sewer

A sewer is an underground network of pipes designed to transport wastewater from buildings and streets to a treatment facility or discharge point. Sewers may carry sanitary wastewater, stormwater, or a combination of both, depending on whether the system is sanitary, storm, or combined. The primary aim is to protect public health and reduce flooding by moving used water away from inhabited areas.

Core components include sewer lines that slope to promote gravity flow, interceptors or trunk mains that collect flows from smaller lines, manholes for access, pumping stations in places with insufficient slope, and treatment facilities where solids are removed and water is cleaned before release.

Sewer systems rely on gravity, so pipe gradients and network layout are crucial. Materials range from ancient clay and cast iron to modern reinforced concrete and plastic (PVC or HDPE). Systems are designed for expected population and rainfall, and may incorporate backflow prevention devices and storm inlets.

Maintenance tackles blockages, which commonly arise from grease, sanitary wipes, and tree roots, as well as infiltration and inflow during heavy rain. Work includes cleaning, rodding, jetting, and sometimes relining or replacing pipes. Poor maintenance can lead to sewer overflows and property damage.

Wastewater undergoes primary settling, secondary biological treatment, and sometimes tertiary processes to remove nutrients and pathogens before discharge or reuse. Sewer management is regulated by environmental and public health authorities, and many cities face challenges from aging pipes and funding constraints, requiring ongoing planning and investment.