Freshwater supports drinking-water systems, agriculture, industry, ecosystems, sanitation, and nearly every aspect of modern life. Yet having water available does not necessarily mean that the water is suitable for every use.
Rivers, lakes, reservoirs, and groundwater can be affected by natural substances and human activities including agriculture, wastewater, urban runoff, industrial activity, mining, leaking storage systems, and aging infrastructure.
That makes water protection a two-part challenge: protect the source whenever possible, and use appropriate treatment when unwanted substances are present.
Quick answer: Water pollution can affect both surface water and groundwater, but the contaminants and risks vary by location. Reverse osmosis is one useful treatment technology because it can reduce many dissolved substances, including salts. It is not, however, the “best” filtration method universally. Activated carbon, ion exchange, sediment filtration, disinfection, distillation, and other technologies may be more appropriate for different water-quality problems.
In This Guide
- How Much Freshwater Is Available?
- What Is Water Pollution?
- Surface-Water Pollution
- Groundwater Contamination
- Water Pollution & Health
- Why Source Protection Matters
- What Is Reverse Osmosis?
- How Reverse Osmosis Works
- What RO Can Reduce
- Limitations of RO
- RO vs. Other Treatment Methods
- Choosing a Home Filter
- Frequently Asked Questions
How Much of Earth's Water Is Freshwater?
Earth contains an enormous amount of water, but the vast majority is saline ocean water.
Only a small share is freshwater, and much of that freshwater is stored in:
- Glaciers
- Ice caps
- Groundwater
A smaller portion occurs in lakes, rivers, wetlands, soil moisture, and the atmosphere.
Avoid the common “only 0.025% of Earth's water is available to humans” simplification. Water availability depends on location, infrastructure, groundwater depth and recharge, water quality, economics, ecosystems, and the intended use. There is no single percentage that accurately represents all water practically available to humanity.
The important point is simpler: usable freshwater is geographically uneven and must be protected from overuse and contamination.
What Is Water Pollution?
Water pollution occurs when substances or organisms change water quality in ways that affect human use, ecosystems, or both.
Potential pollutants include:
- Nutrients such as nitrogen and phosphorus
- Sediment
- Pesticides
- Industrial chemicals
- Petroleum compounds
- Metals
- Salts
- Wastewater-related contaminants
- Microorganisms
Not every substance found in water represents the same level of concern.
EPA distinguishes between primary drinking-water standards, which protect public health, and secondary guidelines, which address mainly aesthetic effects such as taste, color, odor, staining, and deposits. :contentReference[oaicite:2]{index=2}
How Does Surface Water Become Polluted?
Surface water includes rivers, lakes, reservoirs, streams, and other water open to the atmosphere.
Pollution can reach these waters through both direct discharges and diffuse runoff.
Agricultural Runoff
Rainfall and irrigation can carry:
- Soil
- Fertilizers
- Manure
- Pesticides
- Other agricultural materials
into nearby waterways.
Urban Stormwater
Rainfall running across roads, parking lots, rooftops, and developed land can pick up:
- Oil
- Metals
- Road salt
- Trash
- Sediment
- Other pollutants
Wastewater
Municipal and industrial wastewater requires treatment before discharge because untreated or inadequately treated wastewater can affect receiving waters.
Industrial and Legacy Pollution
Some waterways are also affected by historic industrial activity, contaminated sediments, mining, or chemical releases.
Do not use one old national percentage to claim that a particular share of all U.S. rivers or lakes is simply “polluted.” EPA water-quality assessments differ by waterbody type, state, assessment method, pollutant, and designated use. Current local or watershed-specific data are more meaningful than the old figures used in the original article.
Can Groundwater Become Contaminated?
Yes.
Groundwater is naturally protected to some degree by overlying soils and geological materials, but it is not immune to contamination.
Potential sources include:
- Septic systems
- Leaking underground storage tanks
- Landfills
- Industrial sites
- Agricultural chemicals
- Improper disposal of hazardous materials
- Natural geological contaminants
EPA specifically warns that hazardous materials disposed of on the ground can contaminate soil, groundwater, or nearby surface water.
Groundwater problems can also be difficult to correct because contaminated aquifers may move slowly and can require long-term remediation.
Private Wells
Private well owners are responsible for understanding and maintaining their own water source.
If you use a private well, see our Well Water Testing & Treatment Guide.
Can Polluted Water Affect Health?
Yes, but health effects depend entirely on the contaminant, concentration, route of exposure, duration, and the person exposed.
Potential drinking-water concerns can include:
- Microbial contaminants
- Lead
- Arsenic
- Nitrate
- Selected organic chemicals
- Other regulated or locally relevant contaminants
Some contaminants can cause short-term illness, while others pose risks that develop after repeated or long-term exposure.
Taste, smell, and appearance are not reliable safety tests. Some substances mainly cause aesthetic problems, while some contaminants of greater health concern may have no noticeable taste, odor, or color.
Public water systems are subject to enforceable federal drinking-water standards for regulated contaminants.
For an overview of major drinking-water concerns, see our Drinking Water Contaminants Reference Guide.
Treatment Starts With Protecting the Water Source
Water treatment is important, but preventing pollution before it reaches a river or aquifer is often preferable to removing contamination later.
Source-water protection can involve:
- Proper hazardous-waste disposal
- Agricultural nutrient management
- Septic-system maintenance
- Industrial spill prevention
- Protection of wetlands and watersheds
- Stormwater controls
- Groundwater protection
See our Source Water Protection Guide for more information.
What Is Reverse Osmosis?
Reverse osmosis, or RO, is a membrane-based water-treatment process.
USGS describes reverse osmosis as a process in which product water passes through a fine membrane, while salts do not pass through as readily.
RO is widely used in:
- Home drinking-water systems
- Desalination
- Industrial water treatment
- Water reuse
- Commercial applications
CDC also identifies reverse osmosis as one of the technologies used in drinking-water treatment and home filtration.
How Does Reverse Osmosis Work?
To understand reverse osmosis, it helps to start with ordinary osmosis.
Osmosis
A semipermeable membrane allows water to move through while restricting many dissolved substances.
If two solutions with different concentrations are separated by such a membrane, water tends to move toward the side with the higher solute concentration.
This movement creates osmotic pressure.
Reverse Osmosis
RO applies pressure greater than the natural osmotic pressure to the more concentrated side.
This drives water through the membrane in the opposite direction.
The system then produces:
- Product or permeate water: water that passes through the membrane
- Concentrate or reject water: water containing a higher concentration of rejected substances
RO is not simply a microscopic particle strainer. Membrane chemistry, pressure, dissolved substances, charge, water temperature, and system design all influence contaminant rejection.
What Can Reverse Osmosis Reduce?
RO systems can reduce the concentrations of many dissolved substances.
Depending on the membrane and complete system design, these can include:
- Dissolved salts
- Calcium and magnesium
- Sodium
- Selected metals
- Nitrate
- Fluoride
- Selected organic and inorganic contaminants
USGS specifically identifies reverse osmosis as a major method for desalinating saline water.
Household RO performance still varies. For example, USGS research on residential RO units used for arsenic-containing groundwater found considerable differences in real-world removal efficiency among systems and operating conditions.
Never assume that “reverse osmosis” means identical performance across all systems. Look for testing or certification for the exact contaminant and model you are considering.
What Are the Limitations of Reverse Osmosis?
RO is a powerful technology, but it has tradeoffs.
Wastewater
RO produces a concentrate stream containing rejected substances.
The ratio of treated water to reject water varies substantially by system design and operating conditions.
Water Pressure
RO requires sufficient pressure to move water through the membrane.
Some systems use booster pumps when source pressure is inadequate.
Mineral Reduction
RO reduces the concentration of many dissolved ions, including calcium and magnesium.
This is useful when dissolved solids or hardness-related ions need to be reduced, but it also means RO does not selectively retain every naturally occurring mineral.
Maintenance
Most household systems contain multiple stages that can require:
- Sediment-filter replacement
- Carbon-filter replacement
- Membrane replacement
- Tank and system sanitation
Not Every Problem Requires RO
If the main issue is chlorine taste, for example, a properly designed activated-carbon filter may be simpler.
If the primary concern is hard-water scale throughout the house, water softening may be more appropriate.
If the concern is suspended sediment, a particle filter may be sufficient.
Reverse Osmosis vs. Other Water Treatment Technologies
| Technology | Common Strength | Important Limitation |
|---|---|---|
| Reverse osmosis | Broad reduction of many dissolved substances and salts | Reject water, pressure requirements, maintenance, mineral reduction |
| Activated carbon | Chlorine taste and odor; many organic compounds | Does not broadly remove dissolved salts or hardness minerals |
| Water softening | Calcium and magnesium hardness | Not a general-purpose contaminant-removal system |
| Distillation | Separates water from many dissolved substances | Energy use and relatively slow production |
| Sediment filtration | Suspended particles, sand, and rust | Does not remove most dissolved contaminants |
| Ultraviolet disinfection | Microbial disinfection when appropriately designed | Does not remove most dissolved chemical contaminants |
CDC similarly emphasizes matching treatment technology to the specific thing you want the system to remove. Activated carbon can improve taste and odor, while reverse osmosis can address additional dissolved substances.
There is no universally “best” water-treatment technology. The best system is the one that addresses your actual source water and contaminants of concern with documented performance.
Is Reverse Osmosis Better Than Activated Carbon?
They perform different jobs.
Activated carbon is especially useful for the adsorption of many organic compounds and for improving chlorine-related taste and odor.
RO is more appropriate when substantial reduction of dissolved salts and many dissolved ions is desired.
Many RO systems actually include activated-carbon stages because the technologies complement one another.
Is Reverse Osmosis Better Than Distillation?
Neither is universally better.
Both can substantially reduce dissolved substances, but they operate very differently.
RO:
- Uses membrane pressure
- Produces reject water
- Can operate continuously
Distillation:
- Uses evaporation and condensation
- Requires heat
- Can be relatively slow
The right choice depends on:
- Water chemistry
- Energy costs
- Water demand
- Maintenance
- Contaminants of concern
What About Ultraviolet Water Treatment?
UV serves a very different purpose from RO.
Properly designed UV systems can inactivate microorganisms in sufficiently clear water, but UV does not remove dissolved salts or most chemical contaminants.
CDC also notes that UV effectiveness is reduced in cloudy water because particles can shield microorganisms from the light.
This makes UV and RO complementary rather than competing technologies in some treatment systems.
How Should You Choose a Home Water Filter?
1. Identify Your Water Source
Determine whether you use municipal water, a private well, or another source.
2. Identify the Actual Concern
Is the problem chlorine taste, lead, PFAS, nitrate, hard-water scale, iron, dissolved salts, or something else?
3. Review Water-Quality Information
Public-water customers can review their utility's Consumer Confidence Report. Private-well owners should use appropriate water testing.
4. Match the Technology to the Contaminant
Do not buy an RO system simply because RO sounds more powerful, or a carbon filter simply because it is inexpensive.
5. Look for Specific Performance Evidence
Check certification or third-party test data for the exact model and contaminant.
6. Consider Long-Term Maintenance
Include replacement filters, membrane life, water use, electricity where applicable, and maintenance in the total cost.
For a much broader comparison, see our Types of Drinking Water Filtration Systems Guide.
Where Do Gravity-Fed Filters Fit?
Gravity-fed countertop systems take a different approach from reverse osmosis.
They move water through filter media using gravity rather than household water pressure.
Depending on the specific element, these systems can be designed around adsorption and other filtration processes while generally leaving total dissolved solids relatively unchanged.
That makes gravity filtration and reverse osmosis different categories rather than direct one-for-one replacements.
If you are comparing gravity-fed filtration for ordinary household drinking water, see our Berkey® Water Filter Systems Guide and Water Filter Test Results for filter-specific information.
I would not position Berkey® as “better than reverse osmosis.” The appropriate technology depends on the source water and what you want to reduce.
Frequently Asked Questions
Is all surface water polluted?
No. Water quality varies by watershed and pollutant. Surface waters can be affected by runoff, wastewater, industry, agriculture, natural conditions, and other sources, but broad national percentages should not be used to characterize every river or lake.
Can groundwater become polluted?
Yes. Groundwater can be affected by septic systems, agricultural chemicals, leaking storage tanks, landfills, industrial sites, hazardous-material disposal, and naturally occurring substances.
Does polluted water always taste or smell bad?
No. Some contaminants have no noticeable taste, odor, or color. Conversely, some aesthetic substances can produce obvious taste or staining without representing the most serious health risk.
What is reverse osmosis?
Reverse osmosis is a membrane-treatment process in which pressure drives water through a semipermeable membrane while many dissolved salts and other substances are rejected.
Does reverse osmosis remove minerals?
Yes. RO substantially reduces the concentrations of many dissolved mineral ions, including calcium and magnesium.
Does reverse osmosis reduce TDS?
Yes. Reducing dissolved salts and total dissolved solids is one of the major strengths of reverse osmosis.
Does reverse osmosis remove arsenic?
RO can reduce arsenic, but performance varies according to arsenic chemistry, membrane design, water chemistry, maintenance, and operating conditions. Look for contaminant-specific performance data for the exact system.
Does reverse osmosis remove fluoride?
RO membranes can substantially reduce fluoride when the system is properly designed and maintained.
Does reverse osmosis remove chlorine?
Household RO systems commonly use an activated-carbon prefilter to reduce chlorine because chlorine can affect some RO membrane materials. The membrane itself should not be treated as the primary stage for chlorine removal.
Is reverse osmosis better than carbon filtration?
Not universally. RO is more effective for many dissolved salts and ions, while activated carbon is particularly effective for many organic compounds and chlorine-related taste and odor.
Is reverse osmosis better than a water softener?
They serve different purposes. A whole-house softener is designed primarily to control calcium and magnesium hardness, whereas RO is usually used at the point of use for broader dissolved solids reduction.
Does UV remove chemicals from water?
No. UV is used primarily for microbial disinfection. It does not remove most dissolved chemical contaminants.
Does activated carbon create harmful carbon fines?
Some new carbon filters can release small amounts of carbon dust during initial flushing, but this does not make activated carbon inherently harmful to the environment or inferior to reverse osmosis. The two technologies have different functions and environmental tradeoffs.
Does reverse osmosis waste water?
RO produces a reject or concentrate stream. The amount varies substantially by system efficiency, water pressure, membrane type, and operating conditions.
Which home water-treatment method is best?
There is no universal best method. The appropriate choice depends on the source water, contaminants of concern, required treatment capacity, installation, maintenance, and cost.
Final Thoughts
Water pollution is too complex to reduce to a single national percentage or a single “best” filtration technology.
Surface water and groundwater face different contamination pressures. Public water systems use multiple treatment processes and must meet federal requirements for regulated contaminants, whereas private well owners bear more responsibility for testing and maintenance.
Reverse osmosis is an important technology, particularly when the goal is to reduce dissolved salts and a broad range of dissolved substances.
But carbon filtration, water softening, sediment filtration, UV disinfection, distillation, and other technologies each have roles that RO does not replace.
The better question is not “What is the strongest water filter?” It is “What is actually in my water, and which treatment method has documented performance for that problem?”