We define a biological reaction as a reaction mediated by organisms. It encompasses both the organisms and the underlying chemical reactions. To fully apply the knowledge of biological reactions to the treatment of water and wastewater, the chemical nature of these reactions must be given center stage. In other words, to control the process of removing nitrogen by nitrification-denitrification, the intrinsic chemical reactions must be unraveled and fully understood. The organisms only serve as mediators (i.e., the producer of the enzymes needed for the reaction). Thus, on the most fundamental level, nitrogen removal is a chemical process (more accurately, a biochemical process), and the treatment for removal of nitrogen by nitrification-denitrification as used in this textbook is chemical in nature and the process is a chemical unit process. In fact, nitrification-denitrification removal of nitrogen can be effected by purely enzymatic means by providing the needed enzymes externally without ever using microorganisms.
Similar to phosphorus, nitrogen is a very important element that has attracted much attention because of its ability to cause eutrophication in bodies of water. As stated in the chapter on phosphorus removal, the Chesapeake Bay in Maryland and Virginia is fed by tributaries from farmlands as far away as New York. Because of the use of nitrogen in fertilizers for these farms, the bay receives an extraordinarily large amount of nitrogen input that has triggered excessive growths of algae in the water body. Presently, large portions of the bay are eutrophied.
This chapter discusses removal of nitrogen using the unit process of nitrification followed by denitrification. Half reactions are utilized in the discussion of the chemical reactions. Whether or not a particular reaction will occur can be determined by the free energy change of the reactants and products. Thus, half reactions are normally tabulated in terms of free energies. To understand the exact meaning of free energy as it relates to half reactions and thus to nitrogen removal, microbial thermodynamics is discussed. Carbon requirements, alkalinity dose requirements, and reaction kinetics as they apply to nitrogen removal are all discussed. A section on whether or not to remove nitrogen is also included.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.