Reverse-osmosis process water

Identity and separation principle

RO recovery is permeate flow divided by feed flow on the same boundary and time basis. Observed solute rejection is one minus the permeate-to-feed concentration ratio for the same analyte, on the same concentration basis, with nonzero feed concentration. These quantities describe different properties. Permeate crosses the membrane and concentrate remains on the feed side. EPA treatment overview, RO and NF section

Opposing pressure reverses the net water movement of osmosis. Real membranes pass some solute. RO identifies a treatment process, with end-use water quality requiring separate evidence. FDA, Reverse osmosis, definition and operating principle DuPont manual, sections 1.3 and 1.5

Streams and quantity bases

These are membrane-process quantities. Element recovery and overall system recovery have different feed denominators. DuPont defines recovery, rejection, passage, flow and flux separately. DuPont manual, section 1.3. EPA defines recovery over an uninterrupted operating cycle without deducting filtrate subsequently used for backwashing or cleaning. Synthesis: membrane recovery alone does not establish the net water yield of a whole facility. EPA membrane guidance, glossary and section 1.5.

Recovery and observed rejection

The following notation expresses definitions, not operating targets.

Y = Qp / Qf

Here Y is the dimensionless instantaneous recovery fraction. Qp and Qf are simultaneous permeate and feed volumetric flows in the same units. At steady conditions, this equals the corresponding volume ratio. Percent recovery is 100 × Y. DuPont manual, section 1.3.

For a specified solute s, observed concentration rejection is:

Robs,s = 1 − Cp,s / Cf,s

Cp,s and Cf,s are permeate and bulk-feed concentrations of the same analyte on the same chemical and unit basis, such as mg/L of chloride. The feed concentration is nonzero. Percent rejection is 100 × Robs,s. This concentration-ratio definition also appears in Ahmed and colleagues' experimental paper, section 2.4.2. Their sugar-refinery experiment does not establish generic RO performance. Ahmed et al., 2025.

Interpretive synthesis: a rejection value depends on its sample basis. Local feed and permeate samples use a different basis from system-inlet feed and combined product samples. During changing feed composition, unpaired samples need not describe the same separation state. Bulk-feed rejection also differs from a value referenced to concentration immediately at the membrane surface.

Rejection compares concentrations. Recovery compares water quantities. Neither is the percentage of all incoming solute mass recovered in the concentrate. No mass balance or concentration-factor estimate follows without a defined boundary and assumptions about accumulation, reaction, deposition and side streams.

Driving pressure and conditional changes

Net driving pressure is the hydraulic pressure difference across the membrane minus the opposing osmotic-pressure difference:

NDP = ΔP − Δπ

Both terms have pressure dimensions, such as Pa or bar. The differences use feed-side minus permeate-side values at the same location. This is a conceptual local description. A system-average expression requires assumptions about pressure losses and concentration variation. It is not a membrane-area or pump-sizing equation. DuPont's system example explains why hydraulic pressure loss and rising feed-side concentration can reduce driving force along an array. DuPont manual, sections 3.12.1–3.12.2.

Concentration polarization means enrichment near the membrane surface relative to the bulk liquid. The resulting osmotic-pressure difference can differ from one inferred from a bulk-feed sample. EPA membrane guidance, glossary; DuPont manual, section 3.12.1.

DuPont's qualitative comparisons hold other stated variables constant. Higher temperature increases water flux and salt passage. Higher effective pressure increases flux while reducing salt passage. Synthesis: a change in product conductivity alone cannot distinguish a temperature effect from changed feed chemistry or membrane condition. These trends are not universal quantitative corrections. DuPont manual, section 1.3, Figures 6–9 and Table 1.

Membrane and application variants

EPA describes cellulose-acetate and polyamide families with different biological and chemical resistance. Polyamide has limited strong-oxidant tolerance. "RO membrane" therefore does not imply one chemical compatibility. EPA also distinguishes asymmetric construction from composite construction, where the separation layer and support use different materials. Spiral-wound describes module geometry rather than polymer identity. EPA membrane guidance, sections 2.3 and 2.3.1.

Brackish-water and seawater desalination are applications of RO. NF is an adjacent process with different selectivity, including uses for hardness reduction. A shared membrane-process label does not establish interchangeable salt or organic-solute rejection. EPA treatment overview, RO and NF section.

Solute chemistry and measurement limits

Hydranautics describes boron as an example of chemistry-dependent rejection. Charged borate is rejected more readily than uncharged boric acid. Their proportions depend on pH, temperature and salinity. Dissolved carbon dioxide can pass through RO, unlike strongly retained salts. Ammonia and ammonium likewise have different membrane passage behavior. These examples do not establish a rejection value for every membrane or mixture. Hydranautics, RO water chemistry, ammonium, boron and carbon-dioxide entries.

Conductivity reflects the electrical response of ions and depends on temperature. Many organic compounds conduct poorly. A conductivity-derived TDS value uses an empirical conversion factor rather than a complete analysis of individual substances. Synthesis: equal conductivity does not imply equal composition, and low conductivity does not establish low concentrations of every neutral solute, organic substance or microorganism. Conductivity in µS/cm and TDS in mg/L are different quantities. EPA, 5.9 Conductivity, explanatory section. See conductivity of rinse water for measurement terminology.

Applicability and quality limits

Fouling is contaminant accumulation on or within a membrane. Scaling is salt precipitation or crystallization. Concentration polarization is a boundary-layer concentration effect and is not itself proof of deposited scale. Feed variability and these mechanisms limit transfer of performance between waters. EPA membrane guidance, glossary.

FDA's historical technical guide reports bacterial growth in relatively pure RO water and downstream contamination. Synthesis: solute rejection does not establish sterility, absence of endotoxin or preserved quality after storage and distribution. Its historical pharmaceutical examples are not current water-grade specifications. FDA, quality of RO product water.

Scope synthesis: this reference provides no plant design, operating window, treatment recipe or acceptance criterion. It establishes no customer's installed membrane, process-water suitability, potable-water compliance, discharge permission or qualified production capability. End-use composition and microbiological requirements remain separate from the name "RO water".

Source versions and coverage

Sources reviewed on 30 October 2026 PDT. The retrieved document governs version identification when a search snippet differs. No numerical manufacturer performance or operating table is adopted as a generic property.

Coverage gaps include species-specific rejection in mixed industrial waters, aged or damaged membrane behavior, and application-specific organic and microbial quality. No paid standard has been treated as read.