Wastewater treatment is a process that removes pollutants from wastewater. The cleaned water, released into natural water sources, does not harm the environment. Domestic wastewater, also known as municipal wastewater or sewage, is treated at sewage treatment plants. Industrial wastewater is often treated at special industrial wastewater treatment facilities or at sewage treatment plants. When treated at sewage plants, industries usually clean their waste on-site before sending it to the municipal plant. Other types of treatment plants handle agricultural wastewater and leachate, which is liquid from landfills.

The term "wastewater treatment" is often used to describe "sewage treatment."

Common steps in wastewater treatment include separating solids from liquids, such as through sedimentation, and using biological and chemical methods, like oxidation, to clean the water. A final step may involve polishing to further improve water quality. The main waste product from treatment plants is sludge, which is often treated at the same or another plant. If anaerobic treatment is used, biogas may also be produced as a by-product.

Treated wastewater can be reused as reclaimed water. The main goal of wastewater treatment is to make water safe for disposal or reuse. However, before treatment begins, plans for disposal or reuse must be decided to choose the correct treatment method for the wastewater.

Types of treatment plants

Wastewater treatment plants can be identified based on the type of wastewater they process. Different methods are used to treat wastewater, depending on the kind and level of pollution it contains. The treatment process usually includes physical, chemical, and biological steps.

Types of wastewater treatment plants include:

• Sewage treatment plants
• Industrial wastewater treatment plants
• Agricultural wastewater treatment plants
• Leachate treatment plants

Sewage treatment is a type of wastewater treatment that removes harmful substances from sewage to create water that can be safely released into the environment or reused. Sewage comes from homes, businesses, and sometimes pre-treated industrial waste. Many systems exist for treating sewage, ranging from small, local systems (like on-site treatment) to large systems with pipes and pumping stations that move sewage to treatment plants. In cities with combined sewers, stormwater (rainwater) is also sent to treatment plants. Sewage treatment often has two main steps: primary and secondary treatment. Advanced treatment may include a third step called tertiary treatment, which improves water quality further. Secondary treatment reduces organic matter (measured as biological oxygen demand) using biological processes. A fourth step, called quaternary treatment, may also be used to remove tiny pollutants like medicines. This has been used in Sweden.

Many sewage treatment methods use biological processes. Engineers and planners choose methods based on factors like the quality of treated water needed, costs, land availability, energy use, and environmental impact. In areas with few people or limited resources, on-site systems like septic tanks and drain fields are often used. In contrast, advanced systems may include steps like disinfection or removing tiny pollutants.

Globally, about 52% of sewage is treated. However, treatment rates vary widely. For example, high-income countries treat about 74% of their sewage, while developing countries treat only about 4.2%.

Industrial wastewater treatment refers to the process of cleaning wastewater produced by industries. After treatment, this water may be reused or released into sewers or natural water sources. Some industries send their wastewater to sewage treatment plants, while others have their own systems to meet legal requirements for pollution control. Industries that produce highly polluted water, such as those with oil, heavy metals, or chemicals, often use pre-treatment systems to remove harmful substances before sending water to municipal sewers.

Many industries produce wastewater. Recent efforts aim to reduce this waste or reuse treated water in production. Some industries have redesigned processes to avoid creating pollution. Examples of industries that produce wastewater include battery manufacturing, chemical production, food processing, and oil refining. Treatment methods include removing salt, solids, oils, organic matter, acids, alkalis, and toxic materials.

Agricultural wastewater treatment focuses on managing pollution from farms, such as waste from animals or chemicals in runoff. This is especially important for large farms with animals like cows or chickens. Treatment may use systems similar to those used in industry. In areas with open land, ponds or lagoons can be used for lower-cost treatment. Animal waste is often stored in anaerobic lagoons before being spread on fields. Sometimes, wetlands are used to help treat waste naturally.

Agricultural pollution comes from two types: nonpoint source (like soil erosion or chemical runoff) and point source (like waste from animals or farms). Many farms produce nonpoint source pollution through runoff that is not treated in plants.

Leachate treatment plants clean liquid that leaks from landfills. Methods include biological treatment, mechanical filters, carbon filters, electrochemical processes, and special membrane filters.

Unit processes

Wastewater treatment involves several steps to clean water. Physical processes, such as settling or floating, and biological processes, such as oxidation or anaerobic treatment, are used. Some wastewaters need special methods. At the simplest level, most wastewater is treated by separating solids from liquids, usually through sedimentation. By changing dissolved materials into solids, like biological flocs or biofilms, which are then settled or separated, cleaner water is produced.

Phase separation moves impurities into a different liquid phase. This can happen during steps like oxidation or polishing to remove solids. Grease and oil may be reused for fuel or soap production. Sludge in wastewater treatment plants often needs dewatering. How dried solids are handled depends on the type and amount of impurities removed.

Solids like stones, grit, and sand can be removed by gravity if they are dense enough to sink. This is done using grit channels that control water flow to allow heavy solids to settle while lighter ones move forward. Gravity separation is the first step in sewage treatment, called "primary settling tanks" or "primary sedimentation tanks." It is also used for other wastewater types. Denser solids settle at the bottom of still basins. More advanced systems use skimmers to remove floating materials like grease or solids such as feathers or wood chips. Devices like API oil-water separators are designed to separate non-polar liquids.

Oxidation reduces the amount of oxygen needed by wastewater and can remove some harmful substances. Secondary treatment uses chemical reactions to break down organic compounds into carbon dioxide, water, and biosolids. Chemical oxidation is often used to disinfect water.

Secondary treatment, usually biological, removes biodegradable organic matter from wastewater. This step follows a "primary treatment" that separates settleable solids through physical methods. During secondary treatment, microorganisms in aerobic or anaerobic environments break down dissolved and suspended organic matter, measured as biochemical oxygen demand (BOD). These microbes consume organic materials like sugars, fats, and carbon molecules from waste, forming biological solids. Secondary treatment is common in sewage and industrial wastewater treatment.

Advanced oxidation processes remove remaining pollutants after biochemical treatment. Chemical oxidation adds substances like ozone or chlorine to kill bacteria and break down complex organic compounds into simpler ones like water and carbon dioxide.

Anaerobic treatment is used for industrial wastewater and sludge. It occurs in reactors with biofilms that filter solids from wastewater. Microbes convert organic matter into methane and carbon dioxide. Anaerobic systems, like UASB or EGSB, are efficient but may not remove pathogens, nitrogen, or phosphorus. They also produce odors and require extra steps.

Polishing is the final step in advanced treatment, used after earlier methods or independently for some industrial wastewater. Chemical reduction or pH changes reduce chemical reactivity after oxidation. Carbon filters remove remaining impurities by absorbing them onto activated carbon. Sand or fabric filters are commonly used in municipal wastewater treatment.