City water systems collect water from a natural source, clean it through a multi-step treatment process, then push it through a network of underground pipes to reach your tap. The whole system runs on a combination of pumps, gravity, and careful chemistry, with sensors monitoring water quality and pressure around the clock. Here’s what happens at each stage.
Where the Water Comes From
Most cities draw water from one of two sources: surface water (rivers, lakes, or reservoirs) or groundwater (wells drilled into underground aquifers). The choice depends on geography and availability. Some cities blend both. Surface water tends to carry more sediment, algae, and organic matter, so it generally requires more treatment. Groundwater is naturally filtered by soil and rock, but it can pick up minerals and contaminants like arsenic or nitrates along the way.
Raw water is pumped from the source to a treatment plant, sometimes traveling miles through large intake pipes. Many cities also maintain reservoirs to store extra supply for droughts or peak summer demand.
The Five Steps of Water Treatment
Once raw water arrives at the treatment plant, it moves through a standard sequence: coagulation, flocculation, sedimentation, filtration, and disinfection. Not every plant uses every step in the same way, but this is the backbone of the process.
Coagulation and Flocculation
In coagulation, plant operators add chemicals, typically aluminum or iron-based salts, that cause dirt, bacteria, and other tiny particles to clump together. These particles are so small they’d otherwise stay suspended in the water indefinitely. The chemicals give them an electrical charge that makes them sticky.
Next comes flocculation, which is just gentle stirring. The goal is to get those sticky particles to bump into each other and form larger, heavier clumps called flocs. Additional chemicals may be added to help the flocs grow. By the end of this stage, the water looks like it’s full of soft, fluffy sediment.
Sedimentation
The water then flows into large, quiet basins where it sits mostly still. Because the flocs are heavier than water, they sink to the bottom over the course of hours. The settled material, called sludge, is periodically removed. The relatively clear water on top moves on to the next stage.
Filtration
Even after sedimentation, the water still contains particles too small or light to settle out, including some bacteria, parasites, and viruses. Filtration catches these. The water passes through layers of sand, gravel, and activated carbon, each with different pore sizes. Sand and gravel trap physical particles, while activated carbon absorbs dissolved chemicals and removes odors and bad tastes.
Traditional sand filters combined with the earlier coagulation step can remove more than 95% of parasites like Giardia and 99% of coliform bacteria. Some newer plants use ultrafiltration membranes with pores as small as 100 nanometers, which can physically block virtually all bacteria, parasites, and even viruses without relying as heavily on chemical pretreatment. These membrane systems consistently produce water with zero measurable turbidity.
Disinfection
The final treatment step kills any remaining germs. Most plants use chlorine, chloramine (a combination of chlorine and ammonia), or chlorine dioxide. This is also where a critical distinction comes in: the disinfectant isn’t just for the treatment plant. A small amount stays in the water as it travels through miles of pipes to your home, continuing to kill germs along the way. This leftover amount is called a residual disinfectant.
Chlorine is a powerful germ-killer but gets used up quickly, sometimes before the water reaches taps at the far edges of the system. Chloramine lasts longer in pipes and produces fewer chemical byproducts, which is why many utilities have switched to it. The tradeoff is that pipes can develop a slimy layer of bacteria called biofilm, and utilities sometimes temporarily switch back to chlorine to strip that layer away.
How Water Gets to Your Tap
Treated water leaves the plant and enters the distribution system: a vast network of underground pipes called water mains. Large transmission mains carry water to different parts of the city, branching into progressively smaller pipes that eventually connect to individual buildings through service lines.
Pressure is what keeps the water moving forward and flowing when you open a faucet. Most systems rely on a combination of electric pumps and gravity. Pumps push water uphill and into elevated storage tanks or water towers. Once water is raised to a height, gravity does the rest. The weight of the water in a tower creates pressure at the base, and that pressure pushes water through the pipes below. The taller the tower, the higher the pressure. This is the same principle that makes water shoot farther from a hose when you hold the nozzle higher.
Water towers also serve as buffers. During low-demand hours (like the middle of the night), pumps fill the towers. During peak hours (mornings and evenings), the towers release stored water to meet demand without the pumps having to work overtime. This keeps pressure consistent throughout the day.
What the Pipes Are Made Of
The materials running beneath your streets vary depending on when they were installed. Older systems may still have cast iron mains from the early 1900s. More modern installations use ductile iron, PVC (plastic), high-density polyethylene, or concrete. Each material has different strengths: PVC resists corrosion, ductile iron handles high pressure well, and concrete works for very large-diameter mains.
Pipe lifespan depends on the material, soil conditions, and water chemistry, but most infrastructure planning assumes a service life of roughly 50 to 100 years for major mains. The challenge is that many American cities installed their core infrastructure in the early to mid-20th century, meaning large portions of the system are reaching or exceeding that window simultaneously. The EPA issued a rule requiring water systems across the country to identify and replace lead service lines within 10 years, reflecting the fact that there is no safe level of lead exposure.
How Utilities Monitor the System
Modern water systems use a network of sensors and automated controls, often managed through a system called SCADA (supervisory control and data acquisition). Sensors placed throughout the distribution network track two main categories: hydraulic conditions (pressure, flow rate, tank water levels) and water quality (chlorine levels, turbidity, pH).
A pressure sensor at the base of a water tower, for example, tells operators exactly how full the tower is and whether the system has enough pressure. Flow sensors track how fast water moves through mains, which helps identify leaks. If flow in a section spikes without a corresponding increase in customer usage, that’s a sign of a break or unauthorized connection. Water quality sensors detect changes that could indicate contamination or the breakdown of disinfectant residual before the water reaches customers.
Operators use this real-time data to adjust pump speeds, open or close valves, and reroute water when needed. The system also flags alarm conditions automatically, so problems get attention even at 3 a.m.
Preventing Contamination in the Pipes
One risk in any pressurized system is backflow, where water flows the wrong direction and pulls contaminants back into the city main. This can happen during a water main break or a sudden pressure drop. If a garden hose is submerged in a swimming pool during a backflow event, for instance, chlorinated pool water could get sucked into the drinking water supply.
Cities prevent this with backflow prevention devices installed at connection points. The simplest method is an air gap, a physical space between a water outlet and any potential contaminant. For higher-risk connections (commercial buildings, irrigation systems), utilities require mechanical assemblies. A reduced pressure backflow assembly uses two check valves and a relief valve to ensure that if either valve leaks, contaminated water is discharged outside rather than flowing back into the main. Double check valve assemblies handle lower-risk situations where backflow wouldn’t pose a direct health threat.
What Gets Regulated
Public water systems in the U.S. must meet standards set by the EPA under the Safe Drinking Water Act. These standards set maximum allowable levels for dozens of contaminants, from bacteria and lead to industrial chemicals. Utilities test the water regularly and publish annual Consumer Confidence Reports that show exactly what was detected and at what levels.
Standards evolve as science improves. In 2024, the EPA finalized the first-ever limits on PFAS, a group of long-lasting synthetic chemicals found in everything from nonstick cookware to firefighting foam. The new rules set maximum contaminant levels for two common PFAS compounds, PFOA and PFOS, at 4.0 parts per trillion, an extremely low threshold that will require many utilities to install new treatment technology. The EPA’s health goal for both compounds is actually zero, but 4.0 parts per trillion represents the lowest level that can be reliably measured and feasibly achieved.