Water Treatment by Microfiltration and Ultrafiltration

UNESCO-IHE Institute for Water Education, 2601 DA Delft, The Netherlands 

Furthermore, water scarcity has led to the widespread use of desalination to supplement freshwater resources. While reverse osmosis (RO) membranes are used to desalinate seawater and brackish water, MF/UF can be applied as a pretreatment to RO, in an integrated membrane system (IMS). The level of pretreatment, in terms of turbidity and silt density index (SDI), provided by MF/UF was found to be significantly better and more consistent than conventional pretreatment to RO (e.g., coagulation/sedimentation/filtration) for RO feed water. In recent years, the demand for MF/UF as a pretreatment to RO has grown as the need to augment our 

Advanced Membrane Technology and Applications. W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

Microfiltration and ultrafiltration membranes can be made from organic polymers or inorganic materials such as ceramic, glass, or metal or organic polymers. Materials used in MF and UF membrane fabrication are shown in Table 6.1. A number of different techniques are employed to prepare synthetic MF/UF membranes the most important are phase inversion, coating, sintering, and track etching. 

1 Polymeric Membranes Synthetic polymeric membranes can divided into two classes, that is, hydrophobic and hydrophilic. Hydrophihc polymers such as cellulose and its daivatives have been used widely for the manufacture of MF and UF membranes. However, 

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Kennedy MD, Kamanyi J, Salinas Rodriguez SG, Lee NH, Schippers JC and Amy G (2008), Water treatment by microfiltration and ultrafiltration in Advanced Membrane Technology and Applications, New York, John Wiley Sons, 131-170. 

Microfiltration and ultrafiltration (MF/UF) membranes are flexible water treatment tools that can be used in a number of process configurations to meet advanced effluent treatment objectives. MF/UF membranes, when used by themselves, are limited to the removal of particulate and colloidal contaminants however, they can be combined with biological or chemical treatment to remove dissolved contaminants. Furthermore, they represent the ideal pretreatment to reverse osmosis by addressing their main weakness, fouling by particulate materials. 

Microfiltration and Ultrafiltration are the best available technology for water reuse. Two options are available conventional activated sludge followed by tertiary filtration and an integrated membrane bioreactor. Both provide effluent of high quality suitable for treatment by reverse osmosis. The cost of tertiary filtration can be lower than a membrane bioieactor if the water reclamation plant is designed for constant flow and is located at a different site. 

By contrast porous ceramic membranes had found application since the 1960s in the field of large-scale gas diffusion processes for uranium isotope separation. It was only in the 1980s that porous ceramic membranes found other non-nuclear industrial applications, mainly oriented towards microfiltration and ultrafiltration water treatment processes. 

Filtration (water treatment) Refers to the physical separation of particles, colloids, or other contaminants from water by passing the liquid through permeable or semipermeable materials (compare with microfiltration, nanofiltration, reverse osmosis, and ultrafiltration). 

Micro-, ultra-, and nano-filtration can separate smaller particles using media with defined porous sizes (i.e., 10 1—1 pm in microfiltration, 10 2—10 pm in ultrafiltration, and 10 3— 10 2 pm in nanofiltration). Residual colloidal and suspended solids can be removed by microfiltration. Selected salts, most organic compounds, bacteria, protozoan cysts, oocysts and viruses are removed by nanofiltration, so that the treated water will be disinfected. This advanced filtration is used for the treatment of effluents for indirect potable reuse applications such as groundwater injection, water softening, decoloriza-tion, or removal of micropollution. 

Membrane filters are made in a wide variety of pore sizes (Fig. 1). The effective pore size for membranes vary, and membranes can be used in reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). RO membranes are widely used in water treatment to remove ionic contaminations from the water. These membranes have an extreme small pore size and, therefore, require excellent pretreatment steps to reduce any fouling or scaling of the membrane, which would reduce the service lifetime. RO membranes are used by extensive pressures on the upstream side of the filter membrane to force the liquids through the pores. 

Hemodialysis/hemofiltration alone had sales of over US 2200 million in 1998. Reverse osmosis (RO), ultrafiltration (UF) and microfiltration (MF) together accounted for 1.8 billion dollars in sales in 1998. At that time about US 400 million worth of membranes and modules were sold each year worldwide for use in reverse osmosis. About 50% of the RO market was controlled by Dow/FihnTec and Hydranautics/Nitto. They were followed by DuPont and Osmonics. Membranes are apphed during sea-water desahnation, municipal/ brackish water treatment and in the industrial sectors. The market for RO and nanofiltration is growing at a rate higher than 10%/year. The market for desali-... 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

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