List of Figures Fig. 1 - Schematic diagram of a continuous flow biofilm reactor (CFBR) Fig. 2 - Seeding and control operation of all reactors during the first 60 days (Line and symbol represent the simulated results and experimental results, respectively). The symbols and error bars represent the average and standard deviation in the operational data from the four reactors. The fluctuations during the first 10 days were caused by the gradual conversion from suspended biomass to biofilm. Fig. 3 - Experimental and simulated results of CFBRs from days 50 to 200: a) Control operation of Reactor 1 allowed sustained full nitrification and b) DO-limited operation of Reactor 2 gave partial nitrite accumulation when the DO was 2.5 mg/L. Fig. 4 - Experimental and simulated results of CFBRs from days 50 to 200: a) FAinhibited operation of Reactor 3 did not lead to long-term nitrite accumulation and b) FA-inhibited and DO-limited operation of Reactor 4 led to long-term nitrite accumulation. Fig. 5 - Simulated biomass profiles with operational conditions in CFBRs at day 200. Broken straight lines represent the measured biofilm depth at the end of experiments. The table presents the total biomass throughout the biofilm of each reactor. Fig. 6 - NO2- utilization rate of NOB in each biofilm reactor DO measuring port pH measuring port Stirrer Water level Influent Air diffuser effluent Water bath Biofilm Detachable Substratum Handle for detaching substratum Fig. 1 - Schematic diagram of a continuous flow biofilm reactor (CFBR) 70 Concentration (mgN/L) 60 50 40 TAN 30 TNiN TNaN 20 Line: simulated result Symbol: experimental result 10 0 0 10 20 30 40 50 60 Elaped time (day) Fig. 2 - Seeding and control operation of all reactors during the first 60 days (Line and symbol represent the simulated results and experimental results, respectively). The symbols and error bars represent the average and standard deviation in the operational data from the four reactors. The fluctuations during the first 10 days were caused by the gradual conversion from suspended biomass to biofilm. a 70 Concentration (mgN/L) 60 50 40 TAN TNiN TNaN 30 Line=Simulated data Symbol=Experimental data 20 10 0 50 100 150 200 Elapsed time (day) 70 b TAN TNiN TNaN Line: simulated data Symbol: Experimental data Concentration (mgN/L) 60 50 40 30 20 10 0 50 100 150 200 Elapsed time (day) Fig. 3 - Experimental and simulated results of CFBRs from days 50 to 200: a) Control operation of Reactor 1 allowed sustained full nitrification and b) DO-limited operation of Reactor 2 gave partial nitrite accumulation when the DO was 2.5 mg/L. a Concentration (mgN/L) 160 120 TAN TNiN TNaN Line: simulated data Symbol: Experimental data 80 40 0 50 100 150 200 Elapsed time (day) Concentration (mgN/L) b 160 TAN TNiN TNaN Line=simulated data Symbol=Experimental data 120 80 40 0 50 100 150 200 Elapsed time (day) Fig. 4 - Experimental and simulated results of CFBRs from days 50 to 200: a) FAinhibited operation of Reactor 3 did not lead to long-term nitrite accumulation and b) FA-inhibited and DO-limited operation of Reactor 4 led to long-term nitrite accumulation. 18 Measurement: 220m Measurement: 176m Simulation: 171m 15 3 15 Biofilm density (mgVSa/cm ) Simulation: 239m 3 Biofilm density (mgVSa/cm ) 18 12 9 6 Inert AOB NOB 3 0 12 9 6 Inert AOB NOB 3 0 0 100 200 Substratum 300 400 500 600 0 100 Substratum Biofilm depth (m) 300 400 500 600 Biofilm depth (m) a) Reactor 1 b) Reactor 2 18 18 Simulation: 519m 12 9 6 3 Measurement: 587m Inert AOB NOB 15 3 15 Biofilm density (mgVSa/cm ) Measurement: 513m Inert AOB NOB 3 Biofilm density (mgVSa/cm ) 200 12 0 Simulation: 222m 9 6 3 0 0 100 200 Substratum 300 400 500 600 0 Substratum Biofilm depth (m) c) Reactor 3 mg AOB I II NOB Reactor 1 Reactor 2 Reactor 3 Reactor 4 139.5 80.5 137.5 81.4 35.9 11.3 32.9 0.0 100 200 300 400 500 600 Biofilm depth (m) d) Reactor 4 Inert III 147.1 139.2 530.1 218.3 ActiveIV I+II 175.4 91.8 170.4 81.4 TotalV Active/Total NOB/total I+II+III IV/V II/V 322.6 0.54 0.11 230.9 0.40 0.05 700.6 0.24 0.05 299.7 0.27 0.00 Fig. 5 - Simulated biomass profiles with operational conditions in CFBRs at day 200. Broken straight lines represent the measured biofilm depth at the end of experiments. The table presents the total biomass throughout the biofilm of each reactor. Substrate concentration (mgTNiN/L) 30 25 Reactor 20 1 2 3 4 15 Specific utilization rate (mgTNiN/mgVS-day) 0.52 0.21 0.17 0.01 10 0 10 20 30 40 50 60 70 X*t (mgVS/L*day) Fig. 6 - NO2- utilization rate of NOB in each biofilm reactor