of a gas collection hood

with a settler section above it. Gas bubbles cause some granular and flocculent solids (particularly small granules) to rise through the bioreactor and enter the gas-liquid-solids separator. Gas separation occurs in the hood area, thereby allowing some of this suspended material to return directly to the solids blanket. Gas collects in the upper inverted V section of the hood and is removed from the bioreactor. Liquid with some entrained solids flows out of the hood into the settler section where liquid-solids separation occurs. Clarifier effluent overflows the weirs and is discharged while separated solids settle back into the reaction zone. Design of the gas-liquid-solids separation device requires insight into the physical processes occurring there and experience with specific devices in a variety of applications.

Bioreactor dimensions are affected by process loadings, constraints on maximum upflow velocities, wastewater type, and the settling characteristics of the solids that develop in the process."14 "" The solids inventory increases as treatment occurs and new biomass is grown. Consequently, provisions must be made for solids wastage. as illustrated in Figure 13.9. The relative proportions of flocculent and granular sludge can be controlled by the wasting locations used. Bioreactor HRTs in the 0.2 to 2 day range are typical, along with VOL rates of 2 to 25 kg COD/(nr • day), depending on wastewater characteristics and whether granular or flocculent solids develop.

Anaerobic Filter. Anaerobic filter systems use upflow bioreactors that are filled with media. The packing is the same as that used with aerobic plastic media trickling filters (TFs), discussed in Chapter 19. Example media are illustrated in Figure 13.10; the specific surface area is typically 100 nr/m' with a void volume of 90 to 95%. The presence of packing allows for the growth of some attached biomass, but the primary role of the media is to retain suspended growth.:'",s" The media may be thought of as performing like a set of tube settlers, which provide enhanced liquid-solids separation and retention of suspended biomass within the bioreactor. Gas-solids separation is also facilitated within the packed section. Although several types have been used successfully in AF systems, direct comparisons indicate ad-

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.)

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