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　1.8　m　in　height　and　10　cm　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).