Wastewater, commonly known as sewage, is used water that flows from homes, businesses, and industries, including from toilets, sinks, and showers. Untreated sewage contains harmful bacteria, viruses, and pollutants that pose risks to public health and the environment, potentially spreading diseases and damaging aquatic ecosystems.
How Sewage Travels
Wastewater’s journey begins inside buildings, flowing from fixtures into small pipes that connect to larger sewer lines buried beneath streets. Most modern sewer systems rely on gravity, allowing wastewater to flow downhill towards a treatment facility.
In areas where gravity is insufficient or the treatment plant is at a higher elevation, pumping stations are integrated into the system. These stations lift the wastewater to a higher point, enabling it to continue its flow through the main sewer lines until it reaches the treatment plant.
The Wastewater Treatment Stages
At the wastewater treatment plant, sewage undergoes processes to remove contaminants. Preliminary treatment involves screening the incoming wastewater. Mechanical screens remove large physical debris like rags and plastics to prevent equipment damage. Water then enters grit chambers where flow slows, allowing heavier inorganic materials like sand and gravel to settle out.
After preliminary treatment, wastewater moves to primary treatment in large sedimentation tanks (clarifiers). Here, flow slows significantly, allowing organic solids and suspended particles to settle by gravity. This settled material, primary sludge, is collected and removed, while lighter materials like grease and oil float to the surface and are skimmed off.
Secondary treatment focuses on removing dissolved and fine suspended organic matter not settled during primary treatment. This biological process uses microorganisms to consume organic pollutants. In aeration tanks, air is pumped into the water, providing oxygen for beneficial bacteria to thrive and break down organic substances. These bacteria form clumps, called activated sludge, which are separated from the treated water in secondary clarifiers.
Some treatment plants employ tertiary (advanced) treatment for further purification. This stage is important when treated water will be discharged into sensitive environments or considered for reuse. Tertiary treatment can involve processes like filtration through sand or activated carbon beds to remove remaining suspended particles. Nutrient removal processes target nitrogen and phosphorus, which can cause excessive algae growth in waterways. The final step is disinfection, where ultraviolet (UV) light or chlorine is used to eliminate any remaining disease-causing microorganisms.
What Happens to Treated Water and Solids
Following the extensive treatment process, the cleaned water, known as effluent, is typically discharged back into local natural water bodies. This can include rivers, lakes, estuaries, or oceans, depending on the plant’s location and environmental regulations. In some regions, this treated water is also reused for non-potable purposes, such as irrigating agricultural crops, watering golf courses, or for industrial cooling, conserving freshwater resources.
The solid materials removed during the treatment process, known as biosolids or sludge, undergo further processing. This often involves anaerobic digestion, where microorganisms break down organic matter in the absence of oxygen, reducing sludge volume and stabilizing it. After digestion, biosolids are dewatered using techniques like centrifuges or filter presses to remove excess water, significantly reducing their weight and volume. These treated biosolids can then be beneficially reused as a nutrient-rich soil amendment in agriculture, as daily cover at landfills, or for generating biogas, a renewable energy source.
Other Ways Sewage is Managed
While large municipal wastewater treatment plants serve urban and suburban areas, alternative methods exist for managing sewage, especially in rural regions. The most common decentralized system is the septic system, providing on-site wastewater treatment for individual homes or small clusters of buildings.
A typical septic system consists of two main components: a septic tank and a drain field. Wastewater from the home flows into the underground septic tank, where solids settle to the bottom, forming sludge, and lighter materials like grease float to the top, forming scum. Bacteria within the tank partially break down the organic matter. The liquid effluent then flows into the drain field, a series of trenches or beds containing gravel and soil. Here, wastewater slowly percolates through the soil, which acts as a natural filter, removing remaining impurities and pathogens before the treated water recharges the groundwater.
Less common alternatives include composting toilets, which manage human waste without water, and package treatment plants, compact, self-contained units that provide treatment for smaller communities.