What is microfiltration?

Microfiltration is membrane filtration with pores roughly 0.1 to 10 microns, retaining suspended solids, most bacteria and large particles while letting viruses, bacteria spores and everything dissolved pass through. It is commonly used as a pre-filter and in polishing duties. Because viruses and dissolved contaminants pass freely, MF alone is not considered drinking-water disinfection.

What is ultrafiltration?

Ultrafiltration is membrane filtration with pores roughly 0.001 to 0.1 microns (1-100 nanometres), retaining suspended solids, colloids, macromolecules and bacteria; our modules are rated at a nominal 0.01 micron. Dissolved salts, minerals and small organic molecules pass through, so UF softens nothing and desalinates nothing - but an intact UF membrane is a reliable barrier that turns turbid, bacteria-laden water into clear, low-bacteria permeate.

What is tight ultrafiltration?

Tight ultrafiltration sits at the fine end of the UF range (around 1-10 nanometres), blurring into nanofiltration. It removes more natural organic matter, colour and some viruses than standard UF while still operating at low pressures with much higher flux than RO. It is the step between a 0.01-micron drinking-water barrier and true softening.

What is nanofiltration?

Nanofiltration has effective pores near 1 nanometre (cut-offs around 200-1,000 daltons). It removes most divalent ions (so it partially softens water), colour, pesticides and many organic micropollutants, while passing a large share of monovalent salts. Operating pressure falls between UF and RO.

What is reverse osmosis?

Reverse osmosis pushes feed water above its osmotic pressure through a dense semi-permeable film that retains dissolved ions and molecules; pure water (permeate) passes while a concentrate stream carries the retained salts away. RO removes 95-99.9% of dissolved solids, which is why it is the choice for desalination and ultrapure water - and why it needs pretreatment, pressure and a reject stream that UF does not.

MWCO describes a membrane by the size of molecule it retains (in daltons or kilodaltons) rather than by a geometric pore diameter, because UF/MF membranes are polymer networks, not sieves with uniform holes. A 10 kDa MWCO membrane retains most molecules larger than about 10,000 daltons. Nominal pore size and MWCO are related but measured differently - check which one a datasheet quotes.

What is the difference between nominal and absolute filtration ratings?

A nominal rating means the filter captures most particles around that size under the manufacturer’s test conditions, while an absolute rating means validated removal of essentially all particles at or above the stated size (commonly 99.9%+). Two cartridges both labelled "0.01 micron" can perform very differently - the test method behind the number matters more than the number.

What is a hollow fiber membrane?

A hollow fiber membrane is a hair-thin capillary (typically around 1 mm across) whose wall is the filter; thousands of fibers are potted into one pressure vessel, giving an enormous surface area in a small footprint. Water can be driven from inside the fibers to outside or the reverse (outside-in), and the module is backwashed to clean. Almost all modern drinking-water and RO-pretreatment UF uses this geometry.

Reference

Water Treatment Glossary

40 working definitions for the terms that decide equipment specs - membrane processes, system operation, water quality measurements and the certification standards quoted on datasheets. Written for buyers comparing quotes, not for marketing brochures.

Membrane processes

Membrane filtration is sorted by what the membrane lets through. The categories overlap, so always read the rated pore size or cut-off on the datasheet, not just the label.

Microfiltration (MF) What is microfiltration?: Microfiltration is membrane filtration with pores roughly 0.1 to 10 microns, retaining suspended solids, most bacteria and large particles while letting viruses, bacteria spores and everything dissolved pass through. It is commonly used as a pre-filter and in polishing duties. Because viruses and dissolved contaminants pass freely, MF alone is not considered drinking-water disinfection.
Ultrafiltration (UF) What is ultrafiltration?: Ultrafiltration is membrane filtration with pores roughly 0.001 to 0.1 microns (1-100 nanometres), retaining suspended solids, colloids, macromolecules and bacteria; our modules are rated at a nominal 0.01 micron. Dissolved salts, minerals and small organic molecules pass through, so UF softens nothing and desalinates nothing - but an intact UF membrane is a reliable barrier that turns turbid, bacteria-laden water into clear, low-bacteria permeate.Our UF membrane modules Drinking water application
Tight Ultrafiltration (TF) What is tight ultrafiltration?: Tight ultrafiltration sits at the fine end of the UF range (around 1-10 nanometres), blurring into nanofiltration. It removes more natural organic matter, colour and some viruses than standard UF while still operating at low pressures with much higher flux than RO. It is the step between a 0.01-micron drinking-water barrier and true softening.
Nanofiltration (NF) What is nanofiltration?: Nanofiltration has effective pores near 1 nanometre (cut-offs around 200-1,000 daltons). It removes most divalent ions (so it partially softens water), colour, pesticides and many organic micropollutants, while passing a large share of monovalent salts. Operating pressure falls between UF and RO.
Reverse Osmosis (RO) What is reverse osmosis?: Reverse osmosis pushes feed water above its osmotic pressure through a dense semi-permeable film that retains dissolved ions and molecules; pure water (permeate) passes while a concentrate stream carries the retained salts away. RO removes 95-99.9% of dissolved solids, which is why it is the choice for desalination and ultrapure water - and why it needs pretreatment, pressure and a reject stream that UF does not.RO pre-treatment application
Molecular Weight Cut-Off (MWCO) What does MWCO mean?: MWCO describes a membrane by the size of molecule it retains (in daltons or kilodaltons) rather than by a geometric pore diameter, because UF/MF membranes are polymer networks, not sieves with uniform holes. A 10 kDa MWCO membrane retains most molecules larger than about 10,000 daltons. Nominal pore size and MWCO are related but measured differently - check which one a datasheet quotes.
Nominal vs absolute rating What is the difference between nominal and absolute filtration ratings?: A nominal rating means the filter captures most particles around that size under the manufacturer’s test conditions, while an absolute rating means validated removal of essentially all particles at or above the stated size (commonly 99.9%+). Two cartridges both labelled "0.01 micron" can perform very differently - the test method behind the number matters more than the number.

How a filtration system works

The operating vocabulary for pressure-driven membrane systems - what the streams are called, how they foul and how they are cleaned.

Hollow fiber membrane What is a hollow fiber membrane?: A hollow fiber membrane is a hair-thin capillary (typically around 1 mm across) whose wall is the filter; thousands of fibers are potted into one pressure vessel, giving an enormous surface area in a small footprint. Water can be driven from inside the fibers to outside or the reverse (outside-in), and the module is backwashed to clean. Almost all modern drinking-water and RO-pretreatment UF uses this geometry.
Feed, permeate, concentrate What do feed, permeate and concentrate mean?: Feed is the water entering the filter; permeate is the clean water that passes through the membrane; concentrate (or reject/brine) is the stream carrying what the membrane retained. UF in dead-end mode produces no continuous concentrate - retained solids stay on the fiber surface until a backwash ejects them - while RO always produces a concentrate stream that is typically 15-50% of the feed.
Dead-end vs cross-flow filtration What is the difference between dead-end and cross-flow filtration?: In dead-end filtration all feed goes through the membrane and retained matter builds up on it until a periodic backwash removes it - the standard mode for drinking water and RO pretreatment because it wastes almost no water. In cross-flow filtration the feed sweeps along the membrane surface at velocity, continuously carrying retained matter to a drain; it needs much more energy and water but keeps working on heavy solids loads, so industrial concentrate and separation duties use it.
Backwash What is backwashing a membrane?: Backwashing briefly reverses the flow through a membrane - permeate-side water is pushed back through the fibers - to lift the accumulated cake of solids off the surface and restore flux. Our modules run a 5-10 second backwash each cycle and can be flushed with air or chemical solution between campaigns. Backwash water use is typically a few percent of production, far below an RO reject stream.O-A7 filter straw
Clean-in-place (CIP) What is CIP cleaning?: CIP is circulating a cleaning solution - alkaline, acidic, or oxidant - across the membranes without disassembling the system, to remove fouling that backwashing cannot (organics, scale, biofilm). Each membrane material has a maximum chlorine and pH tolerance that defines its CIP recipe, so the cleaning limits on a datasheet matter as much as the pore size.
Flux (LMH) What is membrane flux?: Flux is the permeate volume passing per unit membrane area per hour (litres per square metre per hour, LMH). Rated fluxes assume clean water at 20-25 °C; cold water, foulants and fouling all lower the sustainable value. Sizing tools work backward from the required flow to the membrane area implied by a conservative design flux.Flux sizing calculator
Transmembrane pressure (TMP) What is transmembrane pressure?: TMP is the average pressure difference across the membrane - feed side minus permeate side - and is the force driving filtration. At a constant flow rate, a rising TMP is the primary fouling alarm: more and more pressure is needed to push the same water. Operators track TMP trend, not absolute value.
Silt Density Index (SDI) What is Silt Density Index?: SDI measures how fast a raw water loads a fine test filter, i.e. its colloidal fouling potential; SDI₁₅ (measured over 15 minutes) is the standard. RO elements are typically warrantied for feed SDI below 5, and a well-running UF stage reliably delivers SDI₁₅ under 3, which is why UF - not a 5-micron cartridge alone - is the pretreatment that protects an RO train.RO pre-treatment application
Recovery What is recovery in water treatment?: Recovery is the fraction of feed water that becomes product water. Dead-end UF on clean feed runs above 95% recovery, and our large modules can be flushed nearly water-free; SWRO is usually limited to 35-50% by scaling and brine disposal, and seawater desalination losses mostly come from the RO stage, not the pretreatment.

Water quality terms

The measurements that decide which barrier you need.

Turbidity (NTU) What is turbidity?: Turbidity quantifies how much light scattered by suspended particles makes water cloudy, in Nephelometric Turbidity Units (NTU). It matters because particles shelter microbes from disinfection, so regulators treat turbidity as a proxy for filtration performance; drinking-water standards generally require well below 1 NTU at treatment, and intact UF typically delivers below 0.1 NTU.
Coliform bacteria / CFU What are coliforms and CFU?: Coliforms are a family of common bacteria whose presence indicates that water has been contaminated and pathogens may be nearby - they are indicators, not usually the danger itself. Viable bacteria are counted as colony-forming units (CFU) on a plate; "total count" figures in water analyses are CFU per millilitre or per 100 mL.
Log reduction value (LRV) What is a log reduction?: One log of reduction removes 90% of a contaminant, two logs 99%, three 99.9%, four 99.99% - the multiplier is easier to remember than the percentages. A barrier "validated for 4-log bacteria" cuts a million cells per millilitre to a hundred; NSF protocols for each claim specify the required log value against the hardest challenge organism.
Total Dissolved Solids (TDS) What is TDS?: TDS is the total concentration of dissolved ions (calcium, sodium, chloride, bicarbonate...) in mg/L, usually estimated with a conductivity pen. RO and NF reduce TDS; UF, MF and carbon do not - a TDS meter is therefore the wrong instrument for judging a UF membrane, whose job is particles and microbes, not minerals.
Free chlorine What is free chlorine in water?: Free chlorine is the disinfectant residual left in tap water to keep it safe in the pipes. It tastes and smells unpleasant, damages some taste and odour compounds, and corrodes certain membrane materials - activated carbon removes it, and RO feed water must be dechlorinated upstream of the elements. UF does not remove chlorine.
Dissolved heavy metals Can ultrafiltration remove heavy metals?: UF removes particulate and colloidal metals but passes truly dissolved lead, arsenic or mercury almost freely, because those ions are 100-1,000 times smaller than the pore. Dissolved metals need reverse osmosis, nanofiltration, or selective media such as activated alumina or specialised resins. Any "UF removes heavy metals" claim that ignores this distinction should be read carefully.RO systems
GPD (gallons per day) What does GPD mean on a filter?: GPD is the US gallons-per-day throughput a membrane or element is rated for - the standard unit on RO elements and many UF cartridges. 1,000 GPD is about 3.8 m³/day; multiply by roughly 0.157 to get continuous litres per hour. A "3,000 GPD" quick-connect cartridge is therefore a ~470 L/h device, not a municipal-scale one.

Standards and certification shorthand

What the numbers on certificates actually commit to.

NSF/ANSI 42 What does NSF/ANSI 42 cover?: NSF/ANSI 42 certifies the aesthetic effects of a filter - chlorine taste and odour, and particulate reduction (Class I/III ratings by turbidity performance). A 42 certificate means the claimed particle and chlorine reductions were tested on that exact model; it says nothing about health hazards like lead or bacteria.
NSF/ANSI 53 What does NSF/ANSI 53 cover?: NSF/ANSI 53 certifies health-effect claims: dissolved lead, VOCs, cysts, chlorine byproducts and similar hazards, each tested to a protocol. Filters sold for a specific health risk should carry a 53 listing naming that contaminant - "reduces lead" without an ANSI 53 listing is unverified.
NSF/ANSI/CAN 372 What does NSF/ANSI/CAN 372 cover?: NSF/ANSI/CAN 372 is the materials-safety standard: it verifies the product’s wetted materials contain less than 0.25% lead and evaluates other construction-material safety. It certifies that the filter adds no contamination of its own - it is not a performance standard, which is why we list 42 (performance) and 372 (materials) side by side.
GB 5749-2022 What is GB 5749?: GB 5749 is China’s national drinking water quality standard (current edition 2022), setting limits for microbiological indicators, turbidity, disinfectant residuals and chemical parameters that any treatment serving a Chinese-regulated duty must meet. Our systems are designed to deliver permeate within those limits from typical feed waters.
EU Drinking Water Directive (2020/2184) What is the EU Drinking Water Directive?: The recast EU Drinking Water Directive sets microbiological and chemical limit values for water supplied to households and public buildings across member states, and tightens lead limits for contact materials - the reason EU buyers specify 372-type or equivalent materials documentation from their suppliers.

Units, sizing and site practice

Most specification errors are unit errors, not engineering errors. These are the conversions and field terms that come up when a datasheet, a drawing and a site survey have to agree.

L/h and L/min What is the difference between L/h and L/min?: L/h is litres per hour and L/min is litres per minute, so one litre per minute is sixty litres per hour. Filter capacities are usually published in L/h while plumbing and point-of-use flows are quoted in L/min, which is why a 350 L/h system is also correct when described as about 5.8 L/min. Multiply by 60 when moving from L/min to L/h, and divide by 60 the other way.Convert any flow rate
Cubic metres per hour (m³/h) How many litres per hour is one cubic metre per hour?: One cubic metre per hour equals 1,000 litres per hour, or about 16.7 litres per minute. Municipal and industrial specifications often use m³/h while equipment datasheets use L/h, so a 3 m³/h requirement is the same as 3,000 L/h. A cubic metre per day is a different figure again: 1 m³/day is only about 41.7 L/h, because it spreads the same volume over 24 hours.Flow rate converter
Gallons per minute (US gpm) What is a US gallon per minute in litres?: One US gallon per minute equals 3.785 litres per minute, or about 227 litres per hour. US and some export specifications use gpm, while metric datasheets use L/min or L/h, and the two are not interchangeable with imperial gallons: an imperial gallon is 4.546 litres, so an imperial gpm is about 273 L/h. Always check which gallon a document means before converting.Flow rate converter
bar, psi and MPa How do bar, psi and MPa compare?: One bar equals 0.1 MPa and about 14.5 psi; one MPa equals 10 bar and about 145 psi. Inlet pressure for household and light commercial filtration is usually quoted in bar or MPa, while US equipment is quoted in psi. Our UF modules require at least 0.1 MPa (1 bar, 14.5 psi) at the inlet to run on line pressure, and are rated up to 0.4 MPa.Pressure converter
Micron (µm) What is a micron?: A micron is one thousandth of a millimetre, or one millionth of a metre, written µm. It is the unit used for membrane pore ratings: microfiltration sits around 0.1 to 10 microns, ultrafiltration around 0.001 to 0.1 microns, and our UF elements are rated at 0.01 micron, which is 10 nanometres. Because pore ratings are quoted as nominal or absolute, the number alone does not tell you whether it is a guaranteed cut-off.Pore size converter
Mesh vs micron Is mesh the same as micron?: No. Mesh counts the number of openings per linear inch of a woven screen, so it describes the screen rather than a physical pore diameter, and the same mesh number maps to different micron figures depending on wire diameter and standard. A 100 mesh screen is roughly 150 microns, but not exactly, and the conversion changes between US and other mesh standards. For filtration specifications, ask for the micron rating rather than the mesh count.
ppm and mg/L Are ppm and mg/L the same?: For water analysis they are treated as identical: one milligram per litre is one part per million by mass, because a litre of water weighs about one kilogram. Laboratories report iron, chlorine and hardness in either unit interchangeably. The equivalence breaks down in brines and concentrated solutions where density departs from 1 kg/L, which is why seawater work sometimes quotes ppm by volume instead.
NTU (nephelometric turbidity unit) What does NTU measure?: NTU is the unit in which a nephelometer reports turbidity - the light scattered by suspended particles at a right angle to the beam. It is not a mass concentration and cannot be converted to mg/L, because the reading depends on particle size, shape and refractive index as well as quantity. Low NTU does not prove low dissolved contamination, and high NTU does not identify what the particles are; the value is a stability indicator, not a composition analysis.
Differential pressure (ΔP) What is differential pressure across a filter?: Differential pressure is the pressure drop between the inlet and outlet of a filter, and it is the practical signal that an element is loading. A rising ΔP at constant flow means the element is blinding; a ΔP that no longer recovers after backwash means it is spent. Gauges either side of the housing turn element changes from a calendar guess into a measurement, which is why we recommend fitting them at commissioning.
Element life and service interval How long does a filter element last?: Element life is the volume of water an element can treat before it must be replaced, and it depends on feed-water quality far more than on elapsed time. Two installations with the same daily volume but different turbidity can differ several-fold in interval, which is why a months-based answer without a water analysis is unreliable. Where a spec sheet states a total capacity in litres, use that; where it does not, set the interval from differential pressure readings.Cost out a replacement interval
Housing compatibility (10-inch, 1812) What do 10-inch and 1812 mean for filter housings?: These are housing format codes, not performance ratings. A 10-inch element is about 254 mm long and fits the standard slimline housing used across the industry; 1812 refers to an 18-inch by 12-inch nominal element format used in compact housings. Compatibility depends on outer diameter, end-cap type (flat, fin, DOE or bayonet) and O-ring size, so a photograph of the housing interior and its inner diameter settles the question faster than any model name.Replacement cartridge range
Commissioning flush Why flush a new filter before use?: A commissioning flush discards the first water through a new element, which carries manufacturing residue, carbon fines and air. Flush until the water runs clear - typically several minutes at full flow - and do not use that water for drinking or process make-up. The same applies after an element change, and after any maintenance that opens the housing to the atmosphere.

Specifying for a duty?

Size the membrane in seconds with the flux calculator, or send the term list to our engineers when a datasheet claim needs checking.

Flux sizing calculator Ask an engineer

Membrane processes

Microfiltration (MF)

Ultrafiltration (UF)

Tight Ultrafiltration (TF)

Nanofiltration (NF)

Reverse Osmosis (RO)

Molecular Weight Cut-Off (MWCO)

Nominal vs absolute rating

How a filtration system works

Hollow fiber membrane

Feed, permeate, concentrate

Dead-end vs cross-flow filtration

Backwash

Clean-in-place (CIP)

Flux (LMH)

Transmembrane pressure (TMP)

Silt Density Index (SDI)

Recovery

Water quality terms

Turbidity (NTU)

Coliform bacteria / CFU

Log reduction value (LRV)

Total Dissolved Solids (TDS)

Free chlorine

Dissolved heavy metals

GPD (gallons per day)

Standards and certification shorthand

NSF/ANSI 42

NSF/ANSI 53

NSF/ANSI/CAN 372

GB 5749-2022

EU Drinking Water Directive (2020/2184)

Units, sizing and site practice

L/h and L/min

Cubic metres per hour (m³/h)

Gallons per minute (US gpm)

bar, psi and MPa

Micron (µm)

Mesh vs micron

ppm and mg/L

NTU (nephelometric turbidity unit)

Differential pressure (ΔP)

Element life and service interval

Housing compatibility (10-inch, 1812)

Commissioning flush

Specifying for a duty?