Abstract: Abstract: The universal features of rural domestic wastewater were small quantity, scattered distribution, and great diurnal variation, etc. So it was difficult to centralize, treat, and reuse such wastewater. However, discharging such untreated rural domestic sewage with the characteristics of comparatively high nitrogen and phosphorus concentration to the environment brought about seriously eutrophication. Therefore, it was necessary to research and develop a treatment process which has the advantages of relatively high efficiency, less land occupied, investigated savings, and also easy management and maintenance for nutrient removal from the rural sewage. On the basis of these considerations above, an enhanced denitrifying phosphorus removal system, that is, a two-sludge process integrating anaerobic/anoxic/oxic (AAO) reactor with biological aerated filter (BAF)( AAO-BAF for short), was developed to treat domestic sewage, especially the decentralized sewage in rural areas. In this innovation system, the AAO unit, consisting of nine compartments in sequence, was used mainly for the removal of organic compounds and nutrients without ammonium oxidation, while the BAF unit was mainly responsible for nitrification. The BAF unit, an important component of the two-sludge system, was 1.8m in height and 10cm in diameter, with a light-weight ceramic filled in it. Nine sampling ports were placed along the BAF, marked as B1 to B9 from bottom to top in turn. In this study, a series of experiments were carried out to research the effect of the ammonium volumetric loading on nitrifying characteristics and effluent SS of the BAF. The ammonium volumetric loading, by changing the hydraulic loading (marked as mode 1) and adjusting the effective volume of the BAF unit (marked as mode 2), was varied from 0.43 kg/(m3·d) to 1.2 kg/(m3·d). Mode 1 lasted for 90 d, during which hydraulic loading soared from 1.53 m3/(m2·h) to 4.37 m3/(m2·h). Mode 2 ran for 60 d, and during that period BAF effective volume decreased from 9.8 L to 2.7 L, while hydraulic load maintained 1.53 m3/(m2·h). Throughout the total experimental period, the sludge reflux ratio was 100% maintained, with a nitrification reflux ratio of 300%, and the air-water ratio of BAF unit was set at 3:1~4:1.The results showed that, with the increase of ammonium volumetric loading, the ammonia nitrogen removal efficiency showed a tendency of slowly decreasing first and sharply dropping later during two operating modes. Effluent SS was more sensitive to changes in mode 1 than that in mode 2, which was the most significant difference between the two operating modes. When ammonia volumetric loading varied from 0.43 kg/m3·d to 1.12 kg/(m3·d), the nitrogen removal efficiency was over 81%; and when ammonia volumetric loading increased to 1.21 kg/(m3·d), ammonia removal efficiency decreased sharply to 65% and 68%, respectively. When the ammonia volumetric loading was below 0.74 kg/(m3·d), both effluent SS were lower than 10mg/L; and when ammonia volumetric loading varied from 0.74 kg/(m3·d) to 1.21 kg/(m3·d), effluent SS soared much more sharply during mode 1, at the ammonia volumetric loading of 1.21 kg/(m3·d), effluent SS was 21.8 mg/L and 14.2 mg/L respectively. These findings determined that, to meet the first class of A standard, namely, the standard of ammonia nitrogen concentration less than 5 mg/L, and effluent SS lower than 10 mg/L, BAF unit should be operated at the condition of ammonium volumetric loading below 0.74 kg/(m3d).