In this study, we explored the effects of Fe(II) on the activity of anammox biomass both in short and long terms. With the influent Fe(II) at 0.09 and 0.12 mM, continuous experiments demonstrated that the nitrogen removal rates increased 32.2% and 29.9% compared to that with normal Fe(II) level (0.03 mM). Influent Fe(II) variation could affect the total Fe, heme c levels and hydrazine dehydrogenase activity. At the Fe(II) concentration of 0.09 mM, the total Fe, heme c levels inside anammox cell and hydrazine dehydrogenase activity could increase about 2.0, 2.1 and 2.35 folds as much as that with normal Fe(II) level. However, when the Fe(II) elevated to 0.18 mM, it would cause a mal-effect on anammox bacteria and further deteriorate nitrogen removal performance. It was indicated that the appropriate increase of Fe(II) was beneficial for more heme c synthesis, enhancement of hydrazine dehydrogenase activity, the growth of anammox bacteria. (C) 2013 Elsevier Ltd. All rights reserved.