There are several different techniques of membrane filtration, and those are categorized according to pore size in the membrane. The size of the particles retained by the membranes, thus decreasing with decreasing pore size. All the four membrane techniques listed in the following table and Figure (www.kochmembrane.com) have the pressure difference across the membrane as the driving force for the process. The smaller pore sizes, the higher the needed pressure.
| Membrane type |
Size of pores, μm |
Pressure (bar) |
Flux (l m-2 h-1 bar-1) |
| Micro Filtration (MF) |
0.03 - 10 |
0.1 - 2.0 |
>50 |
| Ultra filtration (UF) |
0.002 - 0.1 |
1 - 5 |
10 - 50 |
| Nano filtration (NF) |
0.001 - 0.01 |
5 - 20 |
1.4 - 12 |
| Reverse smosis (RO) |
0.0001 - 0.001 |
10 -100 |
0.05 - 1.4 |
Membrane processes such as micro filtration (MF) has long been used to provide clean drinking water in areas with poor water resources, but these are relatively new for slurry separation.
Ultra filtration (UF) is made on the liquid separation fraction. It is a type of pre-treatment for the reverse osmosis treatment (RO), both technologies being part of a high tech manure treatment facility where the liquid part is purified up to (or near to) clean water. The ultra filtration process will remove suspended solids as well as bacteria and virus, while small dissolved molecules passes the filtration and can be removed by reverse osmosis, where the pore size of the membranes is smaller.
Schematics of the reverse osmosis process (left) and illustration of a cleaning unit wit reverse osmosis (www.kruger.dk).
Microfiltration and ultra filtration is fundamentally different from reverse osmosis and nano filtration becausethose systems use pressure as a means of forcing water to go from low pressure to high pressure (to control the reverse diffusion through the membrane). MF and UF can use a pressurized system but it does not need to include pressure. The compounds retained in the MF and UF are mainly molecules and colloids, which form a "cake" at the membrane surface.
NF and RO detain mostly ions and the osmotic pressure is the governing parameter for the diffusion of water across the membrane. During NF and RO, it is important to avoid precipitation of solids in the membrane (scaling), as this will cause a pressure drop across the membrane. Therefore, the substance concentration and solubility are limiting factors for membrane plant utilization.
In many membrane plants a flow longitudinally along the membrane (crossflow) is often maintained, to reduce the concentration of substances by the membrane surface. This leads to a reduction of the concentration of retained material, and thereby risk of scaling / fouling decreases.
If the slurry is pressurized on one side of such a membrane, its water content is pushed through the membrane. It is thus possible by the use of membrane filtration to remove ammonia and potassium from the slurry liquid phase. There are many materials that can be used to manufacture membranes, for example cellulose acetate, polyamide and polysulfone. These materials are distinguished by specific characteristics in relation to porosity, pore size and resistance to various substances and environments.
The geometric shape of the membrane is important for internal hydraulic conditions. Moreover, the geometric shape of the membrane affects the physical plant design, and affects how easy the membrane is to be cleaned or recovered.