Raw and Purified Water Quality Testing

Date 2015-06-24 Category Water Supply (Arisu) Updater scaadmin
Seoul Metropolitan Government
Last Update


Testing the quality of both raw and purified water to ensure the highest quality of Arisu

To supply safe and tasty drinking water, the Office of Waterworks at the Seoul Metropolitan Government performs rigorous quality check at each stage of the water supply system from intake and purification to the distribution stage.

Before the Policy Implementation

After the Policy Implementation

  • There was no real-time, inclusive water quality monitoring system in place; issues such as algal bloom and chlorine odor led to public distrust in the drinking water supplied to their homes.

  • Real-time water quality monitoring at each stage of the water supply system

  • Proactive response to the unpleasant odor found in water

  • Real-time release of water quality information to the public

  • Reduction of chlorine odor and stability of water supply to households at the end of water mains

Overview of the Policy

<< Setting up an automatic water quality monitoring system for both source water and raw water at intake points

<< Assuring the optimal quality of purified water

<< Performing water quality testing in the distribution system

<<Rapid response in tarcing harmful substances


Climate change has led to a range of unexpected problems such as torrential rainfall and algae-induced odors. To address these issues and to prepare further against drinking water contaminants, a comprehensive water quality control system was necessary. The new system would strengthen the existing tests at each stage of the water supply system (raw water, purified water), tailored to different kinds of source water. In addition, the chlorine odor had to be removed to encourage more people to drink tap water.

Process of Policy Implementation

Since 1908 when the first modern water purification plant in Ttukdo began operation, Seoul Arisu has undergone tap water quality testing for 14 items such as turbidity, pH, and residues. After the detailed standards for water quality and testing items were stipulated by law on March 11, 1963, the testing has expanded to include a total of 29 substances such as ammoniac nitrogen.

As of 2014, the law stipulates that 59 substances be tested for drinking water such as DBP (disinfection byproducts). On top of this, the Seoul Metropolitan Government has established separate guidelines for a more rigorous review. Expanding the number of regular water quality testing parameters, Seoul introduced 2 additional tests in July 1997 but has now expanded even further to bring the total to 104. The total number of tested items is 163 as recommended by WHO. The result of the stringent tests is made public so that the citizens can build trust in the water supplied to their homes.

Besides the water quality tests, we also introduced the algae alert system in 2000 as a means of reducing odor-causing materials. The system is now supplemented by the odor alert system introduced in 2012 to detect geosmin and 2-MIB. The two measures have been effective in proactively and efficiently tackling the odor issue in Arisu.

Details of the Policy

Water quality control tasks are divided by stages of the water supply process. The Waterworks Research Institute and purification centers undertake testing on raw and purified water, whereas both the Waterworks Research Institute and branch offices perform testing on water in the distribution process. The Office of Waterworks HQ takes charge of operating the Tap Water Assessment Committee, an advisory committee established as per Clause 1, Article 19 of the Enforcement Regulation of the Water Supply and Waterworks Installation Act, and supports the policymaking process.

Setting up an automatic water quality monitoring system for both source water and raw water at intake points

 • Source water (20 points, monthly basis) and raw water (10 points, weekly basis) are subject to water quality testing.

 • 7 items (phenol, ammoniac nitrogen, etc.) are automatically monitored.

 • Biological early warning system (Gangbuk, Amsa, and Pungnap intake stations)

Among 6 reservoirs, the Gwangam reservoir gets water from Paldang (157.3km2) as its source water; the rest including the Gangbuk reservoir get it from Jamsil (6.45km2). We have focused on controlling the quality of Han River’s main and branch streams, which directly affect the quality of raw water, and proactively responded to potential contaminants.

※ In 2013, Paldang and Jamsil recorded BOD of 1.2 ~ 1.6mg/L, which is classified as I-b grade (good).

The Waterworks Research Institute and purification centers conduct testing on both source water and raw water at intake points. In particular, 7 items including phenol and ammoniac nitrogen are subject to automatic, real-time testing.

Moreover, in the Gangbuk, Amsa, and Pungnap (Yeongdeungpo) intake stations, a biological alert system is in operation round the clock to monitor heavy metals and contaminants.


  • Table 1-1 I Water Quality Testing in Source Water and Raw Water

Table 1-1 I Water Quality Testing in Source Water and Raw Water


Tested Sites/Target

Tested Items

Testing Agency



Source Water

16 sites

(Namhan River 8, Bukhan River 8)

15 items

Waterworks Research Institute


Paldang Lower Branch Stream 16points




* Branch Stream: Gungchon, Dosim,Wolmun,Dukso, Hongneung,Sangok

21 items

8 items

Water Quality Monitoring Network



* Dukpung

* Wangsook Stream Water System (Jinjeop, Byeolnae, Jingeon, Guri1, Guri2)

* Kyoungan Stream Water System (Kyoungan, Gwangju, Gonjiam, Opo)

15 items

Water Quality Monitoring Network


Raw Water at

Intake Stations

4 sites

* Gangbuk : Algae Closterium (heavy metal, pesticides)

* Jayang:Green Algae (Heavy metal,Pesticide)

* Amsa : electroactive substances (domestic sewage)

* Pungnap : water flea (insecticide, heavy metal)

Bio Alert Device

Gangbuk, Ttukdo, Amsa,


Real time

Raw Water at

Intake Stations

5 intake points

* CN, phenol, NH3-N, TOC, pH, turbidity (chlorophyll-a)

Automatic Water

Quality Control Device (8 items)

Purification center

Real time

Raw Water at

Intake Stations

10 sites

(6 intake points)

(Han River confluence 4)

* Namhan River : Bolpo-ri, Sinwon-ri

* Bukhan River : Sambong-ri, Jinjoong-ri

158 items

* by law : 31

* by Seoul: 127

(11 items overlapped)

Waterworks Research Institute





Purification center





Assuring the optimal quality of purified water

Automatic water quality monitoring system tailored to each stage of the water supply process

Alert system for algal bloom and odor in source water and raw water

Expansive list of tests (60 stipulated by law + 110 on Seoul’s own list)

We monitor the quality of raw water through analysis, strengthen our disinfection work, and control the turbidity level of filtered water. Through the multi-layered control system, water quality can meet the safety requirement even in the worst case scenario.

As one of the most important indicators of water quality, turbidity is measured round the clock throughout the entire process. We also have measures in place to cater to different needs during the water shortage period such as dry season, rainy season, and winter season.

※ Turbidity level is kept below 0.06 NTU (below 0.1 NTU during the rainy season)

When the alert/warning is issued for either algae bloom or odor, we take a series of measures. First, we conduct water testing more frequently than usual (from every week to every day); and an advanced water treatment facility, the advanced oxidation process (AOP) is optimally operated. Interchlorination is then done in purification stations instead of having pre-chlorination done at intake stations.

Table 1-2 I Algae Alert System: Alert/Warning is issued when the indicated numbers have been reached for both categories for 2 consecutive times

Table 1-2 I Algae Alert System: Alert/Warning is issued when the indicated numbers have been reached for both categories for 2 consecutive times

Algae Warning Items



High-level Warning

Blue-green algae cell count (cell/mL)

1,000 +

10,000 +

1,000,000 +


Since 2012, we have been reducing the amount of chlorine added at reservoirs; instead, we disperse chlorine at several points -- especially in the later stage of process -- to eliminate the chlorine smell. Currently, the chlorine re-dispersion system is used at 17 sites, but the number will be expanded to keep the residual chlorine level between 0.1~0.3mg/L at every faucet.

Reducing the chlorine smell throughout the water supply system

Performing water quality testing in the distribution system

Real-time, automatic water quality monitoring at every step of the distribution system & public disclosure of water quality information

“Arisu Quality Verification System”

“Chlorine Re-dispersion System” at reservoirs

We have redoubled our efforts to improve water quality with a wide range of measures. Recognizing the need to take a step further beyond the legally required tests, we decided to test the water quality at every stage of the water supply process. In addition, we have been carrying out the “Arisu Quality Verification System” and other measures to keep an eye on water quality in the distribution systems. All of the efforts are designed to improve public perception of Arisu and encourage them to drink more tap water.


표 2-3 정수센터 수질검사 현황  


Tested Sites/Targets

Tested Items

Testing Agency

Testing Frequency

Raw Water

6 Purification Centers


Purification Center



Purified Water

6 Purification Centers


Purification Center



Purified Water

6 Purification Centers

8 Sites


(60 legally set, 110 set by Seoul itself)

Waterworks Research Institute


(60 legally set, 6 set by Seoul itself)



Purified Water

Testing new variants of materials

   * Pesticide (3)

   * Industrial Chemicals (1)

   * Radiative Chemicals (1)


(5 more added in 2018)

Waterworks Research Institute


There is a legally required water quality test to be done at faucets in accordance with Article 4 of the Act on the Management of Drinking Water. A total of 450 sites throughout Seoul including households located at the end of the pipe networks are subject to the test. As the testing agency, the Waterworks Research Institute tests on 6 items (bacteria, turbidity, etc.). At least 6,500 sites that were tested in 2013 have all passed the tests.

In addition, every month, 20 old pipes deemed likely to affect water quality undergo testing on 13 items (bacteria, iron, turbidity, etc.). In 2013, 200 pipes went through the testing, and all of them met the requirements for drinking water.

The Waterworks Research Institute also performs quarterly testing on 11 items (bacteria, iron, THMs, turbidity, residual chlorine, etc.) on 120 points throughout Seoul’s water supply system (including purification stations, reservoirs, pressurizing stations, etc.).

In 2013, Seoul’s water met the requirements for all areas.


※ Purification Stations (8), pre-reservoir (26), post-reservoir (26), intersection of the water service area (26), pressurizing stations (8), faucets in households at the end of the pipe network (26), etc.

Furthermore, the residual chlorine level is measured and monitored every day for 113 reservoirs and pressurizing stations. In fact, 188 sites have real-time, automatic monitoring device for 5 items including turbidity and residual chlorine, with the results made public through the “Seoul Water Now System.”

Seoul’s water quality control is not affected by interruptions. When there is water supply facility construction (cleaning of reservoirs and pressurizing stations, pipe repair, etc.), we make sure we check the water quality once again before water is allowed to flow. We were able to ensure the quality of tap water by conducting 1,283 tests in 552 cases of such construction. Branch offices of the Waterworks and private testing agencies also conduct tests on 13,188 water tanks and pipes in large buildings. Equally important, 30,807 drinking fountains in 2,674 sites used widely by the public are subject to weekly checking (monthly for subway, quarterly for school) on 5 items including turbidity.

Our commitment to delivering safe and clean water also led to the introduction of the “Arisu Quality Verification Service,” which is provided to over 300,000 households that are considered low-income, underprivileged, or with problematic water supply environment at home in Seoul every year. To implement this service, we employed 121 civilians (non-public officials) as inspecting agents who are fully trained and dispatched to visit the homes of Seoul citizens to do actual check of the faucets (1st: 5 items including turbidity, iron; 2nd: 7 items including bacteria, ammonia nitrogen). Agents identify the causes of the reported problem and continuously work with the households until the quality improves to the desired level. The service has been highly received by the public, enhancing the satisfaction level.

From 2008 to 2010, 2.6 million households in Seoul had their faucets tested; since 2011, 300,000 households have been having their homes tested every year. In 2013, of the 320,000 households that underwent testing, 450 were found to have poor water quality, and appropriate measures were taken for them.

※ Water quality improvement measures: indoor water pipe repair/replacement (389), switch to direct water supply (50), water tank cleaning (7), etc.

Aside from the various tests, we also run the “Tap Water Assessment Committee” as stipulated by Article 4 of the Water Supply and Waterworks Installation Act and the same ordinance by the Seoul Metropolitan Government. Composed of city council members, professors, and environment experts, among others, the advisory body takes samples of water from 10 points from the intake station to the faucets of homes on 2 purification centers of their choice. The samples are then sent to an independent testing center commissioned to test the water quality on 59 legally required items.

The result report is made available to the public on the website of the Seoul Metropolitan Government and the Committee. This system helped us gain public trust for Seoul’s tap water.

In other words, Seoul has dedicated itself to serving its citizens with safe, healthy, and tasty tap water through multi-layered, stringent water quality tests from intake stations through the production process, all the way to the faucets at home.

  • Table 1-5 | Water Quality Control In Distribution Process 


Table 1-5 | Water Quality Control In Distribution Process
Classification Tested Sites

/ Target
Tested Items Test Frequency Testing Agency
Supply System

Automatic Water


Measuring Device


(Reservoir 87,

pressurizing Station 14)

(Turbidity, pH,

Residual Chlorine etc)
v Branch Office
Reservoir 101 points 12

(General Bacteria etc)
Quarterly Branch Office
Water Process 70 points 12

(THMs etc)
Quarterly Waterworks


Level above comparative

water quality
10 points

(Intake 2, Purification 2,

Direct 2, Water Tanks 2,

Water Purifier Water 2)

(Mineral etc)
Monthly Waterworks


Water Tap Automatic Water

Quality Measuring

91sites 55

(Turbidity, pH,

Residual Chlorine etc)
Real-time Branch Office

Water Tap
450 points

(Regular 434, Old 16)

(12 Old Areas)
Monthly Branch Office
Home Visit

Water Quality

300,000 Households 5

(7 additional items

in case of noncomformity)
Randomly Branch Office
Drinking Fountains 22,590 units

(Schools, Parks,

Trains etc)

(Turbidity, pH,

Residual Chlorine etc)

(Subway monthly)
Branch Office

Water Quality

25 points

(Multi-use facilities)
170 Yearly Waterworks


Others After construction, water quality inspection is conducted on water pipes, stagnated water, water tanks. Citizens do also self-diagnose the water quality etc.
  • Rapid response to tracing harmful substances
    • We are continuously expanding the world's highest level of water quality testing by 2022 to respond to new substances likely to be detected in the Han River water system: Total 340 items

Policy Outcome & Lessons

Today, Seoul is providing healthy and tasty water through systematic and comprehensive water quality testing and monitoring system through the entire process of the water supply system. It is complemented by the real-time automatic water quality monitoring devices and the transparent information disclosure service, all of which have contributed to building public trust in Arisu. The aforesaid measures are made even more effective with the 163 items recommended by WHO as well as the yearly check on the 130 harmful microorganisms that are otherwise not regulated.

We have adopted the “Algae Alert System” and the “Odor Alert System” to deal with problems that directly affect public perception the most and their willingness to drink tap water. The two complementary programs have allowed Seoul to respond preemptively to odor and disinfection taste issues. Another effective measure is the “Chlorine Re-dispersion System,” which is designed to keep the chlorine level in reservoirs between the 0.1~0.3mg/L range. Furthermore, the “Advanced Water Purification Facility” will be completed in all 6 purification stations of Seoul by 2015; this will substantially reduce odor, especially algae-related odor, which in turn will make people more willing to drink tap water.

The relative evaluation method among purification centers has also been effective in upgrading facilities and improving water quality. Each purification center is tasked to do research on a topic of their choice, hold workshops on the research topic, and share their experiences and best practices. Such efforts have contributed to upgrading the purification technology and capacity of water quality experts and professionals.

Applicability of the Policy

Other municipalities have benchmarked Seoul’s systematic, excellent, inclusive water quality control system, which is applied from the source water all the way to the faucets at homes. In particular, the Ministry of Environment and other municipalities have already adopted or are planning to adopt Seoul’s “Odor Alert System,” “Arisu Quality Verification System,” “Chlorine Re-dispersion System,” and harmful microorganisms monitoring practice.


  • Q.How is the standard of Arisu set?
    A. Answer
    In Korea, to meet the standards for drinking water, microorganism, harmful inorganic matter, harmful organic matter, disinfectants, disinfection byproducts, and aesthetic factors have to be tested. The standards protect public health by limiting the levels of contaminants in drinking water. For each contaminant, it sets a health goal, which is the level at which a person could drink 2 liters of water containing the contaminant every day for 70 years without suffering ill health effects; this basically means no health risk.
  • Q.What are the grounds that dictate which items are chosen for testing and how they are tested?
    A. Answer
    Clause 3, Article 4 of the <Water Supply and Waterworks Installation Act> stipulates the following as subject to testing for water quality: harmful microorganisms; substances deemed to have high probability of being detected; substances that may cause social problems, and; items that raised issues internationally. As for the methods and testing guidelines, WHO produces international norms on water quality and human health in the form of guidelines that are used as basis for regulation and standard setting worldwide. We refer to this WHO guideline as well as the practices of other countries.
    (We also use the 30 items included in the recommendations by the Ministry of Environment and 104 items stipulated by the Seoul Metropolitan Government).
    How was the guideline for healthy and tasty water set?
    It was set in December 2010 through sampling events, public survey, public hearing, expert panel, and advisory council as well as outside research, which all began in May of the same year.