"Nitrification is a very important process in wastewater treatment systems performing biological nitrogen removal. The size, footprint and energy consumption of nitrifying activated sludge systems are governed by the requirement of the system to remove ammonia. Ammonia is an important wastewater quality parameter to consider because of its toxicity to aquatic life in receiving water bodies; hence environmental regulations regarding discharge requirements for ammonia are becoming more stringent. Although the complexity of nitrification in activated sludge systems is still being demystified, the possible seeding of biological reactors by raw sewage which contains nitrifying bacteria, has been overlooked so far. Even current best modeling practices such as the International Water Association (IWA) Good Modeling Practices (GMP) for biological wastewater treatment, assume that there is no biomass in raw municipal wastewaters. This study explores the potential of a natural seeding of nitrifiers at full-scale wastewater treatment level. Through the application of high-throughput DNA sequencing, we have shown that raw sewage was indeed supplementing full-scale bioreactors with active nitrifiers. Respirometric assays showed that nitrifying biomass in the studied influents was alive and active, and was capable of reaching full metabolic induction within a few hours. We replicated this phenomenon in the laboratory by adding influent biomass harvested from a full-scale wastewater treatment facility to laboratory-scale sequencing batch reactors (SBRs) operated at washout conditions (low temperature and solids retention time). Addition of influent solids restored nitrification and stabilized the SBR systems. Lastly, we examined the impact of such natural influent nitrifier seeding on the performance of activated sludge models. Incorporating the natural seeding of nitrifiers in the nitrification model, significantly enhanced the performance of the model and its predictive capacity. It is, therefore, recommended that Good Modeling Practices be amended to include the quantification of the level of nitrifiers in influents during wastewater characterization. " --