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 d iscussed based on liquid-circulating flow rate and continuous treatmen t 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 m (3)/(m(3).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%, respec tively, 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 loa ding rate is in the range 0.22 to 0.46 kgBOD/(m(3).day). More than 80% nitrification and denitrification efficiencies can be achieved simult aneously when the both conditions, the aerobic zone ratio being more t han 0.2, and the anoxic zone ratio being more than 0.3, are satisfied. (C) 1998 Published by Elsevier Science Ltd. All rights reserved