How Does a Sewer System Work? Gravity, Then Microbes

The collection network is designed around one number: the self-cleansing velocity, roughly six tenths of a metre per second, which is the flow speed needed to keep solids in suspension. Below it, material settles and the pipe blocks; far above it, the pipe erodes. Achieving that with gravity alone across a flat city is the central design problem, which is why sewers are laid at carefully calculated gradients and why pumping stations exist where the land will not cooperate. The other design decision is whether to combine sewage and rainwater in one pipe or to separate them. Older cities almost all built combined systems, which are cheaper and which overflow into rivers during heavy rain because the treatment works cannot handle the volume, producing the discharges that have become a political issue in several countries. Separate systems keep rainwater out of the treatment stream and cost twice as much to build, which is why retrofitting one into an existing city is close to impossible.

What happens at the end is a sequence of stages, each removing a different fraction:

• •Preliminary treatment, screening out rags, wipes and solid objects and removing grit in a channel where flow slows enough for sand to settle and not for organic matter
• •Primary treatment, holding the flow in large tanks for a couple of hours so that settleable solids sink into sludge and grease floats and is skimmed, removing perhaps half the suspended solids
• •Secondary treatment, which is biological and does most of the work, using bacteria to consume dissolved organic matter in the presence of oxygen, either in aerated tanks holding a suspension of microbes called activated sludge or by trickling the liquid over a bed of stones coated in biological film
• •Secondary settlement, in which the microbial floc settles out and most of it is returned to the aeration tank to keep the population going
• •Tertiary treatment where required, removing nitrogen and phosphorus, which cause algal blooms in receiving waters, and disinfecting with ultraviolet light or chlorine
• •Sludge treatment, usually anaerobic digestion, which reduces the volume, kills pathogens and produces methane that many works burn to run themselves

The activated sludge process, developed in Manchester in 1914, is the central technology and it is a managed ecosystem rather than a machine. Air is blown through a tank containing sewage and a dense population of bacteria, protozoa and rotifers, which consume the dissolved organic material and clump into flocs that can be settled out. The operator's job is to keep the community in the right condition by controlling how much air is supplied, how long the liquid stays in the tank and how much biomass is retained, since too little and treatment is incomplete, too much and the sludge will not settle. Characteristic failures have names: bulking, in which filamentous bacteria overgrow and the sludge floats out with the treated water, and foaming, caused by a different group of organisms. Nitrogen removal requires deliberately alternating oxygen-rich and oxygen-poor zones, since one set of bacteria converts ammonia to nitrate in the presence of oxygen and another converts nitrate to nitrogen gas only in its absence.

The driver in nineteenth-century Europe was epidemic disease and the theory was wrong. Cholera was attributed to bad air, and John Snow's demonstration in 1854 that a London outbreak centred on one water pump was not generally accepted for years. What forced action in London was the Great Stink of summer 1858, when the smell of the Thames made Parliament unusable, and the sewers designed by Joseph Bazalgette were approved within eighteen days. His system intercepted the flow before it reached central London and carried it downstream, and it worked spectacularly against cholera for a reason he did not intend, since removing sewage from the drinking water supply addressed the actual mechanism. Bazalgette is also remembered for doubling the pipe diameter over what the calculations required, on the reasoning that this would only be done once, which is why the system served a city several times the size it was designed for. Similar programmes followed across Europe and North America, and the resulting fall in mortality is the single largest public health gain on record.

The systems handle what they were built for and struggle with what came later. Wet wipes labelled flushable do not disintegrate and combine with fat poured down drains to form the masses that block sewers, of which the largest found in London weighed over a hundred tonnes. Pharmaceuticals, hormones and personal care products pass through conventional treatment largely intact and are detectable in receiving waters, with measurable effects on fish. Microplastics are captured mostly into the sludge, which is frequently spread on farmland. Antibiotic resistance genes concentrate in treatment works, which are being studied as a route by which resistance spreads. And the sector has acquired an unexpected diagnostic function, since wastewater sampling detects viruses, drug use and antimicrobial resistance at community level cheaply and without testing individuals, which became a standard surveillance tool during the pandemic and has been retained. Meanwhile roughly a fifth of the world's population still lacks a safely managed sanitation service, which remains the largest avoidable cause of child mortality from diarrhoeal disease.

Sewers are laid at gradients calculated to keep flow above roughly six tenths of a metre per second so solids stay suspended, and older cities combine sewage with rainwater, which is why they overflow into rivers in storms. Treatment screens and settles the solids, then uses a managed population of bacteria in aerated tanks to consume dissolved organic matter, with nitrogen removed by alternating oxygenated and oxygen-free zones and sludge digested to produce methane. The nineteenth-century systems were built to remove smell and succeeded against cholera for reasons their designers did not understand.