Membrane device configurations
To be effective, membrane polymers must be packaged into aconfiguration commonly called a “device”, “module” or “element”. Themost common element configurations are:
• Plate and Frame
• Capillary Fibre
• Spiral Wound
Plate and frame
This element incorporates sheet membrane stretched over a frame toseparate the layers and facilitate collection of the permeate, which isdirected to a centre tube.
Capillary (hollow fibre)
These elements are similar to the tubular element in design, butsmaller in diameter. They are usually unsupported membrane polymersrequiring rigid support on each end, provided by an epoxy “potting” ofa bundle of the fibres inside a cylinder. Feed flow is either down theinterior of the fibre or around the outside of the fibre.
Manufactured from ceramic, carbon, stainless steel or a number ofthermoplastics, these tubes have inside diameters ranging from aquarter of an inch up to approximately 1 inch (6 to 25mm). The membraneis typically coated on the inside of the tube and the feed solutionflows through the interior (lumen) from one end to the other, with thepermeate passing through the wall to be collected on the outside of thetube.
This element is constructed from an envelope of sheet membrane woundaround a permeate tube that is perforated to allow collection of thepermeate. Water is purified by passing through one layer of themembrane and flowing into the permeate tube. It is by far the mostcommon configuration in water purification applications.
From the perspective of cost and convenience, it is beneficial to packas much membrane area into as small a volume as possible. This is knownas “packing density”. The greater the packing density, the greater themembrane area enclosed in a certain sized device, and generally thelower the cost of the membrane element. The downside of the highpacking density membrane elements is their greater propensity forfouling. The table above summarises this data.
Element configuration Packing Density* Fouling Resistance*
Tubular low high
Capillary Fibre medium high
Spiral Wound low moderate
Plate and Frame low high
*** Membrane area per unit volume element
* Tolerance to suspended solids
Suspended solids, micro-organisms and dissolved organic contaminantsare effectively removed by the “sieving” process; that is, contaminantstoo large to pass through the membrane pores remain in what becomes theconcentrate stream.
For dissolved inorganic (ionic) contaminant removal, the membranetechnologies of nanofiltration and reverse osmosis must be employed,and since the pores of the membranes, although smaller than for themicrofiltration and ultrafiltration membranes, are not small enough tophysically sieve out these contaminants, a different mechanism applieshere.
There is lack of agreement among the experts as to the exact mechanismof this rejection; however, it is known that multivalent salts arerejected to a higher degree than monovalent salts. This characteristicis exploited with nanofiltration membranes which exhibit overall lowersalts rejection than reverse osmosis membranes, but remove multivalentsalts to a much higher degree than monovalent salts. These membranesare also known as “softening” membranes because of their ability toremove the divalent hardness ions of calcium and magnesium, with littleeffect on monovalent ions such as sodium.
The “thin film composite” reverse osmosis polymers that are now on themarket have such high salts rejection characteristics, that bothmonovalent and multivalent salts are rejected to almost the same degree.
“Flux“ is a fundamental characteristic of all membranes, defined as thepermeate rate through a given area of membrane at a specifictemperature and pressure. “Recovery” is defined as that percentage ofthe feed flow rate that passes through the membrane and becomespermeate.
For the process of nanofiltration and reverse osmosis (and to a lesserextent, ultrafiltration), which deal with dissolved materials, aproperty of the solution known as “osmotic pressure” usually becomesthe limiting factor in recovery calculations. Osmotic pressure is acharacteristic of all ionic solutions, and is loosely defined as theresistance of the solvent portion of the solution to passage throughthe membrane. Osmotic pressure is a function of both the particularsolute, as well as its concentration. A specific test is almost alwaysrequired to accurately determine osmotic pressure.
As recovery is increased (typically through the use of a flowrestrictor or concentrate valve), with the resulting decrease inconcentrate flow, the concentration of solute in the concentrate streamincreases, resulting in increased osmotic pressure.
The vast majority of membrane element device and system failures occuras the result of membrane fouling. This fouling is usually the resultof one or more of the following mechanisms:
• Suspended solids in the feed stream resulting from improper feed water filtration.
• Precipitation of insoluble salts or oxides resulting from concentration effects within the membrane device.
• Biofilm resulting from microbiological activity.
These mechanisms cause the membrane surface to become coated withfouling materials that build up in layers. As the layer thicknessincreases, the flow rate across the membrane surface and immediatelyadjacent to it decreases, thereby encouraging more settling ofsuspended solids and increasing the fouling layer thickness – leadingto a vicious circle.
With nanofiltration and reverse osmosis membranes, which reject ioniccontaminants, fouling usually creates a phenomenon known as“concentration polarisation”. The fouling layers inhibit the freemovement of the feed stream away from the membrane surface, and assalts are rejected from the membrane, their concentration at thesurface is higher than in the bulk solution (that portion above thefouling layer).
Since ionic rejection is always a percentage of the salts’concentration at the surface of the membrane, the permeate qualitydecreases as a result of concentration polarisation. This phenomenonmay actually indicate the presence of foulants before a reduction inpermeate rate is detected. The increased salts’ concentration at themembrane surface also promotes precipitation of the salts whosesolubility limit is exceeded as a result of this phenomenon.
The advantage of operating systems at high recoveries is that thevolume of concentrate is small and the flow rate of the feed pump issmaller. The potential disadvantages are numerous:
• Higher concentration of contaminants can result in precipitation and greater propensity for fouling.
• In nanofiltration and reverse osmosis applications, the concentratedsalts will result in higher osmotic pressure, requiring a higherpressure pump and a more pressure resistant system.
• As higher recoveries reduce the quantity of concentrate to bedischarged, the higher concentration of this concentrate stream canitself present discharge problems.
In an ideal RO/NF system, all of the ionic contaminants to be removedare separated by the membrane and exit in the concentrate stream. Butin reality, no membrane is perfect in that it rejects 100% of thesolute on the feed side; this solute leakage is known as “passage”.Expressed as “percent passage”, the actual quantity of solute whichpasses through the membrane is a function of the concentration ofsolute on the feed side.
With reverse osmosis and nanofiltration, the concentration of salts onthe feed side is increased, and therefore, the actual quantity of saltspassing through the membrane also increases.
Many applications demand that, in addition to a minimum concentratevolume, the permeate quality be high enough for reuse. The “catch 22”predicament of permeate quality decreasing as recovery is increased canimpose design limitations. Additionally, the increased osmotic pressureresulting as recovery is increased also imposes a design limit.Generally, pumping pressures in excess of 1000psi (68 bar) areimpractical for most applications.
The design of an effective, economical residential drinking watersystem requires understanding of water chemistry, comprehension of theappropriate treatment technologies and the ability to assemble theminto the optimum total treatment configuration. Hopefully, this articlehas provided some insights into these.
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