Reduction of residual chlorine in the Drinking water in Yokohama City

Shigeo Hiramoto

Manager, Northern Area Construction Division, Waterworks Bureau, The city of Yokohama, 1-1, Minato-cho, Naka-ku, Yokohama, 231-0017 Japan

1. Overview of the waterworks facility in Yokohama The City of Yokohama is the second largest city in Japan with an area of approximately 435

km２ and a population of about 3.65 million. The city’s waterworks system dates back to 1887,

celebrating 122 years in operation in October 2009. With four water filtration plants and pipelines stretching to about 9,000 km in length, the city supplies an average of around 1.2 million m3 of water per day. Providing services in the city’s hilly landscapes with elevations ranging from zero to 150 meters, the Yokohama Waterworks Bureau divides its water distribution areas into twenty-six large blocks, each of which has one distribution reservoir. They are designed to easily control water pressure and the amount of water supplied during normal operations, as well as to minimize the impact of water outage caused by water leakage accidents or renovation work.

The distribution area of each reservoir is also divided by water supply system into two blocks: pressure distribution using pumps and gravity flow distribution. Furthermore, these middle-sized blocks are divided into smaller segments to provide equalized water pressure in mains, thereby avoiding excessive water pressure and reducing the amount of water caused by leakage. The small block system also aims to equalize the time the water stays in the distribution system, so that residual chlorine can be kept at an optimal level.

2. For a supply of safe and high-quality drinking water The Yokohama Waterworks set the goal of

achieving ” Supply of safe and highest quality drinking water” in its “Long-term Vision,” a future vision for Yokohama’s water utility management due to be implemented twenty years from now. It proposes measures necessary to attain this goal, including the improvement of filtration facilities by incorporating advanced filtration technologies and the removal of the odour of chlorine from drinking water (Figure 1).

Elimination of chlorine smell byreduced levels of chlorine

Promotion of improved quality of lead pipes

Promotion of the direct connection of storage tank pipelinesto the city's distribution system

Regular inspection by the waterworkspersonnel on water storage tanks

MeasuresObjectives

active involvement in the maintenance ofwater supply devices

Incorporation of advanced filtrationtechnologies (i.e. membrane filtration andadvanced water treatment)

Supply of safe, high-qualitydrinking water

Removal of chlorine odour byreducing level of chlorine

Replacement of exisiting lead pipes

Direct feed from the the city'sdistribution system instead ofstorage tank system

Regular inspection of water storage tanksby waterworks personnel

MeasuresObjectives

Instruction on the maintenance of watersupply equipment.

Incorporation of advanced filtrationtechnologies (i.e. membrane filtration andadvanced water treatment)

Figure 1: Schematic chart of improvements required to obtain

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3. Efforts for eliminating the chlorine odour (reduction in residual chlorine) According to the poll conducted among the

citizens, about 40% of the respondents were cautious when drinking water directly from the tap, and out of them, 60 % said that they had noticed the odour of chlorine.

The odour of chlorine can be effectively alleviated by reducing the amount of the disinfectant sodium hypochlorite, fed in at water treatment works. However, the Water Supply Law in Japan specifies that a minimum residual chlorine of 0.1mg/ ℓ should be maintained in the tap water to ensure the safety of water for public health.

Water Quality MonitoringS ystem

Analyzers  are installed at  water taps  formonitoring  res idual chlorine  levels

Automatic  water quality analyzer

R eduction of res idual chlorine  levelsat dis tribution reservoirs

FOMA  Network

D istribution reservoir

Maintenance section for water supply

Water intake  opening

Water filtration plant

Dam

Distribution reservoir

Automatic  water quality analyzer

Monthly & daily dataTrends  graph

In response to the citizens’ request for water with a less noticeable chlorine odour, the Yokohama city aims in its “Ten Year Plan” to remove the chlorine odour in the drinking water through methods including the reduction of sodium hypochlorite dosage.

Figure 2: Outline of residual chlorine reduction

With a goal set to reduce the average residual chlorine concentration from the fiscal 2003 level of 0.62mg/ ℓ to less than 0.4mg/ ℓ, the levels requisite both for the safety and quality of drinking water, the city has been conducting the following measures:

1) Improvement of chlorination facilities The chlorine dosing facilities at filtration plants and distribution reservoirs are to be

improved for close control of the residual chlorine levels at water faucets.

2) Replacement of aged pipes In aged water pipes, the residual chlorine is consumed in the reaction with rust inside the pipes, lowering the chlorine levels in the water. This means that an increased amount of sodium hypochlorite is required to sustain the adequate levels of disinfection. The replacement of aged pipes, therefore, should be implemented in due course based on the renovation plans.

3) Installation of automatic water quality analyzers at consumers’ taps

Automatic water quality analyzers installed at citizens’ houses allow for continuous measurement of water quality, providing us the data both to ensure the safety of tap water and examine the results of reduced dosage of sodium hypochlorite at the reservoir. The water quality analyzers have already been installed along streets to measure the water quality in the mains; however, a smaller analyzer set up at home enables the measurement of the actual quality of the drinking water being provided at the consumer’s tap after some chlorine is consumed in the mains.

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4. Experiment overview

legends

Water distr ibution blocks of Yokohama

area  with  water qua lity ana lyzer

T surugamine   blockarea  for future   insta lla tion

An experiment was conducted in the Tsurugamine reservoir and its distribution block from February 4, 2008. The amount of sodium hypochlorite injected into the reservoir was reduced to the ranges enough to lower the chlorine concentration of output water by 0.1mg/ ℓ. Households in the block were asked to cooperate by installing an automatic water quality analyzer and subsequently monitoring the water quality.

The outline of methods and procedures of the experiment is as

follows: (1) Automatic water quality analyzer

1) The automatic water quality analyzer used in the

experiment is a small device comparable in size to a typical outdoor unit of a residential air-conditioner. It can provide continuous measurement of the quality of drinking water in small quantities (at a flow rate of approximately 100 mℓ/min).

Figure 3. Installation of water quality analyzers

Fig. 4 Water quality analyzer

2) The analyzer sends in real time the measured data (residual chlorine, turbidity, water color and temperature) through the NTT FOMA network, allowing for the data collection and storage at the central server. 3) The use of the central server allows for the remote monitoring of the data. When the server detects any abnormal values, it sets off an alarm to inform the water supply maintenance section of the local waterworks office.

(2) Roles of citizens who agreed to monitor the water quality

1) Provide the space for unit installation, electricity and tap water. 2) Observe color, turbidity, odour and taste of the water on a daily basis. Document this and submit a monthly report. 3) Report to the waterworks office in the case of detecting abnormal signs of water quality.

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(3) Installation of automatic water quality analyzer The locations of automatic water quality analyzers

were determined after giving consideration to the following aspects (Figure 5).

P res suredis tribution

Gravity flowdis tribution

Distributionreservoir

Proposed installation

Unit No37

1) Locations allowing the units to homogeneously

cover the whole testing area. 2) Existence of pipe types with a high possibility of

reducing residual chlorine levels 3) The results of the residual chlorine survey conducted

in summer months. Figure 5: Area selection for unit installation

4) The results of residual chlorine simulation using a distribution network analysis

Table 1: Specifications of pipes connected to unit No.37 5) Areas with low residual chlorine levels confirmed

by local water supply maintenance sections In Pipe type Diameter Year installed Pipe type Diameter Yeat installed

ＶＬＧＰ ２０ｍｍ 1983 ＤＩＰ １５０ｍｍ 1977

Water service pipes Distribution pipescorporating the above information, the possible

testing areas were chosen. The final locations of the analyzers were then determined by the type of water supply equipment, field survey results, as well as with the approval of the residents.

For reference, the specifications of the mains and the

water service pipes connected to a household with the analyzer (unit No. 37) are shown in Table 1.

5. Results

The results of residual chlorine values measured in the Tsurugamine reservoir and by the automatic water quality analyzer (unit No.37) are shown in the Figure 6. In the experiment, the residual chlorine level at the reservoir outlet

was reduced by about 0.1mg/ℓ on February 4, 2008. In this chart, the residual chlorine concentration at

2/4 Reduce chlorineresidual level

Residual chlorineconcentration(mg/l)

Residual chlorine at reservoir

Residual chlorine measuredby unit No.37

Data for 01/01/08 - 29/02/08

Unit No. 37

Figure 6: Residual chlorine valuechart

the reservoir (a thin line in the chart) decreased from February 4, from approximately 0.8mg/ℓ to approximately 0.7mg/ℓ. A thick line in the chart, which represents the chlorine concentration measured by the No.37 unit at the consumer’s tap, fell in accordance with this decrease.

Comparing the data collected before and after th

Table 2: Decrease in residual chlorine concentration (average of nine units)

No32～No40 Reduction of residual chlorine No32～No40concentration

Average value of9 units

【February 4, 2008】 Average value of9 units

0.66 0.58(mg/l) (▲0.08mg/l) (mg/l)

e onset of the experiment among nine analyzers set up in the Tsurugamine block, there was a decrease in residual chlorine by an average of 0.08 mg/ℓ (Table 2).

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6. Current status of unit installation

low temperatures inlate August

water temperature

residual chlorine

water colorturbidity

Period of 1 Month: 05/08/08～05/09/08

As from the fiscal 2005, a total of 65 automatic

water quality analyzers are being operated in ten water distribution blocks. Among these, the reduction of residual chlorine has been achieved in three blocks by the fiscal 2008, in which the monitoring by the analyzers is still underway. The city aims to make further efforts to reduce residual chlorine in drinking water to less than 0.4mg/ℓ, thus ensuring the supply of safe and high-quality water to drink.

(Figure 7 shows the examples of data output on the monitoring display for unit No.37) Figure 7: Monitoring display for

unit No.37 7. Challenges lying ahead The issues and tasks to be undertaken in furtherance to the project: 1) Improved and reliable performance of water quality analyzers 2) Surveys on the water in storage tanks atop or inside buildings as well as the provision of

guidance on their proper maintenance.

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1

タイトルReduction of Residual Chlorine in the Drinking Water in Yokohama City

Shigeo HiramotoManager, Northern Area Construction Division,

Waterworks Bureau, The city of Yokohama①

2

Map of JapanMap of JapanSAPPORO

Map of JapanMap of JapanMap of Japan

YOKOHAMAYOKOHAMA

②

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3

Landmark TowerLandmark Tower

Bay BridgeBay Bridge

Mt.FujiMt.Fuji

Port of YokohamaPort of Yokohama

View of View of CCentral Yokohamaentral Yokohama

③

4

Minato Mirai 21 Area

④

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5

Chinaｔown Kaikoukinenkaikan

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Topography Topography of YOKOHAMA CITYof YOKOHAMA CITY

HillHill

TokyoTokyoBayBay

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Overview of The Waterworks Facility

• Area 435 km２

• Population 3.65 million • Water filtration plant ４

• Pipelines 9,000 km • Amount of water supply

1.2 million m3/ day.

⑦

8

Schematic Chart of Improvements Required to Obtain

Elimination of chlorine smell byreduced levels of chlorine

Promotion of improved quality of lead pipes

Promotion of the direct connection of storage tank pipelinesto the city's distribution system

Regular inspection by the waterworkspersonnel on water storage tanks

MeasuresObjectives

active involvement in the maintenance ofwater supply devices

Incorporation of advanced filtrationtechnologies (i.e. membrane filtration andadvanced water treatment)

Supply of safe, high-qualitydrinking water

Removal of chlorine odour byreducing level of chlorine

Replacement of exisiting lead pipes

Direct feed from the the city'sdistribution system instead ofstorage tank system

Regular inspection of water storage tanksby waterworks personnel

MeasuresObjectives

Instruction on the maintenance of watersupply equipment.

Incorporation of advanced filtrationtechnologies (i.e. membrane filtration andadvanced water treatment)

⑧

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9

0

5

10

15

20

25

30

35

40

45

50

構成

比（％

）

H19年度測定H15年度測定H13年度測定H11年度測定H9年度測定H7年度測定

0.1未満

0.1以上

0.2以上

0.3以上

0.4以上

0.5以上

0.6以上

0.7以上

0.8以上

0.9以上

1.0以上

1.1以上

1.2以上

Residual chlorineConcentration

(mg/l)～0.1 0.1～ 0.2～ 0.3～ 0.4～ 0.5～ 0.6～ 0.7～ 0.8～ 0.9～ 1.0～

200720032001199919971995

Residual Chlorine ConcentrationAverage of 6400 Points in Yokohama City

(%)

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Efforts for Eliminating The Chlorine Odour

1) Improvement of chlorination facilities

2) Replacement of aged pipes

3) Installation of automatic water quality analyzers at consumers’ taps

⑩

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Outline of Residual Chlorine ReductionWater Quality MonitoringS ystem

Analyzers  are ins talled at  water taps  formonitoring  res idual chlorine levels

Automatic  water quality analyzer

R eduction of res idual chlorine    levelsat dis tribution reservoirs

FOMA  Network

D is tribution reservoir

Maintenance section for water supply

Water intake  opening

Water filtration plant

Dam

Distribution reservoir

Automatic  water quality analyzer

Monthly & daily dataTrends  graph

⑪

12

Installation of Water Quality Analyzers

legends

Water distr ibution blocks of Yokohama

area  with  water qua lity ana lyz er

T surugamine   blockarea  for future   insta lla tion

⑫

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13

Water Quality Analyzer

Fig. 4 Water quality analyzer

⑬

exterior interior

14

The Locations of Automatic Water Quality Analyzers were determined by

• Locations allowing the units to homogeneously cover the whole testing area.

• Existence of pipe types with a high possibility of reducing residual chlorine levels

• The results of the residual chlorine survey conducted in summer months.

• The results of residual chlorine simulation using a distribution network analysis

• Areas with low residual chlorine levels confirmed by local water supply maintenance sections

⑭

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15

Area Selection for Unit Installation

P res s uredis tribution

Gravity flowdis tribution

Distributionreservoir

Proposed installation

Unit No37

⑮

16

Specifications of Pipes Connected to Unit No.37

Pipe type Diameter Year installed Pipe type Diameter Yeat installedＶＬＧＰ ２０ｍｍ 1983 ＤＩＰ １５０ｍｍ 1977

Water service pipes Distribution pipes

⑯

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Residual Chlorine Value Chart

2/4 Reduce chlor ineresidual level

Residual chlorineconcentration(mg/l)

Residual chlorine at reservoir

Residual chlorine measuredby unit No.37

Data for 01/01/08 - 29/02/08

Unit No. 37

⑰

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Decrease in Residual Chlorine Concentration

No32～No40 Reduction of residual chlorineconcentration

No32～No40

Average value of9 units

【February 4, 2008】 Average value of9 units

0.66 0.58(mg/l) (▲0.08mg/l) (mg/l)

⑱

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19

Monitoring Display for Unit No.37

low temperatures inlate August

water temperature

residual chlorine

water colorturbidity

Period of 1 Month: 05/08/08～05/09

⑲

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Challenges Lying Ahead

The issues and tasks to be undertaken in furtherance to the project:

• Improved and reliable performance of water quality analyzers

• Surveys on the water in storage tanks atop or inside buildings as well as the provision of guidance on their proper maintenance.

⑳

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Thank You for Your Attention

Yokohama water works Bureau

Character of

H a m a p y o n

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