Sewage and wastewater

Sewage and wastewater are terms used to describe the water that has been used in homes, businesses, and industries and that contains waste products. This water flows down drains, toilets, and industrial pipes and collects a mix of substances—everything from human waste and food scraps to soap, cleaning chemicals, pharmaceuticals, and even microscopic plastic particles. Wastewater also includes stormwater, which is rainwater that flows over roads and rooftops, picking up oil, dirt, heavy metals, and other pollutants before entering drainage systems.

The journey of sewage and wastewater begins in our daily lives. When you flush a toilet, take a shower, wash dishes, or run a factory process, the water you use becomes wastewater. In cities and towns, complex networks of underground pipes called sewers collect all this used water and channel it toward treatment plants. In areas without centralized sewage systems, wastewater might be directed to septic tanks or, in less developed regions, sometimes released untreated into the environment.

Wastewater is a problem because of what it contains. The most obvious elements are the organic matter in human and animal waste. When this organic waste enters rivers, lakes, or oceans without being properly treated, it becomes food for bacteria and other microorganisms. As they break down the waste, these microorganisms consume oxygen from the water, depleting supplies needed by fish and other aquatic life. This process is called oxygen depletion and can result in “dead zones”—areas where oxygen levels are too low for most life to survive.

Another big concern with untreated or poorly treated sewage is nutrient pollution, particularly from compounds like nitrogen and phosphorus. These nutrients come from urine, detergents, and fertilizers that wash into sewers. When released into natural waters, they act like fertilizer and encourage the rapid growth of algae in a process called eutrophication. Algal blooms can block sunlight from reaching underwater plants, disrupt aquatic ecosystems, and when the algae die off, the decomposition process further consumes oxygen in the water. Some algal blooms even produce toxins that are dangerous to fish, animals, and humans.

Sewage and wastewater can also carry disease-causing microorganisms such as bacteria, viruses, and parasites. In places where sewage mixes with drinking water sources or is discharged close to where people swim or harvest shellfish, these pathogens can make people sick with illnesses like cholera, typhoid, dysentery, and hepatitis. This is especially dangerous in developing areas without modern sanitation infrastructure.

Another environmental hazard from wastewater is chemical contamination. Everyday products such as cleaning agents, pharmaceuticals, and personal care products often end up down the drain. Many of these chemicals are persistent and not fully removed by typical treatment methods. They can enter rivers and lakes, affecting the health of aquatic organisms and sometimes coming back to affect humans as well. Some chemicals, like hormones and antibiotics, are especially concerning because they can interfere with animal reproduction or contribute to antibiotic resistance, which is a growing public health problem.

Heavy metals such as lead, mercury, and cadmium can also be present in wastewater, especially from industrial sources. When these metals accumulate in the environment, they are toxic to plants, animals, and humans, and they can enter the food chain. Over time, small amounts can build up in living organisms—a process called bioaccumulation—which can have serious health effects.

Even materials we don’t see or think much about, like microplastics, are increasingly found in wastewater. These tiny pieces of plastic come from sources like synthetic clothing, personal care products, and degraded larger plastic items. Wastewater treatment plants remove some, but not all, of these particles. When released, they can accumulate in rivers and oceans, where they are ingested by fish and other animals, disrupting food chains and potentially ending up on our own plates.

When wastewater isn’t managed properly, environmental problems multiply. Polluted water bodies can’t support biodiversity as they should, leading to the loss of species and vital ecosystem functions. Fish kills, the collapse of local fisheries, and the contamination of drinking water supplies are just some of the visible impacts that can result. On a broader scale, when ecosystems are damaged, communities dependent on them for food, water, or recreation suffer too.

Modern wastewater treatment can remove most of these harmful substances, but treatment infrastructure requires significant investment and maintenance. In many developing parts of the world, such systems are lacking, leading to the direct discharge of raw or partially treated sewage into rivers, lakes, and the ocean. Even in wealthier countries, heavy rain can overwhelm sewer systems, causing untreated sewage to overflow into water bodies—a phenomenon called combined sewer overflow.

In sum, sewage and wastewater occur as a byproduct of daily human and industrial activities. If not handled and treated properly, they pose a significant threat to the environment and public health by depleting oxygen in water, fueling harmful algal blooms, spreading diseases, and introducing a range of pollutants and toxins into ecosystems. Dealing with sewage and wastewater effectively is therefore critical to maintaining healthy environments and safe communities.