Illustration of Aerobic Transformations in Sewers

Almeida (1999) made transformation studies of wastewater components in a gravity sewer. The sewer has a length of 7.2 km and a typical retention time of 1.5 hours. An average slope equal to 0.007 and several drops resulted in a sewer dominated by aerobic processes. In addition to the organic components (CODtot, CODsol and BOD), other relevant parameters (ammonia, nitrate, TSS 

FIGURE 5.1. Transformations of protein in wastewater under aerobic and anaerobic conditions (Nielsen et at, 1992). 

TABLE 5.1. Estimated Average Removal Percentages of Selected Wastewater Components in a 7.2-km Gravity Sewer with an Average Retention Time of about 1.5 h. The Sewer Processes are Predominantly Aerobic (Almeida, 1999). 

Although experimental studies under sewer conditions are subject to high variability, it can be concluded that the removal of the components shown in Table 5.1 is mainly attributed to the activity of the heterotrophic biomass. Theoretical considerations and a number of studies like the one performed by Almeida (1999) clearly demonstrate that the heterotrophic biomass is central for understanding aerobic transformations. Studies by Stoyer (1970), Stoyer and Scherfig (1972), Koch and Zandi (1973), Pomeroy and Parkhurst (1973) and Green et al. (1985) have also focused on removal of organic matter in sewers, primarily in terms of BOD and COD. 

The heterotrophic biomass in wastewater is typically not a limiting factor for the aerobic transformations in a sewer. The limitation is typically caused by the supply of the electron acceptor (oxygen), i.e., the reaeration. Contrary to what has been proposed by several authors, it would typically serve no useful purpose to inject biomass (sludge) in a sewer line to enhance the treatment processes unless a significant amount of oxygen is continuously supplied to the wastewater (cf. Example 5.1). 

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