Municipal Water Treatment Explained

Every day, approximately 148,000 public water systems across the United States treat and deliver water to over 300 million people. This remarkable feat of infrastructure transforms raw water from rivers, lakes, and underground aquifers into drinking water that meets EPA safety standards.

But how exactly does this transformation happen? What technologies remove contaminants? And critically, what contaminants can slip through even the best treatment?

This guide takes you inside municipal water treatment plants to understand the science, technology, and limitations of the systems that provide our drinking water.

Most municipal water systems use a multi-barrier approach called conventional treatment. This process has six primary stages, each designed to remove specific types of contaminants:

Stage 1: Coagulation and Flocculation

What Happens:

Chemicals (typically aluminum sulfate or ferric chloride) are added to raw water. These coagulants have positive charges that attract negatively charged particles like dirt, dissolved organic matter, and some pathogens.

The water is then gently stirred, causing these tiny particles to clump together into larger masses called "floc."

What It Removes:

• Suspended sediment and turbidity
• Organic matter (which can create disinfection byproducts later)
• Some bacteria and protozoan cysts
• Heavy metals that attach to particles
• Some phosphates

What It Doesn't Remove:

• Dissolved chemicals (pesticides, pharmaceuticals)
• Viruses (too small)
• Dissolved minerals (arsenic, fluoride, lead)

Stage 2: Sedimentation

What Happens:

Water flows into large settling basins and sits still for several hours. The heavy floc particles sink to the bottom, forming sludge that's removed and treated separately.

Effectiveness:

Removes 80-90% of suspended particles. The clearer the water after sedimentation, the more effective the subsequent filtration will be.

Stage 3: Filtration

What Happens:

Water passes through layers of sand, gravel, and sometimes anthracite coal. These filters trap remaining particles, bacteria, and some chemical contaminants.

Modern plants may use:

• Rapid sand filters: Most common, water flows through quickly
• Slow sand filters: Creates biological layer that consumes contaminants
• Multimedia filters: Multiple layers for better particle capture
• Membrane filtration: Advanced plants use microfiltration or ultrafiltration

What It Removes:

• 99%+ of remaining suspended particles
• Most bacteria (99.9%+)
• Many protozoan cysts (Giardia, Cryptosporidium)
• Some viruses (especially with membrane filters)

What It Doesn't Remove:

• Dissolved chemicals and minerals
• Most pharmaceuticals
• PFAS (unless using specialized activated carbon or membrane filters)
• Nitrates and other dissolved ions

Stage 4: Disinfection

What Happens:

Chemical disinfectants are added to kill remaining pathogens. Common methods include:

Stage 5: pH Adjustment and Corrosion Control

What Happens:

Lime or sodium hydroxide is added to adjust pH (typically 7.0-8.5). This prevents water from corroding pipes and leaching metals like lead and copper into drinking water.

Why It Matters:

This step prevents lead contamination in homes with lead service lines or lead solder. The Flint water crisis occurred largely because the city failed to properly treat water for corrosion control.

Stage 6: Storage and Distribution

What Happens:

Treated water is stored in covered reservoirs or elevated tanks, then distributed through miles of underground pipes to homes and businesses.

Challenges:

• Aging infrastructure: Many U.S. pipes are 50-100+ years old
• Biofilm growth: Bacteria can colonize pipe interiors
• Cross-connections: Rare but serious contamination events
• Pressure fluctuations: Can draw contaminants into pipes through cracks
• Residual disinfectant loss: Chlorine degrades over time/distance

Some municipalities and regions with particularly challenging source water invest in advanced treatment technologies. These are more expensive but remove contaminants that conventional treatment can't:

Granular Activated Carbon (GAC) Filtration

How it works: Water passes through beds of activated carbon that adsorb organic chemicals.

Removes:

• Pesticides and herbicides (90-95%)
• Industrial solvents and VOCs (85-95%)
• Taste and odor compounds (90%+)
• Some PFAS (70-90% depending on type and contact time)
• Disinfection byproduct precursors
• Some pharmaceuticals (variable effectiveness)

Used by: About 20% of U.S. treatment plants, more common in areas with agricultural or industrial contamination.

Reverse Osmosis (RO)

How it works: Water is forced through semipermeable membranes that block contaminants while allowing water molecules through.

Removes:

• Dissolved minerals (fluoride, arsenic, nitrates) 95-99%
• Heavy metals (lead, mercury, chromium-6) 95-99%
• PFAS 90-95%
• Radionuclides 95%+
• Pharmaceuticals 90-99%
• Most organic and inorganic contaminants

Used by: Rare for municipal systems due to high cost and waste. More common in arid regions treating brackish groundwater or for specific contamination issues.

Ion Exchange

How it works: Water passes through resin beads that swap unwanted ions for less harmful ones.

Removes:

• Hardness (calcium, magnesium)
• Nitrates
• Some heavy metals (arsenic, chromium)
• Radium

Used by: Primarily for water softening or nitrate removal in affected areas.

Advanced Oxidation Processes (AOPs)

How it works: Combines ozone, UV light, and/or hydrogen peroxide to create highly reactive hydroxyl radicals that destroy contaminants.

Removes:

• Pharmaceuticals and personal care products (90-99%)
• Endocrine disrupting compounds (85-95%)
• Pesticides (90-95%)
• Taste and odor compounds
• Some PFAS (variable, 30-70%)

Used by: Cutting-edge plants dealing with wastewater-impacted sources or emerging contaminants.

Even with modern treatment, your tap water may contain low levels of contaminants. Here's why:

1. Treatment Is Designed for Regulated Contaminants Only

The EPA regulates about 90 contaminants, but scientists have identified over 60,000 chemicals in use. Treatment plants optimize for regulated substances, not emerging contaminants like:

• Most PFAS compounds (recently regulated)
• Pharmaceuticals (unregulated)
• Microplastics (unregulated)
• Endocrine disruptors (mostly unregulated)
• New pesticides and industrial chemicals

2. Treatment Creates New Contaminants

Disinfection creates byproducts when chlorine reacts with organic matter:

• Trihalomethanes (THMs)
• Haloacetic acids (HAAs)
• Chlorite and bromate (from chlorine dioxide and ozone)

These are regulated, but many plants operate near the legal limits. Some studies suggest even regulated levels pose long-term health risks.

3. Distribution System Contamination

Water leaves treatment plants clean but can pick up contaminants during distribution:

• Lead and copper: Leached from old pipes and fixtures
• Bacteria: Biofilm growth in pipes
• Disinfectant decay: Residual chlorine dissipates over distance
• Pipe breaks: Allow contamination infiltration
• Cross-connections: Rare but serious contamination events

4. Economic and Technical Limitations

Advanced treatment is expensive:

• Activated carbon filtration: $0.10-0.30 per 1,000 gallons
• Reverse osmosis: $1.00-3.00 per 1,000 gallons
• Advanced oxidation: $0.50-1.50 per 1,000 gallons

Smaller or poorer communities often can't afford comprehensive treatment. The EPA sets standards considering "feasibility" alongside health, meaning some contaminants are allowed at levels higher than what's ideal for health.

5. Variable Source Water Quality

Treatment effectiveness varies with source water conditions:

• Seasonal changes (agricultural runoff in spring, algae blooms in summer)
• Storm events (increase turbidity and contamination)
• Upstream activities (industrial discharges, wastewater treatment plant issues)
• Drought (concentrates contaminants)

Treatment is calibrated for typical conditions but may struggle during extreme events.

*Lead primarily enters water from distribution pipes and home plumbing, not source water. Corrosion control at treatment prevents leaching.

Based on system size and source water:

Large Cities (500,000+ people):

• Conventional treatment (coagulation, sedimentation, filtration)
• Chlorine or chloramine disinfection
• Corrosion control
• Possibly: Ozone pre-treatment, GAC filtration, UV disinfection

Medium Cities (50,000-500,000):

• Conventional treatment
• Chlorine disinfection
• Corrosion control
• Less likely to have advanced treatment

Small Towns (3,000-50,000):

• May use simplified treatment (direct filtration, lime softening)
• Chlorine disinfection
• Limited monitoring
• Rarely have advanced treatment

Very Small Systems (under 3,000):

• Minimal treatment (often just disinfection if from groundwater)
• More vulnerable to violations
• Limited resources for upgrades

Find out what your plant uses: Check your Consumer Confidence Report or visit your utility's website. This information should be publicly available.

Why does my water taste like chlorine if it's been treated?

Chlorine taste means treatment is working—residual disinfectant remains to protect water as it travels through pipes. While safe to drink, many people find it unpleasant. A simple carbon filter removes chlorine taste effectively.

If my city has a modern treatment plant, do I still need a home filter?

It depends. Treatment plants remove most contaminants to legal limits, but may not address: emerging contaminants (PFAS, pharmaceuticals), distribution system issues (lead from your pipes), or provide an extra margin of safety for vulnerable populations. Review your water quality report and test your tap water to decide.

Why can't all plants use reverse osmosis like some home filters?

Cost and waste. Municipal RO would cost $2-5 per 1,000 gallons (vs. $0.50 for conventional treatment) and waste 25-50% of water. For a city using 100 million gallons/day, that's millions in additional annual costs and massive water waste. It's only used when absolutely necessary.

What's the difference between chlorine and chloramine?

Chlorine is pure disinfectant that works fast but dissipates quickly. Chloramine (chlorine + ammonia) is more stable and maintains residual protection longer in large distribution systems. Chloramine creates fewer disinfection byproducts but can leach lead from pipes and requires removal before use in aquariums or dialysis.

Can treatment plants remove all contaminants if they want to?

Technically yes, with unlimited resources. Advanced treatment can remove virtually anything, but the cost would be prohibitive—potentially 10-20x current rates. EPA standards balance health protection with economic feasibility. This is why point-of-use (home) filtration makes sense for concerned individuals.

Municipal water treatment is a remarkable achievement that prevents countless illnesses every day. Modern multi-barrier treatment removes or reduces thousands of potential contaminants to safe levels.

However, "safe" doesn't mean "perfect." Treatment has limitations—economic, technical, and regulatory. Understanding what your treatment plant does and doesn't remove helps you make informed decisions about additional home filtration.

Key Takeaways:

• Conventional treatment effectively removes particles, pathogens, and many chemicals
• Disinfection creates byproducts but is necessary to prevent waterborne disease
• Advanced treatment (GAC, RO, AOPs) addresses emerging contaminants but is expensive
• Distribution systems can add contaminants after treatment
• Treatment is designed for general population; vulnerable groups may need extra protection
• Home filtration provides personalized protection beyond municipal treatment