4-Stage vs. 5-Stage Reverse Osmosis: Does the Extra Stage Matter? (2026)

Part of Tampa Bay's tap water has already been through reverse osmosis once. Tampa Bay Water's seawater desalination plant makes up to 25 million gallons a day that way, and it runs the seawater through pretreatment and cartridge filters before any of it touches a membrane. That plant settles the 4 stage vs 5 stage RO question better than a product listing can. The stages in front of the membrane protect it, so the right count depends on what your water needs taken out first.

4-stage vs. 5-stage RO at a glance

Bottom line: Read the stage list on the spec sheet, because the fifth stage is not standard.

What is the difference between a 4 stage vs 5 stage RO system?

The difference between a 4-stage and a 5-stage RO system is one more cartridge, and the reverse osmosis membrane in the middle does the same job in both. The extra cartridge changes what happens to your water before or after the membrane, so the stage list tells you more than the stage count.

A common 4-stage layout runs a sediment prefilter, a carbon prefilter, the RO membrane and a carbon postfilter. Nebraska Extension's guide to home reverse osmosis describes the same basic train, with a storage tank and a separate faucet at the sink.

The fifth cartridge is where brands split. Some 5-stage systems add a second carbon prefilter, often a granular carbon stage followed by a carbon block. Others keep one carbon prefilter and add an alkaline or remineralization cartridge after the membrane, and some sellers count that same cartridge as stage six. We start every reverse osmosis installation with your water test for this reason: the right fifth stage depends on what comes out of your tap.

What does each stage of a reverse osmosis system do?

Each stage of a reverse osmosis system has one job: the sediment filter catches particles, the carbon takes out chlorine and some chemicals, and the membrane removes dissolved minerals and metals. The Nebraska Extension reverse osmosis guide lists arsenic, lead, nitrate, fluoride and sodium among the contaminants the membrane reduces.

What each RO stage handles

The membrane is the part you most want to protect, and the stages ahead of it exist for that. Nebraska Extension says thin-film composite (TFC) membranes deteriorate in chlorinated water, so a system with a TFC membrane needs a carbon prefilter to strip the disinfectant first.

Reverse osmosis has limits. Nebraska Extension notes that hydrogen sulfide, the gas behind a rotten-egg smell, passes through the RO membrane, and it does not recommend RO as the treatment for bacteria.

An under-sink system also makes water at a slow rate. Nebraska Extension puts household output at 10 to 35 gallons a day, and most household systems recover 20 to 30 percent of the water fed to them, with the rest going down the drain.

Why does Tampa Bay's chloramine change the 4 stage vs 5 stage RO answer?

Tampa Bay's chloramine changes the answer because it puts more work on the carbon ahead of the membrane. Tampa Bay Water says its member governments, Hillsborough County, Pasco County, Pinellas County, New Port Richey, St. Petersburg and Tampa, all use monochloramine as their final disinfectant, so an RO system on any of those utilities starts with chloramine at the inlet.

Chloramine is chlorine bound to ammonia. The City of Tampa's 2025 water quality report describes the process: the plant adds chlorine, then ammonia, so the water stays disinfected on its way through the pipes to your house.

Chloramine also needs the right carbon. Tampa Bay Water's monochloramine page says removing it takes a different process from some chlorine-removal methods, and the utility's advice to aquarium owners calls for an activated carbon filter designed to remove monochloramines.

A second carbon prefilter gives chloramine more carbon to pass through before it reaches the membrane, and a carbon block made for chloramine matters more than the count on the box. When we service a St. Petersburg home, the first thing we read under the sink is the label on the carbon cartridges. Our St. Petersburg water quality guide covers the rest of that city's water.

Is a 5-stage RO system better for well water?

A 5-stage RO system is not better for well water when the extra stage is more carbon, because a private well carries no chlorine for that carbon to remove. Nebraska Extension says carbon prefilters on water that is not chlorinated can encourage microbial growth on the membrane, and it recommends a sediment prefilter alone in that case.

Wells in Lutz, Odessa and rural Pasco County can carry hydrogen sulfide, iron and hardness to the membrane. Hydrogen sulfide passes through the RO membrane, so a sulfur smell comes out of the RO faucet unless something upstream removes it. That job belongs to an iron and sulfur system at the well, ahead of anything under the sink.

Nebraska Extension also advises feeding an RO system only coliform-free water, because bacteria can damage the membrane. If your well has never had a bacteria test, test first and pick the stages second.

Does a 5-stage RO system cost more to maintain?

A 5-stage RO system costs more to maintain by one cartridge, and that gap is small next to what decides membrane life: your water. Nebraska Extension says filter replacement intervals depend on how much water you use, its quality and its contaminant levels, and that manufacturers and dealers set the schedule for each system.

The EPA makes the same point in its filter guidance. A reverse osmosis system needs its cartridges and its membrane changed, and a filter works only when you maintain it the way the manufacturer says. In a 4 stage vs 5 stage RO comparison, the schedule you will keep matters as much as the hardware.

Hardness is the cost driver most Tampa homes forget. The City of Tampa reported average hardness of 211 mg/L, or 12.3 grains per gallon, in 2025, and Nebraska Extension says softening relatively hard water ahead of an RO system can extend the life of the membrane.

Watch the faucet as well as the calendar. A slower flow at the RO faucet can mean a clogged membrane, and Nebraska Extension notes that many systems carry a monitor that flags high total dissolved solids. Our reverse osmosis cost guide covers installed pricing in Tampa, and our look at under-sink vs. whole-house RO explains why most homes need only the under-sink unit.

Which should you buy, a 4-stage or a 5-stage RO system?

On Tampa Bay city water, buy the system with more carbon ahead of the membrane: a 5-stage with a second carbon prefilter, or a 4-stage whose single carbon block is rated for chloramine. On a private well, a 4-stage with treatment upstream fits better. Choose a remineralization stage for taste.

Certification matters more than either number. The EPA's guidance on PFAS filters tells you to look for NSF/ANSI 58 certification for PFAS reduction on a reverse osmosis system, and to check that the listing covers the PFAS you want removed. The EPA also notes that current certification standards do not yet show removal down to its newest drinking water limits.

The Whitfield family moved to St. Petersburg from Ohio and ordered a 5-stage system online before their first summer here. Their fifth stage turned out to be an alkaline cartridge after the membrane, with one carbon block in front of it that said nothing about chloramine. They called us asking whether to upgrade to seven stages.

We tested at the kitchen tap, read every cartridge label and swapped the carbon prefilter for a block made for chloramine. The Whitfields kept the alkaline stage because they liked the taste. The membrane stayed, the count stayed at five, and the change that mattered cost one cartridge.

Pick the stages for your water, then compare prices. Tampa Bay Water treats seawater in that order too: pretreatment first, membrane second. More of our drinking water notes live in our reverse osmosis and drinking water articles.

Frequently asked questions

Is a 5-stage RO system better than a 4-stage?

A 5-stage RO system is better than a 4-stage only when its fifth cartridge fits your water. On Tampa Bay city water a second carbon prefilter helps with chloramine, while a remineralization stage changes taste.

What is the fifth stage in a 5-stage reverse osmosis system?

The fifth stage in a 5-stage reverse osmosis system differs by brand. On many systems it is a second carbon prefilter ahead of the membrane, and on others it is an alkaline or remineralization cartridge after it, so check the stage list on the spec sheet.

Do 4-stage and 5-stage RO systems remove PFAS differently?

The stage count does not tell you how well an RO system reduces PFAS. The EPA tells buyers to look for NSF/ANSI 58 certification for PFAS reduction on the specific system and to check that the listing covers the PFAS they want removed.

Which is better for well water, a 4-stage or a 5-stage RO?

For a private well, a 4-stage RO system with treatment upstream fits better than a 5-stage with extra carbon. Nebraska Extension recommends a sediment prefilter alone on water that is not chlorinated, and hydrogen sulfide has to come out before the RO because it passes through the membrane.

How often do you change RO filters and the membrane?

You change RO filters and the membrane on the schedule the manufacturer sets for your system and your water. Nebraska Extension says the right interval depends on how much water you use and what is in it, and a drop in flow at the RO faucet can mean the membrane needs attention.

Do you need a water softener before an RO system in Tampa?

A water softener ahead of an RO system can extend membrane life on hard water, according to Nebraska Extension. City of Tampa water averaged 12.3 grains per gallon of hardness in 2025, so a Tampa home with a softener gives its RO membrane an easier job.

Want an RO system matched to your water?

Not sure what's in your water? Our water test is free. We check your chlorine, hardness and total dissolved solids at the tap and tell you which stages your water needs before anyone talks about equipment.

Free Water Test and Consultation explains your results on the spot. Book a free water test.

Reverse Osmosis Drinking Water is an under-sink system built with the carbon stages your water calls for.

Service and Repair checks the RO system you already own and swaps any cartridge that does not fit your water before you replace anything.

Call or text (813) 729-2125, or book your free water test online. No pressure, no obligation, just answers.

Zach Brownell, ClearQuest Water Solutions - 10+ years installing and servicing water systems across Tampa Bay.

Last updated October 2026