▶ NF membranes have a nominal pore size of approximately 0.001 microns and an MWCO of 1,000 to 100,000 daltons. Pushing water
through these smaller membrane pores requires a higher operating pressure than either MF or UF. Operating pressures are usually
near 600kPa (90 psi) and can be as high as 1,000 kPa(150 psi). These systems can remove virtually all cysts, bacteria, viruses,
and humic materials. (See Figure 2 and Table 1.)
They provide excellent protection from DBP formation if the disinfectant residual is added after the membrane filtration step.
Because NF membranes also remove alkalinity, the product water can be corrosive, and measures, such as blending raw water
and product water or adding alkalinity, may be needed to reduce corrosivity. NF also removes hardness from water, which
accounts for NF membranes sometimes being called“softening membranes.” Hard water treated by NF will need pretreatment to
avoid precipitation of hardness ions on the membrane.
▶ More energy is required for NF than MF or UF, which has hindered its advancement as atreatment alternative.
(NF membrane Filtration)
• NF membranes have been observed to operate on the principle of diffusion rather than sieving as with MF and UF membranes.
▶ RO systems are compact, simple to operate, and require minimal labor, making them suitable for small systems.They are also
suitable for systems where there is a high degree of seasonal fluctuation in water demand.
▶ RO can effectively remove nearly all inorganic contaminants from water. RO can also effectively remove radium, natural organic
substances, pesticides, cysts, bacteria, and viruses. RO is particularly effective when used in series. Water passing through
multiple units can achieve near zero effluent contaminant concentrations.Disinfection is also recommended to ensure the safety
▶ Some of the advantages of RO are:
• High capital and operating costs,
• Managing the wastewater (brine solution) is apotential problem,
• High level of pretreatment is required in some cases,
• Membranes are prone to fouling, and
• Produces the most wastewater at between25?50 percent of the feed.
(RO Membrane & RO Desalination Plant)
• RO removes contaminants from water using a semipermeable membrane that permits only water, and not dissolved ions (such as
sodium and chloride), to pass through its pores. Contaminated water is subject to a high pressure that forces pure water through
leaving contaminants behind in a brine solution.Membranes are available with a variety of pore sizes and characteristics.
(Tech Brief, National Drinking Water Clearinghouse)