1. Introduction
This is the largest purification plant among the three of Osaka city, which include Niwakubo Purification Plant, Toyono Purification Plant. Kunijima Purification Plant is located in the downstream of Yodo River, one of the largest river in Japan and the river has the most stable stream flow of all the large river. The Yodo River is the only one water resource for these three purification plants use as raw water in order to supply service water for the whole city. The river is the confluence of three stream flows: Uji River which rises from the largest lake in Japan Lake Biwa, the Kizu River from Nunobiki Mountains, and Katsura River rising from Tamba Highland then travels through Osaka city before flows into Osaka Bay. Thus the abundant water of the Yodo River has a highly potential utilization for essential demands of the people and city development. The plant divided in to two systems, Shimo-system and Kami-system, see fig. 1.
B: Kami-system
C: Shimo-system
D: Yodo River
E: Intake tower
The Kunijima Purification Plants was constructed in 1913 since the rapidly development of Osaka city and followed by other expansion works. After that the Niwakubo Purification Plant and Toyono Purification Plant were constructed in turn of the years 1957 and 1968 to meet the increasing water demand of the people. In 1969, in order to deal with the continuously growing water demand and contamination,water supply capacity in was increased by 198,000 m3 in Kunijima, and 50,000 m3 in Toyono. And the standard water capacity increased to 2,430,000 m3/day as stated before. The water demand kept increasing until 1970 and reached the maximum at the peak of 2,417,700 m3/day. After that the demand was decreasing. By the year 1975, therefore, all the expansion projects were suspended.
2. Development process
At the begining, before taking a trip around the plant, a conspectus presentation was conducted by Mr. Tanaka, Director Assistant of the Kunijima Purification Plant showing the overview of the plant about the development history, the location, the distribution of Kunijima Purification to the city as well as the link between the three plants and the key point was how the Advanced Treatment Water System is currently implementing there.
Fig. 2. Introduction about Kunijima Purification Plant
In 1965, the city of Osaka launched a distribution pipe maintenance project to eliminate delivery of rust-colored water and counter low water pressure in some areas. This was carried out by replacing aging pipelines and taking other measures deemed necessary to maintain the existing supply networks in good condition. In 1975 a purification plant maintenance project was launched as well to ensure a steady supply of safer and purer drinking water from all purification and distribution plants.
Based on the lesson learnt from the Great Hanshin-Awaji Earthquake of about 7 seismic epicentral in January 17, 1995, the Municipal Waterworks Bureau devised a basic plan named Osaka Minicipal Waterworks Earthquake Prepareness Improvement Plan 21 in March 1996 (Seismic Water Osaka Plan 21). It aims to allow the bureau to put in place stronger earthquakes prepareness measures for its water system in a systematic fashion and do this within a specific timeframe.
The Minicipal Waterworks Bureau is implementing its Third Purification Plant Maintenance Project for equipment in intake stations and purification and supply plants including Kunijima, along with its Fifth Distribution Pipe Maintenance Work for supply and distribution pipelines. In 1992, the bureau launched the Advanced Purification Facility Maintenance Project to improve comprehensively the quality of drinking water. This project aimed to remove foul smell and taste that had been a recurring problem since 1981, and reduce trihalomethane and other organic micropollutants. On March 20, 2000, the bureau began flow to advanced treated drinking water to the entire city area.
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3. Water Quality
The BOD increasing in the downstream in Kunijima and Niwakubo Purification Plants from the 1950s until later of 1960s and now it shows a considerably low value due to the development and improvement of sewer systems in the upper and middle basin. However, in the upstream Purification Plant of Toyono, BOD level have tended to decrease since 1990s. In constrast, from 1980 to 1990, the BOD gradually increasing. The taste and the odor in Lake Biwa, resulting from the mass growth of phyto plankton, has had profound influences on the downstream of the Yodo River since 1980.
4. Purification Facilities
The Kunijima Purification Plant is the largest of the three with a daily supply capacity of 1,180,000 m3. The Plant takes in water from the downstream right bank of the Yodo River, treats it, and distributes it through 15 water mains to the central, north and northwest areas of the city.
As stated above an advanced water treatment system was introduced at the Osaka Municipal Water Works Bureau in March 2000. The advanced water treatment system completely removes foul smell and taste such as musty odors and reduce of trihalomethane to a far lower than that of the conventional system.This system incorporates ozonation and a granular activated carbon (GAC) process into conventional treatment processes, improving tap water quality.
The GAC process is the last stage of the process, and it can effectively remove trihalomethane precursors and other toxic organic substances. GAC treats water through a physical adsorption effect inherent in the GAC itself, and an effect by the microorganisms in the GAC layer.
Fig. 3. Process flow in Kunijima Purification Plant advanced water treatment system
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5. Operating process
The operating process in Kunijima Purification Plant can be descirbed as follow the fig. 3 (see previous page).
1) The plant gets raw water from intake tower at the bank of Yodo River ; then
2) Raw water flows to Grit Chamber see figure 4, by operating this facility, sand is settled down by screen and big dusts are removed from raw water;
3) At Coagula-Sendimentation basin see figure 4, fine colloidal particles in raw water are removed, there have four phases at the Coagula-Sedimentation. Firstly, using aluminum sulfate as coagulation. Secondly, mixing water and coagulation by machine, called flushed mixer. Thirdly, forming large flocks by combining the articles and coagulation using flocculator and at the last, settling down the flock through basin.
Fig. 4. Grit Chamber (right) and Coagula-Sedimentation Basin
1: Adding coagulation
2: Mixing water and coagulation
3: Forming large flocks by combining the articles and coagulation
4: Settling down the flocks
4) Water flows to the rapid sand filters basin (see fig. 5), at here fine turbid matters are removed from settle water and also it removes maganese which is oxide in intermediate ozone contact basin. Filter is sand which diameter is about 0.55mm, and the depth of it is 60cm. The lower of sand, there is gravel, which layer depth is 30cm. After long filtration, turbid matters remain too much and not filtrate so efficiently. So, after 50hours filtration or loss of water head between in and out become 2meters, the filters need to wash. In this technology, the plant use back washing, from bottom to the surface. However the surface of sand is dirtier than bottom of sand filter, so in this case, they add surface washing. Washing water goes to coagula-sedementation basin, and re-treatment. Before applying advanced treatment system, the basin was uncovered, it was very hard to manage, in order to preventing leaking ozone which come from settle water after ozonation, the basin has been covered (see fig. 6)
Fig. 5. Rapid Sand Filters Basin
Fig. 6. Covered Rapid Sand Filters Basin
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6. Advanced Water Treatment System
The Advanced water treament system starts at ozone contact basin (see fig. 7), they set ozone contact basin two points. One is Intermadiate Ozone Contact Basin (IOCB) (see figure 3) between coagula-sedemetation basin and sand filter basin. Aiming to oxidize manganese in settle water. Before introducing it, chlorine was used to oxide manganese. But chlorine causes generating trihalomethan. Manganese itself is items of water quality regulation, and it becomes color matter by reacting with chlorine. The other is Post Ozone Contact Basin (POCB) between rapid sand filter and GAC adsorption basin. The aim is to oxidize organic matters, such as musty odors and trihalomethane, and inactvate microorganism. Inject control is fixed dissolved ozone concentration except for intermediate in summer. Control of IOCB in summer is fixed injection rate. See ozone generator in fig. 8.
Fig. 7. Ozone Contact Basin (left) and GAC Adsorption Basin
Ozone is made from air and by silent electric discharge.
The amount of ozone is 8.8 kg/h (IOCB),11.2 kg/h (POCB).
The concentration of ozone is 20 g/Nm3.
The number of generator is 1 (IOCB), 2 (POCB).
The aim of GAC adsorption basin is removing organic matters from water by adsorption, decomposing and filtration. The light side is GAC through electron microscope. GAC is made from coals.
There are a lot of holes. By these holes, GAC has much adsorption function.
Fig. 8. Ozone generator
Washing GAC is for controlling Metazoans.
Washing method is air washing and back washing.
The Water distribution pond is built underground, which stores clean water.
A cover is installed over the pond in order to keep clean. When distributed amount is small, at midnight, store water, and when amount is large, in the morning and evening, use stock water.
The advanced water treatment facilities above have been introduced into all three purification plants in Osaka City including Kunijima Purification Plant. As we can see from above, this system can completely remove musty odors, and can reduce arelatively large amount of the trihalomethane formed in the conventional intermediatechlorination process. It can also guarantee a higher level of safety against cryptosporidium and other pathogenic microorganisms, resulting in a comprehensive improvement in water quality. Ozonation is performed before and after the rapid sand filtration basin to suppress the generation of bromate ions while making it possible to increase the effects of certain processes.
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7. Earthquake Prepareness
Water supply system of Osaka city serves as an important lifeline by supplying drinking water, domestic water and industrial water to support the comfort of citizens's life and urban system. Hence, ensuring stable flow of water supply at all the times including emergency is the fundimental responsibility of any waterworks system. Ensuring the ability to maintain an adequate supply of water after an earthquake is an especially important issue requiring immediate attention, the responsible office is the Seismic Water Osaka Plan 21 (mentioned in operating process part). More reliable water supply is established by providing comprehensive countermeasure against earthquake disaster from soft and hard side based on promotion of development of the Seismic Water Osaka Plan 21.
The following basic elements comprised in the plan:
1) Improving earthquake resistance of key facilities:
A highly reliable lifeline system to be completed through construction of a water intake system at the collecting point of the assembly.
2) Establishing a water supply and distribution network
3) Improving mutual compatibility among difference distribution systems.
4) Countermearsure against power failure.
5) Expanding emergency material stocking system.
6) Establishing stable water supply routes man-made island.
7) Improving the information and communication system's reliability.
8) Improving earthquake resistance of headquaters.
These are the facilities using in emergency situations, which are shown in figure below:
1: Water truck
2: Water baloon 4m^3
3: Poly bag(3L) Poly container(10L)
Fig. 9. Equipment using in emergency situation
8. Water Usage Management
During the field trip, the students have rose questions about water usage management in Osaka city. In Osaka, water rate are caculated according to the reading of meters installed on consumer premises. The meters are checked monthly for big customers and every four months for others. The rates are paid every month for big customers and every two months for the others. The basic rate for all individual ueses is uniform. A metering system is used to determine excessive comsumption, to which a higher rate applies for domestic and service-industry uses; the rate increases with increased consumption.
Thanks to the study trip, the students had a good chance to see and learn how an advanced water treatment system works in the one of the modernest purification plants in Japan, Kunijima Purification Plant. We also knew how effective of good implementation, maintenance, and management of water purification plant, in order to keep stable supplying adequate water an indispensable demand for the people in their daily life.
References
Report(2006), Water Supply System in Osaka, Osaka Municipal Waterworks Bureau.
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最終更新日 2013年2月22日（金曜）13:52