Three treatment plants, hundreds of millions of gallons and a round-the-clock process keep safe drinking water flowing across the city.
Philadelphia gets its drinking water from the Delaware and Schuylkill Rivers
Three drinking-water plants treat more than 300 million gallons on an average day.
Treatment requires multiple stages, including coagulation, sedimentation, filtration, disinfection, corrosion control and continuous testing.
Aging infrastructure, emerging contaminants such as PFAS and stricter standards require billions of dollars in continuing investment.
A tale of two rivers
Philadelphia’s drinking water begins in the Delaware and Schuylkill Rivers. Before it reaches a faucet, it must pass through a closely controlled sequence of settling, chemical treatment, filtration, disinfection and testing. Water treatment is not a single step that removes everything unwanted from a water supply. It is a sequence of physical and chemical processes; each stage targets different particles, microorganisms or contaminants, and the effectiveness of one stage can determine how well the next performs.
Philadelphia provides a clear example of what that process requires. Water drawn from two working urban rivers must be treated for changing conditions caused by storms, runoff, seasonal temperatures, industrial activity and other pressures before it can safely enter the city’s distribution system. The city operates three drinking-water treatment plants that collectively treat more than 300 million gallons on an average day. The Baxter plant draws from the Delaware River, while the Queen Lane and Belmont plants use the Schuylkill River; once treated, the water travels through approximately 3,100 miles of water mains before reaching customers.
Protecting the source
Treatment begins before water enters a plant. Utilities monitor rivers, reservoirs and surrounding watersheds for runoff, industrial discharges, algal growth, spills and other conditions that could affect water quality. Understanding what is entering the source allows operators to adjust treatment before a problem reaches the distribution system.
Philadelphia’s source watershed begins in New York State and extends approximately 330 miles to the Delaware Bay. The city monitors waterways throughout that watershed, tracks water levels and flow, samples for contaminants and participates in programs intended to reduce stormwater and agricultural runoff. Protecting a source does not eliminate the need for treatment; it reduces the amount and complexity of contamination a treatment plant must manage.
Intake and initial settling
Raw water is drawn from the Delaware or Schuylkill River through an intake and pumped to the treatment plants. Screens and other intake structures keep large debris from entering the system. Once inside, the water is held long enough for sand, silt and other heavy material to settle naturally.
Philadelphia also adds potassium permanganate during this early stage. The chemical is commonly used to control taste and odor compounds and oxidize substances such as iron and manganese. The amount and combination of chemicals can be adjusted as river conditions change. Flexibility is important in this process because surface-water quality is not constant. Heavy rain can wash sediment, organic matter, road pollutants and other material into a river. Drought can reduce flow and concentrate certain contaminants, while warmer temperatures can contribute to algal growth and taste or odor problems. Treatment plants must be designed for changing conditions rather than an average.
Coagulation and flocculation
Many of the particles suspended in river water are too small to settle on their own. Some carry electrical charges that cause them to repel one another. Even when the water appears relatively clear, these microscopic particles can remain suspended.
During coagulation, Philadelphia adds ferric chloride to neutralize those charges. Lime is also used to adjust acidity and create the chemical conditions needed for treatment. Operators must continually monitor the water because the effective dose can change with temperature, turbidity and source-water chemistry.
The water then moves into flocculation basins, where it is mixed gently rather than rapidly. As the suspended material collides, it begins forming larger clusters called floc. The process turns material that would otherwise pass through the plant into particles large and heavy enough to remove.
Coagulation and flocculation illustrate why tap water cannot be treated through a simple filter alone. The plant must first change the physical behavior of particles in the water; only then can settling and filtration remove them efficiently.
Sedimentation
After flocculation, the water enters sedimentation basins where the newly formed particles settle to the bottom. The settled material must be collected and removed; managing those residual solids is another operating responsibility that requires equipment, labor and proper disposal.
Sedimentation reduces the burden on the filters that follow. If too much material reaches the filtration stage, filters can clog more quickly or become less effective; operators therefore measure turbidity, or water cloudiness, throughout the treatment process to confirm that particles are being removed as intended.
Filtration
The clarified water then passes through filters that capture smaller particles left behind after sedimentation. Treatment plants commonly use layers of materials such as sand, gravel or activated carbon; the exact design depends on the source water and the contaminants the utility needs to address.
Filters do not operate indefinitely without attention. They must be monitored, cleaned through a process called backwashing and eventually rehabilitated or replaced. Pumps, valves, instrumentation and control systems must also be maintained to keep water moving at the correct rate.
Disinfection
Once most particles have been removed, the water is disinfected to destroy organisms that could cause disease. Philadelphia uses sodium hypochlorite, a form of chlorine. The city then adds ammonia to create a longer-lasting disinfectant residual that continues protecting the water as it moves through the distribution system.
The disinfectant dose must be carefully controlled. Too little can leave the water vulnerable to microorganisms; excessive amounts can affect taste and contribute to the formation of regulated disinfection byproducts when chlorine reacts with naturally occurring organic matter. Operators must balance immediate microbial protection with the chemistry that develops as water travels through the system.
Philadelphia is also adding ultraviolet disinfection at the Baxter Water Treatment Plant. UV light deactivates bacteria, viruses and parasites without relying on additional chemical disinfectant. It does not replace the plant’s existing treatment process, but provides another protective barrier.
Preparing water for the pipes
Treatment does not end when water is clear and disinfected. Utilities must also manage its chemistry so it does not corrode pipes, dissolve metals or damage the distribution system. Water that meets standards when leaving a plant can still develop problems if it reacts with aging service lines or household plumbing.
Philadelphia adds zinc orthophosphate to form a protective coating inside pipes and reduce corrosion. The city also adjusts fluoride levels for dental health; before water leaves the plant, laboratories test it for approximately 100 regulated contaminants, including microorganisms, metals, nitrates and organic chemicals.
Monitoring continues after the water enters the distribution system. Philadelphia conducts more than 400 tests each month at locations throughout the city to confirm that adequate disinfection remains in the water; operators also watch pressure, flow and real-time water-quality data around the clock.
New contaminants require new treatment
Conventional treatment is highly effective, but it was not designed to remove every modern contaminant. PFAS, for example, can require advanced technologies such as granular activated carbon, ion exchange or high-pressure membrane systems. Installing those systems may require new buildings, pumps, tanks, electrical capacity and equipment for handling spent treatment material.
Philadelphia is testing advanced processes and planning plant upgrades to meet new federal PFAS requirements. This is one reason the cost of water treatment does not remain static. When scientific knowledge improves or regulators establish stricter limits, utilities must add capabilities to plants that may have been designed decades earlier.
The city’s Water Revitalization Plan identified approximately 400 projects involving existing facilities, new facilities and other drinking-water improvements. When developed in 2019, the program carried an estimated cost of $2.5 billion over 25 years. The figure demonstrates how maintaining water quality requires continuing investment rather than the one-time construction of a treatment plant.
Why Philadelphia’s river sources matter
The treatment required by a city depends heavily on where its water originates. Source-water quality, watershed development, geology and federal requirements all influence what a utility must build and operate.
Philadelphia draws from rivers that pass through developed and industrialized areas, making extensive treatment essential. New York City, by comparison, receives much of its water from heavily protected Catskill and Delaware watersheds that are permitted to operate without conventional filtration. Its Croton supply, which serves portions of the Bronx and Manhattan, requires filtration and is treated at a $3.2 billion underground plant capable of processing as much as 290 million gallons per day.
The comparison illustrates why Philadelphia’s treatment system is so important. Water taken from a river serving a densely populated region must pass through several protective barriers before it is ready to enter homes, schools, hospitals and businesses.
Paying for Philadelphia’s water system
The water itself comes from the Delaware and Schuylkill rivers, but treating and delivering it carries a substantial cost. Philadelphia water bills support treatment chemicals, electricity, laboratory testing and the operators, engineers and mechanics who keep the system running around the clock. Rates also pay for water main repairs, pumping equipment, hydrants, meters, service-line replacements and the long-term improvements required to meet new drinking-water standards.
Those rates are not set by the Philadelphia Water Department alone. The department calculates how much revenue it needs to operate, maintain and improve the water, wastewater and stormwater systems, then submits a formal request supported by financial and engineering data. The independent Philadelphia Water, Sewer and Storm Water Rate Board reviews that request and may approve, modify or reject it based on standards established by City Council. Its five members are appointed by the mayor, and the process includes public hearings, technical review and opportunities for customers to comment.
Residential and small-business customers are also represented by a Public Advocate during rate proceedings. Once rates are approved, the Philadelphia Water Revenue Bureau handles billing and collection; assistance programs, including the Tiered Assistance Program, reduce bills for qualifying households based on income.
This oversight does not eliminate the pressure created by rising construction costs, aging infrastructure or new treatment requirements. It does require Philadelphia to document what the system needs, defend its proposed rates in a public process and balance continued investment with affordability. As the city begins upgrading plants and preparing for contaminants such as PFAS, that balance will become increasingly important to keeping safe water both reliable and accessible.
Source:
Philadelphia Water Department. (n.d.). 2024 drinking water quality report. https://water.phila.gov/drops/2024-drinking-water-quality-report/
Philadelphia Water Department. (n.d.). Baxter Water Treatment Plant upgrades. https://water.phila.gov/projects/baxter-water-treatment-plant-upgrades/
Philadelphia Water Department. (n.d.). Water Revitalization Plan. https://water.phila.gov/wp-content/uploads/files/pwd-water-revitalization-plan.pdf
