sewers

A sewer is an underground network of pipes that carries wastewater and, in some cases, stormwater away from buildings and urban areas. Sewers are typically divided into sanitary sewers that transport sewage from toilets and drains, storm sewers that convey rainwater from streets, and, in some cities, combined sewers that carry both. The primary purpose is to protect public health and property by transporting waste and excess water to treatment facilities or suitable outfalls.

Key components of a sewer system include pipes, manholes, inlets, pumping stations, interceptors, and treatment plants. Sewers generally rely on gravity, with intentional slopes to maintain flow; pumping stations lift sewage where gravity alone cannot move it. Materials vary with age and region, including concrete, clay, ductile iron, and plastic pipes.

Design and maintenance considerations focus on capacity, reliability, and safety. Separate sanitary and storm networks help prevent overloading during rain events. Regular inspection and cleaning—often using CCTV surveying and jetting—detects blockages, leaks, and structural issues. Repairs may require excavation or trenchless methods.

Treatment and discharge processes typically involve a wastewater treatment plant where wastewater undergoes screening, primary sedimentation, secondary biological treatment, and sometimes tertiary treatment and disinfection. Sludge is processed separately, and some systems recover energy or nutrients from byproducts.

Challenges include combined sewer overflows during heavy rainfall and aging infrastructure. Cities address these issues with separation of flows, storage tanks, green infrastructure, and expanded treatment capacity. Historically, sewer systems evolved from ancient public works to modern urban networks, now managed by municipalities or utility agencies to meet environmental and public health standards.