This blog post takes a detailed look at the process of purifying sewage and the journey that water takes before returning to nature.
How much water did we use today? Koreans use an average of about 346 liters of water per day for activities like showering, washing hair, doing laundry, washing dishes, and drinking. Therefore, the amount of water used daily by Korea’s population of 50 million people reaches a staggering 17.3 billion liters. Releasing such a massive volume of polluted water directly into nature would cause severe damage to the environment. Just as we cannot use nature’s water directly, nature cannot accept the polluted water we create.
The water we discharge contains various pollutants, including solid residues, organic compounds, pathogens, nitrogen compounds, phosphorus compounds, and heavy metals from households, industrial facilities, and livestock farms. Heavy metals and pathogens threaten the health of ecosystems, while nutrients like organic compounds, nitrogen compounds, and phosphorus compounds become food for aquatic microorganisms, causing eutrophication. Eutrophication refers to a series of events where excessive nutrient consumption by microorganisms depletes oxygen in the water, leading to mass fish deaths. To prevent this ecological imbalance, we purify wastewater at treatment plants to a level nature can handle before returning it.
Sewage treatment plants undergo three main processes. First, when wastewater enters, a preliminary stage removes large floating debris. This involves screening out floating matter or skimming debris from the water surface with poles. This is a measure to prevent equipment breakdown. Next, in the primary treatment stage, the water remains in sedimentation basins for about two hours, allowing solid materials in the water to settle or float due to gravity. Machines in the tanks remove this sludge, eliminating approximately 60% of the solid matter.
The water then moves to the secondary treatment plant. Here, biological treatment occurs, using microorganisms to remove organic matter. Microorganisms consume the organic matter, converting it into cellular components like carbon dioxide or proteins. We either disperse the microorganisms in the water (activated sludge) or attach them to a film for cultivation (trickling filters). Both methods involve a recirculation process, enabling the removal of over 85% of the organic matter.
Tertiary treatment removes remaining heavy metals, pathogens, nutrients, and microorganisms. This can be achieved using activated carbon or through environmentally friendly methods like constructed wetlands. Plants and soil in constructed wetlands absorb nutrients like nitrogen and phosphorus compounds and adsorb heavy metals, naturally removing these substances. Finally, ozone treatment eliminates pathogens, after which the treated wastewater is discharged into rivers.
Thus, the water we use and discard today undergoes three stages of treatment at the sewage treatment plant: primary treatment removes suspended solids, secondary treatment removes organic compounds, and tertiary treatment removes heavy metals, nitrogen compounds, phosphorus compounds, and pathogens before discharge into rivers. While it cannot yet be called completely clean water, it will be purified back into clean water through nature’s self-purification process, making it usable again. We might think sewage treatment plants are not direct tap water supply facilities like water treatment plants, but considering the water cycle, sewage treatment plants play an essential role. Currently, there are about 60 sewage treatment plants in Korea. They treat sewage according to the characteristics of each region and are constantly being researched to improve efficiency.
