Anaerobic microbial processes

The differences in the magnitudes of the parameters shown in Table 3.7 correspond well with the course of (anaerobic) microbial processes in the sewer deposits. Newly deposited sediments not only have the highest pollutant potential but also show the lowest critical shear stress for erosion. Such sediment types may exert the highest impacts on receiving waters from combined sewer overflows. 

The anaerobic microbial processes responsible for the formation of odor-causing substances produce inorganic gases and VOCs. The malodorous inorganic gases are primarily ammonia (NH3) and hydrogen sulfide (H2S). 

Referring to Figure 6.2, it is important and interesting to note that a DO consumption that takes place in a gravity sewer biofilm may proceed with relations to aerobic and anaerobic processes. The anaerobic microbial processes produce... 

The difference between aerobic and anaerobic transformations of wastewater organic matter is crucial. From a basic point of view, however, still related to the sewer systems, aerobic and anaerobic microbial processes have been dealt with in Chapter 3. The aerobic transformations and a corresponding conceptual model were the main subjects of Chapter 5. 

Methanogenesis The anaerobic microbial process that produces methane. 

Reduced sulfur compounds are ubiquitous in aqueous and atmospheric systems (10,11). Natural sources of reduced sulfur species in aqueous environment result from biological reduction of sulfate, anaerobic microbial processes in sewage systems, putrefaction of sulfur-containing amino-acids (12), oxidative decomposition of pyrite (13), and activities of marine organisms in the upper layers of the ocean (14,15). The build-up of sulfides in areas such as the Black Sea is also giving cause for concern (8). 

An alternative explanation to anaerobic microbial processes, for the source of inorganic carbon, is aerobic oxidation of organic carbon and subsequent dissolution of fracture calcites. This explanation was ruled out early in the experiment. Calcites sampled from drillcores at the site (Banwart et al. 1994) exhibited relatively high 5 C values and a very wide range in values (-16%o < 5 C < -6%o), compared to the values determined for dissolved inorganic carbon in the groundwater (-17.4 < C < -15.8). 

Besides being slower, anaerobic treatment is more difficult to manage and can generate by-products that are more mobile or toxic than the original compound, for example, the daughter products of TCE, ie, dichloroethenes and vinyl chloride. It requires a longer acclimation period which means slower startup times in the field. The microbial processes are less well understood, and hence, ate less controlled than for aerobic systems. 

Ronen Z, A Abeliovich (2000) Anaerobic-aerobic process for microbial degradation of tetrabromobisphenol-A. Appl Environ Microbiol 66 2372-2377. 

Knezovich, J. P. Bishop, D. J. Kulp, T. J., et al., Anaerobic Microbial Degradation of Acridine and the Application of Remote Fiber Spectroscopy to Monitor the Transformation Process. Environ. Toxicol. Chem., 1990 pp. 1235-1243. 

The microbial processes often take place under changing aerobic and anaerobic conditions. 

FIGURE 3.2. Outline illustrating the subsystems and occurrence of microbial processes in a gravity sewer under anaerobic conditions. 

A prerequisite condition for the validity of this result is that only aerobic microbial processes proceed. If anaerobic processes take place in the inner part of the biofilm, a reduction in the biomass production will take place, for example, see comments related to Figure 6.2. 

A list of components, constants and system characteristics related to a conceptual description of microbial processes of wastewater in sewers is provided. The list includes symbols for components and parameters relevant for processes under both aerobic and anaerobic conditions (see Chapters 5 and 6). 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

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