Reverse Osmosis

Reverse Osmosis membrane technology is used to produce water from all water sources. Reverse osmosis uses a thin layer of material capable of separating substances from water. Membranes are currently used to treat surface water, fresh and brackish ground water and seawater to achieve various water quality goals. Besides the removal of dissolved ions, reverse osmosis membrane processes are now employed for removal of bacteria, viruses and other microorganisms such as Cryptosporidium and Giardia, and natural organic material, which can result in tastes, odors and color to the water and can react with disinfectants to form disinfection by-products. Reverse osmosis treatment requires high feed water quality in terms of particulate matter, therefore, groundwater will not generally require advanced pre-treatment, whereas surface water may require pretreatment. As technology improvements are made in reverse osmosis systems, such as membrane element and pressure vessels, capital and operating costs of the membrane systems are decreasing.

Nanofiltration

Nanofiltration, also called membrane softening, is now generally recognized as a process distinct from brackish or seawater RO. The term nanofiltration was coined in 1984 to connote greater removal characteristics than microfiltration but much less than those of brackish or seawater RO processes. The distinction is largely one of application, since nanofiltration is generally used to treat fresh water supplies, less than 1,500 TDS.

The softening application utilizes the membranes’ selective removal of divalent calcium and magnesium ions that cause hardness. It should be remembered that all RO membranes exhibit this selectivity, not just nanofiltration membranes. The nanofiltration membrane is mainly used for softening, but is extremely effective in the removal of organics (which cause trihalomethanes when chlorinated), viruses, bacteria and cysts like giardia and cryptosporidium. Other benifets of nanofiltration are effective removal of color, disinfection by-product precursors and micro-organisms. Nanofiltration systems generally operate utilizing concentrate staging, designed in two or more arrays.

Ultrafiltration

The ultrafiltration process is a membrane separation technique that separates components in a liquid stream based upon their molecular weight. Depending on the membrane type, the membrane will reject components with molecular weights from 500 to 500,000 daltons. Large salt crystals, bacteria, suspended solids, organics, and bacteria are very effectively removed using ultra-filtration.

The range of pore size of ultrafiltration membranes is between 2 x 10-3 (0.001) and 1 x 10-1 (0.1) microns. Ultrafiltration falls between reverse osmosis and microfiltration membranes, diatomaceous earth, and depth filters. The ultrafiltration process not only removes components of certain molecular sizes but also all particles larger than the rating of the membrane. This makes ultrafiltration an effective one-step method for both coarse particle and molecular filtration. The ultrafiltration membrane, by virtue of its semi-permeable nature, is capable of separating proteins, colloids, fats, polysaccharides, and suspended solids from smaller molecules such as sugars, salts, and amino acids in the feed stream.

Microfiltration

A microfiltration membrane is characterized by a pore size of 0.1 to 1 micron. Microfilters are symmetric or homogenous materials. Unlike seawater, brackish, and nanofiltration membranes, microfilters have distinct pores. Microfilters are widely used as prefiltration to surface water reverse osmosis plants whether the surface water is a lake, river or seawater. Microfilters are capable of removing suspended solids (turbidity), bacteria, cysts and giardia.

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