Effect of draft tube diameter on nitrogen removal from domestic sewage in a draft tube type reactor

The effect of the ratio of draft tube diameter to reactor diameter (D_i/D_o) on the efficiency of nitrogen removal from domestic sewage is discussed based on liquid-circulating flow rate and continuous treatment data. More than 2.5 minutes of circulation time in the annulus part, which is required to create an anoxic zone, could be maintained under operating conditions in which air flow rate per reactor volume was 2 m³/(m³ · hr) and D_i/D_o was 0.19. When D_i/D_o was set at 0.19, the average total organic carbon (TOC), total nitrogen (TN) and dissolved nitrogen (DN) removal efficiencies were 83.2%, 72.1% and 71.6%, respectively, which were higher than those when D_i/D_o was at 0.26 or 0.36. From these results, it is concluded that 0.19 is the best D_i/D_o for nitrogen removal in a draft-tube type reactor with an effective depth of 4.0m under the treatment condition in which the BOD volumetric loading rate is in the range 0.22 to 0.46 kgBOD/(m³ · day). More than 80% nitrification and denitrification efficiencies can be achieved simultaneously when both conditions, the aerobic zone ratio being more than 0.2, and the anoxic zone ratio being more than 0.3, are satisfied.