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[Water] [Column] All About Water - (1) Measure “Clean Water” and “Dirty Water” with Pollution Indicators!

by Eco Generation | 29-10-2019 16:00 Comments 6 Comments recommendations 0 recommendations

Measure “Clean Water” and “Dirty Water”


with Pollution Indicators!



By Science Columnist Hyung-Ja Kim



Water is an indispensable part of our lives. Water is essential for growing crops, cooling down machines or manufacturing products in factories, chemical reactions that take place in our bodies, etc. The amount of water we can use, however, is decreasing. This is because of water pollution.

The water on the earth is getting dirty for a variety of reasons. Minerals melt in the river bottom, and animal feces or dead creatures also pollute water. The culprit of serious water pollution is people who are the most active. Those we use every day such as shampoo, toothpaste, detergent, and cooking oil and wastewater from factories flow into creeks and pollute rivers and the sea.

Polluted water is inspected for the pollution level, and water purification treatment is conducted. Several indicators are used to determine how clean or polluted the water is: BOD, COD, and DO. BOD stands for Biochemical Oxygen Demand. It is an indicator of the organic pollution and shows how much organic matter is decomposed by microorganisms for 5 days.

BOD Explanation Image

If there is organic matter in the water, microorganisms decompose it. Just like animals eat and digest food, they feed on organic matter. Oxygen is consumed in this decomposition process. If there are a lot of organic matter, the oxygen in the water disappears and the water is polluted. Therefore, the more polluted the water, the more organic matter, and the more oxygen it will need. This can be expressed scientifically as “the higher the BOD, the more polluted the water.” It is expressed as 1 ppm when 1ℓ of water requires 1㎎ of oxygen.

By the way, pesticide ingredients or substances that are difficult to decompose are decomposed slowly by microorganisms. As such, the BOD value can be measured low. IN this case, COD is also used as a pollution indicator for organic matter along with BOD. COD stands for Chemical Oxygen Demand. It is the amount of oxygen consumed when it is decomposed chemically by adding an acid or a base. When an oxidizing agent such as potassium permanganate or potassium dichromate is added to water, the organic matter is oxidized; COD refers to the amount of oxygen required to trigger chemical reactions such as oxidation.

While BOD is a method of measuring the organic matter that organisms can decompose, COD is a method of measuring the organic matter that even organisms cannot decompose. Therefore, it is necessary to measure both BOD and COD in order to determine the pollution level of water in a certain area.

In addition, Dissolved Oxygen (DO, the amount of oxygen dissolved in water) and Suspended Solids (SS) are used to determine legal water quality standards. If there is a lot of oxygen in the water, DO concentration is high. Higher DO means that the water is cleaner. Generally, clean water contains more than 10 ppm of oxygen (5.7-10 ppm for clean streams).

The standard of water quality in South Korea is different for river water, lake water, seawater, and drinking water. For rivers, for example, BOD below 1 ppm is classified as 1st grade water, and below 3 ppm, 6 ppm, 8 ppm, and 10 ppm are classified from 2nd grade to 5th grade each.

1st grade water is clean water that is drinkable after simple treatment, and 2nd grade water is water clean enough to swim. In order to be potable, the water quality needs to be at least 2nd grade water. 3rd grade water is where carp and crucian carp can live, and it can be used for industrial purposes. Water beyond 4th grade should go through a complicated process like “advanced water purification” to be use for drinking.

Water has the ability to purify pollutants by itself. So, up to a certain extent of pollution can be solved by filtration, settlement, and oxidation of water itself. However, the damage is becoming serious as the amount of inflowing pollutants is beyond the self-purification capacity. In this case, only purified water is sent to the river after an artificial purification process. How is sewage and wastewater treated?

There are mainly physical, chemical, and biological methods for treatment of sewage and wastewater. The physical treatment is to remove suspended matter in sewage or wastewater. It is typical to filter out solid materials with larger volumes like plastic bags and to settle materials with large specific gravity including sand. The chemical method is to add chemicals (coagulant) to for better settling in order to remove settling inorganic matter (heavy metal) contained in wastewater through coagulation and settling, or to adjust pH (hydrogen ion concentration index). The biological treatment is usually called secondary treatment. Its purpose is to remove unsettled suspended solids (SS) and soluble organic matter (BOD). These technologies are rarely used independently, but they are generally used in combination.

Water Filtration Process

The treatment process is as follows: First, the sewage and wastewater flowing into the treatment plant goes into a grit chamber. In the grit chamber, heavy and hard matters such as soil, sand, plastic, and wood chips are settled. It separates floating particles heavier than water from water by gravity. Then the water from the grit chamber moves to a settling basin. In the basin, the water is left still for 2-3 hours to filter out slowly sinking pollutants. Then the water discharged from the settling basin moves to the aeration tank and goes through one of the biological methods, activated sludge method.

The activated sludge method is to purify sewage and wastewater by blowing air into the water and let microorganisms proliferate. It is to decompose bad organic matter such as nitrogen and phosphorus, which could not be filtered out by settling. With a lot of air, aerobic microorganisms that like air actively eat up organic matter. The microorganisms that have eaten up the organic matter grow bigger and coagulate, gradually becoming lumps like mud and sinking on the bottom. This coagulated sludge (sediment) absorbs the pollutants of sewage and wastewater. After going through this secondary treatment, more than 90% of BOD and SS are removed.

Filtration Process

There is still a possibility for pathogenic microorganisms to live in the water even after this process. Therefore, the water is sent to chlorination to be disinfected with chlorine. After disinfection, clean water where various bacteria are sterilized is finally supplied to general households through the water pipe or to Han River.

It would be great if we have 1st grade water anywhere in streams, rivers, lakes, and the sea in South Korea. It takes hard work and effort for this. Let's first think about how we can save water. Don’t forget that you can drink clean water only if Han River is clean!



Rachu Khanal

  • Rachu Khanal says :
    Thanks for this great article
    Posted 31-03-2020 07:50

Jasmine Karki

  • Jasmine Karki says :
    Wonderful report!!!
    Posted 31-03-2020 01:39

Sanjay Poudel

  • Sanjay Poudel says :
    Good report...thank you
    Posted 20-03-2020 14:08

Vazira Ikhtiyorova

  • Vazira Ikhtiyorova says :
    Wow, every article I am reading is so informative. I express my sincere gratitude to Tunza!
    Posted 22-02-2020 21:53

Asmita Gaire

  • Asmita Gaire says :
    This is something that every individual should be awared of!
    Thank you so much for sharing among us
    Posted 21-12-2019 04:44

Yushika Subedi

  • Yushika Subedi says :
    Didn't know BOD stands for Biochemical Oxygen Demand.

    Thank you Eco-generation.
    Posted 15-12-2019 19:10

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