Anaerobic systems

Nevertheless, an anaerobic system may be the method of choice under certain conditions (/) contamination with compounds that degrade only or better under anaerobic conditions, (2) low yield aquifers that make pump and treat methods or oxygen and nutrient distribution impractical, (J) mixed waste contamination where oxidizable compounds drive reductive dehalogenation of chlorinated compounds, or (4) deep aquifers that make oxygen and nutrient distribution mote difficult and cosdy. 

Aerobic systems including trickling filters and rotating biological contactors (RBC) are operated in a nonflooded mode to ensure adequate oxygen supply. Other aerobic, anoxic, and anaerobic systems employ flooded reactors. The most common systems are packed beds (anaerobic trickling filter) and fluidized or expanded bed systems. 

Glucose, anaerobic system of solution-phase Np(V) and Pu(V). .. 305-6 Grand Ronde basalt, ground-water... 

Although oxygen was found to be the only oxidant for conversion of coproporphyrinogen III to protoporphyrin IX, anaerobic systems must obviously exist for the biosynthesis of the latter molecule (43). Porphine itself has not been found in nature but spectral lines identical to those of bis-pyridylmagnesiumtetrabcnzoporphine have been detected in interstellar space (53). 

The most widespread biological application of three-phase fluidization at a commercial scale is in wastewater treatment. Several large scale applications exist for fermentation processes, as well, and, recently, applications in cell culture have been developed. Each of these areas have particular features that make three-phase fluidization particularly well-suited for them Wastewater Treatment. As can be seen in Tables 14a to 14d, numerous examples of the application of three-phase fluidization to waste-water treatment exist. Laboratory studies in the 1970 s were followed by large scale commercial units in the early 1980 s, with aerobic applications preceding anaerobic systems (Heijnen et al., 1989). The technique is well accepted as a viable tool for wastewater treatment for municipal sewage, food process waste streams, and other industrial effluents. Though pure cultures known to degrade a particular waste component are occasionally used (Sreekrishnan et al., 1991 Austermann-Haun et al., 1994 Lazarova et al., 1994), most applications use a mixed culture enriched from a similar waste stream or treatment facility or no inoculation at all (Sanz and Fdez-Polanco, 1990). 

For effective biological treatment of dye wastewater, immobilization of bacteria under aerobic anaerobic high-rate reactors should be given special attention. The main cause of effective treatment of these xenobiotics under immobilized condition in high rate reactors is the rapid facile reduction of these compounds to products of lower toxicity [68, 69]. Moreover, the immobilization of anaerobic bacteria and maintenance of a high concentration of biomass in the high rate reactors are factors that improve the tolerance of the anaerobic system to toxic substances [70, 71]. 

Integration between the aerobic and anaerobic systems for wastewater transformations in sewers makes it possible to describe the performance of changing aerobic-anaerobic conditions. Several examples described in Chapter 5, e.g.,... 

Combined aerobic-anaerobic systems, in bioremediation, 25 837 Combined cycle, 10 142-143 Combined cycle fossil fuel plants, 23 236 Combined heat and power (CHP), from biomass, 3 687... 

While 02 serves as the electron acceptor in aerobic biodegradation processes forming H20 as the final product, degradation in anaerobic systems depends on alternative electron acceptors such as sulfate, nitrate or carbonate, which yield, ultimately, hydrogen sulfide (H2S), molecular nitrogen (N2) and/or ammonia (NH3) and methane (CH4), respectively. 

For anaerobic systems, methane gas is an important product. Depending on the type and nature of the biological components, different yields can be obtained for different biodegradable wastes. For pure cellulose, for example, the biogas product is 50% methane and 50% carbon dioxide. Mixed waste feedstocks yield biogas with methane concentrations of 40-60% (by volume). Fats and oils can yield biogas with 70% methane content. 

Macalady, D.L., Tratnyek, P.G., and Grundy, TJ. Abiotic reduction reactions of anthropogenic organic chemicals in anaerobic systems a critical review, J. Contam. Hydrol, l(l) l-28, 1986. 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

The H202 formed may be reduced by catalase, peroxidase or suitable reducing agents. SOD has been identified in many animal and bacterial cells its biological function is to protect tissue against oxygen free radicals in anaerobic systems (reviewed by Farkye, 1992). 

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