Water Filtration Part 1: What Municipal Water Treatment Leaves Behind (And Why You Still Need a Filter)
This post is part of a three-part series on home water filtration.
Your tap water has already been through a lot before it reaches your glass. Municipal treatment plants work around the clock to remove pathogens and particles, making water safe from the bacterial diseases that plagued cities a century ago. Treatment plants were designed to solve the problems of 1900, not 2025. The chemicals we worry about today (PFAS, pharmaceutical residues, industrial contaminants) weren't on anyone's radar when the Clean Water Act passed in 1974. Treatment plants do exactly what they were built to do. What they don't do is remove the full spectrum of contaminants we now know end up in drinking water.
If you've ever wondered whether you need a water filter, the answer depends less on whether your water is "safe" and more on what "safe" actually means. Federal standards set legal limits for certain contaminants, but legal doesn't always mean optimal, especially for chemicals where the science is still catching up. Understanding what municipal treatment does (and doesn't do) helps you decide whether home filtration makes sense for your situation.
How Municipal Water Treatment Works
Most treatment plants in the US follow a conventional four-step process. First, coagulation and flocculation use chemicals like aluminum sulfate to clump particles together. Second, sedimentation lets those clumps settle to the bottom of large tanks. Third, filtration (usually through sand or anthracite) catches remaining particles. Fourth, disinfection with chlorine or chloramine kills bacteria and viruses that could cause disease. Some plants add fluoride for dental health, and some adjust pH to reduce pipe corrosion.
This process reliably removes dirt, sediment, bacteria, viruses, and parasites. It's why we don't see cholera or typhoid outbreaks in the US. The system was designed for microbial threats, not for the synthetic chemicals that now show up in water supplies from agricultural runoff, industrial discharge, or aging infrastructure. Treatment plants can only remove what they're equipped to target.
What Gets Removed (And What Doesn't)
Municipal treatment is effective at removing biological contaminants. Chlorination kills more than 99.9% of bacteria and viruses. Filtration catches parasites like Giardia and Cryptosporidium. The coagulation and sedimentation steps remove suspended solids and some heavy metals that bind to particles. If your concern is foodborne or waterborne illness from pathogens, treated tap water in the US is generally very safe.
The gaps show up with dissolved chemicals. Chlorination doesn't remove PFAS (per- and polyfluoroalkyl substances), the "forever chemicals" used in everything from nonstick cookware to firefighting foam. These compounds pass straight through conventional treatment. A 2023 US Geological Survey study found PFAS in nearly half of all tap water samples tested across the country. Levels were highest near industrial sites, military bases, and wastewater treatment plants, but the chemicals showed up in rural wells and urban systems alike. The EPA established enforceable limits for six PFAS compounds in 2024, though the agency scaled back those regulations in May 2025 and proposed further changes in May 2026, retaining limits only for PFOA and PFOS while extending compliance deadlines to 2031. Most treatment plants still don't have the technology to remove PFAS.
Pharmaceutical residues follow a similar pattern. Trace amounts of antibiotics, hormones, antidepressants, and other medications end up in water supplies through human excretion and improper disposal. Conventional treatment wasn't designed to remove these compounds, and while concentrations are typically very low (measured in nanograms per liter), the long-term health effects of chronic low-level exposure remain uncertain.
Heavy metals present a mixed picture. Treatment plants remove some metals if they're bound to particles, but dissolved metals can pass through. Arsenic, for example, occurs naturally in some groundwater and requires specialized treatment (like oxidation and filtration) to remove. The EPA set the arsenic standard at 10 parts per billion in 2006, down from 50 ppb, but some researchers argue even that level poses cancer risk with decades of exposure.
The Disinfection Byproduct Problem
Chlorine saves lives by killing pathogens, but it creates its own set of chemicals when it reacts with organic matter in water. These disinfection byproducts (DBPs) include trihalomethanes (THMs) and haloacetic acids (HAAs). The EPA regulates total THMs and five HAAs, setting maximum contaminant levels to balance disinfection benefits against byproduct risks.
Research on DBPs has raised concerns about long-term exposure. Some studies link higher DBP levels to increased bladder cancer risk and adverse pregnancy outcomes, though the evidence is still developing. Water systems must stay below federal limits, but those limits are set based on feasibility and cost, not purely on health. If your water has high chlorine taste or smell, DBP levels may be elevated, particularly in the summer when heat increases the chemical reactions that form them.
Some utilities have switched from chlorine to chloramine (chlorine plus ammonia) because it produces fewer DBPs and maintains a disinfectant residual longer in distribution pipes. Chloramine comes with its own trade-offs. It's harder to remove with basic carbon filters, and it can leach lead from pipes and fixtures. If you have fish, chloramine is toxic to aquatic life and must be neutralized in aquarium water.
Lead and Aging Infrastructure
Lead doesn't come from treatment plants. It comes from the pipes that carry water to your home. The US banned lead pipes and solder in 1986, but millions of homes built before then still have lead service lines connecting the water main to the house, or lead solder joining copper pipes inside. Even if your home's plumbing is lead-free, older infrastructure in your neighborhood could be a source.
Lead leaches into water when it sits in pipes, particularly in soft or acidic water that corrodes metal. The longer water sits, the higher the lead concentration, which is why you should run the tap for 30 seconds to two minutes before drinking or cooking if it hasn't been used for several hours. This flushes out water that's been in contact with lead pipes.
Treatment plants add corrosion inhibitors (like orthophosphate) to coat pipes and reduce lead leaching, but the strategy only works if the chemical dose and water chemistry are carefully balanced. The Flint water crisis happened when the city switched water sources without adjusting corrosion control, causing lead to leach from pipes at dangerous levels.
You can't taste, see, or smell lead in water. The only way to know if it's present is to test. Many utilities will test your water for free or low cost. If lead shows up above 15 parts per billion (the EPA action level), filtration is the most practical immediate step while longer-term infrastructure replacement happens. Lead exposure is most concerning for children under six and pregnant women, since it affects brain development.
PFAS: The Contaminant Treatment Plants Don't Address
PFAS became a household name after high-profile contamination cases in communities near chemical plants and military bases. The chemicals are incredibly stable (which is why they're useful in products) and incredibly persistent (which is why they're a problem in water). They don't break down, they don't bind to particles that settle out, and they pass through conventional treatment unchanged.
The regulatory landscape for PFAS has shifted dramatically in the past two years. The EPA set maximum contaminant levels for six PFAS compounds in 2024. In May 2025, the agency announced it would retain enforceable limits only for PFOA and PFOS, the two most studied compounds, while reconsidering regulations for four others: PFHxS, PFNA, GenX, and PFBS. In May 2026, the EPA proposed extending the compliance deadline for PFOA and PFOS from 2029 to 2031 and removing regulations for those four compounds. The practical effect is straightforward: if your water contains any of the other four compounds, your utility has no federal obligation to remove them. Water systems that were preparing to comply with limits for PFHxS, PFNA, GenX, or PFBS can now shelve those plans, shifting the burden to individuals who want those contaminants removed from their drinking water.
Thousands of PFAS chemicals exist beyond these six, and we only test for a handful. Systems that do remove PFAS typically use granular activated carbon or ion exchange, both expensive and not yet universal. If you live near industrial areas, military bases, airports, or wastewater treatment plants, your risk of PFAS exposure is higher. Testing is the only way to know what's in your water. Some utilities have begun testing and publishing results, though not all. Home testing kits are available, though professional lab testing is more reliable. If PFAS is present, specific filtration technologies (activated carbon, reverse osmosis, or ion exchange) can remove it. Standard pitcher filters and basic faucet filters often don't.
When Filtration Makes Sense
Municipal treatment provides a baseline level of safety, but it can't address every contaminant that ends up in tap water. Filtration decisions depend on what's actually in your water, which means testing comes first. You can start by reviewing your water utility's Consumer Confidence Report (CCR), which they're required to send annually or post online. The report lists detected contaminants and whether they exceed legal limits. To find yours, search "[your city or county name] water quality report" or check your water bill for the utility's website. The EPA also maintains a database of water systems at epa.gov/ccr where you can search by zip code.
CCRs have limitations. They report averages over time, not real-time levels. They only include regulated contaminants, not everything that might be present. And they reflect water quality at the treatment plant, not necessarily at your tap, where lead or other contaminants can enter through plumbing.
If you're in a high-risk category (pregnant, young children, immunocompromised) or if your water comes from a private well that isn't covered by federal regulations, testing is even more important. Independent lab testing gives you specific numbers for the contaminants you're concerned about, whether that's lead, PFAS, nitrates, arsenic, or something else.
Once you know what's in your water, you can match filtration technology to the actual problem. Activated carbon removes chlorine, DBPs, VOCs, and some pesticides. Reverse osmosis removes PFAS, heavy metals, nitrates, and dissolved salts. Ion exchange removes hardness and some heavy metals. No single filter removes everything, which is why understanding your water matters.
Filtration also makes sense if you simply don't like the taste or odor of chlorinated water, even if all contaminants are below legal limits. Aesthetic concerns are valid. Carbon filters remove chlorine taste effectively and are inexpensive. If your goal is health protection, start with testing so you're not filtering for the wrong things.
What's Safe Enough?
Federal drinking water standards exist to protect public health, but they're also compromises that balance health risks against cost and feasibility. Maximum contaminant levels (MCLs) are set at levels that are technically and economically achievable for water systems, not necessarily at levels where no health risk exists. For some chemicals, like lead, there is no safe level. For others, the science is still uncertain about what constitutes long-term safe exposure.
You'll often see two numbers in water quality discussions. The MCL is the legal limit. The health goal (called the Maximum Contaminant Level Goal, or MCLG) is the level at which no known or anticipated health effects occur, with a margin of safety. For lead, the MCLG is zero because any exposure carries risk. For some contaminants, the MCL and MCLG are the same. For others, there's a gap. Utilities must meet the MCL, but that doesn't mean the MCLG is achieved.
The decision about whether treated tap water is "safe enough" becomes personal at that point. Some people are comfortable with legal compliance. Others want to reduce exposure as much as practical, particularly for children or vulnerable family members. Filtration gives you that control, but only if you match the filter technology to the contaminants you're actually facing.
Understanding what's in your tap water is the starting point for any filtration decision that's actually grounded in something. From there, the question becomes which technologies remove which contaminants, and then which format delivers those technologies in a way that fits your household and your budget. Those two questions have more specific answers than most filter marketing suggests.
