Enhanced biological nutrients removal using an integrated oxidation ditch with vertical circle from wastewater by adding an anaerobic column.

Compared to conventional oxidation ditches, an integrated oxidation ditch with vertical circle (IODVC) has the characters of concise configuration, simple operation and maintenance, land saving and automatical sludge returning. By the utilization of vertical circulation, an aerobic zone and an anoxic zone can be unaffectedly formed in the IODVC. Therefore, COD and nitrogen can be efficiently removed. However, the removal efficiency of phosphorus was low in the IODVC. In the experiment described, a laboratory scale system to add an anaerobic column to the IODVC has been tested to investigate the removal of phosphorus from wastewater. The experimental results showed that the removal efficiency of TP with the anaerobic column was increased to 54.0% from 22.3% without the anaerobic column. After the acetic sodium was added into the influent as carbon sources, the mean TP removal efficency of 77.5% was obtained. At the same time, the mean removal efficiencies of COD, TN and NH3-N were 92.2%, 81.6% and 98.1%, respectively, at 12 h of HRT and 21-25 d of SRT. The optimal operational conditions in this study were as follows: recycle rate = 1.5-2.0, COD/TN > 6, COD/TP > 40, COD loading rate = 0.26-0.32 kgCOD/(kgSS x d), TN loading rate = 0.028-0.034 kgTN/(kgSS x d) and TP loading rate = 0.003-0.005 kgTP/(kgSS x d), respectively.