Heterotrophs microbial respiration

Heterotrophic microbial respiration in aerobic soil layers, where oxygen is nsed as an electron acceptor ... 

FIGURE 6.26 Relationship between soil oxygen demand (SOD) and heterotrophic microbial respiration by different wetland soils. Each data point represents soil from a different locations in the United States (D Angelo and Reddy, 1999). 

This chapter deals with the microbial transformations of wastewater under aerobic conditions in a sewer network. It emphasizes the transformations of the organic matter and includes processes in both the water phase and the biofilm. Furthermore, transformations of particles in suspension originating from sewer sediments are included. A concept and a corresponding model for the integration of the major microbial processes, i.e., growth of the heterotrophic biomass, the respiration and the hydrolysis, are also dealt with. The basic chemical and biological aspects of sewer processes are focused on in Chapters 2 and 3. The reaeration process is dealt with in Chapter 4. 

Recent research has identified some other microbial routes for denitrification that are not heterotrophic. One, called the anammox reaction, involves the oxidation of ammonium to N2 using either nitrite or nitrate as the electron donor. The second has bacteria using Mn " to reduce nitrate to N2. As noted earlier, N2 is generated by the oxidation of ammonium using Mn02 as the electron acceptor. [Denitrification may also be supported by Fe " (aq) oxidation.] These reactions are summarized in Table 12.2. The overall consequence of these reactions is that ammonium does not accumulate in the pore waters where Mn respiration and denitrification are occurring. 

Aside from adding defined compounds, experimental additions of natural DOM mixtures suspected to vary in lability have helped test ideas about the contribution of various DOM sources to aquatic ecosystems. In a nice example using manipulation of natural DOM sources, Battin et al. (1999) used flowthrough microcosms to measure the relative uptake rates of allochthonous and autochthonous DOM by stream sediments. They documented greater than fivefold differences or more in uptake and respiration, depending on whether the DOM was extracted from soil or periphyton. Moreover, they were able to show, via transplant experiments, several cases where prior exposure to a particular source of DOM increased the ability of that community to metabolize the DOM supplied. There appears to be some preadaptation of microbial catabolic capacity when these stream biofilms were re-exposed to a familiar type of DOM. Similarly, the response of heterotrophic bacteria to carbon or nutrient addition was greatest when the source community was particularly active (Foreman et al., 1998). Kaplan et al. (1996) showed that fixed film bioreactors, colonized on one water source, were unable to rapidly metabolize DOC in water from another source. 

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