The Sewer as a Reactor for Microbial Processes

The three types of sewer networks described represent the main types. In practice, however, sanitary sewers may often appear in partially operated separate sewered catchments, i.e., they may to some extent receive runoff water. Other alternative sewer systems include, for example, the vacuum sewers that are typically small systems, operated locally. 

The sewer is dominated by heterotrophic microorganisms that degrade and transform wastewater components. These processes proceed under redox conditions determined by the availability of the electron acceptor. The importance of the processes for the sewer and the surroundings is not just caused by the removal and transformation of organic substrates — the electron donor—but is also a result of transformation of the electron acceptors exemplified by the formation of hydrogen sulfide from sulfate. 

FIGURE 1.3. Wastewater flow and subsystems for processes within and related to a sewer network. 

In addition to the redox conditions associated with the characteristics of the sewer network as outlined in Table 1.1, a number of other sewer characteristics influence the process conditions. The following examples illustrate the close relations between design and operation characteristics and process conditions  

TABLE 1.1. Electron Acceptors and Corresponding Conditions for Microbial Redox Processes in Sewer Networks. 

---