The Basic Principle of Reverse Osmosis (Nanofiltration)

Reverse Osmosis (Nanofiltration) Principle

Principles of Osmosis and Reverse Osmosis
The phenomenon whereby pure water passes through a semipermeable membrane from the low‑concentration saline side to the high‑concentration saline side is called osmosis. Osmosis demonstrates that when a semipermeable membrane is placed between two compartments, it permits certain substances to pass while rejecting others. Suppose this membrane allows water to pass but blocks salt; if a salt solution is placed on one side and pure water on the other, water will diffuse across the membrane into the salt‑containing compartment, whereas salt cannot cross. The system tends to reach equilibrium—equalizing the concentrations on both sides—and one way to achieve this is for water to flow from the pure‑water side through the membrane into the salt‑solution side. This also shows that osmosis can raise the liquid level in the salt solution until the hydrostatic pressure of the salt‑solution column becomes sufficient to counteract the net flow of water. At equilibrium, the hydrostatic pressure corresponding to the height of the water column is known as the osmotic pressure. Applying an external pressure to this column can reverse the direction of water flow; this is the principle underlying reverse osmosis.

Because salt cannot pass through the membrane, the result of counter‑current operation is the production of pure water from a saline solution. Specifically, an operating pressure is applied to the feed (concentrated) side to overcome the natural osmotic pressure. When this operating pressure exceeds the natural osmotic pressure on the concentrated‑solution side, the natural direction of water‑molecule osmosis is reversed: some of the water molecules from the feed (concentrated) solution pass through the membrane and become purified water on the dilute‑solution side.

Nanofiltration Principle
There is no clear boundary between nanofiltration and reverse osmosis. It derives its name from its ability to reject substances larger than 1 nm (nanometer). Nanofiltration membranes are not perfect barriers to dissolved salts or solutes; the permeance of these solutes through the membrane depends on the salt composition and the specific type of nanofiltration membrane. The lower the permeance, the higher the osmotic pressure across the membrane, bringing the process closer to that of reverse osmosis; conversely, the higher the permeance, the lower the osmotic pressure, and the less significant the influence of osmotic pressure on the nanofiltration process. Nanofiltration performance lies between that of ultrafiltration and reverse osmosis. Organic compounds with molecular weights greater than 200–1,000 can be rejected. In addition, the desalination rate for dissolved salts ranges from 20% to 90%. Operating transmembrane pressures typically fall within the range of 3.5 to 16 bar (50 to 225 psi).

Reverse osmosis (nanofiltration) process
The process of reverse osmosis (nanofiltration) involves continuously applying a certain pressure to the feed side (the concentrated brine side) to overcome the solution’s osmotic pressure, thereby enabling the permeate side (the dilute brine side) to continuously produce permeate (product water).

High-pressure pumps continuously pressurize the feed water as it is conveyed into the system, which comprises pressure vessels and membrane elements. Within the system, the feed water is separated into permeate containing trace amounts of salts and concentrate with a high salt content. The ratio of concentrate to permeate produced from the feed water is regulated by the concentrate‑to‑permeate ratio control. In addition to the pressure vessel, a spiral‑wound module consists of several sets of spiral‑wound membrane elements; typically, one pressure vessel houses 1 to 6 sets of membrane elements, with a maximum of up to 7 sets. As the feed water passes through the next element,
The salt concentration gradually increases, and the concentrate ultimately exits through the last stage, entering the concentrate control valve where it is depressurized. The permeate produced by each membrane element is collected via a common permeate header located at the center of that stage and discharged into a permeate collection line outside the pressure vessel.

Related nouns

Reverse osmosis membrane: A functional semipermeable membrane that allows solvent molecules to pass through while blocking solute molecules is called a reverse osmosis membrane.

Nanofiltration membrane: A functional semipermeable membrane that permits the passage of solvent molecules, certain low-molecular-weight solutes, or low-valence ions is referred to as a nanofiltration membrane.

Membrane element: The smallest unit of a reverse osmosis or nanofiltration process, in which the membrane sheet is assembled with the feed‑water flow channel grid, permeate‑water flow channel material, permeate‑water central tube, and stress‑relief device using adhesives, thereby achieving separation between the feed water and the permeate, is called a membrane element.

Membrane module: The membrane elements are installed inside a pressurized pressure vessel shell to form a membrane module.

Membrane device: A complete, stand-alone membrane unit comprising membrane modules, instrumentation, piping, valves, a high-pressure pump, a security filter, an on-site control panel, and a rack is referred to as a membrane system; the reverse osmosis and nanofiltration processes are carried out using this membrane system.

Membrane system: A complete membrane-based water treatment process, designed to meet specific source‑water conditions and product‑water requirements, and comprising pretreatment, chemical dosing equipment, booster pumps, a storage tank, a membrane module, and interlocked electrical and instrumentation controls, is referred to as a membrane system.

Traffic: Flow rate refers to the feed water flow rate entering the membrane element, typically expressed in m³/h. ³ /h or gpm.

Concentrate flow rate: It refers to the “feed water” flow rate that bypasses the membrane elements in the system. This concentrated stream contains soluble constituents introduced from the feedwater source and is typically expressed in m ³ /h or gpm.

Flux: The flux is expressed as the rate of permeate flow per unit membrane area, typically in units of L/m²·h or gfd.

Dilute solution: The purified aqueous solution serves as the permeate from the reverse osmosis or nanofiltration system.

Concentrated solution: The portion of the solution that does not pass through the membrane, such as the concentrate in reverse osmosis or nanofiltration systems.

Salt content and total solids: Salinity refers to the total amount of dissolved solids in water that can dissociate into ions; the sum of its cations and anions is defined as salinity. The electrical conductivity and resistivity of water are related to salinity: generally, higher salinity corresponds to higher conductivity and lower resistivity.

Acidity: Acidity refers to the content of acidic substances in water that can react with a strong base (such as NaOH or KOH).

Alkalinity: Alkalinity refers to the concentration of alkaline substances in water that can react with a strong acid—typically a 0.1 mol/L standard HCl solution. The alkalinity parameter is often used to assess a water body’s buffering capacity, as well as the solubility and toxicity of metals within it.

Hardness: Water hardness is classified into carbonate hardness and non-carbonate hardness; the sum of the two is referred to as total hardness.

Carbonate hardness: It refers to the salts formed by calcium, magnesium, and other ions in water together with bicarbonate ions. When water is heated, bicarbonates decompose into carbonates; as their solubility decreases, precipitates are formed and released. Consequently, this type of hardness is also known as temporary hardness.

Non-carbonate hardness: It primarily refers to the hardness caused by calcium and magnesium sulfates, nitrates, chlorides, and other compounds. Since these do not precipitate when water is heated to boiling under atmospheric pressure, this type of hardness is also known as permanent hardness.

Electrical conductivity: Electrical conductivity is a numerical measure of a solution’s ability to conduct electric current. Pure water has very low electrical conductivity; when inorganic acids, bases, or salts are present, the solution’s conductivity increases. Electrical conductivity is often used as an indirect indicator of the total ion concentration in water. The electrical conductivity of an aqueous solution depends on the nature and concentration of the ions, as well as the solution’s temperature and viscosity. For every 10°C increase in temperature, electrical conductivity typically rises by approximately 2% to 2.5%; 25°C is commonly specified as the standard temperature for measuring electrical conductivity.

The standard unit of electrical conductivity is S/m (siemens per meter). In practical applications, the commonly used unit is μS/cm.

Turbidity: ISO international standards define turbidity as the reduction in a liquid’s transparency caused by the presence of insoluble substances. Depending on the turbidity standard solution used in the measurement, the resulting turbidity values and their units will vary.

SDI value: SDI (Silt Density Index), also known as the fouling index, is an important parameter for characterizing the quality of feedwater in reverse osmosis systems. Compared with turbidity, it provides a different perspective on water quality; however, SDI values are more accurate and reliable than turbidity measurements. Turbidity is typically determined using spectrophotometry or visual turbidimetry to quantify particulate impurities in water, but these methods cannot accurately measure certain non‑light‑absorbing colloidal particles.

Oxidation-reduction potential ORP: Redox potential (ORP) is a parameter that characterizes the relative concentrations of oxidizing and reducing substances in an aqueous system. Redox potential is typically expressed in millivolts (mV): a positive ORP indicates the presence of oxidizing substances, while a negative ORP suggests the presence of reducing substances.

Organic matter: Organic matter is highly diverse, with its compositional makeup in natural waters exhibiting tremendous variability. At present, there is no reliable direct analytical method for its determination, and several parameters commonly associated with organic matter—such as biochemical oxygen demand, loss on ignition, and total organic carbon—fail to accurately quantify either the total organic content or its specific composition.

Common units and their conversions:

1 in (1 inch) ≈ 25.4 mm
1 in² (1 square inch) ≈ 6.45 cm² ²
1 ft² (1 square foot) ≈ 0.0929 m² ²
1 gallon (U.S. customary) ≈ 3.785 L; 1 GPM (gallons per minute) ≈ 0.227 m³ ³/h
1 GPD (gallons per day) ≈ 3.785 L/D
14.5 PSI ≈ 0.1 MPa ≈ 1 bar (kg/cm²) ²)

The primary metrics for evaluating the performance of reverse osmosis membranes

Desalination rate and salt passage rate

Salt rejection = (Permeate concentration / Feed concentration) × 100%

Desalination rate = (1 − salt concentration of product water / salt concentration of feed water) × 100%

Salt passage rate = 100% - desalination rate

The desalination rate of a membrane element is determined at the time of its manufacture; it depends on the compactness of the ultra-thin desalination layer on the membrane surface—higher compactness yields a higher desalination rate but lower water production. The desalination performance of reverse osmosis for different substances is primarily governed by their molecular structure and molecular weight. For polyvalent ions and complex monovalent ions, the desalination rate can exceed 99%; for simple monovalent ions such as sodium, potassium, and chloride, the rate is slightly lower but still surpasses 98%. Organic compounds with molecular weights greater than 100 can also be removed at rates exceeding 98%, whereas those with molecular weights below 100 are removed less efficiently.

Water production volume

Water production refers to the water‑producing capacity of a reverse osmosis system, that is, the volume of water that passes through the membrane per unit time, typically expressed in tons per hour or gallons per day.

Permeate flux is another key parameter for characterizing the water production capacity of a reverse osmosis membrane element. It refers to the flow rate of permeate per unit membrane area and is typically expressed in gallons per square foot per day (GFD). Excessively high permeate flux can increase the cross‑flow velocity perpendicular to the membrane surface, thereby exacerbating membrane fouling.

Recovery rate

Recovery rate refers to the percentage of feed water that is converted into product water or permeate within a membrane system. It is determined by the quality of the feed water and the specific water‑use requirements. The recovery rate of a membrane system is established during the initial design phase. For reverse osmosis (nanofiltration) membrane modules, the formulas for calculating recovery rate, salt rejection, and desalination efficiency are as follows:

Recovery rate = (Permeate flow rate / Feed flow rate) × 100%

Salt rejection = (Permeate concentration / Feed concentration) × 100%

Desalination rate = (1 − salt passage rate) × 100%