Nitrogen removal performance and bacterial community in a full-scale modified Orbal oxidation ditch with internal nitrate recycle and biocarriers

A modified Orbal oxidation ditch with biocarriers and internal nitrate recycle was used in a full-scale municipal wastewater treatment plant, and nitrogen removal performance and microbial community were investigated. The results indicated that ammonia nitrogen removal efficiency increased from 80.5% to 95.4% after increasing oxygen supply and adding biocarriers in inner channel, and biocarriers played an important role in ammonia nitrogen removal. Internal recycle could enhance total nitrogen (TN) removal in outer channel, with TN removal efficiency of modified Orbal oxidation ditch reaching 76.5 %, which was higher than that of the traditional Orbal oxidation ditch (68.7 %). The pyrosequencing results revealed that bacterial community of the activated sludge in inner, middle and outer channels were very similar, but differed to that of the biofilm. The relative abundance of Nitrospira was 15.3 % in biofilm and less than 1.5 % in activated sludge. Nitrospira was enriched in the biofilm, thereby ammonia nitrogen removal was enhanced.