Anoxic and Anaerobic Conditions

Details of the anaerobic degradation of chloroalkanes and chloroalkenes have been discussed in detail in Chapter 7, Part 3, and the application of reductive processes to bioremediation has been examined under denitrifying, sulfidogenic, and methanogenic conditions. An important observation 

A valnable comparison was made of the relative effectiveness of (a) augmentation with a cnltnre containing Dehalococcoides ethenogenes, (b) stimnlation of the natural dechlo-rinated population by addition of lactate and mineral nntrients, and (c) a recircnlation control withont amendment (Lendvay et al. 2003). The hrst procedure rapidly affected complete dechlorination, whereas the second was effective only after a lag period, and in both the hrst and second treatments the population of Dehalococcoides increased. 

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Garrido, J.M., Mendez, R., and Lema, J.M., Simultaneous urea hydrolysis, formaldehyde removal and denitrification in a multifed upflow filter under anoxic and anaerobic conditions, Water Res., 35, 691-698, 2001. 

FIGURE 2.2. Microbial biomass and substrate relations as applied to wastewater in sewer systems under aerobic, anoxic and anaerobic conditions and involving an external electron acceptor. 

Example 2.2 shows how the oxidation of organic matter (the electron donor) is balanced by using the production of electrons as the central element. The cases in Examples 2.3 to 2.5 are balances for electron acceptor reductions under aerobic, anoxic and anaerobic conditions, respectively (all are relevant for processes in wastewater of sewer networks). 

As indicated in Figure 3.4, the covalent bond, i.e., two common shared electrons, between two carbon atoms in the complex molecule is cleaved when initiated by the exoenzymes. The highly reactive intermediates that are formed react and produce new and stable bonds resulting in two new molecules that may undergo further hydrolysis. Hydrolysis is, thus, an important initial step in the transformation of complex organic matter present in a form that cannot directly be used at substrate. Hydrolysis is a process that—with different reaction rates — proceeds under aerobic, anoxic and anaerobic conditions. It is important to note that hydrolysis takes place without use of an electron acceptor. 

As(V)) and arsenite(As(III)) are the most abundant forms of arsenic (Smith et ah, 1998). In soils and water systems, As(V) is dominant under aerobic condition and As(III) under anoxic and anaerobic conditions. But, because the redox reactions between As(V) and As(III) are relatively slow, both oxidation forms are also found in soils regardless of the pH and Eh (Masscheleyn et al., 1992). Reducing soil conditions (Eh < 0 mV) greatly enhances the solubility of arsenic, and the majority of soluble arsenic is present as As(III). 

Aesoey, A., M. Storfjell, L. Mellgren, H. Helness, G. Thorvaldsen, H. Oedegaard, and G. Bentzen (1997), A comparison of biofilm growth and water quality changes in sewers with anoxic and anaerobic (septic) conditions, Water Sci. Tech., 36(1), 303—310. 

The integrated aerobic-anaerobic WATS model has changed this situation. As an example, it is possible to use the model in a gravity sewer with changing aerobic and anaerobic conditions. As previously stressed, a number of in-sewer processes still need to be dealt with. Examples are the anoxic transformations and the processes related to the extended sulfur cycle, particularly, the oxidation of sulfide and the emission of hydrogen sulfide into the sewer atmosphere, including its further oxidation at the sewer walls. Combined use of empirical and conceptual models is still needed. 

It is not only the properties of the compound in question that influence its behaviour but also the environmental and operational conditions it is subjected to (temperature, pressure, pH, redox conditions), as well as the particular configurations of the (biological) reactors (in particular, their partitioning into compartments featuring different conditions mainly aerobic, anoxic and anaerobic), SRT and, to variable extents, HRT. Moreover, in chemical processes, reagent... 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

Corrosion occurs in the aqueous enviromnent because protons and oxygen, which are readily available in nature, are reduced to more stable products. They require electrons to reduce, which are supphed by the base metals and other electropositive species. In fact, this is the reason why only few metals, such as Pt and Au, are found in their native state. These metals, therefore, are called noble metals. The remaining metals exist as minerals in their oxidised state. In anoxic or anaerobic conditions, reducing agents such as sulfur and nitrate can lead to corrosion, as evidenced during microbial corrosion (Smith etal.,20 ). 

Processes reported in Table 1 are typically anaerobic (AN). In agreement with the observations reported by Wuhrmann et al. [49], azo-dye bioconversion occurs with the standard organism and other facultative or obligatory aerobic bacteria in exclusively anoxic conditions. Different methods can be used to establish the required anaerobic conditions. A common procedure is simply sparging oxygen-free gas... 

Concerning the reduction step of the redox reaction, the heterotrophic microorganisms may use different electron acceptors. If oxygen is available, it is the terminal electron acceptor, and the process proceeds under aerobic conditions. In the absence of oxygen, and if nitrates are available, nitrate becomes the electron acceptor. The redox process then takes place under anoxic conditions. If neither oxygen nor nitrates are available, strictly anaerobic conditions occur, and sulfates or carbon dioxide (methane formation) are potential electron acceptors. Table 1.1 gives an overview of these process conditions related to sewer systems. 

Anoxic conditions require the absence of DO and the presence of nitrates. Such conditions are typically only found when artificially implemented. The aerobic and anoxic pathways of organic matter degradation are identical. The addition of nitrate to wastewater is widely used as a control measure to avoid anaerobic conditions in sewers (Section 6.2.7). 

There is — in addition to the use of nitrate for control of anaerobic conditions in sewers — a potential for anoxic treatment in terms of removal of organic matter. The anoxic treatment is an alternative to aerobic treatment. An advantage is that the addition of nitrate is simple compared with the injection of oxygen. However, a NUR value that is of the same order of magnitude as the actual OUR value—compared in units of electrons transferred—is crucial to obtain a relatively high removal rate of organic matter. For this and other reasons it is important to compare aerobic and anoxic transformations (cf. Example 5.5). 

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