The activated sludge process employing sponge-like carrier is said to have the effect of improving the settleability of activated sludge, however, either of continuous and quantitative evaluation at real wastewater treatment plants or investigation of the mechanism based on the microbial evaluation of activated sludge and carrier sludge has not been carried out.

　In this study, a wastewater treatment plant located in a cold climate region was selected, and long-term settleability of the activated sludge of two lanes with and without the carrier was evaluated and compared with the microbial characteristics of the activated sludge. In addition, experiments using the activated sludge and carrier of the plant were carried out for the mechanism investigation. As a result, followings were clarified; settleability of the activated sludge during low temperature period was degraded in the lane without carrier, but it was good and stable in the lane with carrier. Further, in the lane without carrier, filamentous bacteria were in the dendritic form connecting several activated sludge flocs, while in the lane with carrier, the filamentous bacteria were folded and contained in a single floc. From the experiment results, the morphology of the filamentous bacteria was thought to be influenced by that protozoa which existed abundantly in the carrier predated bacteria of the activated sludge.

　Reduction of ammonia nitrogen concentration in treated wastewater may be discussed in the future in terms of the adverse effects of ammonia on aquatic life. Many wastewater treatment plants may require reduction measures when the target value of ammonia nitrogen concentration in treated wastewater is set to low concentrations. In this study, a post-nitrification process was evaluated for its treatment performance of ammonia nitrogen in secondary effluent, aiming to implement it in small-scale treatment plants. Equipped with carriers and controlled using an ammonia nitrogen sensor, the process was operated by carrier fluidization methods of aeration mixing or mechanical mixing to compare the performance of each method. In each method of aeration mixing and mechanical mixing, ammonia nitrogen removal efficiency was 95% and 93%, nitrification rate per total biological reactor volume was ranged from 1.5 to 7.5 mg-N/(L・h) and 2.5 to 8.3 mg-N/(L・h), and required aeration volume per removed ammonia nitrogen by nitrification was 0.80 m³/g-N and 0.60 m³/g-N, respectively. In terms of reduction of ammonia nitrogen concentration in secondary effluent ranging same as this study, mechanical mixing method showed better efficiency in reducing aeration for nitrification. Nitrifying bacteria in the carriers were detected in relatively high proportions by microbial community analyses using a next-generation sequencer. Implementation of this technology in a small-scale wastewater treatment plant with biological anaerobic-aerobic filters was estimated to require a foot-print of approximately 12% of the plant’s biological reactor volume.