Page 296 - ro membanes

GLOSSARY 279
Pretreatment Process that includes one or more source water treatment technologies (e.g., screening, coagulation, sedimentation, filtration, chemical addition etc.) that aim to remove foulants from the saline source water prior to RO separation in order to protect the membranes and improve desalination plant performance.
Product water Low-salinity (fresh) water usually with TDS concentration of 500 mg/L or less produced by the desalination plant and suitable for distribution system delivery. For the desalination plant permeate to be converted to product water, it has to be disinfected and conditioned for corrosion and predetermined water quality requirements.
Recovery The ratio of desalinated low-salinity water (filtrate or permeate) flow to feed water flow of filtration system. Reverse osmosis Pressure driven movement of water through a semipermeable membrane from the side of the
membrane with more concentrated solution to that of a less concentrated solution.
Salinity The concentration of total dissolved solids in water.
Salt passage The ratio of the concentration of salt/s (ions) in permeate and the concentration of the same salts (ions)
in the feed seawater; typically, salt passage is expressed as a percentage of the feed water concentration of salts. Salt rejection The ratio of salts (ions) removed (rejected) by the RO membrane to the same salts (ions) in the saline
source water; salt rejection is equal to 100% minus the salt passage.
Scale Mineral deposits formed on the surface of membrane and/or membrane matrix as a result of concentration
(saturation) of the mineral/s to a level at which they form insoluble amorphous or crystalline solids.
Scale inhibitor See antiscalant.
Scaling Process of scale formation on the surface or in the matrix of RO membrane.
Semipermeable membrane A membrane that has structure that allows small molecules, such as water, to pass while
rejecting a large portion of the salts contained in the feed water.
Silt A sedimentary material consisting of very fine particles intermediate in size between sand and clay.
Silt density index (SDI) A dimensionless parameter widely used to quantify the potential of seawater or brackish
water to cause particulate and colloidal fouling of RO membranes.
Specific permeability (flux) The capacity of membrane material to transit flow also named specific flux; expressed
as the membrane flux normalized for temperature and pressure, in liters per square meter per hour per bar
(Lmh/bar).
Spiral-wound element An RO or NF membrane element that consists of membrane leafs wound around a central
permeate collection tube and including feed and permeate spacers, antitelescoping devices and a brine seal. Stage A set of pressure vessels installed and operated in parallel.
Subsurface intake Intake located below the ground surface collecting source water from groundwater aquifer. Ex-
amples of subsurface intakes are vertical, horizontal, and slant wells and infiltration galleries.
Suspended solids Particulate solids suspended in the water.
Thin-film composite membranes (TFC) Semipermeable membranes manufactured principally for use in water
desalination systems that can be considered as a molecular sieve constructed in the form of a film from two or
more layered materials.
Total dissolved solids (salinity) A measure of the total mass of all dissolved solids contained in the water.
Total suspended solids The concentration of filterable particles in water (retained on a 0.45 mm filter) and reported
by volume.
Train (rack, skid) A membrane system that consists of rack housing a number of pressure vessels that have a
common feed, permeate, and concentrate piping and control equipment, and can be operated independently; an
RO system or MF or UF membrane system consists of multiple trains operating in parallel.
Transmembrane pressure (TMP) The driving force that transmits product water through the membrane. Troubleshooting A process of diagnosing and correcting the source of a problem.
Turbidity A measure of concentration of suspended solids in water, which is determined by the amount of light
scattered by these solids.
Ultrafiltration Filtration through membranes of pore size between 0.01 and 0.05 mm.
Uniformity coefficient The ratio of the 60th percentile media grain diameter to the effective size of the filer media. Variable-frequency drive (VFD) A type of adjustable-speed drive used in electromechanical devices to control motor
speed and torque by varying motor input frequency and voltage.
Viscosity A tendency of fluid to resist flow (movement) as a result of molecular attraction (cohesion).