This type of filtration includes filters that retain particles on the micron size range, with molecular weights higher than 100.000 Daltons.
It is used to remove suspended solids and bacteria from water. Although viruses are smaller than the membrane pore size, some viruses are retained as they stay attached to bacteria. It significantly reduce water turbidity.
Retention takes place due to the pass of water through a porous material or a membrane; and it could occur by two different configurations: dead end or cross-flow. Dead end filters operate the same way as a multimedia filter: solids are accumulated on the surface and between the filter media, while a continuous flow of water is obtained (until media is saturated). Cross-flow configuration (membrane type filters) produce two different streams: one portion flows through the membrane, and is called permeate water, and the other portion flows along the surface of the membrane flushing away the solids, and is called concentrate or reject.
Dead end filter cartridges are commonly used as a pretreatment to feed water to a reverse osmosis unit or nanofiltration, and often used after a multimedia filter. These filters only retain part of the suspended solids but do not retain dissolved salts, color, viruses, etc.
Maintenance is quite simple for it only requires the replacement of the filter once it's saturated, which is usually observed by a high pressure drop across the filter. A reasonable time is around three months of operation, but it varies with the amount and size of the particles.
The filter is made of Polyethylene, cellulose, fiberglass, nylon, etc. and the cartridge housing can be made of stainless steel, plastic, acrylic, depending on water corrosion potential, and working pressure. There is a great variety of filters including sanitary, multiple, absolutes, nominal, etc. An absolute filter of 5 microns, for example, retains 99.99% of the particles of that size, while a nominal filter of 5 microns, retains around 90%. This difference is mainly due to the fact that the retention size of a nominal filter is a statistical value, which is the result of a Gaussian distribution of the pore size; however the retention limit of an absolute filter is the result of a laboratory test, and indicates to the size of the biggest particle that passes through it.
Membrane filters, on the other hand, operate differently: water flows tangentially to the surface of the membrane and divides into two flows, one that passes through the membrane (permeate), and one that runs parallel to the surface. This last one helps remove the solids that do not pass through the membrane, and receives the name of concentrate or reject for it accumulates all the particles that are to be rejected. Nevertheless, on same applications, like juice production, this flow can be the actual product and the previously called permeate, the reject. In order to effectively make water filter trough the membrane, pressure is applied (generally by the use of a water pump); operating pressure ranges between 1 and 25 kg/cm2, depending on the feed water quality. Operating pressure determinates not only the kind of filter to be selected, but the construction material to allow constant operation without any complication.
Usually, hollow-fiber membranes are used for microfiltration applications since it allows the lowest operating pressure, and therefore, lower energy consumption. The hollow-fiber membrane tube range from 0.5 to 8 mm diameter, and water flows inside-out or the opposite way (outside-in) depending on the supplier. Advantages and disadvantages are observed on each configuration. On the first case (inside-out) there is no risk of deformation of the fiber, that can occur on the other case, in which pressure can compact fibers reducing its capacity to filter the fluid. However, when filtration is outside-in, all the particles are accumulated on the outer side of the fiber, which makes removal (cleaning) much simpler than on the first case.
The amount of water that passes through the membrane is proportional to the applied pressure and the membrane surface, and inversely proportional to the thickness of the fiber. Microfiltration membranes can be made of different materials as long as compatibility with the fluid is verified, as well as with the cleaning chemicals. Cellulose acetate (CA), PVC, Polyacrylonitrile, polycarbonate, and polysulfone are the most common. This last one is specially interesting for it resist up to 93°C (200 °F), pH from 0.5 to 13, and a great variety of chemical agents.
Microfiltration membranes have pore sizes from 0.3 to 10 microns and due to thier open construction the concentration polarization effect is not as notorious as it is on nanofiltration or reverse osmosis membranes.
Cleaning includes a counter-current displacement of particles (backwash), although usually another method is necessary to properly clean the membrane. Same particles remain attached to the surface of the fiber due to a different mechanism: adsorption (mainly organic compounds or microorganisms). This requires a chemically enhanced backwash; different chemicals are used depending on the application. Membranes are cleaned in-situ, therefore no removal from the unit is needed, greatly simplifying the cleaning procedure.