Membrane Fouling Control
For optimal performance, feedwater to the RO system should be treated to remove gross amounts of solids, and to prevent fouling of the RO system.
Various unit operations - collectively called pretreatment - upstream of the RO system, must be carried out on most waters. The types of unit operations used in the pretreatment system are determined by the chemical nature of the membrane as well as the system configuration.
Typically, pretreatment will consist of filtration to remove large particles, and one or more of the following operations:
Adjustment
of solubility parameters to prevent precipitation of sparingly soluble salts
(scaling) as a result of concentrating action of the RO process
Coagulation
of colloidal matter
Chemical
treatment to prevent biological growth
The need for pretreatment is determined from the feedwater composition, the conversion of the RO system and the solubility of the particular salt.
Membrane fouling is the main cause of permanent flux decline and loss of product quality in RO systems. The cause and prevention of fouling depends greatly on the feedwater being treated, and appropriate control procedures must be devised for each plant. Fouling control involves pretreatment of the feedwater to minimize fouling control as well as regular cleaning to handle nay fouling that still occurs.
If pretreatment is inadequate, fouling will occur rapidly, cleaning will be less effective and the need for cleaning will increase. In any case, frequent cleaning (more than once a month) should not be substitute for pretreatment if long life and efficient operation are desired.
In general, fouling can be divided into 4 principal categories: scale, silt, bacteria, and organic compounds. More than 1 category may occur in the same plant.
Scale
Scale is caused by precipitation of dissolved metal salts in the feedwater on the membrane surface. As water is removed from the feed into the permeate, the concentration of salts in the feed increases until its solubility limit is exceeded. The salt then precipitates out on the membrane surface as scale. A measure of the tendency of a particular feedwater to produce scale is the concentration factor, as given by:
where the Recovery Rate is the ratio of the product water flow rate to feedwater flow rate:
The relationship between the brine solution concentration factor and the water recovery rate is shown in the Figure.
Pretreatment for scale prevention is usually used for all membranes and RO configurations. With plants that operate below a concentration factor of 2 (i.e. 50% recovery rate), scaling is not normally a problem. For many brackish water RO systems, the recovery rate is normally 80 - 90%. The salts that most commonly form scale are: calcium carbonate, calcium sulfate, silica complexes, barium sulfate, strontium sulfate and calcium fluoride.
Calcium carbonate scaling is by far the most common, and it is easily controlled. Acid addition to prevent calcium carbonate scaling is used for both brackish water and seawater.
Brackish water may also require addition of anti-scalant such as sodium hexametaphosphate. Anti-scalant interfere with the precipitation of the salt and maintain the salt in solution even when the solubility limit is exceeded. Alternatively, ion exchange softening can be used to prevent scaling by precipitation of calcium sulfate, barium sulfate, strontium sulfate and calcium fluoride.
For some brackish RO systems, lime softening (with lime or soda ash) is used in the pretreatment to prevent scaling and permit operation of RO system at high conversion (85 - 90%).
Silica (SiO2) scaling may be a problem with some brackish waters because once formed, silica scale is difficult to remove by cleaning.. Typical pretreatment include conversion control, increasing temperature to increase solubility, and lime/soda ash softening.
All RO devices are very sensitive to fouling by ion corrosion products, so materials of construction must be carefully considered. The most effective way to minimize corrosion in an RO plant is to limit the use of metal. Non-metallic materials should be employed on all wetted parts wherever they are practical and economical. For low pressure piping, plastic materials such as polyvinyl chloride, polyethylene, fibre-reinforced polyester and epoxy fibreglass are frequently used. Stainless steels (304, 316) are commonly used for high-pressure pumps and pipings in brackish water RO systems.
Silt
Silt is formed by suspended particulates of all types that accumulate on the membrane surface. Typical sources include organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate matter.
An indicator of likelihood of silt fouling in feedwater is the silt density index (SDI) of the feedwater. The SDI is an empirical measurement of the time required to filter a fixed volume of water through a standard 0.45-mm pore size microfiltration membrane. Suspended material in the feedwater that plugs the filter increases the sample filtration time, producing a higher SDI.
The maximum tolerable SDI varies with membrane module design. Spiral wound modules generally require an SDI of less than 5, whereas follow fine-fibre modules are more susceptible to fouling and require an SDI of less than 3.
To avoid fouling by suspended solids, some form of feedwater filtration is required. Groundwater usually has very low SDI values, and cartridge filtration is often sufficient. However, surface water or seawater may have an SDI of up to 200, requiring flocculation, coagulation, and deep-bed multimedia filtration before RO treatment.
Biofouling
Biological fouling is the growth of bacteria on the membrane surface. If biological fouling is a problem, the feedwater is usually chlorinated. This is normally required for surface waters (brackish as well as seawater). Well waters are not chlorinated unless Cl2 is required to prevent biological degradation of the membrane. The susceptibility of membranes to biological fouling is a strong function of the membrane composition.
Cellulose acetate membranes are easily attacked by bacteria. Feedwater for such membranes needs to be sterilized. Polyamide membranes are also susceptible to bacterial attack, but thin-film composite membranes are generally quite resistant. Chlorination can be used for cellulose acetate membrane, but be followed by dechlorination for polyamide and other membranes, usually by addition of sodium metabisulfate. In ultrapure water production, sterility is often maintained by UV sterilizers.
Organic Fouling
Organic fouling is the attachment of materials such as oil or grease to the membrane surface. It is common in RO systems used for treating industrial effluent streams. Filtration or carbon adsorption can be used to remove organic materials from feedwater.
An example of a complete pretreatment flow scheme for seawater RO plant is shown in the Figure where the water is controlled for pH, scale, particulates and biological fouling.
The feedwater is first treated with chlorine to sterilize
the water and to bring it to a pH of 5 - 6. A polyelectrolyte is added to flocculate
suspended matter, and 2 multi-layer depth filters then removed the suspended
matters. The water is dechlorinated by dosing with sodium bisulfite, followed
by an activated carbon bed. The water is then filtered through a 1 - 5 mm cartridge
filter before being fed to the RO modules.
