The Role of Coagulants in Reducing Colloidal Fouling
Coagulant Technical Guide
Autopsies show that colloidal and particulate fouling is the most frequent cause of RO system performance loss and reduced element life. Colloidal and particulate foulants include:
• Colloidal silica
• Metal silicates
• Alum floc (aluminum hydroxide)
• Iron floc (ferric hydroxide)
Chemists differentiate between colloids and particulates based upon their size. The consensus is that colloids have diameters less than one micron, while particulates are larger than one micron. Macromolecules are mainly organic colloids and are present in virtually all-natural water sources. These include humic and fulvic acids, polysaccharides, and proteins. Microbial activity is the primary source of organic macromolecules.
Colloids and particulates are drawn to membrane surfaces by permeate convection, the rate of deposition being proportional to permeate flux. To minimize colloidal and particulate fouling rates, reverse osmosis membrane manufacturers developed empirical flux limits for use in system design.
Figure 1 illustrates the effect of colloidal fouling on RO system flow. The blue line represents a system with effective media filtration. The gray line is for a system having ineffective filtration. Sharp increases in flow occur after each cleaning.
COAGULATIONS AND FILTRATION
Multimedia Filtration (MMF) is the most economical means of removing colloids from RO feed water. However, to be effective, in-line coagulation is required upstream of the filters. Without coagulation, media filters typically remove only 30 to 50 percent of feed water colloids. With coagulation, colloid removal rates of 95 to 99 percent are typical. Coagulants are either metal salts or organic polymers:
The most common metal coagulants are aluminum sulfate (alum) and ferric chloride or sulfate. Added to water, metal salts go through a series of hydrolysis reactions that result in the formation of positively charged hydrates that operators of ultrafilter and microfilter systems are increasingly using iron containing coagulants to reduce colloidal and particulate fouling, especially in systems used to treat municipal and industrial wastewaters.
Organic polymers have molecular weights ranging from about 70,000 to 500,000 and most contain quaternary amine groups giving them positive electrostatic charges.