Clean is not necessarily pure: Why treated wastewater often remains harmful

When I open a tap to fill my water bottle, I am grateful to live in a part of the world that offers this luxury. I am aware that not everyone can easily access clean water. Water pollution constantly requires attention in our urbanized world because wastewater can come from many households, businesses, and factories. This wastewater can contain all kinds of pollution and contamination, such as viruses, excess nutrients, chemicals, radioactive materials, and heat. Due to the negative effects of pollutants in wastewater, sewage treatment plants have been built in many parts of the world. These plants remove pollutants and contaminants from the incoming wastewater. Unfortunately, these plants are not always able to remove all types of pollutants. For example, metals, pharmaceuticals, personal care products, cleaning agents, and tiny plastic particles are very difficult to remove. This means that discharged wastewater can still contain a mixture of pollutants and contaminants. These contaminants can have various consequences, for example for aquatic animals, as they are constantly surrounded by small amounts. The degree to which aquatic animals like minnows are affected depends on other factors such as water temperature. Physiology The physiology of fish in treated wastewater is only affected at warm temperatures (20°C). Fish physiology includes the normal functioning of their bodies and body parts. The physiology of minnows is affected because their metabolic rate in warm water is about five times higher than in cold water (4°C). A high metabolic rate means that chemical processes occur more quickly. This is the case both when they are resting and when they are very active. When minnows are also exposed to treated wastewater, their metabolic rate increases even further, by about six to seven times compared to fish in cold water. The higher metabolic rate results in minnows needing more energy to stay alive and their organs being more stressed. Additionally, the volume of red blood cells decreases compared to the total blood volume. This is unhealthy because red blood cells transport oxygen to the cells, meaning that fewer red blood cells result in less oxygen available to the cells. Taken together, these consequences affect the overall condition and health of the body. Behavior The behavior of fish in wastewater is affected by both warm and cold water. At warm temperatures, minnows exposed to a lot of treated wastewater become less bold: they stay longer in their shelters. This can be problematic as metabolism consumes more energy in warm water. Consequently, staying in their refuges can reduce their energy levels and fitness. At low temperatures, minnows tend to spend less time with other fish in their school when surrounded by a lot of treated wastewater. This can be critical because when they are alone, it is harder to keep an eye on predators; when they are near other fish, it is less likely they will be eaten by other fish. If the fish follow each other to good feeding spots, it is also harder for them to find food when they are less social. This is especially critical for fish exposed to wastewater in winter when food is scarce. What you can do: - Dispose of (toxic) substances in appropriate containers instead of flushing them down the toilet or drain - Dispose of medications correctly to reduce pharmaceutical pollution - Use natural instead of chemical cleaning products - Use as little as possible when using personal care products like sunscreen About the Author Dr. Erlijn van Genuchten is an internationally recognized sustainability and cybersecurity expert. As a member of the United Nations Economic Commission for Europe Task Force on Digitalization in the Energy Sector, she brings her expertise at the highest level.