The excessive deposition of ammonium (reactive nitrogen) in the environment has led to losses of biodiversity and the eutrophication of ecosystems. Anthropogenic sources contribute twice the natural rate of terrestrial reactive nitrogen and provide about 45% of the total amount of it produced annually on Earth. Recently, a biological process that anaerobically metabolizes ammonium and facilitates iron reduction, termed Feammox, was discovered. The use of Feammox activity together with hollow fiber membrane bioreactors (HFMB), for which the latter are based on the formation of biofilms of bacterial communities, constitutes an efficient and sustainable method for the removal of ammonium from agriculturally derived wastewater. To implement the use of HFMB with Feammox activity, the formation of Feammox bacterial biofilms from wastewater sludge samples from a brewery was evaluated. The cultures were enriched with two different carbon sources, namely, sodium acetate and sodium bicarbonate; then, ferrous iron and ammonium concentrations, which were used as indicators of reactive nitrogen removal, were measured. The measurements revealed that the ammonium removal level reaches 20.4% when sodium acetate is used as carbon source. Moreover, an increase in the ferrous iron concentration of +Δ84.6 mg/L was observed, indicating that Feammox activity had been generated. Biofilm formation was observed under Feammox conditions on the hollow fibers. These results showed that Feammox bacteria can form biofilms and efficiently remove ammonium from wastewater, constituting an essential feature with which to scale up the process to HFMBs. Overall, these results contribute to a better understanding of the Feammox process that can be used to implement these processes in agriculture and thus progress towards a more sustainable industry.