Membrane Glossary: 60+ Water Treatment Terms

This comprehensive glossary defines over 60 essential terms used in membrane technology for water and wastewater treatment. Bookmark this page as your quick reference guide for understanding membrane specifications, processes, and performance parameters.

A

Activated Sludge: A biological wastewater treatment process using aeration to promote microbial growth that degrades organic pollutants. In MBR systems, membranes replace the secondary clarifier in the activated sludge process.

Air Scouring: The process of introducing coarse bubbles along the membrane surface to create turbulence and prevent fouling. Air scouring is the primary fouling control mechanism in submerged MBR systems.

Asymmetric Membrane: A membrane with a dense, thin selective layer on one side supported by a porous substrate. Most UF and MF membranes are asymmetric, with the selective layer determining separation performance.

B

Backwash (Backpulse): Reversal of flow through a membrane to remove accumulated foulants. Typically performed every 10-30 minutes for 30-60 seconds in UF and MBR systems.

Biofouling: Fouling caused by the growth of biofilms (bacteria, algae, fungi) on the membrane surface. One of the most challenging forms of fouling to control and remove.

BOD (Biochemical Oxygen Demand): A measure of the amount of dissolved oxygen consumed by microorganisms when decomposing organic matter in water. MBR systems typically achieve 98-99% BOD removal.

Bubble Point: The minimum pressure required to push air through the largest pores of a wetted membrane. Used as a membrane integrity test and quality control parameter.

C

CEB (Chemically Enhanced Backwash): A backwash procedure where chemical cleaning agents (typically sodium hypochlorite or citric acid) are added to the backwash water for more effective fouling removal.

CIP (Clean-In-Place): A chemical cleaning procedure performed without removing membranes from their housing. CIP involves soaking membranes in cleaning solution to remove accumulated foulants.

COD (Chemical Oxygen Demand): A measure of the total quantity of oxygen required to oxidize all organic material in water. COD includes both biodegradable and non-biodegradable organics.

Concentration Polarization: The buildup of rejected solutes near the membrane surface, creating a concentrated boundary layer that reduces effective driving pressure and can promote scaling or fouling.

Crossflow Filtration: A filtration mode where the feed flows parallel to the membrane surface, sweeping away accumulated solutes and particles. Used in most pressurized membrane systems.

D-F

Dead-End Filtration: A filtration mode where all feed water passes through the membrane. Simpler than crossflow but more prone to fouling. Used in some UF applications with low-turbidity feed.

Desalination: The removal of dissolved salts from water, typically using RO membranes. Used for seawater and brackish water treatment to produce fresh water.

Effluent: Treated wastewater discharged from a treatment plant. MBR effluent quality is typically superior to conventional activated sludge effluent.

Flat Sheet Membrane: A membrane configured as flat panels assembled in plate-and-frame modules. An alternative to hollow fiber configuration used in some MBR applications.

Flux: The volume of permeate produced per unit membrane area per unit time. Expressed as LMH (liters per square meter per hour) or GFD (gallons per square foot per day).

FOG (Fats, Oils, and Grease): Common wastewater contaminants from food processing, restaurants, and industrial sources. FOG must be removed or reduced before membrane treatment to prevent severe fouling.

Fouling: The accumulation of unwanted materials on the membrane surface or within its pores, reducing flux and increasing TMP. Types include organic, biological, inorganic (scaling), and colloidal fouling.

G-L

GFD (Gallons per Square Foot per Day): A unit of membrane flux commonly used in North America. 1 GFD equals approximately 1.7 LMH.

Hollow Fiber Membrane: A membrane configuration using thin tubular fibers (0.5-2.0 mm diameter) bundled in modules. Offers highest packing density and can be backwashed. TheWay Membranes specializes in PVDF hollow fiber modules.

Hydrophilic: Water-attracting surface property. Hydrophilic membranes resist organic fouling better than hydrophobic membranes. PVDF membranes are modified to be hydrophilic for water treatment applications.

Integrity Test: A test to verify that a membrane module is intact and providing the expected barrier performance. Methods include pressure decay test, vacuum hold test, and diffusive air flow test.

LMH (Liters per Square Meter per Hour): The standard international unit for membrane flux. Typical MBR design flux is 15-25 LMH for municipal wastewater.

Log Removal Value (LRV): A measure of pathogen removal efficiency. 1 log equals 90% removal, 2 log equals 99%, 3 log equals 99.9%, etc. UF membranes typically achieve greater than 4 log bacteria removal and greater than 2 log virus removal.

M-P

MBR (Membrane Bioreactor): A wastewater treatment process combining biological activated sludge treatment with membrane filtration. Produces high-quality effluent suitable for reuse in a compact footprint.

Microfiltration (MF): Membrane process with pore sizes of 0.1-10 microns. Removes suspended solids, bacteria, and some protozoa. Operates at low pressure (0.1-2 bar).

MLSS (Mixed Liquor Suspended Solids): The concentration of suspended solids (biomass) in the aeration tank. MBR systems operate at higher MLSS (8,000-15,000 mg/L) than conventional systems (2,000-4,000 mg/L).

MWCO (Molecular Weight Cut-Off): The molecular weight at which 90% of a solute is rejected by the membrane. Used primarily for NF and UF characterization. Expressed in Daltons (Da).

Nanofiltration (NF): Membrane process with pore sizes between UF and RO (approximately 1 nm). Removes divalent ions, organic molecules, and color. Operates at 5-20 bar pressure.

NTU (Nephelometric Turbidity Unit): The standard unit for measuring turbidity (cloudiness) of water. MBR permeate typically has turbidity below 0.2 NTU.

Outside-In Flow: A membrane flow direction where feed water flows from outside the hollow fiber to inside. Standard in submerged MBR and many UF applications. TheWay membranes use outside-in flow.

Permeability: Membrane flux normalized to TMP, expressed as LMH/bar or LMH/kPa. Declining permeability indicates fouling and the need for cleaning.

Permeate: The treated water that passes through the membrane. In MBR systems, permeate is the final effluent product.

PVDF (Polyvinylidene Fluoride): A fluoropolymer widely used as membrane material for its excellent chemical resistance, mechanical strength, and durability. The material of choice for UF and MBR membranes.

R-T

Recovery Rate: The percentage of feed water converted to permeate. UF systems typically achieve 90-95% recovery; RO systems achieve 40-85% depending on feed salinity.

Rejection Rate: The percentage of a specific contaminant removed by the membrane. Calculated as (1 - Permeate Concentration / Feed Concentration) x 100%.

Retentate (Concentrate): The portion of feed water that does not pass through the membrane, containing the rejected contaminants at higher concentration.

Reverse Osmosis (RO): A membrane process that uses high pressure to force water through a semi-permeable membrane, removing 95-99.5% of dissolved salts and virtually all suspended matter.

Scaling: The precipitation of inorganic salts (calcium carbonate, calcium sulfate, silica) on the membrane surface. Controlled through antiscalant dosing, pH adjustment, and limiting recovery rate.

SDI (Silt Density Index): A standardized test measuring the fouling potential of water. Critical for RO feed water quality assessment. UF-treated water typically has SDI below 2.

SRT (Sludge Retention Time): The average time activated sludge remains in the biological reactor. MBR systems operate at longer SRT (15-30 days) than conventional systems, improving treatment of slowly biodegradable compounds.

Submerged Membrane: A membrane module immersed directly in the process tank, operating under vacuum to draw permeate through. The standard configuration for MBR systems.

TDS (Total Dissolved Solids): The total concentration of dissolved inorganic and organic substances in water. Measured in mg/L or ppm. RO membranes remove 95-99.5% of TDS.

TMP (Transmembrane Pressure): The pressure difference across the membrane driving filtration. For UF and MBR, typically 5-50 kPa. Rising TMP at constant flux indicates fouling.

TSS (Total Suspended Solids): The total amount of particulate matter suspended in water. MBR permeate typically contains less than 1 mg/L TSS.

Turbidity: A measure of water cloudiness caused by suspended particles. Measured in NTU. Membrane-filtered water typically has turbidity below 0.2 NTU.

U-Z

Ultrafiltration (UF): Membrane process with pore sizes of 0.01-0.1 microns. Removes particles, bacteria, viruses, and colloids. Operates at low pressure (0.5-5 bar). TheWay Membranes specializes in PVDF UF hollow fiber membranes.

Water Reuse (Reclaimed Water): The practice of treating wastewater to a quality suitable for beneficial reuse. MBR technology produces effluent quality meeting most reuse standards for irrigation, industrial, and environmental applications.

ZLD (Zero Liquid Discharge): A treatment approach that recovers all water from a waste stream, leaving only solid residue. Typically uses UF pretreatment followed by RO, evaporation, and crystallization.

This glossary is maintained by TheWay Membranes. For membrane technology questions not covered here, contact our engineering team at info@thewaymembranes.com.