Membrane Filtration

Membranes are thin and porous sheets of material able to separate contaminants from water when a driving force is applied. Once considered a viable technology only for desalination, membrane processes are increasingly employed in both drinking water and wastewater treatment for removal of bacteria and other microorganisms, particulate material, micropollutants, and natural organic material, which can impart colour, tastes, and odours to the water and react with disinfectants to form disinfection by-products (DBP). As advancements are made in membrane production and module design, capital and operating costs continue to decline.

Adapted from SAGLE & FREEMAN (2004)

Water treatment processes employ several types of membranes. They include microfiltration (M-F), ultrafiltration (U-F), reverse osmosis (R-O), and nanofiltration (N-F) membranes. Microfiltration membranes have the largest pore size and typically reject large particles and various microorganisms. Ultrafiltration membranes have smaller pores than microfiltration membranes and therefore, in addition to large particles and microorganisms, they can reject bacteria and soluble macromolecules such as proteins. Reverse osmosis membranes are effectively non-porous and, therefore, exclude particles and even many low molar mass species such as salt ions, organics, etc. Nanofiltration membranes are relatively new and are sometimes called “loose” reverse osmosis membranes. They are porous membranes, but since the pores are on the order of ten angstroms or less, they exhibit performance between that of reverse osmosis and ultrafiltration membranes.

Filtration spectrum of reverse osmosis, nanofiltration, ultrafiltration, microfiltration and particulate filtration relative to the pore size of the common material. Source: RADCLIFF (2004)

How membrane filter works with water

Membrane processes have become more attractive for potable water production and advanced wastewater treatment in recent years due to the increased stringency of regulations, water scarcity, and decreasing costs of membranes. Membrane processes have excellent separation capabilities and show promise for meeting many of the existing and anticipated drinking water standards (TECHBRIEF 1999).

Although there are a number of different methods of filtration that incorporate membrane technology, the most mature is pressure driven membrane filtration. This relies on a liquid being forced through a filter membrane with a high surface area. There are four basic pressure driven membrane filtration processes for liquid separations. These are, in ascending order of size of particle that can be separated: reverse osmosis, nanofiltration, ultrafiltration and microfiltration.

The use of reverse osmosis is well-established for desalination of drinking water and the production of deionized water for process use. Ultrafiltration and microfiltration, which-unlike reverse osmosis and nanofiltration-utilize porous membranes, are now becoming increasingly common in water and wastewater treatments.

Membrane processes are very versatile: for example ultrafiltration and microfiltration are efficient for turbidity suspended particles and microbial removal, nanofiltration and ultrafiltration membranes for organics (e.g. DBP precursors, micropollutants) and viruses separation, and reverse osmosis process for desalination and removal of small size organic contaminants.