Wastewater is a complex mixture of used water flowing from residences, commercial buildings, industries, and sometimes stormwater runoff. This water contains dissolved and suspended contaminants, including organic matter, nutrients, and various microorganisms. Treating this water is necessary to protect public health and prevent environmental contamination of rivers, lakes, and oceans. The overall process transforms the contaminated water into an effluent clean enough to be safely returned to the natural water cycle.
Primary Treatment: Initial Screening and Settling
The first stage of treatment is a physical process designed to remove large, easily separable solids from the incoming water. Wastewater first passes through mechanical bar screens, which block and collect large debris such as rags and plastics that could damage equipment downstream. Following screening, the water flows into grit chambers, where the velocity is intentionally slowed down. This reduction allows heavy, inorganic materials like sand and gravel, known as grit, to settle out by gravity.
Next, the water enters large sedimentation tanks, often called primary clarifiers, designed for minimal turbulence. The slow flow allows smaller suspended organic solids to settle to the bottom over about two hours. Mechanical scrapers continually move this settled material, called primary sludge, toward a central collection point. This physical separation typically removes between 40% and 60% of the total suspended solids. Primary treatment leaves behind dissolved organic matter and pathogens, requiring further purification.
Secondary Treatment: Biological Breakdown and Purification
The secondary stage transitions from physical separation to a biological process, harnessing microorganisms to consume remaining dissolved organic contaminants. Water flows into large aeration basins where it is mixed with a concentrated community of bacteria and protozoa, known as activated sludge. Compressed air is injected into the mixture, providing the necessary oxygen for these aerobic microorganisms to thrive. This constant aeration and mixing ensures the microbes come into maximum contact with the organic matter.
The microorganisms metabolize organic compounds, such as sugars, fats, and proteins, breaking them down into harmless byproducts like carbon dioxide, water, and new microbial biomass. This process significantly reduces the Biochemical Oxygen Demand (BOD), which measures the oxygen required by microbes to break down the waste. After several hours in the aeration basins, the mixture moves to secondary clarifiers. The flow slows, allowing the newly formed microbial clusters, or floc, to settle out by gravity, separating from the cleaner water.
A portion of this settled microbial biomass is recirculated back to the aeration tanks to continuously seed the process; this is the namesake “activated” sludge. The biological treatment stage is effective, removing about 85% of the organic matter and suspended solids remaining after primary treatment. The water exiting the secondary clarifiers is clear but still contains fine suspended particles, dissolved nutrients like nitrogen and phosphorus, and potentially harmful pathogens.
Advanced Treatment and Final Disinfection
Following the secondary stage, advanced treatment steps are often implemented to meet stringent water quality standards before the water is returned to the environment. Advanced treatment typically involves tertiary filtration, where the water passes through fine granular media, such as sand or membranes, to strain out remaining microscopic particles and suspended solids. This polishing step ensures the water is clear, which is beneficial for the next stage of pathogen removal. These processes can also remove excess nutrients, such as nitrogen and phosphorus, preventing harmful algal blooms in receiving waterways.
The final step is disinfection, which is the targeted inactivation or destruction of any remaining pathogens. Treatment plants commonly use chemical methods, such as adding chlorine or its compounds, which are effective and widely adopted. Because chlorine can be toxic to aquatic life, a dechlorination step is often required before the water is discharged. Physical methods like ultraviolet (UV) light are also used, where high-intensity UV lamps disrupt the pathogens’ DNA, preventing them from reproducing without adding chemicals.
Managing Biosolids and Residual Waste
The purification process generates a large volume of solid material, or sludge, collected from both the primary and secondary clarification stages. This material is managed and processed to reduce its volume and stabilize its organic components. Stabilization often occurs in large, enclosed tanks called digesters, where microorganisms break down the sludge in the absence of oxygen through anaerobic digestion. This process reduces pathogens and produces methane-rich biogas, which can be captured and used as a renewable energy source to power the treatment plant.
The stabilized product is referred to as biosolids, a nutrient-rich, semi-solid material. Biosolids are dewatered using processes like filter presses or centrifuges to reduce moisture content, making them easier to handle and transport. Once treated to meet regulatory standards, these biosolids are commonly applied to agricultural land as a soil amendment and fertilizer. Other options for final management include landfill disposal or incineration.