Đang tải... (xem toàn văn)
ABSTRACT The Odaiba Marine Park is a typical sightseeing place that represents Tokyo. But unfortunately the Odaiba Marine Park is not in comfortable condition for the people dabbling in seawater, because of the increase of fecal coliforms due to the CSO (Combined Sewer Overflow) system after heavy rain. Tokyo Metropolitan Government started the ocean area purification experiment with EBARA Corporation. The filtration and the ultraviolet ray disinfection were adopted for the seawater purification process. The purification zone in the Odaiba Marine Park was partitioned with two oil fences, and the water from the purification plant was continuously drained off, at 5,000m3/day for three consecutive months. The purified seawater was discharged from July to October in 2003. Moreover, the effectiveness of photoreactivation, seen as the most challenging task when adopting disinfection by ultraviolet ray, is reported as the preliminary study conducted in 2002. The model tests, using seawater mixed with untreated sewage resulted in either a lower photoreactivation rate of the coliform groups, or further inactivation of the groups. This suggests that the coliform groups of the freshwater-origin are affected by salt density in the seawater with inhibition of the photoreactivation.
Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 63 - Seawater Purification Experiment and Consideration about Photoreactivation of Coliforms in the Inside of Seawater R. HATA *1 , K. SASAKI *2 , K. TSUTSUMI *3 , M. AKATSU *4 *1 Engineering Department, Municipal Water Works Division, Ebara Corp. 1-6-27 Kohnan Minato-ku, Tokyo 108-8480, JAPAN (E-mail: hata.ryosuke@ebara.com) *2 Applied Chemistry Lab., Ebara Research Corp.4-2-1 Honfujisawa, Fujisawa-shi, Kanagawa 251-8502, JAPAN (E-mail: sasaki.kenichi@er.ebara.com) *3 Engineering Department, Municipal Water Works Division, Ebara Corp. 4-2-1 Honfujisawa, Fujisawa-shi, Kanagawa 251-8502, JAPAN (E-mail: tsutsumi.kaori@ebara.com) *4 Engineering Department, Municipal Water Works Division, Ebara Corp. 1-6-27 Kohnan Minato-ku, Tokyo 108-8480, JAPAN (E-mail: akatsu.miki@ebara.com) ABSTRACT The Odaiba Marine Park is a typical sightseeing place that represents Tokyo. But unfortunately the Odaiba Marine Park is not in comfortable condition for the people dabbling in seawater, because of the increase of fecal coliforms due to the CSO (Combined Sewer Overflow) system after heavy rain. Tokyo Metropolitan Government started the ocean area purification experiment with EBARA Corporation. The filtration and the ultraviolet ray disinfection were adopted for the seawater purification process. The purification zone in the Odaiba Marine Park was partitioned with two oil fences, and the water from the purification plant was continuously drained off, at 5,000m 3 /day for three consecutive months. The purified seawater was discharged from July to October in 2003. Moreover, the effectiveness of photoreactivation, seen as the most challenging task when adopting disinfection by ultraviolet ray, is reported as the preliminary study conducted in 2002. The model tests, using seawater mixed with untreated sewage resulted in either a lower photoreactivation rate of the coliform groups, or further inactivation of the groups. This suggests that the coliform groups of the freshwater-origin are affected by salt density in the seawater with inhibition of the photoreactivation. KEYWORDS Combined Sewer Overflow, ocean area purification, ultraviolet rays disinfection, photoreactivation 1. INTRODUCTION Odaiba Marine Park is one of few sandy beaches in the Tokyo Metropolitan area, welcoming many visitors and children to play in the water during summer. The Park, however, is not in a preferred condition for those people, due to chronic appearance of red tide from spring to summer. Also, the deterioration of the water quality is observed after a heavy rain due to influence of sewage effluents. Tokyo Metropolitan Government (Bureaus of Environment, Port and Harbor, and Sewerage) and Ebara Corporation have been jointly conducting an experimental research of sea area purification of 3 year, since 2003. The sea area purification experiment was performed in a partition of Odaiba Marine Park and the effect of the purification has been investigated. In the year of 2003, purified seawater discharge and investigation were conducted from July, 18 to October, 31 in total amount of 106 days. One partition of the sea area of Odaiba Marine Park was elaborated as a target for purification experiment and silt fence was used to define the boundary of this purification zone. Filtration and ultraviolet ray disinfection was adopted as the purification method. In a purification plant that was build neighboring the Ariake West Canal and distancing 1 km to the Odaiba Marine Park, seawater was filtrated to increase the UV ray transmittance and coliform groups were inactivated by a medium pressure UV lamps. This purified seawater was pumped at a rate of 5,000m 3 /day and discharged into the purification zone in Odaiba Marine Park. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 64 - Sea bathing water quality standard was taken as reference to establish the target values for the experiment. The target values in the purification zone were set as follow: fecal coliforms low than 10count/100mL and COD Mn low than 5mg/L. The target treatment value for the purification plant was set as follows: inactivation rate of fecal coliforms higher than 99.99% or less than 10count/100mL and COD Mn low than 5 mg/L. Additionally, a tendency that the number of microorganisms was found to decrease after the photoreactivation is reported here. This is a result of photoreactivation experiment conducted as preliminary tests of UV disinfection performed in the year 2002. 2. METHODS 2-1. Seawater purification experiment Figure 1 shows the overall configuration of the purification facility. Seawater was purified in a plant located in the Ariake Water Reclamation Center, Tokyo Metropolitan Bureau of Sewerage. The purified seawater was supplied from the purification plant at a flow rate of 5,000 m 3 /day to the purification zone distancing 1 km in Odaiba Marine Park. The purification zone consists of a partition of approximately 7,000 m 2 in area (approx. 10,000 m 3 in volume). The basic flow for the treatment method was filtration+disinfection. The filtration facility is composed by biofilters, in which the SS-COD Mn is decreased by the SS removal and D-COD Mn is decreased by the biological treatment effects; resulting in the improvement of UV transmittance through the seawater and reduction of fecal coliforms counts. The facility adopted for the disinfection was the ultraviolet ray that has less impact to the ecosystem. Specifications of the purification equipment and the UV disinfector are shown in Tables 1 and 2, respectively. The target water qualities in the purification plant and the purification zone are shown in Tables 3 and 4, respectively. The target values for water quality were selected taking as a reference the "sea bathing water quality standard" established by the Ministry of the Environment, Japan. The water quality rank [A], was selected for all items with exception of COD Mn , in which rank [B] was adopted. Please, note that the highest rank for water quality is [AA] followed by [A] and [B]. Table 1. Spec of the purification equipment Table 2. Spec of the UV disinfector Overall plant Throughput 5,000 to 7,500 m 3 /day Not raining Under rain Config. Ф3800 × 2 tower in parallel Required inactivation rate (%) 98.3 99.95 Filtration device using biofilters Filtration speed LV = 300 to 450 m/day Throughput (m 3 /day) 5,000 5,000 Type Medium pressure UV transmittance (%) 90 80 Calculation parameters D10 value* (mJ/cm 2 ) 30 15 No. of UV lamps 6 lamps × 2 blocks Total: 12 lamps For 6-lamp configuration 99.7 99.90 UV disinfector Power 24 kw Inactivation rate (%) For 12-lamp configuration 99.9990 99.9998 *D10 represents the UV dose required for 90% inactivation (i.e. log (survival rate) of -1). Ariake West Canal Filtration device Disinfection device Supply Pipes Seawater intake Purification plant Discharging Partitioning Odaiba Marine Park Figure 1. Facility configuration Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 65 - Table 3. Target water quality in the purification plant Table 4. Target water quality in the purification zone Figure 2 shows the flowchart of the seawater purification plant. Seawater was supplied from the Ariake West Canal and a 30mm open mesh net was used in the sluice gate to prevent the drawing of eventual garbage and jellyfishes. Additionally, a 2mm open mesh screen was provided on the inflow side of the primary seawater tank. After filtrated, the seawater was aerated in the filtrated seawater tank increasing the DO level before discharging. Therefore, the oxygen deficiency was avoided in the seawater at purification zone. Figures 3 and 4 show the top view and cross-section view of the purification zone at Odaiba Marine Park. Inside the park, the purified seawater is delivered to the purification zone by a water pipe buried under the beach and is gradually poured out from perforated discharge pipes laid under the sea bottom. The purification zone was sectioned with a double fence structure consisting of dropping and raising curtain oil fences. The fences are "silt fences," which are used for offshore works, each having curtains that allow the passage of water, but avoid the passage of turbid particles. Water quality investigations were conducted by sampling seawater at points indicated with green X marks in Fig. 3 (three points inside and one point outside of the purification zone). Item Fecal coliforms count COD Mn Target Inactivation of 99.99% or more or 10 CFU/100 mL or less 5 mg/L or less Item Fecal coliforms count Oil film COD Mn Transparency Target 100 CFU/100 mL or less Not found 5 mg/L or less over 1 m Figure 2. Flowchart of seawater purification plant U V Ariake West Canal Primary seawater tank Filtration equipment × 2 Filtrated seawater tank (Air) Ultraviolet disinfector Odaiba Marine Par k Purified seawater tank Ariake WWTP. Primary seawater tank Backwashed water tank Screen Figure 3. Purification zone top view 送水管 自立式オイルフェンス 垂下式オイルフェンス LWL HWL 平均 水位 放流管 管理室 水際線200m 70m 送水管 自立式オイルフェンス 垂下式オイルフェンス LWL HWL 平均 水位 放流管 管理室 水際線200m 70m Dropping curtain oil fence Raising curtain oil fence Laying-under-the-ground dischar ge pipe Care house Average tide level Water pipe Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 66 - 2-2. Preliminary photoreactivation test The present investigation performed between October and November, 2003 was conducted as preliminary test of seawater purification. Here, the behaviors of fecal coliforms that outflow by CSO were studied. Test water samples were made collecting seawater from the canal and mixing with 0.5 to 1.6% of raw sewage. Raw sewage was obtained from the Ariake Water Reclamation Center. The samples were UV-treated (25 to 39 mJ/cm 2 ), using a compact test device with medium-pressure UV lamps. For the following photoreactivation treatment; the UV-treated samples were enclosed in sterilized glass petri dishes, 50 mm in internal diameter and 15 mm in depth, with quartz caps. They were immediately placed outdoors and left there for one to two hours, subject to sunlight of 27,000 to 57,000 LUX. Then, the number of coliforms and fecal coliforms in the samples were counted. 3. RESULTS 3-1. Seawater purification experiment Since the summer of 2003 was cooler than the usual, water temperature was kept low in general and raw water quality deterioration due to the red tides was not observed too much. The occurrence of CSO by the rain was sporadically seen, but as the filtration facility removed almost 80% of the coliforms, UV disinfector could be operated with a sufficient margin of reserve in overall. Table 5 summarizes the calculated UV dose of the disinfector, based on the UV transmittance of influent seawater (filtered seawater), during the experimental period. Although the twelve UV lamps originally installed were reduced to six on September 1, the UV dose become excessive along the experimental period. Table 5. UV dose from the UV disinfector 12 lamps 6 lamps Period July 18 to Sept. 1 Sept. 2 to Oct. 13 UV transmittance [%] 92 (86 - 94) 94 (91 - 97) UV dose [mJ/cm 2 ] 166 (119 - 194) 106 (80 - 138) Table 6 lists the average values of water quality during the experimental period at: the raw water colleted at Ariake West Canal, purification plant discharge, three points inside and one point outside of the purification zone at Odaiba Marine Park. Figure 5 shows the profile of coliforms and fecal coliforms count in the raw water, filtered water and purified seawater during the experimental period. Figure 4. Purification zone cross section LWL HWL 70m 送水管 オイルフェンス カーテン 放流管 砕石 LWL HWL 70m70m 送水管 オイルフェンス カーテン 放流管 砕石 Oil fence Curtain Discharge pipe Crushed stone Water pipe Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 67 - Table 6. Water qualities in and out of the experimental zone (Ave. during the experiment) Items Raw water Purified Inside zone (average.) Outside the zone Ave. 141 291 90 95 Transparency [deg.] Range 25 - 280 85 - 300 12 - 280 20 - 250 Ave. 2.3 0.5 3.9 3.0 Turbidity [deg.] Range 0.8 - 17 0.2 - 2.0 1.1 - 26 0.8 - 15 Ave. 4.8 0.7 7.1 6.0 SS [mg/L] Range 1.5 - 30 0.2 - 3.7 1.8 - 28 2.1 - 15 75% value 2400 <3.6 2,100 11,000 Coliforms count [MPN/100 mL] Range <36 110,000 Max. <3.6 * <36 190,000 Max. <36 1,100,000 Max. 75% value 176 <2 217 730 Fecal coliforms count [CFU/100 mL] Range <2 - 19,800 <2 * <2 - 31,100 <2 - 35,200 Ave. 3.0 2.5 3.4 3.7 COD Mn [mg/L] Range 1.2 - 5.4 1.2 - 5.2 1.7 - 11 1.6 - 9.9 *) Coliforms and fecal coliforms were not detected in the purified seawater during the whole experimental period. As a result of investigation of water quality in the purification plant, the coliforms and fecal coliforms was not detected in the purified seawater, and the UV disinfection was found to be sufficient for inactivation. Referring to COD Mn , the low values found in the raw water quality during the experimental period also contributed and the targets values set in Table 3 for the purification plant was practically achieved. Referring to the result of the water quality in the purification zone, if we compare the number of fecal coliforms (75%value) inside and outside the purification zone, inside the purification zone was 217 counts/100mL against 730counts/100mL outside the purification zone. Furthermore, if we look to the profile of fecal coliforms, the number of fecal coliforms clearly increased after the rain due to the influence of CSO. The influence of CSO occurred predominantly after the rain rather than during the rain and depending on the rainfall conditions, this influence to the water quality was hold for several days. If we compare the recovery tendency of water quality between inside and outside of the purification zone, clear tendency for fast recovery inside the purification zone was observed. From these results, we can conclude that the purification facility and methodology used this year allowed to achieving a certain level of purification efficiency. However, the number of fecal coliforms (75%value) inside of purification zone was 217 counts/100mL and the values set in Table 4, 100count/100mL were not satisfied. This was found to be due to the inflow of sea water to the purification zone caused by the tides and also due to the inflow during the high tide at the end of silt fences located at the water’s edges. Concerning to the seawater inflow from the edges of silt fence, a provisional wall was installed at the end of the experimental period of this year, with excellent results. Such result will be surely reflected in the experimental methodology of next year. Following, the appearance frequency of each rank of sea bathing water quality standard inside of the purification zone are illustrated in Table 7. The numbers of days that could be graded as rank “A” was 47day (44% of total days) outside against the 67days (64% of the total days) inside the purification zone. The ranked days were 20 point higher inside rather than outside. On the other hand, the number of days that was graded as “bathing prohibited” at inside and outside of purification zone were 11days (10%) and 21 (20%), respectively. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 68 - Table 7. Fecal coliforms count index in the sea bathing water quality standard Bathing allowed Sea bathing water quality standard Approriate (A) Allowed (B･C) Bathing prohibited Fecal coliforms count (CFU/100 ml) 100 Max. 101 - 1000 1001 Min Frequency Day % Day % Day % Inside the experimetal zone 67 64 28 26 11 10 Outside the experimetal zone 47 44 38 36 21 20 (Experimental period:106 days in total) 3-2. Preliminary test of photoreactivation The results are shown in Tables 8 and 9. The coliforms inactivation rate by UV treatment in four experiments was in the range of 99.78 to 99.96%, which is similar to the rate set in the UV treatment guideline for sewage. With photoreactivation for one to two hours after the UV treatment, the coliform groups in the 1 10 100 1,000 10,000 100,000 ふん便性大腸菌群数[個/100ｍL] 原水 ろ過水 浄化海水 0 20 40 60 80 100 120 140 160 7/18 7/28 8/7 8/17 8/27 9/6 9/16 9/26 10/6 10/16 10/26 降雨量[mm] 1 10 100 1,000 10,000 100,000 7/18 7/28 8/7 8/17 8/27 9/6 9/16 9/26 10/6 10/16 10/26 ふん便性大腸菌群数[個/100mL] 区域内 区域外 図5 降雨量とふん便性大腸菌群数の関係 Rainfall [mm] Fecal coliforms [ count/100 mL] Fecal coliforms [count/100 mL] Primary water Purified seawater Filtered water In the zone Out of the zone Figure 5. Rainfalls vs. fecal coliforms counts Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 69 - experimental run 1 and 2, in which 0.5% raw sewage was added, shows a tendency to stopping the increase of its population to 2-6 times larger than that immediately after the UV treatment. In the experimental run 3, in which 1.6% of the raw sewage was added and UV irradiation was 31 mJ/cm 2 , the increase of population stopped at 2-5 times larger. On the other hand, at the run 4, in where the UV dose was reduced to 25 mJ/cm 2 , showed only a slight population growth from 240 to 270 count/100 mL. This preliminary tests showed that the run 4 that has a low inactivation by UV resulted in small photoreactivation. Even the run 1, in which a high inactivation by UV was achieved, resulted in a population increase of coliforms of only 6 times. The fecal coliforms showed the following tendency: undetected immediately after UV treatment and 17 count/100 mL or lower after photoreactivation in the experimental run 1 and 2. This was found to be due to the low amount of fecal coliforms in the raw water and due to the high UV inactivation rate, which was 99.8%. Assuming that the amount of fecal coliforms are 4000 count/100 mL in the raw water, and the inactivation rate is 99.8%, the fecal coliforms amount after the UV treatment should be 8 count/100 mL. Since, the analytical detection limit of the fecal coliforms is 17 count/100 mL; the amount increase should be within two times of the original amount, even if the photoreactivation takes place. However, according to the results in the experimental run 3 and 4, the surviving fecal coliforms immediately after the UV treatment was 50 count/100 mL and after the photoreactivation the fecal coliforms could not be detected. That means, the number of fecal coliforms fell below 17 count/100 mL. In this photoreactivation test; seawater was mixed with raw sewage that contains huge amount of freshwater-origin coliform groups. This test showed a tendency that the coliforms from freshwater origin has a poor capacity for photoreactivation or even that the coliforms are inactivated in seawater environment. The results suggest that coliform groups living in fresh water are influenced by the salt concentration and the photoreactivation are inhibited when discharged in seawater. Table 8. Photoreactivation test results (coliforms) Coliform groups MPN/100 mL Run Raw sewage added amount (%) Date UV dose (mJ/cm 2 ) Raw water Disinfected seawater Inactivation rate (%) Disinfected seawater, photoreactivated for 1 hr Disinfected seawater, photoreactivated for 2 hr 1 0.5 2002.10.10 38.8 2300 3.6 99.84 23 23 2 0.5 2002.10.10 31.1 2300 3.6 99.84 7.4 15 3 1.6 2002.11.19 31.2 110000 43 99.96 93 210 4 1.6 2002.11.19 25.0 110000 240 99.78 240 270 Table 9. Photoreactivation test results (fecal coliforms) Fecal coliform groups CFU/100 mL Run Raw sewage added amount (%) Date UV dose (mJ/cm 2 ) Raw water Disinfected seawater Inactivation rate (%) Disinfected seawater, photoreactivated for 1 hr Disinfected seawater, photoreactivated for 2 hr 1 0.5 2002.10.10 38.8 4000 <17 >99.58 <17 <17 2 0.5 2002.10.10 31.1 4000 <17 >99.58 <17 <17 3 1.6 2002.11.19 31.2 23000 50 99.78 <17 <17 4 1.6 2002.11.19 25.0 23000 50 99.78 <17 <17 4. CONCLUSION Concerning to the performance of seawater purification plant, the number of coliforms and fecal coliforms were always under the detection limit during the whole experimental period. Other water quality items were also satisfactory. Referring to the results of the purification zone, during the 106 day starting from July, 18 to October, 31 2003, as the result of seawater purification and discharge of 5000m 3 per day, the water quality inside the purification zone was found to be improved rather than outside. Also, inside the Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 70 - purification zone, the peaks in the number of the fecal coliforms after the occurrence of CSO were lower rather than outside, and a faster tendency for recovery was observed. Concerning to the results of preliminary experiments of photoreactivation, the sewage coliforms of fresh-water origin, when UV disinfected in seawater environment followed by photoreactivation, do not demonstrated a outstanding photoreactivation effect such as usually observed in effluents of sewage treatment. The fecal coliforms, in particular, showed a tendency to be inactivated by the photoreactivation process. 5. TASKS IN FUTURE Concerning to the seawater purification plant, the UV irradiation dose was found to be excessive in this experiment, once the removal rate of coliforms could be kept at high values at the filtration facility, even with water quality deterioration. Additionally, there was a considerable difference in the number of coliform groups between the occurrence of CSO and fine weather when CSO do not affects. Therefore, keeping the UV irradiation dose constant can result in unnecessary electric power consumption. Optimal irradiation dose according to the water quality should be found out and a control system for the irradiation dose should be still established. Referring to the purification zone, in view to satisfy the water quality target for longer periods; improvement of the device to prevent the water infiltration from the edges of silt fences; experiment with purified seawater discharge increased to 7,5000m 3 per day; and water flow investigation to clarify the movements and retentions situation of purified sea water will be carry out. Concerning to the tendency found out in the preliminary experiment of photoreactivation; test in the seawater with occurrence of CSO will be continued and clarification of the influence of photoreactivation on the sea area with inflow water from the CSO and sewage treatment effluents are being planned. In addition, in view to understand the causes of the suppression of photoreactivation of coliform groups in seawater, we will investigate the behavior of the coliform groups upon photoreactivation/dark field activation condition by conducting model experiments with artificial seawater. Investigation of bacterial flora change should also be conducted to understand the influence to the coliform groups from domestic wastewater when discharged into the seawater environment. 6. REFERENCES 1) T.Nakazato(2004). Water21, June 2004.pp. 42-43. . 2 hr 1 0.5 2002.10.10 38 .8 230 0 3. 6 99.84 23 23 2 0.5 2002.10.10 31 .1 230 0 3. 6 99.84 7.4 15 3 1.6 2002.11.19 31 .2 110000 43 99.96 93 210 4 1.6 2002.11.19. value 176 <2 217 730 Fecal coliforms count [CFU/100 mL] Range <2 - 19,800 <2 * <2 - 31 ,100 <2 - 35 ,200 Ave. 3. 0 2.5 3. 4 3. 7 COD Mn [mg/L] Range