Cleaning membrane

Likewise, to the inevitable phenomena of membrane fouling, all membrane based filtration processes require periodic cleaning. Without a safe practical, reproducible, cost effective and efficient cleaning procedure, the viability of cross-flow filtration may be highly questionable. Membrane cleaning process must be capable of removing both external and internal 

A careful choice of cleaning solutions and procedures will extend the service life of the membrane. In many polymer membrane filtration systems. 

Product losses during cleaning may be important especially when high recoveries ( 95%) are required and the desired product is located in the retentate phase. Additional product loss will occur in the fouled membrane elements. These combined losses may range from 0.5% to 3% which is significant when recovering high value-added product. 

Generally speaking, all microfiltration and uluafiltraiion applications require some form of periodic, disruptive cleaning to remove the foulants and thereby restore the membrane flux close to its initial level. The cleaning solutions and the procedures used industrially vary with the type of fouling encountered. 

The choice of the acid type and strength can be crucial to maintain the structural and chemical integrity of the membranes. For example, while silver membranes can withstand strong acids such as 10% hydrofluoric acid, they are not resistant toward nitric or sulfuric acids. On the contrary, hydrofluoric acid is not recommended for alpha-alumina membranes. 

A membrane system consists of many membrane modules which, in turn, are made of several membrane elements. Both ends of a membrane element are sealed with such materials as enamels or ceramic materials. The connections between elements and between elements and the housing or pipings are typically made from plastics or elastomers for liquid phase applications. 

A simplified material selection guide has been provided to determine the general chemical compatibility between the membrane (and its accessory components) and the process su eam. The guide is primarily for relatively low temperature applications. 

Finally, mechanical properties of a porous membrane can be significantly reduced by the presence of pores in the matrix. Both pore size and porosity have large impacts on the mechanical properties. Available mechanical properties of porous inorganic membranes are scarce and the strength measurement techniques arc not specified. More systematically acquired mechanical properties of porous inorganic membranes are needed. 

Finally, fouling can be removed to a certain extent by chemical cleaning. Changes in solution chemistry can minimise fouling far from isoelectric point, low 1), as can low pressure operation, high crossflow velocity, selection of modules, and hydrophilic membranes (Fane (1997)). 

An oventiew of the application and volume treated by membrane processes is given in Table 3.2. The increase in recent years is obvious. At this stage, there is still more RO than MF and UF being used in potable water treatment (Wiesner et al (1992)). 

Process Pore Size [nm] Operating Pressure [MPa] Application Municipal Drinking Water Production (1994)1 Kd-i] Municipal Drinking Water Production (1998)2. [mM-i] 

NF micropores 2nm 0.1 - 1.5 surface ground waters of high hardness and organics 500 000 1 000 000 

RO non-porous ( ) 5-8 (2x osmotic pressure) desalination of sea- and brackish water 3 000 000 10 000 000 

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Reverse Osmosis Membrane Cleaning. Citric acid solutions are used to remove iron, calcium, and other cations that foul ceUulose acetate and other membranes in reverse osmosis and electro dialysis systems. Citric acid solutions can solubilize and remove these cations without damaging the membranes (94—96). 

The type of membrane cleaning required depends on both the type and degree of fouling experienced, but typically it is either organic (bacterial slimes, natural organics, or process foulants and nutrients) or inorganic (silica, carbonate, sulfate, or phosphate deposits). 

Continued decline in performance indicates a membrane cleaning or compatibihty issue. The adequacy of the cleaning step is determined by the recovery of at least 80 percent of the initial normalized water flux. Although some variability in water flux is typical, any consistent dechne reflects an inadequate cleaning procedure. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Reverse osmosis membrane process, 27 637 Reverse osmosis membrane cleaning citric acid application, 6 647 Reverse-osmosis membranes, 75 811, 825 development of, 75 797 Reverse osmosis models, 27 638-639 Reverse osmosis permeators, 76 19 Reverse osmosis seawater desalination process, 26 85 Reverse osmosis systems blending in, 26 80-81 brackish and nanofiltration, 26 80-83 Reverse osmosis technology... 

Keywords Additional discharge streams Antisealants Chemical treatment Coagulation/flocculation Concentration factor Dechlorination Filtration High recovery Membrane cleaning Pretreatment Recovery rate... 

The majority of the discharge from a desalination processes is concentrated brine from the membrane process, and this may contain quantities of treatment chemicals used. Treatment of water is necessary in all desalination plans for variety of reasons feed water treatment, membrane protection, membrane cleaning, permeate treatment and concentrate treatment prior to discharge. Although non-chemical treatment is possible, chemical treatment is widely practiced. 

The hydrodynamic shear forces at the membrane surface tend to reduce the boundary layer and keep the membrane clean. 

Liberman, B. (2004) Methods of direct osmosis membrane cleaning online for... 

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