| In the bio-denitrification process in the municipal wastewater treatment, more and more research indicates that the nitrification-denitrification can be processed simultaneously in the outer ditch of Orbal oxidation ditch process. Compared with traditional bio-denitrificaiton process, simultaneous nitrification-denitrification process can reduce the number and size of the reaction equipments, decrease the oxygen supply amount then save energy, decrease or even need no dosage of carbon sources, and save expenditures for dosages, etc. However, from the view of the structure and shape of the Orbal oxidation ditch, (i) the round or ellipse tri-ditch type may be difficult to plane layout for the engineering with narrow land locations; (ii) the independent sedimentation tank occupies relatively large area of land; (iii) central island counts some areas which enlarges inefficient land demand; and (iv) the backflow of sludge and nitrified water from the sedimentation tank relies on dynamic system, which increase the cost of construction and operation. Therefore, the modified Orbal oxidation ditch model is put forward, which is based on analysis of spatial features of bio-denitrification process of Orbal oxidation ditch, with reservation of its simultaneous nitrification-denitrification characteristics in the low oxygen condition in the outer ditch, and with reference of racetrack style configuration and backflow ways in the Carrousel oxidation ditch, as well as in combination of sideditch solid-liquid separation technique in the sideditch integrated oxidation ditch. In comparison with typical Orbal oxidation ditch, the modified Orbal oxidation ditch illustrates more concisely and compactly with no need to construct an independent sedimentation tank, which can decrease 1/3 to 1/2 of land occupation; and the backflow of sludge and nitrified water can be hydraulically processed, which saves the energy consumption for dynamic backflow. In the experiments of modified Orbal process model, main research has been emphasized on the effects of some operational parameters, such as DO, F/M, C/N, HRT and SRT, on its operational performance. The results indicates, with the same level of other parameters, (i) the TN removal can reach 80.0% and 70.9% with the control of dissolved oxygen under the levels of 0.3-0.6mg/l and 0.6-0.9mg/l, respectively; (ii) the TN removal upgrades from 80%, 83% to 84.1% with the decrease of sludge load in the system from 0.1, 0.07, to 0.05gCOD/gMLSS·d; (iii) the nitrogen removal only reaches 58% when C/N is 4, but can reach more than 80% when C/N is at or over 7, which indicates that the TN removal increases when higher C/N ratio; (iv) when HRT increases from 12h, 18h to 24h, the TN removal also increases from 78.6%, 82.0% to 83.0%; (v) the TN removal average in the system increases from 75.6%, 78.5% to 83.0% with the increase of sludge age from 18d, 20d to 24d; (vi) the low oxygen conditions in the modified Orbal oxidation ditch model do not decrease the effect of COD removal, and the dissolved oxygen gradient with the distribution of 0-2-1mg/l in the system ensures the COD removal reaching the range of 80%-86%; and (vii) the phosphate removal, however, is not good enough, which varies from 40% to 50%, even lower than 35% in occasional situations, and the phosphate concentration in the outflow varies in the range of 1.28-2.38mg/l, which dissatisfies the discharge standard with the level of 1.0mg/l. Finally, the bio-dynamical analysis has been made with the experimental results in the modified Orbal oxidation ditch system, the maximum specific velocity of substrate removal Vmax = 6.96d-1, and the organic contaminant degradation coefficient Ks = 108.8mgCOD/d; the reaction order in of NH3-N degradation n=1.0, and the reaction velocity constant k=0.23.
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