FAQ's > Reverse Osmosis.

What is inorganic scale in relation to reverse osmosis membranes?

An increased amount in the concentration of dissolved inorganic salts at the membranes surface usually results in inorganic scale formation. Precipitation is liable to occur if the ionic product of a dissolved salt exceeds its solubility. This could then go on to fouling of the membrane surface. An average finding shows that precipitated salts are calcium carbonate, barium sulphate and calcium sulphate. Traditionally, to control deposition in brackish water and seawater operations, mineral acids and polyphosphate acids were used. This method proved that neither acid’s were entirely satisfactory in all possible situations.

Serious supply problems and safety hazards to plant operators are able to occur when mineral acids are used. Another problem that the use of acid presents is that acid needs to be accurately dosed to minimise corrosion and other large problems such as scale formation (sulphuric acid = sulphuric scale formation). SHMP is not thought to be the most affective route to fighting sulphate deposition. As it is a phosphate based addictive, it can become a source of nutrients for bacteria. This can lead to the membrane becoming fouled. As this product is actually distributed as a solid, it can be difficult to dissolve to the correct dosage.

What is colloidal fouling in relation to reverse osmosis membranes?

Iron, either soluble form or insoluble form is the main cause of colloidal fouling from within the feed water. The nature of the iron present depends on numerous contributing factors, the main one of which is the PH balance of the water. Although pre-filters can remove iron oxides, this can be proven ineffective, as the particles can be very small. If the PH of the water rises too much, soluble iron can form insoluble iron oxides. In reverse osmosis systems, the PH rises from the feed to the reject so that soluble iron can precipitate throughout the system.
How is a reverse osmosis membrane manufactured?
The manufacturing process of a reverse osmosis membrane mainly focuses on the production of a porous material. The pricing structure is dependant on two different factors. One factor is he raw material itself. Another factor is the ease in which the size or size distribution of the pores can be introduced. Depending on the material of the porous membrane, this process can be more difficult to introduce pores into the membrane. For example, Inorganic membranes are formed by compressing and sintering of fine powders onto a pre-prepared porous support. This type of membrane formation is usually a very expensive method, especially if the membrane thickness needs to be consistent or if the pores need to be fine. Micro filtration or ultra filtration membranes made from Zirconium and/or Titanium may be extremely costly, often exceeding £1000 per m2. As an alterative there is homogenous polymeric membranes. These membranes are considerably cheaper costing less than £10 per m2 to produce. Alongside this huge advantage, there are some disadvantages. They can be limited in their permeability, is porosity and mechanical strength. These problems may cause the membrane to be unsuitable as the membrane may not be strong enough to deal with certain procedures or the permeability of the product may be a problem as it is too restricted.

What is the structure of membrane materials?

Chemically homogeneous, physically isotropic
Physically isotropic, chemically heterogeneous
Physically anisotropic, chemically heterogeneous
Physically and chemically heteroge neo us/an isotropic
What materials form membranes?
Ceramic polymeric ion-exchange symmetric micro porous
Supported liquid
Integral
Asymmetric
Micro porous
Asymmetric composite
What example materials are used?
Alumina, silicate, graphite, metals extruded silicone rubber
PTFE, polyethylene, polypropylene, polycarbonate
Functionalised polymeric materials cellulose derivatives, polyamide, polysulphone
Hydrophobic liquid in silicone rubber
Cellulose derivatives, polyamide, polysulphone
Ultra thin layer on micro porous polysulphone support

Polymeric micro porous membranes are usually manufactured using a technique called phase inversion. A process called gelation is used. This is where a solution of the polymer is put into water to produce the micro porous membranes. This technique then goes on to produce a skin layer; this then creates an integral anisotropic micro porous membrane. This is the type of membrane that is often used in the technique of reverse osmosis.

Supported liquid membranes have proven to be affective for procedures such as gas separation. Gas separation is where the improved mass of transport of gasses through liquids over that attainable in solids becomes important. Although this method has proven to be affective, it has not yet been manufactured on a large scale and is not the most common method for commercial use. The technology is being modified and improved by trying to accomplish liquid separations. Liquid separations are achieved by a combination of a high-velocity, hydrophobic immobilised in a polymer matrix, and the supported liquid is liable to contain a carrier. This is a component that has a chemical reaction and a reversible reaction with the desired component in the liquid mixture and therefore assists its transportation process through the membrane.

Another membrane that can be produced is an ion exchange membrane. This is by fictionalisation of a homogeneous polymer film or more simply by immobilising powered ion-exchange resins in an inert resin matrix. This last method is often favoured by manufacturers from china this is because the materials that are produced are less selective and has inferior mass support properties compared with the more expensive homogenous materials.What is the future for membrane technology?
The application of membrane technology is undoubtedly expanding and progressing at an incredible rate. The reasoning behind this is because the product is becoming increasingly cheaper and cheaper. Also legislation towards the environment are becoming tighter and tighter. Because of these environmental stipulations, water filtration systems in general are now in a high demand. Because of this membrane technology is so economic, it is usually the favoured approach.
The range of available membrane materials is huge. There are a large amount of membranes that are of chemical composition or of physical structure, but the most important property is the mechanism by which separation is actually achieved. On this basis, membranes may be referred to as either porous or dense. Porous membranes let more particles through, also particle that are of a lager size. Dense membranes on the other hand are less permeable and let fewer particles through and only particles of a smaller size. This process can be altered depending on how pure the water needs to be.

What is the membrane separation process?

Reverse Osmosis (RO) or Hyoer filtration Ultra filtration (UF) – separated by good quality of the separation of both large, dissolved solubility and diffusion rates of water molecules and suspended colloidal particles and dissolved species water.

Electro dialysis (ED) Micro filtration (MF) – separation is achieved by the process of differing ionic separation of suspended solids from water size, charge and charge density of solute ions, using ion exchange membranes.
Gas Transfer (GT) – in the process of gas transfer, gas is from molecular form, transferred under a concentration gradient into water.

What is a Reverse Osmosis membrane?

There are various different definitions that can be given to the word membrane, one definition is- “ An intervening phase separating two phases and/or acting as an active or passive barrier to the transport of matter between phases” (European `society of Membrane Science and Technology)
Another definition is “ an inter phase separating two homogenous phases and affecting the transport of different chemical components in a very specific way” (Prof Solt, School of Water Sciences, Canfield)
The final definition of a membrane is – a material that only allows some substances to pass through easily than others, this would be substance of a smaller size than the permeability of the membrane. This in turn is the basis of the separation process.

Membrane structure is an important aspect of the membrane; the main objective of the manufacturing of membranes is to create a product that can withstand a good proportion of mechanical strength. It is also important that the membrane can maintain a high throughput of a desired permeate with a high level of selectivity. These last two parameters are mutually counteractive. This is because a high level of selectivity can usually only be obtained by using a membrane with small pores and thus a high level of hydraulic resistance, or low permeability. The permeability of a material increases by increasing the level of pores within the material. This implies that a high level of material porosity is desirable. The overall membrane resistance is directly proportional to its thickness. Finally, selectivity will be compromised by a broad pore size distribution. Its sands to reason that the optimum physical structure for any membrane material is based upon, a thin layer of material and a narrow range of pore size and high porosity.

What is concentration polarisation in relation to Reverse Osmosis?

Concentration polarisation (CP) is the term that describes the tendency of the solute to accumulate at membrane solution interference within a concentration boundary layer or stringent liquid film. This layer contains near stringent liquid, since at the membrane surface itself the liquid velocity must be nil. This implies that the only mode of transport within this layer is diffusion, which is around two orders of magnitude slower than convective transport in the bulk liquid region. Rejected materials thus build up in the region adjacent to membrane, increasing their concentration over the bulk value. This build up occurs exponentially with increasing flux. This thickness of the boundary layer, on the other hand, is determined entirely by the system hydrodynamics, decreasing in thickness when turbulence is promoted.

For pressure driven processes, the greater the flux, the greater the build up of solute at interface. The greater the solute build up, the higher the concentration gradient. The steeper the concentration gradient, the faster the diffusion. These mass transfers are all in dynamic equilibrium with one another. CP increases the propensity for sparingly soluble solutes to precipitate out into the membrane, forming a gel layer, as well as generally increasing the concentration of colloidal or suspended material at the membrane surface. Furthermore, CP increases the permeation of the rejected materials through the membrane because of the increase in the trans-membrane concentration gradient generated. For RO, CP raises the effective osmotic pressure at the membrane surface interface, increasing the required trans-membrane pressure for operation. This is thus always desirable to suppress CP by promoting turbulence.

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