Afbr

The AFBR is an expanded-bed reactor that retains media in suspension from drag forces exerted by upflowing wastewater. Part D in Figure 7.28.2 shows the process schematic. Fluidization of the media particles provides a large surface area where biofilm formation and growth can occur.

The media particles have a high density resulting in a settling velocity that is high enough so that high-liquid-velocity conditions can be maintained in the reactor. However, the media particles' overall density decreases as biomass growth accumulates on the surface area. The decrease in density can cause the bioparticles to rise and be washed out of the reactor. To prevent this situation, the reactor controls fluidized-bed height at a required level by wasting a corresponding amount of overgrown bioparti-cles. The wasted bioparticles can then be received by a mechanical device that separates the biomass from the wasted media particles. The cleaned particles can then be returned to the reactor, while the separated biomass is wasted as sludge.

The AFBR combines a suspended-growth system and an attached-growth system since biomass growth attaches to the media particles which are suspended in the waste-water. The reactor recycles a portion of the effluent flow ensuring uniform bed fluidization and sufficient substrate loading.

An advantage of the AFBR is that it employs small flu-idized media that provide a high biomass holdup in the reactor, reducing hydraulic retention time. The AFBR also prevents bed-clogging and high-pressure drops—complications associated with anaerobic filters. Due to the flexibility provided by bed-height control in an AFBR, a constant biomass concentration can be maintained in the reactor independent of substrate loadings (Shieh and Keenan 1986). Another advantage of the AFBR is that it is insensitive to variations in influent pH, temperature, and waste loading because it maintains a high biomass holdup and completely mixed conditions inside the reactor.

Some commercially available AFBR processes include the ANITRON system developed by Dorr-Oliver, Inc.; the BIOJET process, which employs an AFBR with an enlarged top section; and the ENSO-FENOX process, which combines an AFBR with a trickling filter. The AFBR has been applied to a variety of industrial treatment processes with substrates such as molasses, synthetic sucrose, sweet whey, whey permeate, glucose, and acid whey.

Since high circulation rates are used in AFBR operation (especially for treating high-strength industrial wastewater) the reactor can be designed as a completely mixed, het-erogenous process. Because most of the active biomass is retained in the fluidized-bed, the contribution of suspended biomass growth to overall reactor performance is insignificant. Thus, the first term in Equation 7.28(1) can be removed, and the remaining expression calculates the overall substrate utilization rate in an AFBR as follows:

Environmental engineers can use Equations 7.28(3) through 7.28(6) to determine the effectiveness factor associated with an AFBR in a similar manner as for the anaerobic filter. To determine the reactor biomass concentration (X), they can use solid-liquid fluidization correlations. These correlations link the particle concentration in the fluidized state to the measurable physical characteristics of a fluidized-bed process. The following Richardson-Zaki correlation is widely used:

where:

U = superficial upflow velocity of the wastewater through an AFBR, distance/time Ut = bioparticle terminal settling velocity, distance/time

= bed porosity n = the expansion index

Shieh and Chen (1984) propose two empirical correlations relating Ut and n to the Galileo number (NGa) that defines the physical characteristics of an AFBR as follows:

where:

Pp = bioparticle density, mass/volume p = wastewater density, mass/volume g = gravitational acceleration, distance/time2

[i = wastewater dynamic viscosity, mass/time-distance pm = media density, mass/volume

P = biofilm moisture content rm = support media radius, length rp = bioparticle radius, length

The following equation calculates the AFBR biomass concentration:

The choice for media types should be based on the following media characteristics:

• A large surface area for microbial growth

• A large void space to accommodate the accumulation of biological and inert solids and minimize short-circuiting

• Inertness to biological and chemical reactions

• Resistance to abrasion and erosion

Small media should be used since they provide large surface-to-volume ratios, and thus, a greater surface area for biofilm growth without increasing reactor volume. Small media are also easier to fluidize, reducing the circulation requirements which decreases the shearing effects and allows a more quiescent environment for optimal biofilm growth. Silica sand, anthracite coal, activated carbon, stainless-steel wire spheres, and reticulated polyester foams are some of the media that can be considered for AFBR applications. Yee (1990) reports on the effects of various media types on AFBR performance and kinetics.

At a standard temperature and pressure, 1 lb of BOD removed yields 5.62 ft3 methane. Anaerobic decomposition processes are summarized in Figure 7.22.4.

A portion of the waste material is used by the anaerobic biosystem as a source of energy and in the synthesis of new bacterial cells. Cell synthesis is affected by the type of waste being treated, but generally, for every pound of BOD destroyed by the anaerobic process, approximately 0.1 lb of new cells is produced compared to 0.5 lb in the aerobic process. Therefore, the sludge or solids buildup is less in the anaerobic system. When anaerobic lagoon contents are black in color, this indicates that the lagoon is functioning properly.

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