Filtration Solutions

Filtration is the mechanical or physical process used to separate contaminants from fluids or gases by the imposition of a medium through which only the fluid or gas can pass. The fluid or gas that passes through is called the filtrate. Various and oversized contaminants in the fluid or gas are entrapped, but the separation is still not complete. The filtrate may still contain fine particles (depending on the pore size and filter efficiency). Various filtration solutions or systems are also used to describe some biological processes, particularly in water treatment and sewage treatment in which undesirable constituents are removed by absorption into a biological film grown on or in the filter medium as in slow sand filtration.

Applications

  • Filtration is generally used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas or a supercritical fluid. Depending on the application, either one or both of the components may be isolated.
  • Filtration, as a physical process is widely used in chemistry for the separation of materials of different chemical composition. A solvent may be chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into the solution and pass through the filter medium, while the other will be retained. Chemists use this technique to purify various compounds.
  • Filtration is also important and widely used as one of the unit operations of chemical engineering. It may be simultaneously combined with other unit operations to process the feed stream, as in a biofilter, which is a combined filter and biological digestion device.
  • Filtration differs from simple sieving, where separation occurs at a single perforated layer in the sieve. In sieving, particles that are too large to pass through the holes of the sieve are retained by the sieve. In filtration, a multilayer lattice design medium retains those particles that are unable to follow the tortuous path through channels of the medium. Oversize particles will in most cases form a cake layer on top of the filter medium improving filter performance for a time until finally blocking the filter lattice, preventing the fluid phase from crossing the filter. Commercially, the term filter is applied to membranes where the separation lattice is so thin that the surface becomes the main zone of particle separation, even though some of these products might be described as sieves.
  • Filtration differs from adsorption, where it is not the physical size of particles that causes separation but the effects of surface charge. Some adsorption devices containing activated charcoal and ion exchange resin are commercially called filters, although filtration is not their principal function.
  • Filtration differs from removal of magnetic contaminants from fluids with magnets (typically lubrication oil, coolants and fuel oils), because there is no filter medium. Commercial devices called “filter magnets or magnetic filters” are sold, but the name reflects their use, not their mode of operation.
  • Alternative filtration solutions or systems are employed but one effective solution using centrifuges is centrifugation — instead of filtering the mixture of solid and liquid particles, the mixture is centrifuged to force the (usually) denser solid to the bottom or side, where it often forms a firm cake. The cleaned liquid can then be decanted. This method is especially useful for separating solids which do not filter well, such as gelatinous or fine particles. These solids can clog or pass through a standard filter.