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Organic loading

Projected volumetric organic loading:

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Although this determination was simplistically developed for a single tower, multiple towers are typically designed by engineers to provide beneficial system redundancy. In similar fashion, while this design estimate was based on an average daily flow, engineering designs are typically based on higher likely flows, such as the facility's daily peak flow, in order to physically accommodate these types of hydraulic transient events.

The third and fourth design factors listed in Table 16.2, which are somewhat secodary in importance relative to the preceding loading terms, refer to the typical depths and levels of recirculation provided with these fixed-film systems. Older, rock-media designs were seldom built with media depths beyond 2 to 3 m, but many newer, plastic-media units are considerably taller (with depths up to 10+ m) and are designed to handle much higher loads. As for the notion of incorporating a recycle stream, this capability is considered necessary with influent flows that tend to have extreme lows (e.g., smaller towns, daytime-only industrial operations), such that the biofilm can be kept wetted during periods with influent lulls rather than having it dry out.

Extending beyond these heuristic design criteria, there are also a number of empirical models used to qualify the reaction-rate kinetics and expected reactor performance levels with attached-growth reactors. These models, typically named in honor of the responsible author or group, include the following variations: Eckenfelder, Rose, NRC, Howland, Schulze, Germain, Velz, and modified-Velz. Each of these models is considered to have its own merits, but the modified-Velz model, given in equation (16.1), is one of the more widely used strategies. As is the case with many of these competing models, the modified-Velz model follows a first-order format:

Cout exp(fcDQr-20) THLn

where

Cout = outlet substrate concentration (mg/L) Cin = inlet substrate concentration (mg/L) k = empirical reaction-rate constant

Substrate (CBOD5) Concentration (mg/L) 20 40 <50 80 100 120

Substrate (CBOD5) Concentration (mg/L) 20 40 <50 80 100 120

Figure 16.10 Representative pattern of exponential substrate removal relative to attached-growth biotower depth.

D = media depth (m) qt-20 = temperature correction factor (relative to 20°C) THL = hydraulic loading (based on influent plus recycle flow) n = empirical loading constant

Figure 16.10 accordingly presents a representative substrate removal profile derived with this model, as might be derived for a particular range of media heights relative to a given set of constant values for k, n, and O, In turn, this type of model does provide an informative characterization of biofilm performance, at least on a qualitative basis. However, on closer review, it would also appear that these types of models bear an inherent degree of fuzziness on several important issues.

The parameters of the modified Velz equation are sometimes reported with high precision (e.g., n = 0.4332; CH2M-Hill, 2000). However, it should be recognized that these types of documented constants reflect cumulative, averaged values derived from real-world biofilms whose actual site- and point-specific metabolic activities undoubtedly vary to a significant degree, depending on time, location, season, and so on. Indeed, these constants could well vary from reactor top vs. reactor bottom locations, and even from one substrata depth within a biofilm to another.

Yet another level of "fuzziness" exists with the total hydraulic loading rate (THL). The THL depends on influent flow and cross-sectional media area, but it does not actually account for the manner in which the influent is applied. For example, two parallel systems of equal design, construction, and hydraulic loading could have identical THLs, yet have sizably different levels of performance. In this case, the issue of "how" the influent is applied represents an important factor, as opposed to simply "how much," relative to both the rate of movement of the rotating influent distributor and the corresponding pulsing of flow across the media. With each successive radial pass, this rotating arm distributes a liquid stream whose downward passage across the media appears as a moving wave whose cyclic repetition follows that of the arm's travel speed. This pulsing and wavelike movement of liquid into and across the depth of the media would have an amplitude (i.e., height of the wave) that increased as the rotating arm was slowed. In turn, higheramplitude waves would impose higher levels of physical shear and drag against the face of the biofilm, to an extent that would probably yield thinner attached-growth depths.

Undoubtedly, though, one of our largest levels of uncertainty with understanding attached-growth systems is our appreciation of the biofilm complexity in terms of biological makeup, physical conformation, and related metabolic behavior. Attached-growth system models akin to the modified-Velz equation implicitly suggest that biofilms are built and behave as homogeneous surfaces, with uniform levels of depth, kinetic activity, and structural composition, but this is hardly the case. Instead, biofilms comprise a highly nonhomogeneous, three-dimensional matrix with a decidedly complex physical structure. In short, attached-growth biofilms are configured as a densely arrayed microcosm of life, composed largely of bacteria but with several higher life-forms, including protozoans, rotifers, and nematodes, and possibly even fungus and insect forms as well.

This complexity within biofilm structures starts at an early stage during an initial period of startup and adhesion, where a preliminary layering of bacterial cells (i.e., a mono-layer) binds to a clean media surface within a relatively short period of time (i.e., minutes to hours). This adhesive phenomenon stems from the presence of a sticky coating of exo-cellular polymeric materials secreted by bacterial cells growing under conditions of limited substrate availability, thereby allowing them to bind not only to media surfaces but also to other cells. In time, this monolayer coverage subsequently extends in thickness, progressively melding new and old cells as well as various enmeshed inorganic precipitates and particulate solids, to a point where the monolayer eventually reaches depths ranging from a few hundred to several thousand micrometers.

An apt analogy for mature biofilm growth is that of a forest canopy, with overarching branched limbs (newly grown cells) stretched across open glades (internal cavities gouged out by ongoing microbial sloughing and fluid shear) and underlying brush (deeper deposits of dead or inert cells). However, even then, this canopy will be apt to collect an added assortment of intertwined solids and cells, including a variety of higher life-forms living within the biofilm as well as entrapped wastewater solids. Given the chemical complexity of this film, yet another group of enmeshed, precipitated solids could well be wrapped into this matrix, including such materials as sulfide-, hydroxide-, phosphate-, or carbonate-based precipitates of iron, manganese, calcium, magnesium, and aluminum.

Of course, commensurate with this increased microbial depth, substrate and product transport through the film also becomes considerably more constrained, to a point where most biofilms eventually take on a vertically stratified layering. For those microorganisms living on the outer, aerobic edge of this biofilm (depicted schematically in Figure 16.11), their proximity to energy-rich substrates, nutrients, and oxygen received from the overlying bulk solution will allow them to reach the highest rates of aerobic metabolic activity. Figures 16.6 and 16.7 depict the type of bacterial growth found at this topmost, aerobic layer, including the considerable presence of filamentous forms, which give the biofilm its inherent outer roughness.

In particular, Figures 16.12 and 16.13 depict progressive enlargements of an extensive overlying growth of a filamentous Beggiatoa-type growth. Organisms such as this are often found living at this top-level biofilm region, catabolically using sulfide diffusing to the surface from the underlying anoxic-anaerobic (i.e., sulfate-reducing) zone. Strictly aerobic higher life-forms (e.g., most protozoans, rotifers, worms) are also commonly seen in this outer region, grazing steadily through this relatively active region.

Moving downward into the biofilm, however, metabolic uptake will progressively deplete the available substrates and nutrients, imposing metabolic limitations that progressively retard the activity of these lower cells. Although no exact threshold can be predicted for the point at which the oxygen will be exhausted, interstitial measurements taken with microscale probes suggest that this point will occur roughly in the neighborhood of 150 mm.

Figure 16.12 Highly filamentous biofilm morphology at top surface (~800x).
Figure 16.13 Biofilm surface close-up with dense Beggiatoa-type filaments (~2000x).

Moving deeper into the biofilm, therefore, the aerobic metabolic viability of the cells will decline as a direct consequence of diminished substrate and oxygen availability. Oxygen, in particular, declines steadily in availability as depth increases, to the point where it is depleted and anaerobic conditions begin. One such zone of transition from an overlying aerobic top layer to an underlying anoxic and/or anaerobic zone is depicted in Figures 16.14 and 16.15.

With oxygen reaching limiting conditions within the subsurface biofilm strata, anoxic respiration will accordingly deplete whatever alternative electron acceptors (i.e., nitrates, nitrites, etc.) might be present, eventually leading to sulfate reduction and a metabolic release of sulfides. Tucked just below the overlying Beggiatoa filaments shown in Figures 16.14 and 16.15 is a distinctly different type of bacteria, Desulfovibrio, which

Figure 16.14 Biofilm transition zone between aerobic top and anaerobic bottom (~2000~).
Figure 16.15 Biofilm transition zone (aerobic to anaerobic) close-up (~4000x).

is responsible for this sulfate reduction. These lower cells are more tightly packed and have a characteristic comma-shaped curvature. The last photograph in this series, shown in Figure 16.16, provides a close-up look at this group of cells.

The sulfides produced at this lower level will often scavenge and precipitate a variety of metals (probably dominated by iron sulfide, FeS, but undoubtedly including many other metal species), leading to a sort of cementation behavior that further inhibits chemical transport at these lower depths. Whereas optimal activity would be achieved with limited biofilm depths, therefore, the aging process experienced with biofilms leads to progressive metabolic constraints at the lower depths. Buried at a depth that limits their access to key

Figure 16.16 Biofilm anaerobic bottom zone close-up with Desulfovibrio-type bacteria (~4000x).

substrates and nutrients, as well as hampered by transport problems brought about by entrapped and precipitated solids, the underlying cells must inevitably shift to anaerobic (or at least anoxic) life-styles.

However, when viewed in terms of microbial diversity and potential treatment efficacy, the circumstance of having adjacently layered aerobic and anaerobic strata within a biofilm does appear to provide a potentially beneficial arrangement. Indeed, these strata could well complement one another, such as a reductive transformation of what might otherwise comprise recalcitrant organics (e.g., chlorophenolics, nitrophenolics, aniline dyes) within the anaerobic sublayer, followed by a concluding oxidation within the overlying aerobic layer.

At least in theory, it would also appear that it might hypothetically be possible to develop a beneficially layered sequence of nitrifying and denitrifying biofilms, such that nitrates produced in the oxidative overlying strata would then be directly inside reduced the lower anoxic-anaerobic depth. However, this conceptual scheme would be difficult to attain since the slow-growing nitrifiers are prone to overgrowth and submergence by faster multiplying heterotrophs, at which point competition for oxygen and other essential nutrients severely constrains their viability. As a result, nitrifying biofilm systems designed for oxidation of wastewater ammonia-nitrogen are usually configured as stand-alone processes, supplied with a pretreated wastewater whose biodegradable organic levels available for heterotrophic growth have been largely removed, and provided with specially designed media offering a far higher specific surface area more conducive to these slow-growing lithotrophs.

Once these attached-growth systems have been placed into use, though, and their mul-tilayered biofilm configurations have been established, operating personnel have surprisingly few monitoring and control options with which they might try to improve or optimize performance. Merely measuring the attached mass or depth of biofilm within a reactor is quite difficult, although in rather rare instances, provisions have been made to track biofilm mass using removable sample coupons removed from inside the reactor.

Other than shifting reactor loading patterns by taking biofilm towers off- or online, the most significant control factor is probably that of altering the rotation speed for the distributors. In recent years, there has been a distinct shift to slow down the rotational speed of these distributing arms, dropping the number of rotations per minute (rpm) from values of 1 to 2 by a factor of 10 or more (i.e., to 0.1 or even lower rpm rates). Slowing down these distributing arms while maintaining the same influent waste flow means that the hydraulic loadings applied at the point of release are considerably higher. This increase subsequently raises the level of hydraulic shear as it moves downward across the face of the biofilm, and the added shear helps to keep the biofilm thinner and more active than would be the case with a much slower-moving water film.

Notwithstanding these operational constraints with control and monitoring, though, several significant developments have occurred with attached-growth media materials and configurations over the past several decades. In the realm of media options, rock media largely gave way to lighter plastic media with far higher specific surface areas midway through the twentieth century, and today's vendor options include a wide range of modular and random-packed versions. Table 16.3 provides specific technical details regarding several of these newer media forms compared against the older rock option. These newer plastic media provide several times more surface area per unit volume, which in turn yields considerable improvements in biofilm growth and loading capacities. In addition, plastic media are distinctly lighter and have far higher void fractions (i.e., the

TABLE 16.3 Attached-Growth Media Options

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