wastewater

Wastewater is used water that has been altered by human activities and contains substances that may pose risks to human health and the environment. It includes domestic wastewater from households, municipal wastewater from industries, commercial facilities, and inflow from streets and stormwater systems in combined sewers. Industrial wastewater may contribute hazardous chemicals, heavy metals, and high concentrations of organic matter.

Wastewater management involves collection through sewer networks, treatment, and safe discharge or reuse. Sanitary sewers carry wastewater from buildings, while storm sewers collect rainfall; combined systems mix stormwater with wastewater in some cities. The treatment objective is to reduce contaminants to levels that protect public health and aquatic ecosystems.

Treatment typically proceeds in stages. Preliminary treatment uses screening and grit removal to protect downstream equipment. Primary treatment allows solids to settle, producing a liquid effluent with reduced suspended solids. Secondary treatment employs biological processes, such as activated sludge or biofilms, to biodegrade organic matter and to substantially reduce biochemical oxygen demand. Tertiary or advanced treatment may include filtration, nutrient removal (nitrogen and phosphorus), and disinfection (chlorination, UV, or ozonation). Sludge generated during treatment is thickened, stabilized, and often digested anaerobically to reduce volume and recover energy.

Treated wastewater may be discharged to rivers, lakes, or coastal waters, or reused for agricultural irrigation, industrial processes, or groundwater recharge. Compliance with water quality standards and permits governs discharge and reuse.

Global challenges include aging infrastructure, inadequate access to sanitation, and the need to minimize environmental and public health risks while maximizing resource recovery. Wastewater treatment is an evolving field that increasingly emphasizes energy recovery, resource recovery, and the mitigation of emerging contaminants.