Biofouling or biological fouling is the undesirable accumulation of microorganisms, plants, algae, and/or animals on wetted structures. Biofouling is especially economically significant on ships' hulls where high levels of fouling can reduce the performance of the vessel and increase its fuel requirements.Biofouling is also found in almost all circumstances where water based liquids are in contact with other materials. Industrially important examples include membrane systems, such as membrane bioreactors and reverse osmosis spiral wound membranes cooling water cycles of large industrial equipments and power stations. Biofouling can also occur in oil pipelines carrying oils with entrained water especially those carrying used oils, cutting oils, soluble oil or hydraulic oils.

The time period usually elapsing between the attainment of supersaturation and the appearance of the first crystals is called the induction time. It is considerably influenced not only by the initial conditions and process conditions, like supersaturation, state of agitation, temperature and seeding, but also by the measurement techniques used. The first changes in the system's physical properties due to the formation of the solid phase may be followed by the appearance of the first visible crystals. The measured induction time is generally a complex quantity made up of several components. It is often measured by visual observation or by particle size analyzer. Induction time varies within a large range (< 5 s up to 1000 s), at high supersaturation it can be very short, i.e. less than 1 sec.

Nanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter.
Scaling or precipitation fouling involves crystallization of solid salts, oxides and hydroxides from solutions. Precipitation fouling is a very common problem in boilers and heat exchangers operating with hard water and often results in limescale. Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of solids (usually crystals).
A seed crystal is a small piece of single crystal / polycrystal material from which a large crystal of the same material typically is to be grown. The large crystal can be grown by dipping the seed into a supersaturated solution, into molten material that is then cooled, or by growth on the seed face by passing vapor of the material to be grown over it.

A mixture of metal ions in a solution can be separated by precipitation with anions such as Cl-, Br-, SO42-, CO32-, S2-, Cr2O42-, PO42-, OH- etc. When a metal ion or a group of metal ions form insoluble salts with a particular anion, they can be separated form others by precipitation. We can also separate the anions by precipitating them with appropriate metal ions.

There is no definite dividing lines between insoluble salts, sparingly soluble, and soluble salts, but concentrations of their saturated solutions are small, medium, and large. Solubility products are usually listed for insoluble and sparingly soluble salts, but they are not given for soluble salts. Solubility products for soluble salts are very large.

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The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances.

glossary