Turn on a tap and water appears. The infrastructure behind that instant is a system that took over a century to build, requires constant maintenance, and in many cities is quietly approaching a crisis of aging pipes and underinvestment.
Here's the full journey.
Quick answer
Tap water usually moves from a river, reservoir, or aquifer to a treatment plant, then through coagulation, settling, filtration, disinfection, storage, and pressurized distribution pipes. The surprising part is how much of the system is about pressure management and pipe maintenance after treatment is finished.
Source water
Municipal water comes from two places: surface water (rivers, lakes, reservoirs) or groundwater (aquifers accessed via wells). Most large cities rely primarily on surface water because the volumes required exceed what most aquifers can sustainably supply.
Reservoirs serve two purposes: storage and gravity. A reservoir at elevation stores months of supply as a buffer against drought, and its height creates the pressure gradient that pushes water through the distribution system without continuous pumping. New York City's Catskill and Delaware watershed system, for example, supplies over 1 billion gallons per day largely by gravity — the reservoirs sit at enough elevation to pressurize the system for most of the city without pumping.
Treatment: the four main stages
Coagulation and flocculation. Chemicals — typically aluminum sulfate or ferric sulfate — are added to the water. These cause fine suspended particles (silt, algae, bacteria) to clump together into larger particles called "floc." This is why treatment plant water appears milky when chemicals are first added and then clarifies.
Sedimentation. The water slows in large settling basins, and the floc sinks to the bottom. The clarified water at the top moves on. The settled sludge is periodically removed.
Filtration. The clarified water passes through beds of sand, gravel, and often activated carbon. This removes remaining fine particles and begins adsorbing dissolved organic compounds that cause taste and odor issues. Activated carbon filtration is why treated water tastes different from river water even though it comes from the same source.
Disinfection. Chlorine (or chloramines, ozone, or UV light) kills remaining pathogens. Chlorine also provides residual protection — it remains in the water through the distribution system to kill bacteria that might enter through pipe cracks or construction work. This residual chlorine is what gives tap water its faint taste in some cities.
The distribution system
Treated water enters a distribution network of pipes ranging from large transmission mains (a meter or more in diameter) down to the small service lines entering individual buildings. The system is pressurized to ensure water flows uphill, to the top floors of buildings, and to maintain positive pressure that prevents contamination from entering cracked pipes.
Pressure is managed through elevated storage — water towers. A water tower filled overnight at low-demand hours provides the pressure head for daytime peak demand without requiring treatment plants to vary their output dramatically. The water tower's height, not a pump running in real time, is what pushes water up to the fifth floor of a building.
The pipe problem
Here's where the comfortable narrative of managed infrastructure meets a difficult reality. A large fraction of distribution pipes in developed-world cities are cast iron pipes laid in the late 19th and early 20th century. They have a design life of roughly 100 years. Many are well past it.
The American Society of Civil Engineers' infrastructure report cards have consistently rated US drinking water infrastructure at C or D grades, reflecting a backlog of deferred pipe replacement estimated in the hundreds of billions of dollars. Old pipes lose water to leaks — US systems lose an estimated 20-30 billion gallons per day to leakage. Old pipe joints corrode and can leach lead into the water, as Flint, Michigan demonstrated catastrophically in 2014-2015.
The challenge is that pipe replacement is invisible, expensive, and politically unrewarding. Politicians who fund a new stadium get credit. Politicians who replace pipes under a street get nothing — the water continues to flow, which is what it was supposed to do anyway.
The water in your glass is safe and clean. Getting it there involves infrastructure that is simultaneously impressive in its engineering and concerning in its age.
For nearby infrastructure pieces, read how sewage treatment works, how traffic lights are decided, and how the power grid stays balanced.