Biological reactors, conventional

Another problem with conventional fermenters concerns foaming. In traditional systems, the introduction of large quantities of gas into the vigorously agitated fermentation liquor often produces great quantities of foam in the reaction vessel. Biological reactors are particularly susceptible to foaming because of the surfactant properties of most biomolecules. This foam severely limits the usable volume of the vessel and can render the fermentation process inoperable and microbially contaminated when the gas flow exit lines become filled with foam. All of these problems have a substantially adverse influence upon the yield and cost-eflectiveness of conventional fermentation processes. 

The design of conventional biological reactors is very similar to those of gas-liquid, slurry, and polymerization reactors outlined in other chapters. As a matter of fact, biological reactors are the most versatile of all reactors, since such a reactor can carry two or three phases, the liquid can be Newtonian or non-Newtonian, the solids can be heavy or light, and the reaction mixture can be simple or complex. A biological reactor, however, carries certain distinct features ... 

For a conventional mechanically agitated biological reactor, the information provided for aqueous gas-liquid and gas-liquid-solid systems in Sections II, III, and VII is applicable here. For power consumption, the most noteworthy works are those by Hughmark (1980) (see Eqs. (6.15) and (6.16)) and Schiigerl (1981). For gas-liquid mass transfer, the relationship kLaL = (P/V, ug) is applicable for biological systems. The relationships (6.19) and (6.20) are also valuable, and their use is recommended. The most generalized relation for kLaL is provided by Eq. (6.18). The intrinsic gas-liquid mass transfer coefficient is best estimated by Eq. (6.23). For liquid-solid mass transfer, the use of the study by Calderbank and Moo-Young (1961) (Eqs. (6.24)-(6.26)) is recommended. For viscous fluids, Eq. (6.27) should be used. 

Biological reactors — which are designed to replace conventional bio-active beds and activated sludge tanks may be classified as being borderline between UF and membrane reactors. The processes in question will be analyzed later. 

Attempts to install MF or UF membranes in conventional biological reactors (of activated sludge type) with recirculation are also promising. Such membranes prevent the outflow of the biomass, thus improving the efficiency of the system. 

Recently developed applications of activated carbon to wastewater treatment involve the addition of PAC to conventional activated sludge aeration tanks. The combination of PAC with biological process is often times referred to as the PACT or PAC-activated sludge process (3). The PACT process has attracted a great deal of interest because it is a method by which the performance of a waste treatment facility may be improved in various areas. Technical advantages that can potentially be achieved by adding PAC to biological reactors include ... 

In wet-air oxidation, the aqueous mixture is heated under pressure ia the presence of air, which oxidi2es the organic material. The efficiency of the oxidation process is a function of reaction time and temperature. The oxidation products are generally less complex and can be treated by conventional biological methods (31). The reactor usually operates between 177 and 321°C with pressures of 2.52—20.8 MPa (350—3000 psig). 

Three-phase fluidized bed reactors are used for the treatment of heavy petroleum fractions at 350 to 600°C (662 to 1,112°F) and 200 atm (2,940 psi). A biological treatment process (Dorr-Oliver Hy-Flo) employs a vertical column filled with sand on which bacderial growth takes place while waste liquid and air are charged. A large interfacial area for reaction is provided, about 33 cmVcm (84 inVirr), so that an 85 to 90 percent BOD removal in 15 min is claimed compared with 6 to 8 h in conventional units. 

Many potential applications are under study. Miniature chemical reactors could be used for portable applications in which they provide advantages of rapid startup and shutdown and of increased safety (intensification by requiring only small quantities of hazardous materials). The development of chip-scale chemical and biological analysis systems has the potential to reduce the time and cost associated with conventional laboratory methods. These devices could be used as portable analysis systems for detection of hazardous chemicals in air and water. There is considerable interest in using a microreactor to provide in situ production of hydrogen for small-scale fuel-cell power applications by conducting a reformation reaction from some liquid hydrocarbon raw material (e.g., methanol). 

The design and operation of a bioreactor are mainly determined by biological needs and engineering requirements, which often include a number of factors efficient oxygen transfer and mixing, low shear and hydrodynamic forces, effective control of physico-chemical environment, easy scale-up, and so on. Because some of these factors can be mutually contradictory, it is difficult to directly employ a conventional microbial reactor to shear-sensitive plant tissue cultures. 

In this section, we first evaluate the design of conventional nonaerated and aerated stirred slurry reactors. Since the design of mechanically agitated gas-liquid reactors has already been discussed in Section II, here the main emphasis is placed on the effects of solids on the design parameters. We subsequently illustrate some special-purpose slurry reactors used in the chemical and petrochemical industries. Novel slurry reactors used in biological or polymeric industries are discussed in Sections VI and VII, respectively. 

The design characteristics outlined in Table XXV allow this reactor to treat various kinds of waste water biologically in extremely short periods. For example, a waste water of initial BOD, = 850 mg/1 had its value reduced to 30 mg/1 during a mean residence time of 17 min. Compared with a conventional concrete basin, the volume of the reciprocating bioreactor required for purification of a given flow rate can be reduced to approximately 1/30 or 1/60. 

A comparison between the performance of conventional biological waste-water treatment basins and the reciprocating bioreactor is given in Table XXVI. As shown, the conversion rate in the reciprocating reactor can be as high as more than four times that obtained in a conventional bioreactor. 

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