Sewer systems

A sewer system collects used water from homes, institutions and industry and carries it to treatment. Without one, wastewater soaks into the ground or runs in open drains, contaminating groundwater and spreading disease. Planning a sewer system decides what flows it carries, how it is laid out and how it is kept working.

A separate system carries sanitary sewage in one network and stormwater in another, so treatment plants receive a fairly steady, concentrated flow. A combined system carries both in one pipe, cheaper to build in dense old cities but overloaded in storms, which leads to combined sewer overflows of diluted sewage into rivers. A partially separate system admits some roof and yard runoff. Indian practice generally favours separate systems for new work.

Sewers usually run by gravity, following ground slopes, from small house connections to laterals, branch sewers, mains and trunk sewers or outfalls. Where ground is flat, pumping stations lift sewage to a higher level. Manholes at every junction, change of direction, slope or size, and at regular spacing, allow inspection and cleaning. Sewers must be ventilated, because sewage gives off hydrogen sulphide, which is toxic and corrodes concrete and metals above the water line. Sewer gas also makes confined-space entry dangerous; mechanised cleaning and strict safety procedures are essential, and manual entry without them is prohibited.

Asset management keeps a sewer system working: a digital map of every pipe and manhole, CCTV surveys to grade condition, planned cleaning of known silting reaches and trenchless rehabilitation such as cured-in-place lining where pipes have cracked.