Wastewater, often called sewer water, is any water whose quality has been negatively affected by human use, originating from homes, commercial businesses, and industries. This complex liquid contains suspended solids, dissolved organic matter, nutrients like nitrogen and phosphorus, and various microorganisms. The ultimate goal of treatment is to clean this water so it can be safely returned to the environment, protecting public health and aquatic ecosystems. This process involves a series of physical, biological, and chemical steps designed to progressively remove contaminants before the water rejoins the natural water cycle.
Preliminary and Primary Treatment
The treatment process begins with preliminary steps focused on removing large, non-degradable debris that could damage equipment or interfere with later stages. Wastewater first flows through screens, which mechanically remove items such as rags, plastics, and coarse paper products. Following this, the water enters specialized grit chambers, where the flow velocity is reduced to allow heavy, inert materials like sand and gravel to settle out. This prevents accumulation in pipes and tanks.
Once debris and grit are removed, the water moves into the primary treatment stage, which relies on sedimentation. The water is directed into large sedimentation tanks, often called clarifiers, where the flow is significantly slowed down. This allows organic solids and suspended particles to settle to the tank bottom due to gravity. Up to 70% of the suspended solids and 40% of the organic load (BOD) can be removed during this physical separation. The settled material, known as primary sludge, is scraped from the bottom, while lighter materials like grease and oils are skimmed off the surface.
Secondary Treatment
Since primary treatment cannot remove dissolved and colloidal organic matter, secondary treatment employs biological methods. This stage utilizes beneficial microorganisms to consume remaining organic pollutants in a controlled environment. The most common method is the activated sludge process, which involves transferring the water into large aeration tanks where air or pure oxygen is continuously injected. This oxygen-rich environment encourages the rapid growth of aerobic bacteria and other microbes.
These microorganisms cluster into biological flocs, or “activated sludge,” that actively consume organic compounds such as sugars, fats, and proteins. The microbes convert the dissolved organic matter into energy, carbon dioxide, water, and new cell material, effectively reducing the biochemical oxygen demand by up to 90%.
After aeration, the mixture flows into a secondary clarifier, where the heavy microbial flocs settle to the bottom, separating from the cleaner liquid. A portion of this settled biological sludge is recycled back to the aeration tank to maintain an active population of microbes, while the excess is removed for further processing.
Tertiary Treatment and Disinfection
Following secondary clarification, the water may still contain specific pollutants, such as nutrients or trace contaminants. Tertiary treatment is employed to remove non-biodegradable substances like nitrogen and phosphorus, which can cause environmental problems like eutrophication. Specialized biological or chemical processes are used, such as adding metal salts to precipitate phosphorus, or creating anoxic zones for bacteria to convert nitrate into harmless nitrogen gas.
The water may also pass through advanced filtration systems, such as sand or granular media filters, to remove any remaining fine suspended particles. The final step before discharge is disinfection, which targets any remaining pathogens. Common methods include exposing the water to ultraviolet (UV) light, which prevents reproduction, or introducing chemical agents like chlorine or ozone. The resulting clean liquid, known as the final effluent, is then discharged into a receiving water body, treated to meet stringent regulatory standards.
Managing Solids and Byproducts
Throughout the treatment stages, a considerable volume of solid material, called sewage sludge, is generated and must be processed. The combined sludge from the clarifiers is highly organic and contains a large amount of water, which must be reduced to make it manageable. This material is stabilized through digestion, where it is placed in large, sealed tanks called digesters.
Anaerobic digestion, occurring without oxygen, uses specific bacteria to break down the organic matter. This process significantly reduces volume, removes pathogens, and stabilizes the material to reduce odor.
Following stabilization, the sludge is subjected to dewatering processes to further reduce its liquid content. This can be accomplished using mechanical devices like belt presses or centrifuges, or by allowing it to dry on beds. Dewatering reduces the weight and volume, making the material easier to transport.
The resulting product, now referred to as biosolids, is a nutrient-rich, soil-like material. Biosolids can be safely reused as a soil amendment or fertilizer for non-food crops, or they may be disposed of in a landfill or incinerated, depending on quality and local regulations.