Reverse osmosis — under-counter and whole-home

Reverse osmosis (RO) forces water through a semipermeable membrane that removes a broad range of dissolved contaminants — including many that other technologies don't address, such as PFAS, nitrate, arsenic, lead, and dissolved solids. It produces the highest-purity water of the common residential treatment technologies. An RO system is a multi-stage architecture: the membrane itself is the constant, and the stages before and after it are chosen based on what the specific water needs and what the household wants the treated water to do. RO comes in two configurations: point-of-use (under-counter), which treats a single tap, and point-of-entry (whole-home), which treats all the water entering the house. This article explains how RO works, what each configuration involves, what to consider in running one, and the factors that inform the decision.

Why this matters

Reverse osmosis is the residential treatment technology that produces the highest purity, and it removes contaminants that most other technologies leave behind. For households on the region's softer surface-water municipal supplies, RO is the technology that delivers a meaningfully higher level of purity beyond what municipal treatment provides. For households with specific contaminants that RO addresses well — PFAS, nitrate, arsenic, lead, dissolved solids — it can be an effective targeted solution. For households that want RO-quality water across all household uses, whole-home configurations deliver that at every tap.

This article is for anyone considering RO, comparing the point-of-use and point-of-entry configurations, or wanting to understand what RO does and doesn't do before making a decision.

How reverse osmosis works

The core of RO is straightforward. Water is pressed against a semipermeable membrane with pores small enough to block most dissolved substances while letting water molecules through. The purified water (the "permeate") passes through and is collected; the water carrying the rejected contaminants (the "concentrate" or "reject") is sent to drain.

That single-line description covers the physics. Building an RO system around that membrane involves selecting the stages that surround it — stages before the membrane to protect it and prepare the water for it, and stages after the membrane to make the treated water fit whatever the household wants to do with it.

The membrane and its pressure source

The membrane needs pressure to force water through it. That pressure can come from household water pressure alone, or from a pump that boosts pressure at the membrane feed. Higher and more consistent feed pressure improves membrane performance and efficiency (recovery ratio — the share of feed water that becomes permeate rather than reject). Household water pressure from a street connection or a well is often sufficient for point-of-use RO but may be marginal or inadequate for point-of-entry systems that need to feed a larger membrane at higher flow rates. Feed-side pumps address this.

Pretreatment — protecting the membrane

Pretreatment is whatever the specific water needs before it reaches the membrane. The membrane is expensive and sensitive; keeping it clean and undamaged is what makes RO systems economical to run and long-lived. Common pretreatment stages include:

• Sediment filtration to remove particles that would foul the membrane surface.
• Carbon filtration to remove chlorine and chloramine, which damage RO membranes over time. This is essential — municipal water with a chlorine residual will degrade an untreated membrane relatively quickly. Carbon also removes taste, odor, and many organic contaminants ahead of the membrane.
• Anti-scalant dosing to prevent hardness minerals and silica from precipitating on the membrane. Anti-scalant keeps these dissolved constituents in solution as they pass through the membrane, where the RO then rejects them along with everything else. For whole-home RO on hard water, anti-scalant is typically the preferred approach — it uses less water than softening pretreatment, adds no salt to the wastewater stream, and addresses silica scaling that softening does not. Softening as pretreatment remains an option when a household is already running a softener for whole-house hardness treatment, in which case the softener does double duty.

Which pretreatment stages are included, and in what configuration, depends on the specific water. A well with hardness and iron needs a substantially different pretreatment train than a municipal supply with a chlorine residual and no significant hardness. A water test is the input that determines what pretreatment is needed.

Post-treatment — making the permeate fit its use

The water leaving the membrane is very pure — RO removes beneficial minerals along with contaminants, which is what makes it pure and also what makes it taste different. Post-treatment is whatever's needed to make that permeate fit what the household wants to do with it. Common post-treatment stages include:

• Remineralization to add back minerals for taste (and, in some cases, health preferences).
• Soda ash injection to raise pH — RO permeate is often mildly acidic, which can affect taste and, in whole-home applications, interact with plumbing.
• UV disinfection as a post-membrane safeguard against any microbial breach downstream of the membrane.
• Storage — a small pressurized tank in point-of-use systems, a large cistern in whole-home systems — to buffer between the membrane's steady production rate and household on-demand use.
• Repressurization to deliver stored water to the tap or throughout the house at usable pressure. Point-of-use tanks are pressurized during fill; whole-home systems typically use a variable-speed pump to deliver cistern water to the house at consistent pressure.
• Final polishing filtration to refine taste immediately before delivery to the tap.

As with pretreatment, which post-treatment stages are included depends on what the water needs and what the household wants. A whole-home RO delivering water to every tap has different post-treatment requirements than a point-of-use system delivering to one kitchen faucet.

Configuration is the point

The membrane is the constant. Everything else is configuration. Two households with different water and different needs may run RO systems that share the membrane and share very little else in their pretreatment and post-treatment trains. This is a design feature of RO, not a complication — it's what lets the same core technology serve situations from a small under-counter unit on chlorinated municipal water to a whole-home system on a hard, iron-bearing well.

What RO removes — and what it doesn't

RO is effective across an unusually broad range of contaminants. It substantially reduces:

• Dissolved solids and salts
• PFAS
• Nitrate
• Arsenic
• Lead and many other heavy metals
• Fluoride
• Many disinfection byproducts (in combination with the carbon stages)
• Most dissolved minerals, including the calcium and magnesium that constitute hardness

This breadth is what distinguishes RO. Most other residential technologies are more specific — carbon for chlorine and organics, softeners for hardness, UV for microbes. RO addresses many categories of contaminant at once.

What RO does not address as effectively:

• Dissolved gases pass through the membrane. Hydrogen sulfide (rotten-egg odor) is not reliably removed by RO and needs separate treatment.
• Some small, neutrally charged molecules are less efficiently rejected than charged ions.
• Microorganisms — while RO membranes physically block bacteria and most viruses, RO is not certified as a disinfection method, and a system that develops a breach or contamination downstream of the membrane could pass microbes. Where disinfection is the goal, UV is the appropriate technology (and is a common post-treatment addition).

RO also removes beneficial minerals along with contaminants. The resulting water is very pure, which some people find tastes "flat" and which has prompted remineralization as a common post-treatment stage. The health significance of the removed minerals is minor for most people (dietary minerals come overwhelmingly from food, not water), but the taste difference is real and is a matter of preference.

Point-of-use and point-of-entry — two configurations

The same RO architecture scales from a single tap to a whole house. The two configurations differ in what they serve, what pretreatment and post-treatment the specific installation calls for, and how they handle pressure and storage.

Point-of-use (under-counter) RO

A point-of-use RO system treats water at a single point — most commonly the kitchen sink — and delivers it through a dedicated faucet. It produces a few gallons of purified water per day, buffered by a small storage tank under the sink, for drinking and cooking.

Powered or non-electric. Most point-of-use systems are electric — a pump boosts feed pressure to the membrane, which improves recovery ratio (less water sent to drain per gallon of permeate), speeds production, and often eliminates the need for as large a storage tank. Non-electric point-of-use systems are also available and are the right choice in some situations: off-grid installations, backup or resilience-focused setups, or locations without power at the installation point. Non-electric systems rely on household water pressure alone to drive the membrane, produce more slowly, and require a storage tank to buffer that slower production against on-demand use. Most households with typical installation conditions choose electric; non-electric is a considered choice for specific circumstances.

Pretreatment. Point-of-use pretreatment typically includes sediment and carbon pre-filtration built into the unit. Chlorine removal is essential; softening ahead is uncommon at the point-of-use scale since the membrane sees limited volume.

Post-treatment. Post-treatment at point-of-use is typically a final carbon polishing stage, with remineralization increasingly common on newer systems.

Wastewater. Older point-of-use systems sent several gallons to drain per gallon produced; newer efficient designs — particularly those with feed-side pumps — have substantially improved this ratio.

Installation. Moderate — connects to the cold water line, drain, and a dedicated faucet. Fits under a standard kitchen sink.

Point-of-entry (whole-home) RO

A point-of-entry RO system treats all the water entering the house, so that every tap, fixture, and appliance receives RO-purified water — for drinking, cooking, bathing, laundry, ice, and every other household use.

Powered. Point-of-entry RO is essentially always powered, for practical reasons. Whole-home demand requires higher production rates than household water pressure alone can drive through an appropriately-sized membrane; feed-side pumps raise pressure at the membrane to achieve good recovery, and delivery-side pumps (typically variable-speed) repressurize treated water from the storage cistern for household distribution. Non-electric point-of-entry would require an impractically large reservoir and would still deliver marginal performance.

Pretreatment. More substantial than at point-of-use, because the volume is larger and the membrane is larger. Sediment and carbon pre-filtration are standard. Where the source water is hard, anti-scalant dosing is the typical approach — it uses less water than softening pretreatment, produces no salt discharge, and addresses silica scaling that softening does not. Where a whole-home softener is already installed for other reasons, the softener addresses hardness ahead of the RO as an incidental effect of its main job. Iron and other specific conditions call for additional pretreatment tailored to the water.

Post-treatment. Because whole-home RO water goes to every use in the house, post-treatment considerations are broader. Storage in a cistern (typically hundreds of gallons) buffers production against household demand peaks. A variable-speed delivery pump repressurizes cistern water for the house. Remineralization and pH adjustment (often soda ash) are typical, since demineralized water throughout the house can affect taste and interact with plumbing. UV disinfection is common as a post-membrane safeguard.

Wastewater. Proportionally larger than point-of-use because the volume treated is much larger. Reject water volume is a real operational consideration and one of the reasons anti-scalant is preferred over softening pretreatment when either would work (anti-scalant reduces the water and salt burden significantly).

Installation. Substantial — a point-of-entry system integrated into the home's main water line, with the cistern, feed pump, delivery pump, and configured pretreatment and post-treatment train. Requires dedicated utility space and integration with the home's plumbing and electrical.

Comparing the two

Both configurations use the same core technology; they differ in scale, scope, and configuration.

Point-of-use treats the water you consume directly. Point-of-entry treats everything, for households that want RO purity at every tap in the house. Both deliver the same purity of water at the membrane; the difference is scope, configuration, cost, and operation.

RO on the region's water

For households on the softer surface-water municipal supply common in and around Durham and the broader region, RO adds purity beyond what municipal treatment provides. Municipal water in these systems is treated to federal standards and relatively low in hardness, and RO delivers a further level of contaminant reduction on top of that — including PFAS and other contaminants that municipal treatment may not fully address, along with any residuals from disinfection chemistry and dissolved substances below regulatory levels. Chlorine removal ahead of the membrane is essential (carbon pretreatment); softening or anti-scalant pretreatment is generally unnecessary on soft municipal water. Households choose RO in this context for the higher purity itself, for specific contaminant concerns, or for both.

For households with a specific contaminant that RO addresses well — PFAS in the Cape Fear basin, nitrate or arsenic on a well, elevated dissolved solids, lead concerns — RO can be an effective targeted treatment. Configuration choice (point-of-use for consumption water only, or point-of-entry for whole-house coverage) depends on whether the contaminant is a concern only for consumption or across all household uses.

For households on harder well water considering RO, pretreatment matters more: the membrane lasts longer and performs better when the water feeding it is not scaling it, which is why anti-scalant dosing is typical for whole-home RO on hard water, and why softening as pretreatment applies primarily when a softener is already installed for other reasons. Iron in well water is a separate pretreatment consideration — precipitated iron fouls RO membranes as surely as it fouls softener resin, and iron removal is standard pretreatment ahead of RO where iron is present.

Costs over time

RO has several cost components worth understanding before installing:

• Purchase and installation. Point-of-use systems are the lower end; point-of-entry systems are substantially higher because of the membrane capacity, cistern, pumps, and configured pretreatment and post-treatment train.
• Filter and membrane replacement. Pre-filters and post-filters are replaced periodically (typically every 6–12 months depending on use and water quality); the membrane lasts longer (typically several years) but eventually needs replacement. These are the recurring costs.
• Water and energy. RO sends some water to drain, which shows up on a metered water bill. Powered systems (essentially all point-of-entry, and most point-of-use) use electricity for feed pumps and, in point-of-entry systems, for delivery repressurization. Newer efficient systems and anti-scalant pretreatment reduce the wastewater component but do not eliminate it.
• Anti-scalant. Where anti-scalant is used as pretreatment, the dosing consumable is a recurring operational cost — small compared to salt for softening, but real.

We'll cover the full total-cost-to-own framework for treatment systems in a dedicated article. For RO specifically, the point is that it's a system with ongoing filter, membrane, water, energy, and (where applicable) anti-scalant costs that factor into the decision alongside the initial cost.

Factors in the decision

Several factors typically inform the decision about whether RO fits your household and, if so, which configuration:

Your goals. Are you primarily addressing a specific contaminant, seeking higher overall purity for consumption water, wanting whole-house RO purity for all uses, or some combination? Different goals point toward different configurations.

Which taps matter to you. If the goal is protection for drinking and cooking, point-of-use delivers that focused result. If the goal is RO-quality water at every tap in the house — for bathing, laundry, ice makers, coffee makers throughout the house, kitchen and bathroom sinks alike — point-of-entry delivers that broader result.

Your specific water. For contaminant-driven decisions, testing confirms what RO would address and at what levels. For general-purity decisions on already-good municipal water, the choice is more about scope than about problem-solving.

Household composition and use patterns. Households with infants, immunocompromised members, or specific health considerations may weight the purity case differently than adult-only households. Households with high water use have proportionally larger operating costs for whole-home systems.

Space and installation. Point-of-use fits under most kitchen sinks. Point-of-entry requires dedicated utility space (including cistern footprint) and integration with the main water line. Your home's infrastructure shapes what's practical.

Pretreatment requirements. On harder water, anti-scalant dosing is typical whole-home pretreatment; softening applies when a softener is already installed for other reasons. Additional pretreatment for iron or other specific conditions adds to initial and operating cost.

Power availability and resilience preferences. For point-of-use, electric configurations are the common default; non-electric is a considered choice for off-grid, backup, or specific site conditions. For point-of-entry, powered operation is a practical requirement.

Cost and maintenance tolerance. Both configurations have ongoing filter, membrane, water, and (where applicable) pump and anti-scalant costs. Comfort with periodic maintenance and cost planning shapes which configuration and which system within a configuration fits.

When professional advice makes sense

Professional input is most useful when sizing a whole-home system (membrane capacity, cistern size, pumps, pretreatment and post-treatment configuration), when determining pretreatment requirements for harder or well water (anti-scalant vs. softening, iron removal), when a confirmed contaminant requires verification that RO is the right approach (some contaminants have better-suited or lower-cost treatments), or when integrating RO into a larger treatment train.