Ultrafiltration is used for the separation of suspended solids, colloids, bacteria and virus. This technique uses membranes with pore size between 1-100nm.
|TECHNIQUE ||MWCO* (Da) |
|Microflitration ||>105 |
|Ultrafiltration ||103-105 |
|Nanofiltration ||102-104 |
|Reverse osmosis ||102 |
* MWCO = molecular weight cut-off
There are 4 membranes geometries:
Spiral wound module: this design tries to maximize surface area in a minimum amount of space. It is the less expensive but more sensitive to pollution due to its manufacturing process. It consists of consecutive layers of large membrane and support material in an envelope type design rolled up around a perforated steel tube.
Plate and frame module: it is normally used for bad quality water. They are set up with a stack of membranes and support plates.
Tubular membrane: Generally used for viscous or bad quality fluids. These modules do not need a preliminary pre-treatment of the water. As the feed solution flows through the membrane core, the permeate passes through the membrane and is collected in the tubular housing.
The main drawback is that the system is not very compact and has a high cost per m2 installed and it is not very compact. Diameter’s tube is generally between 4 and 25mm.
Hollow fiber membrane: The modules contain several small (0.6 to 2 mm diameter) tubes or fibers. As the feed solution flows through the open cores of the fibers, the permeate is collected in the cartridge area surrounding the fibers. It can carry out the filtration in two ways, either “inside-out” or “outside-in”
Comparison between the different types of geometry (Source: Water Treatment Handbook, Degrémont Suez, 7th edition):
| ||Tubular ||Hollow fiber ||Plate and frame ||Spiral wound |
|Reverse Osmosis, Nanofiltration ||yes ||yes ||yes ||>95% |
|Ultrafiltration, Microfiltration ||yes ||>95% ||yes ||yes |
|Cleaning ease ||+ ||+++ ||- ||+ |
|Pre-treatment need ||+++ ||+ ||+ ||- |
|Recovery rate ||+ ||+++ ||+ ||++ |
|Module size ||- ||++ ||+ ||++ |
|Cost per m2 ||- ||+++ ||+ ||+++ |
+++: obvious advantage
There are two ultrafiltration module configurations: