Fluidized Bed Bioreactors. Fluidized bed bioreactors (FBBRs) can be operated with any of the three biochemical environments, and the nature of that environment determines what the bioreactor accomplishes. Fluidized bed systems for denitrification were among the earliest developed because all materials to be reacted were present in a soluble state. However, through use of pure oxygen as a means of providing dissolved oxygen at high concentration, aerobic fluidized beds soon followed. Their chief purpose is removal of soluble organic matter, but they are also used for nitrification. Finally, anaerobic fluidized bed systems were developed for the treatment of soluble wastewaters.4 The key characteristic of fluidized bed systems is their ability to retain very high biomass concentrations, thereby allowing small bioreactor volumes to be used. This is accomplished by using very small particles, which provide a large surface area per unit volume, as the attachment media for biofilm growth. Maintenance of the particles in a fluidized state by control of the upflow velocity ensures better mass transfer characteristics than can be achieved in other attached growth systems. The major use of FBBRs has been for industrial wastewater treatment. This type of attached growth bioreactor is discussed in Chapter 21.
Rotating Biological Contactor. The rotating biological contactor (RBC) is a modern application of an old idea for the removal of soluble organic matter and the conversion of ammonia to nitrate. Microorganisms growing attached to rotating discs accomplish the desired objectives by the same mechanisms used in suspended growth systems, but in a more energy efficient manner because oxygen transfer is accomplished by the rotation of the discs, which are only half submerged. These bioreactors have been popular for the treatment of both domestic and industrial wastewaters, typically at smaller installations. RBCs are discussed in Chapter 20.
Trickling Filter. As indicated in Table 1.2, trickling filter (TF) is the name given to an aerobic attached growth bioreactor in the shape of a packed tower. Until the mid 1960s, TFs were made of stone, which limited their height to around two meters for structural reasons. Now TFs are made of plastic media much like that used as packing in absorption and cooling towers and are self-supporting to heights of around seven meters because of the greater void space and lighter weight of the media. The primary use of TFs is for removal of soluble organic matter and oxidation of ammonia to nitrate. Traditionally, TFs have been used for municipal wastewater treatment in small to medium size installations desiring minimal operating expense. However, since the introduction of plastic media they have also found use as pre-treatment devices preceding other biochemical operations. This is because they have the ability to reduce the waste concentration at relatively low operating cost, a bonus when aerobic treatment is being employed. Trickling filters cause relatively little degradation of insoluble organic matter and should not be used for that purpose. They are covered in Chapter 19.
Packed Bed. Packed bed bioreactors utilize submerged media with a particle size on the order of a few millimeters. They are designed and operated with flow either upward or downward. Because of the small particle size, packed beds act as physical filters, as well as biochemical operations. Their primary use is for conversion of soluble inorganic matter, particularly nitrification and denitrification, depending on the biochemical environment provided. They are also used to remove soluble organic matter, especially at low concentrations. They are discussed in Chapter 21.
Anaerobic Filter. Even though the name anaerobic filter (AF) suggests the use of media like that in a packed bed, this is not the case. Rather, an AF is a packed tower containing plastic media like that in a TF. Unlike a TF, however, an AF is operated under submerged conditions to maintain the microbial community in an anaerobic state. Its primary use is to treat high strength soluble wastewaters by converting the bulk of the organic matter to methane. Biomass grows attached to the solid media in the tower, and flow may be from either direction. If flow is upward, suspended biomass may accumulate and have to be removed periodically. Even though AFs are attached growth systems, their anaerobic character is the determinant of their behavior. Anaerobic filters are discussed in Chapter 13.
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