A Concept for the Aerobic Microbial Transformations of Wastewater in Sewers

2 A CONCEPT FOR THE AEROBIC MICROBIAL TRANSFORMATIONS OF WASTEWATER IN SEWERS 

Although the investigations of both Raunkjaer et al. (1995) and Almeida (1999) showed that removal of COD — measured as a dissolved fraction — took place in aerobic sewers, a total COD removal was more difficult to identify. From a process point of view, it is clear that total COD is a parameter with fundamental limitations, because it does not reflect the transformation of dissolved organic fractions of substrates into particulate biomass. The dissolved organic fractions (i.e., VFAs and part of the carbohydrates and proteins) are, from an analytical point of view and under aerobic conditions, considered to be useful indicators of microbial activity and substrate removal in a sewer. The kinetics of the removal or transformations of these components can, however, not clearly be expressed. Removal of dissolved carbohydrates can be empirically described in terms of 1 -order kinetics, but a conceptual formulation of a theory of the microbial activity in a sewer in this way is not possible. The conclusion is that theoretical limitations and methodological problems are major obstacles for characterization of microbial processes in sewers based on bulk parameters like COD, even when these parameters are determined as specific chemical or physical fractions. 

It is considered important to identify a rather simple, however, generally well-accepted concept for the microbial transformations that can be extended when further knowledge on sewer processes exists. Although details may be missing in a simple description, advantages in terms of possibilities for a sound 

FIGURE 5.3. Biomass and substrate relations as applied to aerobic wastewater processes in sewer systems. 

Compared to the traditional BOD and COD removal concept, which considers organic matter as degradable in a fictitious removal process, the concept described has moved to highlight biomass as being the real active component, depending on the nature and availability of organic substrates and electron acceptor. The heterotrophic biomass is, therefore, in terms of its activity, the central component of such a concept. 

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