How Reverse Osmosis Works

Yes — reverse osmosis is one of the most effective methods available for removing PFAS (per- and polyfluoroalkyl substances), commonly known as "forever chemicals," from tap water. The RO membrane filters contaminants down to .001 microns, rejecting PFOS and other PFAS compounds at a rate of 90–99%.

RO filtration also removes a broad spectrum of other harmful contaminants, including:

• Lead (95–98% rejection) — shop lead reduction filters
• Fluoride (85–92% rejection)
• Pesticides and herbicides (up to 99%)
• Chlorine and chloramines (up to 98%) — shop chloramine reduction filters
• Arsenic V (92–96%), nitrates (60–75%), sulfates (96–98%) — shop RO membranes that target dissolved solids

Curious exactly what's lurking in your local water supply? Start with What's In Your Drinking Water, then review the complete RO Contaminant Removal List to see removal rates for 65+ contaminants. Ready to protect your family? Shop RO systems or use our Filter Finder if you already own a system and need replacement filters.

A residential reverse osmosis system filters tap water through multiple stages, each targeting a different class of contaminant:

1. Sediment Pre-Filter: The first line of defense. A sediment filter traps dirt, rust, sand, and silt — protecting the delicate RO membrane from damage caused by particulate matter.
2. Carbon Block Pre-Filter: Water next passes through a carbon filter, which removes chlorine, chloramines, volatile organic compounds (VOCs), and other chemicals that would degrade the RO membrane over time and affect taste.
3. RO Membrane: The heart of the system. Your home's water pressure (typically 40–100 psi) pushes water through a semi-permeable RO membrane with pores as small as .001 microns. Clean, purified water passes through to a storage tank; dissolved contaminants and impurities are captured and flushed down the drain. No electricity or chemicals are required.
4. Post-Carbon Polishing Filter (4-stage systems): As water leaves the storage tank on its way to your faucet, it passes through a final carbon polishing filter that removes any residual taste or odor, delivering crisp, clean-tasting drinking water. Browse our RO filters and membranes to find the right replacement for your system.

Purified water is stored in a holding tank (typically 3–4 gallons) so clean water is ready the moment you turn on the tap. Want to go deeper? See our full guide: What is Reverse Osmosis? Ready to shop? Browse our top-rated RO drinking water systems or read our Best RO System Reviews to find the right fit for your home.

Municipal tap water is treated to meet EPA safety standards — but "meeting standards" doesn't mean your water is free of contaminants. Depending on where you live, your tap water may still contain:

• Lead leached from older pipes and fixtures — removable with a lead reduction filter or RO system
• PFAS "forever chemicals" from industrial and agricultural sources
• Chlorine and chloramines added during the disinfection process — shop chloramine filters or carbon filters
• Nitrates from fertilizer runoff, particularly in agricultural regions
• Pesticides, pharmaceuticals, and other dissolved solids — targeted by VOC reduction filters

The EPA allows trace levels of dozens of contaminants in public drinking water — levels considered statistically "safe" but not necessarily comfortable for every household, especially those with children, pregnant women, or immunocompromised family members.

The first step is to know what's in your water. Review Is Tap Water Safe to Drink? and What's In Your Drinking Water for a breakdown by contaminant type. For households that want comprehensive protection, a reverse osmosis drinking water system removes up to 99% of 65+ contaminants and delivers better-tasting water than most bottled water brands — for just pennies per gallon. Not sure which system is right for you? Read our RO Buyer's Guide or browse best-rated RO systems.

Yes — most under-sink RO systems are designed for DIY installation and don't require a plumber or special tools. The basic process involves:

1. Connecting the system's inlet tubing to your cold water supply line using a simple saddle valve or inline valve
2. Running a drain line from the system to your sink drain to carry away rejected contaminants
3. Drilling a small hole in your sink or countertop and installing the dedicated RO faucet
4. Placing the storage tank and filter housing under the sink
5. Flushing and sanitizing the system before first use per the owner's manual

Most homeowners complete the full installation in 1–2 hours. ESP Water provides step-by-step RO installation instructions and owner's manuals for every system we carry. You can also connect your RO system to your refrigerator's water dispenser or ice maker — see our guide: How to Connect an RO System to a Refrigerator. If you ever run into issues, our US-based technical support team is available Monday–Friday to help troubleshoot.

For most households, an under-sink reverse osmosis system is the smarter, more cost-effective choice. Here's how the two compare:

Because most water used at home goes toward bathing, laundry, and flushing toilets — not drinking — filtering 100% of your home's water supply with an expensive RO system is rarely necessary or practical. For whole-home protection against bacteria and viruses, most households are better served by pairing an under-sink RO system with a whole house UV purification system.

Still weighing your options? Read Is a Whole House RO System Worth It? and compare the pros and cons of reverse osmosis before deciding.

One of the biggest advantages of owning an RO system is how affordable it is to maintain. Here's a breakdown of the typical annual maintenance costs:

Total annual cost: roughly $50–$100, depending on your system, water quality, and household usage. By comparison, the average household spending $30–$50/month on bottled water pays $360–$600 per year — for water that's often no more pure than what your RO system produces for pennies per gallon.

To keep costs down, avoid proprietary systems that require expensive OEM-only filters. Many ESP Water customers convert their existing RO system to use our affordable, high-quality replacement filter kits. We also offer filter subscription plans with automatic delivery and reminder emails — so you never miss a change. Learn exactly what's involved in maintaining your RO system year-round, or follow our step-by-step guide: How to Change RO Filters and Membranes. Need to order replacements now? Shop all RO replacement filters and parts, or use our system identification tool if you're not sure which filters fit your model.