Undissolved Solids — Turbidity
Not all particles in water are dissolved. Even though they may be solid, many undissolved solids can remain ‘in suspension’ without sinking to the bottom of the vessel that contains them. We see this with immersion-brewed coffee such as a French Press or a cupping; even though the coffee may be left for as long as 20 minutes, it can retain a cloudy, turbid appearance. Turbidity is the term used to describe the total mass of suspended solids in water, and these suspended solids are called colloids.
As a rough guide, particles smaller than 1 micron in size are usually too small to settle out in a solution. A key to how colloids remain dispersed in water is their very high surface area compared with their mass. Particles of sand will not form colloids in water, whereas particles of clay will. A single gram of clay particles can have 1000 times more external surface area than the same mass of coarse sand.
An additional quality helps particles remain in suspension. It derives from a scientific principle that also affects mineral ions. Almost all soil particles have a negative charge. This negative charge attracts a layer of cations that forms around it. This layer might be a mixture of hydrogen cations (H+) as well as magnesium (Mg2+) and calcium (Ca2+) cations. This layer in turn attracts anions, forming a double layer, or bilayer. The bilayer produces a force that repulses other such colloids. This force prevents particles from amalgamating and helps to keep them in suspension.
A negatively charged colloid surrounded by a bilayer of ions
The turbidity of water and the incidence of disease are directly correlated. Colloids tend to attract bacteria and other toxic organic matter that adsorb to the surface of the colloid. Because of this health risk, local authorities tend to prioritise the removal of colloids before distributing water into the public water network.