E-Andrew Sindt Creative Component S11.pdf

has a very low cBOD concentration and a significant nitrate concentration. A supplemental source ofcarbon and separate anoxic process following the oxic process is required for achieving nearly completenitrogen removal.In an effort to reduce chemical and operating costs (e.g., aeration costs), and to ensure completenitrogen removal before discharge to lakes or streams, single sludge denitrification needs to achieve twogoals that seem to be in conflict with each other. First, the system must provide aerobic conditions thatallow for full nitrification in order to generate the nitrate required for denitrification. Second, the singlesludge system must reserve a sufficient amount of organic electron donor for anoxic denitrification(Rittman and McCarty, 2001). Engineers have developed three basic strategies to handle this problem(Rittman and McCarty, 2001). These strategies include biomass storage and decay, classicalpredenitrification, and simultaneous nitrification and denitrification.2.2.1.1 Biomass Storage and DecayBiomass storage and decay relies on the electron equivalents that are sequestered by microorganismsthrough growth and synthesis. These electron equivalents were originally derived from the cBOD andcan be released to propel denitrification through endogenous respiration. This process is also known asa Wuhrmann biomass decayer (Wuhrman, 1964).As shown in Figure 2.1, biomass storage and decay starts with an aerobic tank where nitrification occursand cBOD is partially oxidized and partially stored through biomass synthesis. The resulting nitrate andbiomass are then sent to the anoxic tank where denitrification takes place, driven by the endogenousrespiration of biomass.Figure 2.1- Biomass storage and decay (Rittman and McCarty, 2001)Two major reasons prevent this approach from being implemented as a stand-alone process fornitrogen removal. First, the kinetics of endogenous respiration are slow, which then requires a highmixed liquor suspended solids (MLSS) concentration and a long hydraulic retention time (HRT) in the15