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Technique Nitrification by autotrophic bacteria requires more expensive operating strategies in a WWTP since nitrifying bacteria are slow growing and have lower oxygen utilization kinetics than aerobic heterotrophs. Therefore, traditional activated sludge process operations usually run with longer retention time, intense aeration, and lower COD to keep nitrifying bac- teria active, or activated sludge processes are replaced with advanced biological processes such as sequencing oxic-anox- ic batch reactors. However, achieving the performance by changing process- es or operations still relies on the activ- ity of indigenous nitrifying microbiol- ogy. Indigenous heterotrophic bacteria are capable of degrading the rbCOD, but are less efficient at converting less biodegradable and slowly hydrolysable COD to rbCOD which requires various enzymes. Due to the lower hydrolysis rate, non-biologically degraded COD ultimately yields more biosolids per pound of COD, which requires addi- tional treatment and disposal cost. In addition, increased aeration energy is required to treat wastewater due to the presence the other non-beneficial microbiology. The Significance Of Biochemical Processes In Wastewater Collection System Figure 1 (prior page) illustrates the environment in the sewer line, which is simplified as the bulk water phase, the biofilm phase, the biofilm/ sediment phase, and the sewer atmosphere. Biochemical processes take place in each phase of the sewer. The sewer atmosphere is the only source of the oxygen (O2) that dissolves into the wastewater in the sewer except the dissolved oxygen (DO) coming in with the influent wastewater. The hydrogen sulfide gas (H2S) and other odor causing volatile organic compounds (VOCs) emit from the bulk liquid phase to the sewer atmosphere based on the respective Henry's constant for each compound and the characteristics of the wastewater. The biochemical transformations in the bulk liquid phase, the biofilm phase or in the biofilm/sediment phase of the sewer network are initiated by microorganisms (mainly dominated by the large population wateronline.com ■ Water Online The Magazine, Wastewater Edition 11 of heterotrophic bacteria). The most important processes are the hydrolysis of hydrolysable COD and the utilization of rbCOD. Heterotrophic bacteria can improve the influent wastewater quality by changing the ratio of hydrolysable COD to rbCOD, and can reduce the influent loads to the WWTP though microbial metabolisms described earlier. This increase in the rbCOD fraction can further enhance the nitrogen (N) and the phosphorus (P) removal in the WWTP.