Reverse Osmosis Water Filtration: Membrane Technology, Stages, TDS Rejection Rates, and Storage Tank Design

Volume I  ·  August 2026

Reverse osmosis is a membrane separation process in which water is forced through a semipermeable membrane under pressure, leaving dissolved solids behind. The membrane permits water molecules to pass while rejecting 95–99% of dissolved ions, including sodium, chloride, calcium, magnesium, sulfate, nitrate, and heavy metals such as lead, arsenic, and chromium. It is not a filtration process in the mechanical sieving sense — the pores in an RO membrane are on the order of 0.0001 microns, approximately 500,000 times smaller than the diameter of a human hair, and the separation mechanism is governed by solution-diffusion rather than size exclusion. Water molecules dissolve into the polyamide active layer, diffuse through it under the applied pressure gradient, and desorb on the permeate side; dissolved ions, whose solubility and diffusivity in the polymer matrix are orders of magnitude lower, are rejected at the membrane surface and carried away in the concentrate stream. This analysis examines the membrane chemistry, the multi-stage configuration of a residential RO system, the performance metrics that govern rejection efficiency, and the hydraulic components — storage tank, flow restrictor, and permeate pump — that determine production rate and waste ratio.

The RO Membrane: Thin-Film Composite Construction

The membrane in every modern residential RO system is a thin-film composite (TFC) membrane, consisting of three laminated layers wound into a spiral-wound element. The topmost layer — the active rejection surface — is a polyamide film approximately 0.2 microns thick, formed by interfacial polymerization of m-phenylenediamine and trimesoyl chloride directly on the surface of a microporous polysulfone support layer. The polyamide is crosslinked at the molecular level, producing a dense polymer network through which water can diffuse but hydrated ions cannot. The polysulfone support layer, approximately 40 microns thick, provides mechanical strength without contributing to the rejection mechanism. The bottom layer is a nonwoven polyester fabric web, roughly 120 microns thick, that functions as the structural backing.

The spiral-wound configuration compresses the largest possible membrane area into the smallest possible cylindrical housing. A flat sheet of membrane is folded over a permeate collection tube to create a membrane leaf, with a feed-channel spacer mesh — a coarse plastic netting — placed between the folded membrane surfaces to create a gap through which feed water flows. The membrane leaf and feed spacer are then wound around the central permeate tube, creating alternating layers of feed channel, membrane, permeate channel, membrane, and feed channel. The permeate spacer — a finer mesh — carries filtered water inward toward the central collection tube. A standard residential RO membrane element, measuring 1.8–2.0 inches in diameter and 10–12 inches in length, contains approximately 3–5 square feet of membrane surface area and is rated for 36–100 gallons per day of permeate production under standard test conditions: 60 psi feed pressure, 77°F water temperature, and 500 ppm NaCl feed concentration with 15% recovery. Actual production in a residential installation at 40–50 psi and 50–60°F well water will be 30–50% of the rated value — a reduction driven by both the lower net driving pressure and the higher water viscosity at lower temperatures.

The TFC membrane replaced cellulose triacetate (CTA) membranes that dominated residential RO from the 1970s through the early 1990s. CTA membranes are cast as a single asymmetric layer from cellulose acetate polymer and are inherently chlorine-tolerant, permitting direct operation on chlorinated municipal water without a carbon pre-filter for dechlorination. The trade-off is a lower rejection rate — 88–94% for sodium chloride versus 96–99% for TFC — and susceptibility to biological degradation from bacteria that metabolize the cellulose acetate substrate. TFC membranes, by contrast, are destroyed by free chlorine at concentrations above 0.1 ppm; the polyamide layer oxidizes, and rejection performance collapses within days to weeks of continuous chlorine exposure. This is why every TFC-based RO system includes a carbon pre-filter upstream of the membrane: the carbon chemically reduces free chlorine to chloride ions, which the membrane then rejects normally. The Aquatic Life RO Buddie and the iSpring RCC7 are representative TFC-based under-sink RO systems that include carbon pre-filtration for chlorine removal.

Osmotic Pressure and the Driving Force Equation

Reverse osmosis works against the natural osmotic pressure of the feed solution. Osmotic pressure — the pressure that must be applied to prevent water from flowing across a semipermeable membrane from the low-concentration side to the high-concentration side — is proportional to the total dissolved solids concentration according to the van't Hoff relation: π = iCRT, where i is the van't Hoff factor (approximately 2 for NaCl, which dissociates into two ions in solution), C is the molar concentration, R is the gas constant, and T is the absolute temperature. For typical tap water at 500 ppm TDS (approximately 0.0086 M NaCl), the osmotic pressure is roughly 6 psi. This may appear negligible, but it increases as water is recovered from the feed stream. At 50% recovery — meaning half of the feed water becomes permeate and half becomes concentrate — the concentration of dissolved solids on the feed side doubles, and the osmotic pressure at the membrane surface increases to approximately 12 psi. At 75% recovery, it reaches 24 psi. The net driving pressure for permeation is the applied feed pressure minus the osmotic pressure minus any pressure losses through the feed spacer channel. A system operating at 50 psi feed pressure with a feed TDS of 500 ppm has a net driving pressure of approximately 50 − 6 = 44 psi. The same system at 75% recovery drops to 50 − 24 = 26 psi — a 41% reduction in driving force that manifests as a proportional reduction in permeate production rate.

This relationship explains why residential RO systems are designed for low recovery rates — typically 15–25% for under-sink units — and why they produce 3–5 gallons of concentrate for every gallon of permeate under typical operation. The low recovery rate keeps the osmotic pressure at the membrane surface low, maintaining high net driving pressure and high rejection efficiency. Increasing recovery by restricting the concentrate flow to produce less waste simultaneously reduces rejection efficiency and permeate quality; a system optimized for 95% rejection at 20% recovery may deliver only 88% rejection at 50% recovery. The concentrate-to-permeate ratio, often mischaracterized as a fixed "waste ratio" by marketing departments, is a design parameter set by the flow restrictor, and the optimal value depends on feed water quality: high-TDS feed water requires a higher concentrate flow rate to prevent scaling and maintain rejection; low-TDS feed water can tolerate a tighter flow restrictor and a lower waste ratio without sacrificing rejection.

Measurement of Rejection Rate

Membrane rejection rate is calculated as R = (1 − Cp/Cf) × 100, where Cp is the TDS of the permeate and Cf is the TDS of the feed. The measurement requires a calibrated conductivity meter — not a TDS meter that estimates TDS from conductivity using a fixed conversion factor (typically 0.5 or 0.7), which introduces error when the ionic composition of the feed is unknown. A conductivity meter calibrated to a known NaCl standard provides a direct measurement in microsiemens per centimeter (µS/cm), which can be converted to ppm NaCl using the factor 0.5 µS/cm per 1 ppm. An RO membrane that reduces feed water from 500 ppm TDS to 10 ppm permeate is operating at 98% rejection. The same membrane reducing 500 ppm to 25 ppm is at 95% rejection — a difference of 3 percentage points that represents a 2.5× increase in permeate TDS. The rejection rate is highest for divalent ions (calcium at 96–99%, sulfate at 98–99%) and lowest for monovalent ions (sodium at 94–98%, chloride at 94–97%) and uncharged species (silica at 80–90% depending on pH, dissolved CO₂ at essentially 0% when present as carbonic acid). Nitrate rejection is highly pH-dependent: at neutral pH, nitrate (NO₃⁻) is rejected at 85–95%, but below pH 6 the rejection rate can drop to 60–70% as the membrane surface charge becomes less negative.

The practical consequence of the ion-specific rejection profile is that RO permeate is not chemically pure water. It typically contains 5–25 ppm TDS, with the residual dissolved solids dominated by monovalent ions that passed through the membrane and dissolved gases — CO₂, oxygen, and nitrogen — that are not rejected by the membrane at all. The dissolved CO₂ in permeate forms carbonic acid, lowering the pH of RO water to 5.5–6.5 and producing the slightly acidic, flat taste that consumers often describe as "empty" or "dead." This is the rationale for the post-carbon polishing filter and, in premium systems, the remineralization stage discussed below.

System Stages: The Four-Stage Configuration

A residential under-sink RO system is not a single filter but a sequence of independent filtration stages, each serving a chemically distinct function. The standard four-stage configuration — the minimum that adequately protects the membrane from chlorine and particulates while polishing the permeate for taste — is:

Stage 1: Sediment pre-filter. A melt-blown or pleated polypropylene cartridge, typically 5-micron nominal rating, that captures suspended solids — sand, silt, rust particles, and pipe scale — before they reach the carbon block and membrane. A 5-micron rating removes particles visible to the naked eye (human visual resolution is approximately 40 microns) but passes colloidal clays and fine silt that range from 0.5–2 microns. A 1-micron sediment pre-filter provides more protection for the downstream carbon block and is standard on higher-quality systems such as the APEC ROES-50. The sediment filter is the first stage to clog and the stage that determines the system's maintenance interval on water with high suspended solids.

Stage 2: Carbon block pre-filter. An extruded carbon block filter, typically with a nominal pore size of 5–10 microns, that performs three chemical functions: dechlorination of the feed water by catalytic reduction of free chlorine (HOCl and OCl⁻) to chloride ions (Cl⁻) at the carbon surface; adsorption of dissolved organic compounds — trihalomethanes, chloroform, pesticides, herbicides, and volatile organic compounds — that contribute taste and odor; and, to a lesser extent, adsorption of heavy metals that react with the activated carbon surface. The dechlorination function is non-negotiable for TFC membranes: a carbon block that is exhausted and passing chlorine will destroy the membrane downstream. Carbon block capacity for chlorine removal is approximately 2,000–3,000 gallons of water chlorinated at 1–2 ppm free chlorine, after which breakthrough occurs — a figure that varies with flow rate, contact time, and water temperature. Systems with a granular activated carbon (GAC) cartridge instead of a carbon block have lower contact efficiency and less chlorine capacity per unit volume; a carbon block is preferred for the critical dechlorination function.

Stage 3: RO membrane. The TFC spiral-wound membrane in its own pressure vessel — a sealed cylindrical housing with feed, permeate, and concentrate ports. Feed water enters at one end, flows axially through the feed spacer channel, crosses the membrane, and exits as two streams: permeate from the central collection tube and concentrate from the opposite end of the housing. The membrane housing includes an automatic shut-off valve (ASO) — a hydraulically actuated diaphragm valve that senses the pressure differential between the feed line and the permeate line after the storage tank. When the storage tank reaches approximately 60–65% of feed pressure — typically 30–40 psi at the tank with 50–60 psi feed — the ASO closes, stopping feed flow through the system. This is the mechanism that prevents the system from running continuously when the tank is full. The ASO is purely hydraulic; it contains no electrical components, no float switch, and no solenoid.

Stage 4: Post-carbon polishing filter. A granular activated carbon or carbon block cartridge, typically 10-inch, installed downstream of the storage tank and upstream of the dispensing faucet. This filter has no dechlorination function — the water passing through it is RO permeate, which is chlorine-free — and exists solely to adsorb residual dissolved organic compounds that may have passed through the membrane or leached from the storage tank bladder, and to remove any taste or odor imparted by the tank materials. The post-carbon filter is also the final particulate barrier before the faucet; it captures any carbon fines shed by the pre-carbon filter that bypassed the membrane. The post-carbon filter is replaced on the same schedule as the pre-filters — typically every 6–12 months — because its adsorption capacity is limited even though the contaminant load in permeate is low.

Premium systems add a fifth stage — remineralization — in which permeate passes through a calcite (calcium carbonate) or corosex (magnesium oxide) filter that dissolves a small quantity of mineral back into the water. This raises the pH from 5.5–6.5 to 7.0–7.5 and adds approximately 10–30 ppm of calcium or magnesium, improving taste. The remineralization cartridge does not alter TDS rejection percentage; it adds TDS to the permeate after the membrane, and systems with a remineralization stage must be TDS-tested at a sampling port between the membrane and the remineralization cartridge — not at the faucet — to obtain the true membrane rejection rate. The iSpring RCC7AK and APEC ROES-PH75 are representative five-stage systems with alkaline remineralization.

Storage Tank and Pneumatic Delivery

Domestic RO membranes produce permeate at rates of 2–10 gallons per day under residential conditions — far too slow for on-demand dispensing from a faucet that flows at 0.5–1.0 GPM. The storage tank resolves this mismatch: the membrane fills the tank over hours, and the tank delivers a batch of water in seconds when the faucet is opened.

A residential RO storage tank is a steel or fiberglass pressure vessel internally divided by a butyl rubber diaphragm or bladder. The lower chamber contains compressed air at a factory pre-charge pressure, typically 5–7 psi when empty. The upper chamber receives permeate from the membrane. As permeate enters, the air bladder compresses, and the air pressure rises according to Boyle's Law (P₁V₁ = P₂V₂). When the faucet is opened, the compressed air expands, forcing water out of the tank. The tank does not require a pump because the air bladder provides the discharge pressure. The tank's usable volume — the drawdown — is determined by the air-side pre-charge pressure and the shut-off pressure of the ASO. For a 4-gallon tank with an internal volume of approximately 3.2 gallons, pre-charged to 7 psi and operating with an ASO that shuts off at 40 psi tank pressure (60% of 65 psi feed), the drawdown volume is: V_usable = V_tank × [1 − (P_precharge / P_shutoff)] = 3.2 × [1 − (7 + 14.7) / (40 + 14.7)] = 3.2 × [1 − 21.7/54.7] = 3.2 × 0.603 = 1.93 gallons. Approximately 1.3 gallons of stored water are not recoverable — the residual volume that remains when the tank air pressure equals the discharge line pressure. This is why a 4-gallon RO tank delivers approximately 2 gallons of water before refilling begins.

The tank pre-charge pressure must be checked and adjusted when the tank is empty of water. A tank pre-charge that is too high reduces drawdown by limiting how much water can enter before the ASO shuts off; a pre-charge that is too low reduces discharge pressure and may allow the diaphragm to stretch beyond its elastic limit, causing permanent deformation. A low-pressure tire gauge reading 0–20 psi — the same type used for bicycle tires — is the correct tool for checking RO tank pre-charge pressure at the Schrader valve on the tank's air side.

Flow Restrictor and Recovery Rate

The concentrate (waste) line of an RO system is fitted with a flow restrictor — a capillary tube or a precision-orifice fitting that limits concentrate flow to a specific rate, typically 250–420 mL/min for a 50 GPD membrane. The flow restrictor performs three simultaneous functions. It maintains the pressure differential across the membrane by preventing the feed water from short-circuiting directly to drain. It sets the recovery rate — the fraction of feed water that becomes permeate — by controlling the ratio of permeate flow to concentrate flow. And it provides the cross-flow velocity across the membrane surface necessary to sweep rejected ions away from the membrane, preventing concentration polarization — the accumulation of a high-concentration boundary layer at the membrane surface that raises the local osmotic pressure and reduces both permeate flux and rejection.

The flow restrictor is sized to the membrane's rated production. A 50 GPD membrane with a 4:1 concentrate-to-permeate ratio requires a flow restrictor that passes approximately 4 × 50 gallons per day = 200 gallons per day of concentrate, or 526 mL/min. A 100 GPD membrane at the same ratio requires 1,052 mL/min. Installing a 50 GPD flow restrictor on a 100 GPD membrane produces concentrate starvation: insufficient cross-flow velocity, increased concentration polarization, reduced rejection rate, and faster membrane scaling. Installing a 100 GPD restrictor on a 50 GPD membrane produces unnecessarily high waste flow without improving rejection. The flow restrictor is a matched component, not a universal part.

A permeate pump — a hydraulically driven piston pump powered by the hydraulic energy of the concentrate stream — can reduce the waste ratio from 4:1 to approximately 1:1 or 1.5:1 without compromising rejection, by using the energy that would otherwise be dissipated across the flow restrictor to boost permeate production. The permeate pump is plumbed in the permeate line between the membrane and the storage tank; the concentrate stream drives a piston that forces permeate into the tank at elevated pressure, overcoming the tank's back-pressure and increasing the net driving pressure across the membrane. In installations with feed pressure below 40 psi — where the ASO may cycle prematurely and permeate production drops sharply — a permeate pump is the most effective single upgrade and can increase daily production by 50–80% while reducing wastewater volume by 60–75%. The Aquatec ERP-500 permeate pump is the industry-standard retrofit pump for residential RO systems with low feed pressure.

Booster Pumps for Low Feed Pressure

When feed pressure is below 40 psi — common in well-water systems with pressure tanks set to 30/50 psi cut-in/cut-out, or in municipal systems at the end of long distribution mains — an RO membrane cannot develop sufficient net driving pressure to produce rated output. A booster pump — an electrically driven diaphragm pump, typically 24 VDC, installed on the feed line upstream of the pre-filters — raises feed pressure to 60–80 psi, restoring production to the membrane's rated capacity. The booster pump is controlled by a pressure switch that cycles the pump on when the ASO is open and off when the tank is full. Unlike a permeate pump, which recaptures hydraulic energy from the concentrate stream, a booster pump consumes electrical energy — approximately 1–2 amps at 24 VDC, or 24–48 watts — and adds a powered component to an otherwise passive hydraulic system. Booster pumps are paired with a pressure switch and a transformer plugged into a GFCI-protected outlet under the sink; the electrical components must be installed with drip loops and positioned above any potential standing water.

Whole-House Reverse Osmosis

Scaling RO from an under-sink point-of-use system to a whole-house point-of-entry system changes every design parameter. A whole-house RO system must produce 200–500 gallons per day for an average four-person household — 4–10 times the output of a single under-sink membrane — which requires multiple membrane elements in parallel, a high-pressure booster pump (typically 1–2 HP, 220 V), and an atmospheric storage tank of 200–500 gallons with a repressurization pump that delivers treated water to the household plumbing at 40–60 psi. The atmospheric tank breaks the direct connection between membrane production and household demand: the RO system fills the tank at its own production rate, and the repressurization pump — a multistage centrifugal pump controlled by a pressure switch — draws from the tank and delivers water to the house on demand. A float switch in the atmospheric tank controls the RO booster pump; when the tank is full, the RO system shuts down.

Whole-house RO introduces a water chemistry problem that does not exist at the point-of-use scale: RO permeate is corrosive to copper plumbing. Water with TDS below 25–50 ppm — the typical output of a properly functioning RO membrane — is aggressive and will leach copper from household pipes, producing blue-green staining at fixtures and eventually pinhole leaks. Whole-house systems must include a remineralization or pH correction stage — typically a calcite contactor, a sodium hydroxide injection system, or a blended bypass that mixes a fraction of untreated water back into the permeate stream to raise TDS to 50–80 ppm and stabilize pH above 7.0. The remineralization stage is a consumable: calcite media dissolves over time and requires replenishment at intervals determined by water usage and the target TDS increase. For a household using 200 gallons per day with a target TDS increase of 40 ppm, approximately 0.07 pounds of calcite are dissolved per day, or 25 pounds per year — a maintenance interval of 6–12 months for a typical 50-pound calcite contactor.

Membrane Fouling and Scaling

RO membranes degrade from two distinct mechanisms. Fouling is the accumulation of particulate matter, organic films, and biological growth on the membrane surface, reducing permeate flux by blocking the feed spacer channels and creating a hydrodynamic boundary layer that promotes concentration polarization. Scaling is the precipitation of sparingly soluble salts — calcium carbonate, calcium sulfate, barium sulfate, and silica — directly onto the membrane surface when their solubility product is exceeded in the concentrate stream. Both mechanisms reduce production rate and, in advanced stages, reduce rejection. Scaling is quantified by the Langelier Saturation Index (LSI) of the concentrate stream; an LSI above +2.0 at the membrane surface indicates a high scaling risk that mandates either reduced recovery (more waste flow to dilute the concentrate) or feed-water pretreatment with a water softener upstream of the RO. A water softener removes calcium and magnesium — the cations that combine with carbonate and sulfate to form scale — and replaces them with sodium, which has a solubility product orders of magnitude higher and does not precipitate on the membrane surface.

Membrane sanitization — flushing the system with a food-grade sanitizer such as hydrogen peroxide or a proprietary RO membrane cleaner — is recommended at each filter change interval for systems on untreated well water or surface water, and annually for systems on chlorinated municipal water where the carbon pre-filter has suppressed biological growth in the feed line. Without periodic sanitization, heterotrophic bacteria that survive the carbon pre-filter — which removes chlorine but does not sterilize — can colonize the permeate side of the membrane and the storage tank, producing a biofilm that imparts an earthy taste and, in extreme cases, reduces permeate production by coating the permeate spacer. A system sanitized at annual filter changes will maintain production rate indefinitely, limited only by the gradual irreversible fouling of the membrane over its 3–5 year service life.