Sanitary sewer

• 1. Introduction

  ›

  A sanitary sewer is one of the least glamorous things a city can build, and also one of the most consequential. Beneath your feet right now, in virtually every modern city, an underground network of pipes and tunnels is silently moving waste away from homes and businesses toward treatment plants. Without it, the animal feces that once piled on city streets, the chamber pots emptied into those same streets, the runoff mingling with raw sewage in old combined drains would still define daily urban life.

  But the sanitary sewer is not simply a pipe. It is a decision, an engineering choice with enormous trade-offs. Cities that built their drainage systems before sewage treatment existed made one choice. Cities that urbanized in the mid-20th century or later made a very different one. And many old cities are still living with systems they built generations ago.

  What separates a sanitary sewer from every other underground pipe? Why do some cities have separate systems for sewage and rainwater while others still combine them? And what happens when the infrastructure ages and the cracks begin to spread?

• 2. Separated From The Storm

  ›

  The central logic of a sanitary sewer is separation. A sanitary sewer carries sewage from houses and commercial buildings but deliberately excludes stormwater. Storm drains handle surface runoff separately, channeling it directly to nearby waterways rather than to treatment plants.

  This separation solves a specific engineering problem. Sewage treatment is less effective when wastewater is diluted with rainwater. When heavy rain or snowmelt hits a combined system, the surge can exceed what a treatment plant can handle. The overflow goes untreated into the environment. Keeping the two streams apart avoids this entirely.

  The trade-off is cost. Building two separate underground systems is more expensive than building one. For cities deciding how to manage their drainage, the calculation has historically come down to the cost of providing adequate treatment during heavy rain events versus the capital outlay of two networks. Many cities resolved this by building combined sewers first, before sewage treatment plants even existed, and then never replacing them. Suburbs built later, with treatment already established, more often chose separate sanitary sewers from the start.

• 3. How The Network Works

  ›

  Gravity does most of the work in a conventional sanitary sewer. Pipes from individual buildings, called laterals, feed into branch sewers that typically run beneath streets. Those branch sewers discharge by gravity into trunk sewers at junctions called manholes. Larger cities may also have interceptors, which gather flow from multiple trunk sewers before it reaches a treatment facility.

  Manholes are usually made of precast concrete and can be cylindrical, eccentric, or concentric depending on the site. Beyond providing access for inspection and maintenance, they serve as vents for sewer gases and allow changes in pipe direction that straight runs cannot accommodate.

  Designing the size of a sewer involves calculating the population it will serve over the system's anticipated lifetime, estimating per capita wastewater production, and accounting for the peaks caused by daily routines. Minimum pipe diameters are set to prevent blockage by solid materials flushed down toilets. Gradients are chosen to maintain flow velocities high enough to keep solids from settling inside the pipe. Industrial wastewater flows also factor into the calculation, though storm runoff does not.

• 4. When Gravity Is Not Enough

  ›

  Not every neighbourhood sits at an elevation that allows gravity to carry sewage to a treatment plant. When a gravity sewer would have to run uphill, a force main or rising main takes over. Force mains are pumped systems, typically built from welded steel or high-density polyethylene jointed to resist internal pressure. A lift station, essentially a sewer sump, collects sewage and pumps it to a higher elevation before it continues its journey.

  Force mains also serve as primers for inverted siphons, which carry sewage underneath rivers or other obstructions. The pump may discharge into another gravity sewer or directly into a treatment plant, depending on the layout.

  Where even a force main is impractical, a pressure sewer can connect an individual property to the nearest gravity main. A macerator pump installed in a pumping well close to the property grinds up solids and ejects the waste through a small-diameter high-pressure pipe. This approach suits difficult or remote properties that would otherwise require costly excavation for a conventional lateral connection.

• 5. Alternative Configurations

  ›

  Effluent sewer systems, sometimes called septic tank effluent drainage or solids-free sewer systems, take a different approach to the problem. Septic tanks at each residence or business remove most of the solids before the remaining liquid, the effluent, is sent onward to a centralized treatment plant or a distributed local system. Because the bulk of the solid waste is already removed, the treatment plant serving an effluent system can be substantially smaller than a conventional one. The pipes used are narrow, typically between 1.5 and 4 inches in diameter, and because the waste stream is pressurized, they can be laid just below the ground surface following the natural contour of the land.

  Simplified sanitary sewers, most common in Brazil and other developing countries, use small-diameter pipes, typically around 100 millimetres, often laid at relatively flat gradients of around 1 in 200. The investment cost can be roughly half that of a conventional sewer. The maintenance burden, however, is generally higher.

  In low-lying communities where gravity drainage is impractical, vacuum sewers use differential atmospheric pressure to pull wastewater through pipelines ranging from 125 millimetres to 280 millimetres in diameter toward a central vacuum station.

• 6. Keeping The Pipes Alive

  ›

  All sewers deteriorate with age. But sanitary sewers carry a specific vulnerability that combined sewers and storm drains do not share: infiltration and inflow. When joints leak or pipes crack, groundwater seeps in and excessive stormwater can enter, backing up raw sewage before it reaches treatment. Keeping infiltration at acceptable levels demands a higher maintenance standard than simply preserving structural integrity.

  For decades, the only remedy for a cracked or damaged sewer pipe was expensive excavation, removal, and replacement, typically followed by street repavement. In the mid-1950s, a method was invented using two units positioned at each end of a damaged run, with a special cement mixture between them pulled from one manhole to the next under high pressure, sealing cracks rapidly as it cured. A modern version of this approach uses epoxy resin to create a pipe within a pipe, relining the aging structure from the inside.

  Pipe bursting offers another option. A new pipe, typically made of PVC or ABS plastic, is drawn through the old one behind an expander head that breaks apart the old pipe as the new pipe follows. Both relining and bursting are most effective on trunk sewers; repairing lines with lateral connections is complicated because provisions must also be made to reconnect those laterals without allowing groundwater to infiltrate through the junctions.

  Blockage prevention adds a further layer of management. Campaigns and regulations, such as requirements for grease interceptors at certain commercial customers, aim to keep the pipes clear before problems develop.

• 7. A History Underground

  ›

  Sanitary sewers emerged from combined sewers built in an era when animal-powered transport filled city streets with dung. Night soil collection from chamber pots was impractical in densely populated areas, and surface runoff was expected to flush waste off streets and underground to somewhere distant. These early combined systems were not designed with treatment in mind; treatment came later, as expanding populations made the consequences of untreated discharge impossible to ignore.

  As sewage treatment became necessary, the cost of handling the diluted waste that flows through combined systems, requiring greater volumes and more pumping capacity, pushed newer communities toward separate systems. Communities that urbanized in the mid-20th century or later almost uniformly built separate sanitary sewers and storm drains. Older cities, having already invested in combined infrastructure, largely left those networks in place.

  In the United Kingdom, a sanitary sewer carried the alternative name of foul sewer, a plainspoken label that reflects exactly what it was designed to carry and what it was designed to keep out of sight. The terminology has changed, but the underlying division, clean runoff one pipe, human waste another, remains the governing logic of modern urban water management wherever cities have had the opportunity to build it in from the start.