Practical planning of oxidation ditch process using axial flow pump for aeration

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1 \ Indian Journal of Engineering & Materials Sciences Vol. 5, August 1998, pp Practical planning of oxidation ditch process using axial flow pump for aeration Masuo Ozaki", Hideo Nakasone" & Tsuneo Tanaka" adepartment of Civil Engineering, Maebashi Institute of Technology, 460 Kamisadori, Maebashi, GUIl)IIla,Japan bfaculty of Agriculture, Ibaraki University, Ami Chuoh, Inashiki, Ibaraki, Japan 'Department of Civil Engineering, Gumma University, Tenjin, Kiryu, Gumma, Japan Received 7 February 1997 An oxidation ditch process uses falling water as aerator with the falling water being caused by a pump. Submergible motor pump (D300mm, 7.5kWh) was used in the previous experiments and it was changed to axial flow pump (0500mm, 3.7kWh) this time. Several points regarding the pump change and introducing this oxidation ditch process into a practical use are considered in the present paper. The rotation speed of the axial flow pump (250 rpm) was lower than that of the submergible motor pump (1,600 rpm). Floc diameter using the axial pump was found to be larger than that of the submergible motor pump. It is found that these differences had no serious effects on the experimental results carried out after However, larger floc diameter is ascribed to the mixed liquor entering into sedimentation tank drawn from the water surface of the ditch. If the mixed liquor is drawn at a point near the bottom, the treatment results will be the same as the experimental results before 1994 and the scum will not enter sedimentation tank. The results revealed that a baffle and a shower are necessary to prevent scum formation on the water surface of the ditch. Small air bubbles attached on the floes of sludge, since air and water mixed strongly with the pump and the falling water. Other parameters such as length of the ditch, suitable interval of intermittent aeration, control of scum formation, and the nitrogen and phosphorus removal are also investigated. Oxidation ditch process was developed in The Netherlands for the first time' and the aerator used in the beginning was horizontal rotor and the ditch was masonry. After that many aerators were developed" At present, the oxidat'ion ditch processes are widely used all over the world. The authors started the oxidation ditch process experiments using the falling water as an aerator since The first equipment used was of smaller capacity (70 L/day) and it was aimed to treat animal waste water. After the completion of the experiment, a concrete oxidation. ditch (V=60m 3 ) was constructed to treat human waste water and investigations were started in The experimental results obtained in 1991 and 1992 have already been presented". In this paper, fundamental aspects of this oxidation ditch process are discussed with a special emphasis on the idea that the pump can be used for aeration and it can successfully cause mixed liquor flow in a ditch. Some differences in treatment process are also investigated. Meanwhile, this paper supplies some important information which could not be presented in the earlier work". In addition, the authors would like to discuss the practical use of this oxidation ditch. Experimental Plant and Planning Condition The outline of the experimental plant is shown in Fig, 1. The ditch width is 2.0m, the water depth is 1.0 m, and the center-line length of the ditch is 30.0m. Two final sedimentation tanks are located inside of the ditch. The submergible motor pump that was used before 1994 is shown in Fig. 2 and the axial flow pump which was used after 1994 is shown in Fig. 3. Abilities of both submergible motor pump and axial flow pump are shown in Table 1. The diameter of the axial pump was greater than that of the submergible pump. The power consumption and the rpm of the submergible pump were higher than that of the axial flow pump. The width, depth and length of I

2 168 INDIAN 1. ENG. MATER. scr., AUGUST 1998 Fig. I--Dutline of the experimental plant Fig. 3-Axial flow pump Table I---Characteristics of pumps Pump Power consumption Pump diameter Pump discharge Pump rotation Submergible 7.5 kw.h 300mm 12.0ml/m 1,600 rpm Axial flow 3.7 kw.h SOOmm 12.5 mj/m 250 rpm Fig. 2-Submergible motor pump this oxidation ditch process are highly dependent on pump ability used. The minimum velocity needed in the ditch was 0.1 mls and the needed area of the ditch is 2.0 m". If the width is 2.0 m, the depth will be 1.0 m, because the amount of treatment waste water is 60 m'/d. The oxygen demand of oxidation ditch process was 1.5 times more than that of influent BOD found in experimental results". The oxygen supply by the falling water was also calculated using Eq.( I)' In the case of falling height hi.2m, unit discharge q<235 m 2 /h.m In r =5.39 h UJ x q-0363 x H03lO... (I) where r =deficit ratio at 20 0 e and given as r =(C- C,)/(C<-C2); C = saturation concentration of oxygen in water (mgil); C, and C 2 = DO concentrations of upstream and downstream of the weir (rng/l), respectively; and h = height of falling water (rn), q = discharge per metre of the nappe (m 2 /h. m), H = tail water depth (m). In calculation of Eq.( I), H = (2/3) x h when H >(2/3) x h is important. Also, if the falling water is divided into multiple streams, the aeration efficiency increases further". The increased deficit ratio due to the separation of the nappe is expressed in Eq. (2). E = In r211.1 I In r20.. (2) where E is the rate of increase due to separation of the nappe, r211s is the deficit ratio at 20 0 e with the separated nappe, and r20 is the deficit ratio at 20 0 e without the separation of the nappe. The value of the E was 2.7 at maximum as obtained previously by Nakasone'. Using the axial flow pump shown in Table 1, and, assuming that the DO concentration of upstream is 2.0 mg/l and h is 0.6 m, H = (2/3) x 0.6 since H >(2/3) x h, and the nappe is divided, the deficit ratio at 20 e 0 is r20 = And deficit ratio decreases according to the concentration of MLSS and it was 85% when MLSS concentration was 3,000 rng/l'. Then the C 2 is 4.7 mgil since r]o.1 is 1.664[=( )/(8.84- C 2 )] and additional quantity of DO is 2.7 x 11.5 x 1440 = 44.7 kg-os/d. Since the oxygen consumption of the oxidation ditch process is 1.5 times of the influent BOD (60m 3 /d x 200mglL x 1.5 = 18kg-0 2 /d), the oxygen

3 OZAKI et al.: OXIDATION DITCH PROCESS 169 supply is about 2.5 times of the oxygen demand. Therefore, the minimum velocity is a restriction factor in this oxidation ditch process. Hence, 2.5 times more waste water could be treated with this axial flow pump in Table I. Then high oxygen concentration was always achieved in this oxidation ditch process, because the oxygen transfer decreases according to the increase of upstream DO concentrationt Ci), since driving force(=cs-c,) decreases. Results and Discussion The submergible motor pump was replaced by the axial flow pump and experiments were started in September 1994, when MLSS concentration was more than 3,000mglL. Twelve data were taken in 1995 and the comparison of average data taken between 1991 and 1995 are given in Table 2. As a result of pump change from the submergible motor pump to the axial flow pump, the experimental results showed that the energy input is about half, the removal rates were nearly the same. The reason why such results were observed was that the intake point near the bottom was changed to the surface of the ditch. Then light floc and scum influenced into sedimentation tank and light pin floc flew out into the effluent. The differences of intake points are shown in Fig. 4. The change of the pump did not affect the nitrogen and phosphorus removal as shown in Table 2 since nitrogen and phosphorus does not have direct relation with the floc size. For organic materials such as BOD, COD and SS, however, the removal results were affected by the floc. size. Since contact chances between the microbes and the waste water of small floc size were greater than of big floc size. The smaller the diameter of floc " the greater the relative contact area between waste water and microbe. Therefore, a position of the intake point will be important and it should be near the bottom of the ditch. Different diameters of floc according to the change of the pump rotation are exhibited in Fig. S. Other results observed in a horizontal rotor (Plant A) are also described. Fig. 5 shows that the number of floc size larger than 400 urn in diameter is much higher in Plant A. The diameter of the experiments conducted in 1995 is clearly larger than that of the experiments of The flocdiameter larger than 300 microns appeared in the experiments performed in This change of diameter will very much influence the CENTER 'ElL OXIDATION IJlTClI Fig. 4-Difference of intake points and the intake equipment 8 8 N N 8 8 N N "" Floc size (14m) Fig. 5--Distributional sketch of floc diameter Table 2-{:omparison of data between 1991 and 1995 Parameters Influent Effluent Removal rate Influent Effluent Removal rate BOD(mgIL) % % COD(mgIL) % % SS(mgIL) % % ph T-N(mgIL) % % T-P(mg/L) % % \

4 170 INDIAN 1. ENG. MATER. SCI., AUGUST 1998 sedimentation velocity in the sedimentation tank as shown in Fig. 6. It had been hoped that the sedimentation velocity would increase by the increased diameter of the floc, but it did not happen. Hence, it is suggested that the change of floc size was also caused by the appearance of different kinds of microbes shown in Table 3. Since the water was mixed vigorously by the pump and the falling water, much foam was formed following the falling water. Fine bubbles also attacked to floes which would eventually float on the surface of the ditch. To prevent the scum and bubbles formation, some measures were necessary to be taken in this process. This was Table 3-Test results of microbe Name of microbe Appearance(N/mg) Exp Exp Ciliata Vo/ticella Epistylis Opercu/aria 0 17 Aspidisca Eup/otes 0 17 Tokophrya 25 0 Spirostomum 0 34 Sarcodia Amoeba Arce lla Eug/ypha Mastigophora Peramera 0 68 Matazoa Rotaria 0 68 Co/urella ]:-10 o -20.S; 1-30 fo -e- Exp Exp PlantA e-, <, 15 -SO 1_ 60 o Time elapsed (min) easily overcome by means of putting the shower in front of the aerator and setting the plastic board in downstream of the weir as shown in Fig. 7. The board should be placed after the roll of falling water. Though the distance from the wall is dependent on the discharge, more than 3.0 m is necessary. The direction of shower should also be given a special attention. Table 3 shows that Vorticella appeared to a much higher value in 1995, which concluded that this oxidation ditch process had become nearly equal to other mechanical aeration using some kinds of rotors. From these experimental results, treatment conditions suggest that the pump using high rpm was favorable rather than using low rpm. It could also prevent bulking since the chains of Hyphomycetes were broken with the impeller of the pump in this oxidation ditch process. The density of floc is larger than that observed in other mechanical aerators, which is convenient for treatment even if the floc diameter is small. As it is well known, the nitrogen removal of the oxidation ditch process shows favorable results. There are some kinds of nitrogen removal methods. In general, making the anoxic zone before aerator, intermittent aeration, and the batch process are commonly used for an oxidation ditch process. In this oxidation ditch process, falling water caused by axial flow pump is used as an aerator. This is due to the fact that the length of the ditch is short and it is difficult to make anoxic zone in front of the aerator. The DO concentrations according to the flow direction of the center in the ditch were measured and the results are shown in Fig. 8. The DO concentrations in Fig. 8 were measured while a mixed liquor temperature was 22 e. As already explained, It must be difficult to make the anoxic zone in front of the aerator in this oxidation ditch process. To remove nitrogen, intermittent aeration was only available. The oxygen 3m Fig. 6---Sedimentation velocity in the ditch Fig. 7--Control of scums and foams in ditch

5 \ OZAKI ct al : OXIDATION DITCH PROCESS Mo -v-, a \ GG I -, DO CONCENTRATION (mg/l)......_ S Fig. 8--DO concentration of the ditch center 1.1 -S I o MLSS (mg/i) Fig.l0-0xygen transfer reduction due to MLSS concentration Fig. Cf--DO concentration and aeration patterns for good denitrification consumption was ImglL in 5 min, since it takes 5 min for one circulation. Therefore, the oxygen consumption rate is 12 mglllh. In this case, in order to create the anoxic condition in oxidation ditch, 36 min are necessary. Since the oxygen consumption rate decreases according to the decrease of temperature, the time schedule of intermittent aeration was favorable as shown in Fig. 9 (ref. 4). The deficit ratio at 22 C calculated from the DO concentration in Fig. 8 is 1.448[ =( )/( )]. The deficit ratio calculated from the Eqs (1) and (2) was r2ijs=l.958. This deficit ratio was changed into at 22 C from Eq. (3). In r, =In r2()[ ( T - 20 C)] (3) The value of2.002 could be verified while water is clean. Therefore, some revision is necessary, it can be done using Fig. 10 (ref. 7). The dissolved oxygen concentration presented in Fig. 8 was measured when MLSS concentration was 3,200 mgll. In this MLSS concentration, the deficit ratio at 20 C decreased about 16%. Then deficit ratio rso. is l.645 = l.958 x 0.84 and r22s =l.701 from Eq. (3). Relative error for calculated value is 14.9 % «l )/l.701). This error would be rather satisfactory even though conventional DO meter was used for DO measurement. Phosphorus removal was done with coagulant precipitation into the influent waste water directly in this oxidation ditch process. The coagulant quantity of precipitation was Fe: P = Imol : I mol. Great change was not observed by the change of pump 'as shown in Table 2. Therefore, the coagulant precipitation of FeCh is available in this oxidation ditch process. The large amount of sludge increase due to coagulant precipitation was not observed since sludge in the ditch was always broken with the pump impeller and the increase of sludge was very small. Conclusion The axial flow pump does not break easily. The on-off operation and maintenance of the pump are much easier. Aerator using the pump is very cheap compared with the mechanical aerator. It is proposed, therefore, that this oxidation ditch process will benefit especially in the developing countries. The authors will then need to consider practical aspect if a plan is made for oxidation ditch process. Based on the findings of this study, the following conclusions are drawn: I. The pump rpm became low (250 rpm) by means of the change of the pump. The removal condition did not improve. This situation might have happened from another reason such as the change of the intake point of mixed liquor. 2. The removal condition in the case of small floes caused by a high speed impeller improves, because bulking was prevented by the fact that the hyphomycetes are broken into pieces with the high speed of the impeller of the pump. 3. To prevent scum accumulation in front of the pump, a plate and a shower are necessary as shown in Fig. 7. /

6 \ 172 INDIAN J. ENG. MATER. SCI., AUGUST It is enough to supply oxygen which is 1.5 times the amount of the influent BOD in the oxidation ditch process. The oxygen transfer by falling water is calculated by Eqs (1)-(3). The calculated values match the measured values fairly well. 5. The nitrogen removal is performed well by the intermittent aeration as shown in Fig The phosphorus removal is performed well by precipitation of FeCh into the oxidation ditch directly. References 1 CEP Consultants Ltd, Oxidation ditch technology, Proc of Int Conf, Edinburgh, UK, (1982). 2 Hoshikuma Y, J Uater Waste, 26(1)(1989) Nakasone H, Study on animal waste water treatment, Proc of Annual Conference of JSIDRE, (1979), Nakasone H, J Environ Eng, ASCE, 113(1)(1987) Nakasone H, Ozaki M, Abe I & Fujisaki M. Trans JSIDRE, 143 (1989) Nakasone H & Ozaki M, J Environ Eng, ASCE, 121 (2) (1995) Van der Kroon G T M & Scharman A H, Weir aeration part 2, H 2 0, No.22, (1969), /

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