The use of a membrane, or more properly, a semipermeable membrane, to separate substances when a driving force is applied across the membrane. Once considered a viable technology only for desalination, membrane processes are increasingly employed for the removal of bacteria and other microorganisms, particulate material, and natural organic material which can impart color, tastes, and odors to water and react with disinfectants to form Disinfection Byproducts (DBP). Due to their greater effectiveness, waste stream disposal, at up to 15-25 percent (and higher for the RO process) of the total treated water volume, is a significant problem with membrane treatment systems. Pressure-driven membrane filtration processes include:
- Microfiltration (MF) – Loosely defined as a membrane separation process using membranes with a pore size of approximately 0.03 to 10 micros, a molecular weights cutoff (MWCO) of greater than 100,000 daltons, and a relatively low feedwater operating pressure of approximately 100 to 400 kPa (15-60 psi). Representative materials removed by MF include sand, silt, clays, Giardia lamblia and Cryptosporidium cysts, algae, and some bacterial species. MF is not an absolute barrier to viruses; however, when used in combination with disinfection, MF appears to control these microorganisms in water.
- Ultrafiltration (UF) – Involves the pressure-driven separation of materials from water using a membrane pore size of approximately 10,000 to 100,000 daltons, and an operating pressure of approximately 200 to 700 kPa (30-100 psi). UF will remove all microbiological species removed by MF (partial removal of bacteria), as well as some viruses (but not an absolute barrier to viruses) and humic materials.
- Nanofiltration (NF) – Membranes which 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 600 kPa (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. 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 further treatment may be required. NF also removes hardness from water, which accounts for NF membranes sometimes being called “softening membranes.”
- Reverse Osmosis (RO) – This process 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 the membrane, leaving contaminants behind in a brine solution. 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.