Water Treatment Glossary: 60+ Key Terms Explained

This glossary covers the essential terminology used across reverse osmosis, desalination, membrane filtration, and industrial water treatment. Each definition is written for water treatment professionals, system designers, and procurement teams who need precise, technical language without unnecessary simplification.

A

Alkalinity: The capacity of water to neutralize acids, expressed as mg/L of calcium carbonate (CaCO3) equivalent. Alkalinity is primarily contributed by bicarbonate, carbonate, and hydroxide ions. In RO system design, alkalinity influences scaling potential and determines whether acid dosing or antiscalant injection is required to prevent calcium carbonate precipitation on membrane surfaces.
Antiscalant: A chemical additive injected into RO feed water to inhibit the formation of mineral scale deposits on membrane surfaces. Antiscalants work by interfering with crystal nucleation and growth of calcium carbonate, calcium sulfate, barium sulfate, silica, and other sparingly soluble salts. Dosing rates are typically 2–5 ppm and are calculated based on feed water analysis and system recovery rate.
ASTM: American Society for Testing and Materials. ASTM standards govern testing methods for water quality parameters, membrane performance, and material specifications used in water treatment equipment manufacturing and system design.

B

Backwash: The process of reversing flow through a filter bed to lift, expand, and flush accumulated solids from the media. Backwash is performed on multimedia filters, activated carbon filters, and softeners at regular intervals determined by differential pressure or timer settings. Effective backwash requires 50–100% bed expansion and typically runs for 10–15 minutes.
Brackish Water: Water with total dissolved solids (TDS) between 1,000 and 15,000 ppm, falling between freshwater and seawater in salinity. Common sources include inland wells, estuarine water, and agricultural runoff. Brackish water RO (BWRO) systems operate at 150–400 psi with recovery rates of 65–85%.
Brine: The concentrated reject stream discharged from an RO system, containing the dissolved salts and impurities removed from the feed water. Brine TDS is determined by feed water TDS and system recovery rate. A seawater RO system operating at 45% recovery produces brine at approximately 60,000–65,000 ppm TDS. Brine disposal is a significant environmental and regulatory consideration in plant design.
BWRO (Brackish Water Reverse Osmosis): A reverse osmosis system designed specifically for treating brackish water with TDS between 1,000 and 15,000 ppm. BWRO systems use lower-pressure membranes and pumps compared to seawater systems, operating at 150–400 psi with higher recovery rates of 65–85%.

C

Cartridge Filter: A disposable or cleanable filter element housed in a pressure vessel, used as the final pre-treatment stage before RO membranes. Standard ratings are 5 microns nominal or absolute. Cartridge filters protect high-pressure pumps and membranes from particles that pass through upstream multimedia or UF filtration.
CIP (Clean-in-Place): A chemical cleaning procedure performed on RO membranes without removing them from the pressure vessels. CIP involves recirculating a cleaning solution (alkaline, acidic, or enzymatic) through the membrane elements at low pressure and elevated temperature to dissolve foulants, scale, and biofilm. CIP frequency ranges from monthly to annually depending on feed water quality and pre-treatment effectiveness.
Concentrate: See Brine. The terms concentrate and brine are used interchangeably in RO system operation to describe the reject stream containing concentrated dissolved solids.
Conductivity: A measure of water's ability to conduct electrical current, expressed in microsiemens per centimeter (uS/cm). Conductivity is directly proportional to dissolved ion content and serves as a real-time proxy for TDS measurement. The approximate conversion is TDS (ppm) = conductivity (uS/cm) x 0.5 to 0.7, depending on the specific ions present.
Cross-Flow Filtration: A filtration mode where feed water flows parallel to the membrane surface rather than perpendicular to it. The tangential flow sweeps rejected contaminants along the membrane surface to be discharged as concentrate, preventing rapid buildup of a fouling layer. All spiral-wound RO and NF membranes operate in cross-flow mode.

D

DAF (Dissolved Air Flotation): A pre-treatment process that injects micro-bubbles of air into the feed water, causing suspended solids, algae, and oil to float to the surface where they are skimmed off. DAF is commonly used ahead of RO pre-treatment when the feed water source is subject to algal blooms or contains emulsified oils.
Desalination: The process of removing dissolved salts and minerals from seawater or brackish water to produce freshwater suitable for drinking, irrigation, or industrial use. The two primary commercial desalination technologies are reverse osmosis (membrane-based) and thermal distillation (evaporation-based). Reverse osmosis accounts for approximately 65% of global installed desalination capacity.
Differential Pressure (dP): The pressure drop measured between the feed inlet and concentrate outlet of an RO system or individual pressure vessel. Rising differential pressure indicates fouling or scaling of the membrane elements and is a key trigger for CIP cleaning. Normal dP for a clean system is 5–15 psi per vessel; CIP is typically initiated when dP increases by 15% above the baseline value.

E

EDI (Electrodeionization): A water polishing technology that combines ion exchange resins with an applied electrical field to continuously produce high-purity deionized water without chemical regeneration. EDI is typically installed after a two-pass RO system to achieve resistivity of 15–18 megohm-cm for pharmaceutical, semiconductor, and power generation applications.
ERD (Energy Recovery Device): A mechanical device that captures hydraulic energy from the high-pressure brine discharge of an SWRO system and transfers it to the incoming feed water. ERDs reduce the energy required by the high-pressure pump by 50–60%. Common types include pressure exchangers (isobaric), turbochargers, and Pelton wheels. ERDs are standard equipment on all seawater desalination systems above 10,000 GPD.

F

Feed Water: The raw or pre-treated water supplied to an RO or NF system for desalination or purification. Feed water quality—including TDS, temperature, pH, SDI, turbidity, and specific ion concentrations—determines membrane selection, operating pressure, recovery rate, and pre-treatment requirements.
Flux: The rate of permeate production per unit of membrane area, expressed as gallons per square foot per day (GFD) or liters per square meter per hour (LMH). Design flux determines how hard the membrane is working. Higher flux increases production but accelerates fouling. Typical design flux for BWRO is 14–18 GFD; for SWRO, 10–14 GFD.
Fouling: The accumulation of unwanted material on the membrane surface or within the membrane pores, reducing permeate flow and rejection performance. Fouling types include colloidal fouling (silt, clay), organic fouling (humic acids, proteins), biological fouling (biofilm), and scaling (mineral precipitation). Pre-treatment is designed to minimize fouling, and CIP procedures are used to restore fouled membranes.
FRP (Fiber-Reinforced Plastic): The standard material for RO pressure vessels. FRP vessels provide the strength to contain operating pressures of 150–1,200 psi while resisting corrosion from both feed water and cleaning chemicals. Vessels are manufactured to ASME Section X standards with ANSI/NSF 61 certification for potable water applications.

G

GPD (Gallons Per Day): The standard unit of measurement for RO system capacity in North America. One GPD equals approximately 0.00379 cubic meters per day. Metric equivalent: 1 m³/day = 264.2 GPD. System capacities range from 100 GPD for point-of-use units to millions of GPD for municipal desalination plants.
GPM (Gallons Per Minute): Flow rate measurement used for specifying pump capacity, pre-treatment filter sizing, and pipe sizing. 1 GPM = 0.227 m³/hr. To convert system GPD capacity to feed pump GPM, divide GPD by 1,440 and adjust for the system recovery rate.

H

Hardness: The concentration of dissolved calcium and magnesium ions in water, expressed as mg/L CaCO3 equivalent. Hard water causes scaling in RO systems, boilers, and heat exchangers. Water with hardness above 120 mg/L is considered hard. Hardness is managed in RO systems through antiscalant dosing, acid injection, or upstream water softening.
HMI (Human-Machine Interface): A touchscreen display or computer terminal that provides the operator interface for monitoring and controlling an automated water treatment system. The HMI displays real-time process data, alarm status, trend graphs, and allows operators to adjust setpoints, start/stop equipment, and initiate CIP cycles.

I

Ion Exchange: A water treatment process in which dissolved ions are exchanged for other ions of similar charge using synthetic resin beads. Common applications include water softening (exchanging calcium and magnesium for sodium), demineralization (exchanging cations for hydrogen and anions for hydroxide), and selective ion removal (nitrate, arsenic, fluoride).

L

LSI (Langelier Saturation Index): A calculated index that predicts whether water will tend to deposit calcium carbonate scale (positive LSI) or dissolve existing calcium carbonate (negative LSI). LSI is calculated from pH, temperature, TDS, calcium hardness, and alkalinity. In RO system design, the concentrate-side LSI must be maintained below +0.5 through antiscalant dosing or acid injection to prevent scaling.
LMH (Liters per Square Meter per Hour): The metric unit for membrane flux. 1 LMH = 0.589 GFD. Design flux values are specified in LMH in most international projects and membrane manufacturer literature.

M

Membrane: A semi-permeable barrier that allows water molecules to pass while rejecting dissolved salts and other contaminants. In RO and NF applications, membranes are thin-film composite (TFC) polyamide sheets wound into spiral-wound elements. Membrane selection is determined by feed water salinity, required rejection rate, operating pressure, and fouling propensity.
MF (Microfiltration): A membrane filtration process using membranes with pore sizes of 0.1–10 microns. MF removes suspended solids, bacteria, and turbidity but does not reject dissolved salts or viruses. MF is used as pre-treatment ahead of RO or as standalone treatment for particle removal in process water applications.
MMF (Multi-Media Filter): A pressure filter vessel containing layers of graded filter media—typically anthracite, sand, and garnet—arranged from coarse to fine. MMFs remove suspended solids and reduce turbidity and SDI in RO feed water. They are backwashed periodically to flush accumulated solids from the media bed.

N

NF (Nanofiltration): A membrane process operating between reverse osmosis and ultrafiltration, using membranes with molecular weight cutoffs of 200–1,000 daltons. NF selectively rejects divalent ions (calcium, magnesium, sulfate) at 90–98% while passing monovalent ions (sodium, chloride) at 30–70%. Applications include water softening, color removal, and partial demineralization.
Normalized Permeate Flow: Permeate flow adjusted to standard temperature (25 degrees C) and pressure conditions, allowing comparison of membrane performance over time regardless of seasonal temperature changes. A decline in normalized flow indicates membrane fouling; a decline in normalized salt rejection indicates membrane degradation or damage.

O

Osmotic Pressure: The natural pressure that must be overcome to force water through an RO membrane against the concentration gradient. Osmotic pressure is proportional to feed water TDS: approximately 1 psi per 100 ppm TDS for brackish water and approximately 400 psi for standard seawater at 35,000 ppm. The operating pressure of an RO system must exceed the osmotic pressure to produce permeate.

P

Permeate: The purified water that passes through the RO or NF membrane. Permeate quality is measured by TDS or conductivity and is typically 95–99.8% lower in dissolved solids than the feed water, depending on membrane type and operating conditions.
pH: A logarithmic scale measuring the hydrogen ion concentration of water, ranging from 0 (strongly acidic) to 14 (strongly alkaline), with 7 being neutral. RO membranes typically operate within a pH range of 2–11 for cleaning and 4–9 for continuous operation. Feed water pH affects scaling potential, membrane rejection, and chemical compatibility.
PLC (Programmable Logic Controller): An industrial computer used to automate the control of water treatment equipment. PLCs manage pump starts/stops, valve sequencing, chemical dosing, alarm logic, and communication with SCADA systems. Allen-Bradley, Siemens, and Schneider Electric PLCs are commonly used in water treatment applications.
PPM (Parts Per Million): A unit of concentration equivalent to milligrams per liter (mg/L) for dissolved substances in water. Used to express TDS, individual ion concentrations, and chemical dosing rates. Seawater is approximately 35,000 ppm TDS; drinking water standards require TDS below 500 ppm.
Pre-Treatment: The series of treatment steps applied to raw water before it enters the RO system, designed to protect membranes from fouling, scaling, and degradation. Pre-treatment may include multimedia filtration, ultrafiltration, activated carbon, water softening, cartridge filtration, chemical dosing (antiscalant, chlorine removal), and pH adjustment.
Pressure Vessel: A cylindrical FRP or stainless steel housing that contains the spiral-wound membrane elements and withstands operating pressure during RO or NF operation. Standard pressure vessels hold 1–8 membrane elements. BWRO vessels are rated to 600 psi; SWRO vessels are rated to 1,200 psi.

R

Recovery Rate: The percentage of feed water converted to permeate, calculated as (permeate flow / feed flow) x 100. Higher recovery means less water is wasted as brine but increases the concentration factor and scaling risk. Typical recovery rates: BWRO 65–85%, SWRO 35–50%, NF 75–90%.
Rejection Rate: The percentage of a specific dissolved constituent removed by the membrane, calculated as (1 - permeate concentration / feed concentration) x 100. Modern RO membranes achieve 95–99.8% salt rejection depending on membrane type, operating conditions, and the specific ion being measured.
Reverse Osmosis (RO): A pressure-driven membrane separation process that forces water through a semi-permeable membrane, rejecting 95–99.8% of dissolved salts and contaminants. RO is the dominant technology for desalination, water purification, and industrial water treatment worldwide. Systems range from small point-of-use units to municipal plants producing millions of gallons per day.
RO Train: A complete, independent RO processing unit within a larger plant, consisting of its own high-pressure pump, membrane array, instrumentation, and controls. Large plants use multiple trains for redundancy (N+1 design), maintenance flexibility, and load matching. Each train can be operated, cleaned, or maintained independently of the others.

S

Salt Passage: The percentage of dissolved salts that pass through the membrane into the permeate, calculated as (permeate TDS / feed TDS) x 100. Salt passage is the complement of rejection: a membrane with 99.5% rejection has 0.5% salt passage. Increasing salt passage over time indicates membrane degradation or O-ring seal failure.
SCADA (Supervisory Control and Data Acquisition): A computer-based system that monitors and controls water treatment processes from a central location. SCADA collects data from PLCs and field instruments, displays process status on operator workstations, logs historical data, generates alarms, and can provide remote access for off-site monitoring and troubleshooting.
SDI (Silt Density Index): A standardized test (ASTM D4189) that measures the fouling potential of RO feed water by timing the flow of water through a 0.45-micron filter at 30 psi. SDI below 3.0 is required for reliable RO membrane operation. Pre-treatment systems are designed to consistently achieve the target SDI value regardless of seasonal feed water variability.
Silica: Silicon dioxide (SiO2) dissolved in water, present as reactive (dissolved) or colloidal forms. Silica scaling on RO membranes is difficult to remove and can cause permanent damage. Concentrate-side silica concentrations must be maintained below 120–150 ppm (reactive) through antiscalant dosing and recovery rate limitation.
Spiral-Wound Element: The standard membrane element configuration for RO and NF applications. Flat sheet membranes are wound around a central permeate collection tube with feed spacers between the membrane leaves to create a compact, high-surface-area element. Standard sizes are 4040 (4-inch x 40-inch) and 8040 (8-inch x 40-inch).
SWRO (Seawater Reverse Osmosis): A reverse osmosis system designed to desalinate seawater with TDS of 30,000–45,000 ppm. SWRO systems operate at 800–1,200 psi with recovery rates of 35–50%. Energy recovery devices are standard equipment at all but the smallest capacities to reduce specific energy consumption from 8–14 kWh/m³ to 3–5.5 kWh/m³.

T

TDS (Total Dissolved Solids): The total concentration of all dissolved inorganic and organic substances in water, expressed as mg/L or ppm. TDS is measured by evaporation at 180 degrees C or estimated from conductivity. Classifications: freshwater below 1,000 ppm, brackish water 1,000–15,000 ppm, seawater 30,000–45,000 ppm. WHO drinking water guidelines recommend TDS below 600 ppm.
TFC (Thin-Film Composite): The dominant membrane construction for modern RO and NF elements. TFC membranes consist of three layers: a polyester support web, a microporous polysulfone interlayer, and an ultra-thin (0.2 micron) polyamide active layer that provides salt rejection. TFC membranes cannot tolerate chlorine exposure above 0.1 ppm.
Turbidity: A measure of water clarity caused by suspended particles scattering light, expressed in nephelometric turbidity units (NTU). RO feed water should have turbidity below 1.0 NTU. Pre-treatment filters and clarifiers reduce turbidity from raw water levels of 5–100+ NTU to acceptable levels for membrane treatment.

U

UF (Ultrafiltration): A membrane filtration process using membranes with pore sizes of 0.01–0.1 microns. UF removes bacteria, viruses, colloids, and high-molecular-weight organics while passing dissolved salts. As RO pre-treatment, UF consistently produces feed water with SDI below 2.0 regardless of raw water variability, making it increasingly standard for large desalination plants.
UV (Ultraviolet Disinfection): A chemical-free disinfection method that uses ultraviolet light at 254 nm wavelength to inactivate bacteria, viruses, and protozoa by damaging their DNA. UV is used as post-treatment after RO or as a disinfection barrier in distribution systems. UV dose is expressed in mJ/cm²; the standard dose for drinking water disinfection is 40 mJ/cm².

V

VFD (Variable Frequency Drive): An electronic motor controller that adjusts pump motor speed by varying the frequency of the electrical supply. VFDs allow RO system flow and pressure to be adjusted precisely without throttling valves, reducing energy consumption by 20–50% at partial loads. VFDs also provide soft-start capability that reduces mechanical stress on pumps and membranes.

W

Water Hammer: A sudden pressure surge caused by rapid valve closure or pump shutdown in a pressurized piping system. Water hammer can damage RO membranes, pressure vessels, and piping. Prevention measures include slow-closing valves, VFD-controlled pump ramp-down, and pressure relief devices on the high-pressure side of the system.

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