SLUDGE TREATMENT USING GAS-LIQUID TWO-PHASE SWIRLING JET AND OZONE

Transcription

1 ISTP-6, 5, PRAGUE 6 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA SLUDGE TREATMENT USING GAS-LIQUID Moriyoshi Shitara *, Manabu Iguchi **, and Taku Tamamori * *Huens Co., Ltd., Obihiro, Hokkaido 8-8, Japan, **Division of Materials Science and Engineering, Hokkaido University, Hokkaido Moriyoshi Shitara - huens@par.odn.ne.jp, Phone , fax Keywords: sludge treatment, gas-liquid two-phase jet, ozone, swirl motion, agitation, mixing time Abstract A sludge treatment using a gas-liquid twophase swirling jet and ozone was newly proposed. A swirl motion of a gas-liquid twophase jet appeared in a cylindrical bath under certain injection conditions, and the bath was strongly agitated in the presence of the swirl motion. The swirling jet had a very excellent effect on the ozone reaction with sludge. The intensity of agitation was quantitatively evaluated by means of mixing time. An empirical equation was derived for the mixing time. Sludge was satisfactorily decomposed by this method, and, as a result, BOD 5 and suspended solid (SS) in the sludge were significantly decreased. Introduction Sludge from sewage treatment facilities is increasing year by year. Several treatment methods have been proposed to reduce the sludge amount ). For example, dewatering, incineration, and drying are very common among them. Recycling of sludge also has been tried by many researchers and engineers. However, further reduction of the sludge amount is still necessary in many countries. An effective treatment method of sludge has been expected. Recently, sludge treatment using ozone has been considered to be one of the effective methods ). In the conventional method, an ozone-air mixture is directly introduced into the reaction tank through a perforated plate. The intensity of agitation is usually weak in this system because bubbles are driven only by the buoyancy forces acting on them. The ozone reaction efficiency in the conventional method is therefore not so high because of its limited mixing intensity ). In this study, a sludge treatment using a gas-liquid two-phase swirling jet and ozone was investigated. When an airwater mixture was injected into a cylindrical bath through a centered bottom nozzle, a swirl motion of a gas-liquid two-phase jet appeared in the bath under certain injection conditions, and the bath was strongly agitated in the presence of the swirl motion 3,). Mixing time was introduced to evaluate the intensity of agitation quantitatively 5). In previous studies, information on the mixing time for a bath agitated by a bottom blowing bubbling jet or a bottom blowing liquid jet was obtained 6-). The mixing time in the presence of the swirl motion was much shorter than that in the absence of it. Unfortunately, the mixing time of a cylindrical bath agitated by a swirl motion of a gas-liquid two-phase jet is not known. It was experimentally revealed in this study. The swirling jet brought a very excellent effect on the ozone reaction with sludge ). The results were presented in this paper. Experiment Figure shows a schematic diagram of the experimental apparatus. For investigating the

2 Moriyoshi Shitara, Manabu Iguchi, Taku Tamamori mixing time, T m, of a cylindrical bath agitated by a swirl motion of a gas-liquid two-phase jet, three test vessels made of transparent acrylic resin were used. The inner diameters of the vessels were D =3, and 39mm. The corresponding heights of the vessels were H=, 5 and 7mm, respectively. De-ionized water was filled to a predetermined depth, H L, in each vessel, drained from four nozzles settled on the bottom of the vessel, and supplied into the bath through a centered bottom nozzle again. Therefore, the water was circulated with a pump. The liquid flow rate, Q L, was changed from to 5cm 3 /s by means of a frequency inverter equipped with the pump. Air was injected into the bath through a pipe connected upstream of the nozzle. The air flow rate, Q g, was adjusted from to 5cm 3 /s with a mass flow controller. After the swirl motion of the gas-liquid two-phase jet appeared in a cylindrical bath, an aqueous KCl solution was poured onto the bath surface as a tracer. The concentration of the aqueous KCl solution was mol/l. The ratio of the amount of de-ionized water in the bath to the aqueous KCl solution was. The mixing time, T m, was measured with an electric conductivity meter. The sensor of the electric conductivity meter was placed mm apart from the side wall of the vessel and mm apart from the bottom of the vessel. A well-known criterion was chosen to determine the mixing time, T m, as shown in Fig.. Meanwhile, for investigating the decomposition of sludge, the cylindrical test vessel was made of transparent acrylic resin. It had an inner diameter, D, of 38mm and a height, H, of mm. Sludge was filled to a depth, H L, of 35mm. Accordingly, the aspect ratio, H L /D, was kept at.9. A mixture of ozone and air was injected into the bath. The sludge was drained from four nozzles placed on the bottom of the vessel and circulated with a pump. The liquid flow rate, Q L, was varied from to 67cm 3 /s. Ozone was artificially generated by making use of an ozone generator. 3 Results and discussion 3. Mixing time in the presence of gas-liquid two-phase swirling jet Figure 3 shows the generation regions of swirl motions of a liquid jet, a gas-liquid two-phase jet, and a bubbling jet, respectively. The inner diameter of the vessel, D, was mm. The x- axis indicates the total flow rate, Q (= Q L + Q g ). Two types of swirl motions occurred depending on the aspect ratio, H L, /D. One is called the shallow-water wave type and the other is called the deep-water wave type ). In the following, the swirl motion of the deep-water wave type will be mentioned. Figure shows the measured values of the mixing time, T m, in the presence of the swirl motions of the three types of jets. The inner diameter of the vessel, D, was mm. The mixing time, T m, in the presence of the swirl motion became short as the liquid flow rate, Q L, increased. All the measured values of the mixing time were compared with the following Eq.() in Fig.5. /T mgl =/T mg + /T ml () where T mgl, T mg, T ml are the values of the mixing time in the presence of the swirl motions of a gas-liquid two-phase jet, a bubbling jet, and a liquid jet, respectively. Equation () can satisfactorily predict the measured values of the mixing time. 3. Sludge treatment using gas-liquid twophase swirling jet and ozone Figure 6 shows a photograph of sludge being treated by gas-liquid two-phase swirling jet and ozone. It was found that the bath was strongly agitated in the presence of the swirl motion. This processing was named the ozone injection with swirl motion. Figure 7 shows changes in SS of sludge before and after the treatment by the two methods. One is the treatment of ozone injection with swirl motion and the other is the treatment using an ozone-air mixture introduced directly into the bath through a perforated plate. This processing was named the ozone aeration. When the treatment time was hours, SS became 66.7% of its initial value of 63mg/L

3 SLUDGE TREATMENT USING GAS-LIQUID for the ozone aeration, while it became only 7.5% for the ozone injection with swirl motion. Figure 8 shows changes in BOD 5 of sludge after treatment by the two methods. When the treatment time was 7 hours, the degradation of BOD 5 was 99.6% for the ozone injection with swirl motion and 3.% for the ozone aeration. The ozone reaction efficiency for the ozone injection with swirl motion was found to be very excellent. This is because the contact efficiency of SS with ozone was highly enhanced, the mean diameter of bubbles of ozone and air mixture was significantly decreased, and the residence time of bubbles became long in the presence of the swirl motion. Figure 9 shows a photograph of sludge after treatment by the two methods. Some of the sludge was not decomposed for the ozone aeration, while the sludge was almost completely decomposed for the ozone injection with swirl motion. Conclusions () Equation () was useful for predicting the measured values of the mixing time. () Sludge was effectively decomposed by a gas-liquid two-phase swirling jet and ozone, and, as a result, BOD 5 and SS in the sludge were significantly decreased. Nomenclature T m : mixing time (s) T mgl : mixing time in the presence of the swirl motion of a gas-liquid two-phase jet (s) T mg : mixing time in the presence of the swirl motion of a bubbling jet (s) T ml : mixing time in the presence of the swirl motion of a liquid jet (s) D : bath diameter (m) d nen : nozzle diameter (m) H : height of test vessel (m) H L : bath depth (m) Q : total flow rate (Q g + Q L ) (cm 3 /s) Q g : air flow rate (cm 3 /s) Q L : liquid flow rate (cm 3 /s) BOD 5 : biochemical oxygen demand (mg/l) SS : suspended solid in sludge(mg/l) References () Journal of Resource and Environment, Vol.39-5, Environmental Communications Co. Ltd., Tokyo, (3). () M.Shitara,M.Iguchi,K.Takano,T.Tamamori, H.Shitara and T.Maruyama: Materials Transactions, (3),56. (3) M.Shitara, M.Iguchi, T.Tamamori, J.Yoshida and D.Iguchi: Tetsu-to-Hagane, 9-6(), 35. () M.Shitara, M.Iguchi, T.Tamamori and K.Takano: Tetsu-to-Hagane, 9-6(), 35. (5) Y.Sasaki, M.Iguchi and S.Yokoya: CAMP- ISIJ, 7(), 3. (6) Y. Takatsuka and M.Iguchi: Journal of Materials Processing and Manufacturing Science, 8(), 65. (7) Y.Takatsuka and M.Iguchi: ISIJ Int., (),. (8) K.Abe and M.Iguchi:ISIJ Int., (), 58. (9) Y.Takatsuka and M.Iguchi: Tetsu-to- Hagane, 88(), 85. () J.Yoshida, D.Iguchi and M.Iguchi: Tetsu-to-Hagane, 9-6(), 37. () M.Shitara, M.Iguchi, and T.Tamamori: Tetsu-to-Hagane, 9-(5),. () M.Iguchi,S.Hosohara,T.Koga,R.Yamagu chi and Z.Morita: ISIJ Int.,33(993), 37. Swirl motion Water or Sludge Pump Electric conductivity meter Personal computer Cylindrical vessel Mass flow controller Regulator Compressor or Ozone generator Fig. Experimental apparatus. 3

4 Moriyoshi Shitara, Manabu Iguchi, Taku Tamamori Output voltage (v) 5 3 (+.5)V F (-.5)V F V F T m 3 Time (s) Fig. Definition of mixing time, T m Q g : Q L = : Experimental Calculated 6 Q = Q g + Q L (cm 3 /s) Q g : Q L = :3 HL / D.5.5 Q=QL QL:Qg=3: QL:Qg=: QL:Qg=:3 Q=Qg 6 Experimental Calculated 6 Q = Q g + Q L (cm 3 /s) 3 Q (cm 3 /s) Fig.3 Generation regions of swirl motions of jets (D=mm). Fig.5 Comparison between experimental and calculated mixing time values in the presence of swirl motion (D=mm). 5 3 Q=Qg Qg:QL=3: Qg:QL=: Qg:QL=:3 Q=QL 6 Q Q L Q g (cm 3 /s) Fig. Mixing time in the presence of swirl motion of jet (D=mm). Fig.6 Photograph of sludge being treated by gas-liquid two-phase swirling jet and ozone.

5 SLUDGE TREATMENT USING GAS-LIQUID SS (mg/l) 8 6 Before Before After After Swirl motion Fig.7 Changes in SS of sludge after treatment by the two methods (treatment time=hours). 3 BOD5 (mg/l) Swirl motion 6 8 Treatment time (t/h) Fig.8 Changes in BOD 5 of sludge after treatment by the two methods. Swirl motion Fig.9 Photograph of sludge after treatment by the two methods. 5