Engineering textbooks tell a simple story about nitrification and denitrification. Classic nitrification-denitrification theory begins with the bacterial genera Nitrosomonas and Nitrobacter performing ammonia and nitrite oxidation, respectively. Then facultative or obligate anaerobic bacteria denitrify by oxidizing organic carbon with nitrate. Recent advances in environmental microbiology have revealed previously unknown bacteria and pathways in the nitrogen cycle that tell a far more complex story. Classic theory has been successful for technologies that employ fast-growing bacteria, such as activated sludge, for almost a century. In contrast, nitrogen transformations in treatment wetlands are only partially explained by classic theory because they are ideal environments for slow-growing bacteria. Recently discovered bacterial processes, such as Anammox and heterotrophic nitrification, can be native to treatment wetlands. Other known nitrogen-cycle bacteria in nature occupy ecological niches similar to those that can exist in treatment wetlands, but their role in denitrification remains unexplored in a treatment context. The experience of treatment wetlands demonstrates that classic theory is no longer valid as a general model. We propose a broader model of nitrogen transformations in treatment wetlands that integrates recent discoveries. This general model is intended as a conceptual tool for those working with nitrogen pollution abatement.
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