Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) with irreplaceable significance are comprehensive indicators for evaluating water quality. Effective control of water pollution depends on the rapid determination of these indicators. In the study, the rapid determination of BOD was carried out by activated sludge aeration degradation method. The BOD quantification was achieved by measuring the COD change before and after microbial degradation (BOD ΔCOD). Singlefactor experiment was employed to investigate the effect of operating parameters on the determination of BOD. In addition, the interactions of main variables were evaluated by Box-Behnken Design, and the results showed that the interaction term of reaction time and initial pH value had a significant effect. The optimum conditions for determination of BOD were reaction time of 273.68 min, dosage of activated sludge of 8.72 mL, and pH of 7.18. Under the optimal conditions, the correlation experiment showed that BOD ΔCOD and BOD 5 exhibited a good linear relationship in the concentration range of 0-500 mg/L, and the correlation coefficient R 2 was 0.996. It indicated that the method realized not only the rapid determination of BOD, but also the integrated determination of COD and BOD.

Abstract. Based on a 380mm × 280mm bloom caster mold, the level fluctuation of steel-slag interface in the mold was simulated by the VOF model of commercial software Fluent. The effects of current intensity and frequency of EMS (electromagnetic stirring) on the level fluctuation in the mold were studied. The results show that whether or not with EMS, the maximum level fluctuation site of the mold occurs in the vicinity of the submerged entry nozzle. Compared with casting without EMS, molten steel flows horizontally rotatably under the action of the electromagnetic force by electromagnetic stirring, so the impact depth of molten steel decreases, then the level fluctuation slightly reduces, and the maximum level fluctuation value in the wide direction and the narrow direction of the mold, reduce from 4.24mm and 4.14mm to 4.04mm and 3.73mm respectively. With increasing intensity and frequency of current, the mold level fluctuation rises and the distribution uniformity of the level fluctuating amplitude worsens. The maximum level fluctuation enlarges by 0.18mm with raising the current intensity from 450A to 550A, but it enlarges by 0.79mm with 600A current intensity. The maximum level fluctuation enlarges by 0.15mm with raising the current frequency from 1.5Hz to 2.0Hz, but it quickly enlarges by 0.78mm with 2.5Hz current frequency. When the current strength and frequency are not more than 550A and 2.0Hz, level fluctuations are 4.00mm or less, which can meet requirements for controlling the bloom surface quality. IntroductionElectromagnetic stirring (EMS) is widely used for its metallurgical effect on inclusion removal, the elimination of subsurface bubbles, reducing central segregation and improvement of central equiaxed grains. In recent years, the EMS technology has been researched by a number of metallurgical scholars. Liang Zhou et al [1][2][3] have tested the magnetic field of M-EMS by using a Gauss meter, qualitatively analysed the characteristics of the electromagnetic field, and the relationships of current intensity with stirring speed, molten steel flow, fluid velocity distribution were discussed. Miaoyong Zhu et al [4] have simulated the three-dimensional flow field and temperature field of a bloom mold, researched the influence of stirring intensity on the flow field and temperature field of molten steel, but dealt with the mold free surface as a plane of symmetry or no shear stress free wall. Furthermore, research on the level fluctuation was not done. At present, studies of the free surface mostly focuses on slab molds [5,6], and research on bloom molds is rarely reported [7,8]. Jiaquan Zhang et al [7] have studied the level fluctuation of the free surface under different nozzle hole angles by building a

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