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"Plastic media sheets are typically configured in ~60 x ~60 x ~120 cm bundles. Source: Adapted from Metcalf and Eddy (2003).

"Plastic media sheets are typically configured in ~60 x ~60 x ~120 cm bundles. Source: Adapted from Metcalf and Eddy (2003).

portion of bed volume open to liquid and air flow rather than being occupied by the media itself), which subsequently helps to avoid problems with solids clogging that historically tended to affect the older rock media units.

As compared to the original stationary media mode, there have also been a number of recent developments based on rotational or fluidized media movement options. One such version shown in Figure 16.17 is called the rotating biological contactor (RBC). RBC

Figure 16.17 Attached-growth rotating biological contactor system: (a) covered tanks and gantry; (b) interior media view.

Figure 16.17 Attached-growth rotating biological contactor system: (a) covered tanks and gantry; (b) interior media view.

reactors are built as cylindrical media bundles with densely packed, rotating plastic sheets which may either be partially or fully submerged into the waste stream. During rotation of the partially submerged units, the attached biofilm is exposed sequentially to the waste-water and to the air, while submerged units may be operated in an aerated or even an anoxic or anaerobic mode.

Over the past two decades, several new strategies for attached-growth processing have been developed, including two options where the involved biofilms are maintained as suspended, millimeter-scale microbial clusters rather than using a traditional sheetlike configuration. The first such approach is typically aerobic, with small-diameter, inert media particles (typically, plastic or low-density fired-clay beads, sand, or activated carbon granules, etc., in the size range 1 to 4 mm) used as a biofilm carrier, with varied media densities used alternatively to secure either settleable or buoyant behavior. Because of the elevated biomass densities found in the latter options, their elevated oxygen uptake rates will require high-level aeration, such that these operations are commonly referred to as biological aerated filter (BAF) systems. These types of small media biofilm systems are typically operated in an up-flow operating regime, possibly with partial or full fluidi-zation of the media during its operation on either a continuous or an intermittent basis and with intermittent backwashing being used (possibly with coaeration) to flush and extract entrapped solids. There are several advantages with the latter type of BAF reactor. First, the media usually has a cumulative surface area that is far higher than would be attainable had the same bed been packed with standard plastic media, and this increased surface area raises the mass of biofilm able to grow inside the reactor. To some extent the magnitude of this increase is tempered by the fact that physical rubbing of granules against one another keeps the biofilm thinner than what is usually seen in conventional trickling filter systems. However, the cumulative effect of having a denser and more active biomass is that the reactor loadings can be increased dramatically. In fact, volumetric organic loadings several times higher than that of standard fixed-film systems (Rittman and McCarty, 2001) have been reported with many of these newer BAF biofilm systems (e.g., at levels measured in the range 5 to 10 kg CBOD5/day • m3).

Yet another innovative quasi-attached-growth concept uses suspended granular biofilm clusters whose dense, multimillimeter microbial matrix develops in a self-aggregated bead configuration rather than relying on the preceding inert support materials. These granulated biofilm clusters are then suspended in an upflow anaerobic sludge blanket (UASB) process, where incoming soluble COD is rapidly fermented to gaseous methane and carbon dioxide, which then rises and is released from the reactor through an overlying inverted-cone degasification cover. Internal recirculation of the granular biofilm cluster blanket is then achieved by upwelling lift provided via wastewater flow and initial gas buoyancy followed by subsequent downward settling of the denser microbial beads after gas detachment. Here again, these UASB systems carry extremely high granular biofilm densities, to the point where their permissible loadings with industrial-type soluble organics are among the highest possible with wastewater operations, ranging from 12 to 20 kg CBOD5/day • m3 (Metcalf and Eddy, 2003).

Yet another hybrid approach for these biofilm systems couples attached-growth with suspended-growth processes using a trickling filter/solids contactor (TF/SC) scheme. These designs include a solids aeration tank placed between the biofilm tower and clarifier, with settled solids drawn from the clarifier underflow being recycled back to the intermediate solids contactor unit to enhance the uptake and separation of fine particulates. These intermediate tanks are typically not all that large, with HRTs usually below

1 hour, and their suspended solids levels are usually not all that high compared to conventional suspended-growth reactors at 700 to 1500 mg/L, but their ability to facilitate improve bioflocculation elevates the overall efficiency of the TF/SC process beyond that of standard biofilm processes, with expected removal efficiencies for BOD and suspended solids 10 to 15% higher than those of standard biofilm processes.

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