An iron-manganese co-oxide filter film (MeOx) coated on filter sand has been shown be a good catalyst for the oxidation of ammonium in the treatment of groundwater. However, its catalytic activity decreases gradually when it is used to remove ammonium from surface water. In this study, phosphate was added to sustain the ammonium removal activity of MeOx in a pilot-scale filter system for surface water treatment. The results showed that the ammonium removal efficiency of the MeOx filter without phosphate dosing decreased gradually, and just 27.69% removal efficiency was obtained at 6.1 degrees C. After dosing with 30 mu g/L phosphate, the ammonium removal efficiency remained high after continuous operation for 176 days and reached 85.74%, even at 6.1 degrees C. It was found that both the biological nitrification activity and the chemical catalytic activity of MeOx were markedly enhanced after phosphate dosing (compared with that without phosphate dosing). Through inactivation by ozone, the biological nitrification and chemical catalytic activity were distinguished. It was found that chemical catalytic activity accounted for around 67.59% of the total ammonium removal. Aluminum was found to accumulate on the surface of MeOx and combine competitively with its active sites. This may be the primary reason for the decrease in the catalytic activity of MeOx. With phosphate dosing, the accumulation of aluminum decreased by 60.53% and the chemical combination of aluminum and MeOx was suppressed, thus sustaining the catalytic activity of MeOx.