Figure 10.4.GIF

Microfiltration (MF)

Microfiltration membranes have pores ranging from 0.1 to 2.0 microns. This process is less common with waste treatment processes because permeate from a microfilter is generally unacceptable for discharge. In some cases, this membrane process may be used in conjunction with settling agents, polymers, activated carbon, and other chemicals that assist in the retention of waste constituents. When using this membrane, care must be taken to prevent membrane pore blockage by waste streams components through the selection of the proper membrane type for the specific plant wastes.

Ultrafiltration (UF)

The process of ultrafiltration is the most common membrane-based wastewater treatment process. It uses a membrane with pore sizes ranging from 0.005 to 0.1 micron. Particles larger than the pores in the membrane, such as emulsified oils, metal hydroxides, proteins, starches, and suspended solids, are retained on the feed side of the membrane. Molecules smaller than the pores in the membrane, such as water, alcohols, salts, and sugars pass through the membrane. This filtrate, which is treated water, is often referred to as permeate.

Ultrafiltration membranes are rated on the basis of molecular weight cut-off (MWCO) and range from 1,000 to 500,000 MWCO (daltons). The use of MWCO is only an approximate indication of membrane retention capabilities and should be used with guidance of the membrane manufacturer.

Nanofiltration (NF)

Nanofiltration uses a membrane pore size between UF and RO. These membranes are effective in removing salts from a waste stream by allowing them to pass into the permeate while concentrating other components such as sugars, nitrogen components, and other waste constituents causing high BOD/COD in waste streams.

Reverse Osmosis (RO)

Reverse osmosis is the tightest membrane process in that it allows only water to pass through the membrane, retaining salts and higher molecular weight components. RO membranes are used for tertiary treatment producing water with low BOD/COD and of near-potable water quality. RO permeate may be recycled throughout the plant and reused for various plant processes. RO is normally used as a post-treatment process following coarser filtration processes such as ultrafiltration.