Membrane filtration is a collective name for various separation processes that differ strongly in character. Membrane filtration can be applied to various qualities of feed water: groundwater, surface water, process water or even effluent water.
The membrane in the water purification plant acts as a filter that selectively permits the passage of water under a certain operating pressure while denying the passage of certain substances carried in the water. The membrane consists of polymers of specific separation properties.
The membrane divides the feedwater into two separate flows: a permeate flow (also referred to as the product) and a concentrate (also referred to as retentate). The ratio between these flows is called the recovery. Recovery is an indication of the yield of the separation process.
Membrane filtration can be separated into micro- and ultrafiltration on the one hand, and nanofiltration and reverse osmosis (also referred to as hyperfiltration) on the other.
This distinction is based on the type of polymer and two factors that determine the performance of the membrane: the selectivity and the productivity. The selectivity is expressed in retention or separation factor, and the productivity is expressed in flux or permeability.
When membrane filtration is applied to the removal of particles, suspended matter and micro organisms, use is made of micro- or ultrafiltration membranes. The open nature of these membranes enables a high productivity to be maintained at low operating pressures. The recovery ratio of this process is 80% or higher.
When salts or pesticides need to be removed from the water, nanofiltration of reverse osmosis is applied. The pressures required for nanofiltration and reverse osmosis can be 10 times or more higher than those for micro- and ultrafiltration, while at the same time the productivity is lower. The maximum recovery is 85%, however, this is seldom achieved.
Both membrane processes can be connected in series in order to strip the feedwater of both suspended matter and salts. This process generates water of high quality, suitable for every type of industrial use. The choice for the type of membrane process will vary per project, and is determined by a large number of aspects, for example the (varying) quality of feedwater, the desired quality of the product water, the use of filtration catalysts (chemicals), the degree of membrane fouling, the use of membrane cleaning agents, etc. Taken together, selecting the right membrane process and proper operation of the plant is work for highly trained specialists.