How clean is the water we use before it returns to nature after sewage treatment?

Koreans use an average of 346 liters of water per day, and this contaminated water is purified through primary, secondary, and tertiary treatment at sewage treatment plants. The water is then discharged into nature to remove suspended solids, organic chemicals, heavy metals, and pathogens, and is recycled into clean water through nature’s self-correcting process.

 

How much water did we use today? The average South Korean uses about 346 liters of water per day, including showering, washing hair, doing laundry, washing dishes, and drinking water. That’s a whopping 17.3 billion liters of water per day for 50 million South Koreans. If we were to release this amount of polluted water into nature, nature would be severely damaged. Just as we can’t use nature’s water as it is, nature can’t accept the polluted water we create.
The water we throw away contains a variety of pollutants, including solid waste from households, industries, and livestock farms, organic chemicals, pathogens, nitrogen compounds, phosphorus compounds, and heavy metals. Heavy metals and pathogens threaten the health of ecosystems, while nutrients like organic compounds, nitrogen compounds, and phosphorus compounds feed microorganisms in the water, causing eutrophication. Eutrophication is a process in which excessive nutrient consumption by microorganisms leads to a lack of oxygen in the water, which causes fish kills. To prevent this imbalance in the ecosystem, we rely on sewage treatment plants to clean the water to a level that nature can handle.
Sewage treatment plants go through three main stages of treatment. First, when sewage comes in, there’s a preliminary stage to remove bulky floatables. This is done by sifting through a sieve or sweeping the surface of the water with a stick to collect and remove scum. Then, in primary treatment, the water spends about two hours in the sedimentation basins, where the solids in the water sink or float away by gravity. Machines in the basins remove this debris, removing about 60% of the solids.
The water then moves to a secondary treatment plant. Here, it undergoes biological treatment, which uses microorganisms to remove organic matter. Microorganisms eat organic matter and convert it into cellular building blocks like carbon dioxide and proteins. We either release the microorganisms into the water and let them graze (activated sludge) or grow them on film (trickling filters). Both methods are subjected to recirculation, which allows us to remove more than 85% of the total organic matter.
Tertiary treatment removes any remaining heavy metals, pathogens, nutrients, and microorganisms. This can be done using activated carbon or naturally through artificial wetlands. The plants and soil in the wetlands absorb nutrients, such as nitrogen and phosphorus compounds, and adsorb heavy metals, removing them naturally. Finally, after ozonation to kill pathogens, the sewage is discharged into a stream.
The water we use and dispose of today is first treated in a sewage treatment plant to remove suspended solids, second to remove organic compounds, and third to remove heavy metals, nitrogen compounds, phosphorus compounds, and pathogens before being discharged into the stream. The water is not yet completely clean, but through nature’s self-cleaning process, it will be purified and made available for our use. We may think that sewage treatment plants are not a direct tap water supply like water treatment facilities, but they play an essential role when considering the water cycle. Currently, there are more than 60 sewage treatment plants in Korea, which treat sewage according to the characteristics of each region and are constantly being researched to increase their efficiency.