The oxidation of ammonia coupled with the reduction of iron is a unique pathway mostly reported in soils and sediments. An anaerobic sludge from a piggery wastewater treatment plant had been acclimated to an $NH_4{^+}/Fe^{3+}$-rich environment to secure an enrichment culture and investigate an anaerobic ammonia oxidation coupled with an iron reduction. The enrichment culture showed an average pH of 6.8 and the concentration of mixed liquor volatile suspended solid was measured as 1,120 mg/L. The mol ratio of oxidized $NH_4{^+}$ and reduced $Fe^{3+}$ was 0.33 mol $NH_4{^+}$/mol $Fe^{3+}$. It was suggested that the culture acclimated to $NH_4{^+}/Fe^{3+}$ contained the anaerobic ammonia oxidizing bacteria as well and thus $NH_4{^+}$ was fully oxidized to $NO_3{^-}$ by the bacterial consortia. In a batch experiment using the culture, the oxidation of $NH_4{^+}$ was increased as the initial concentration increased. However, it was suspected from the experimental results that other iron reducing bacteria had grown under the environment applied for the enrichment culture. As a result, it was observed that heterotrophic and autotrophic iron reducers were competing for $Fe^{3+}$.