MICROTSCM (07/09)
Page 3
REV 2.4
3.0
Theory of Operation
In a liquid form, water molecules move around each other at a fast rate. One affect of this
fast movement is the ability to suspend matter. This phenomenon is called “Brownian
Motion”. It occurs when microscopic particles are maintained dispersed in suspension due
to their random bombardment by the fast movement of water molecules. Typical particles
found in raw water entering WTP have finely divided clay particles and organic matter
collectively called silt.
A second phenomenon which stabilizes the suspension is the surface charge of the
suspended matter. When a salt such as sodium chloride is place in water, complete
dissolution occurs. This system reaches a stable energy level when the individual sodium
and chloride ions (Na+ and Cl-) are separated in the water phase by being surrounded by
water molecules.
In the case of large pseudo salts, e.g. Aluminosilicates (clay), only partial dissolution takes
place due to incomplete breakdown of the crystal to individual ions. The structure of these
clays is similar to silica or sand except that random silicon atoms in the crystal are
replaced by aluminum atoms in the cage structure, causing the clay to swell and crack
between adjacent aluminum atoms in the crystal. Thus a clay particle is formed with a size
of less than 1 micron with a negative charge. This particle is small enough to be
maintained in suspension by Brownian Motion. The particles in the suspension repel each
other due to their surface charge, preventing them from coming together and
agglomerating, or flocking to form a larger particle, which would settle out. The result is
an energetically stable system and is the reason why the particles remain dispersed.
The counter ions (say sodium for the sake of argument) are separated from the large cage
structure because they are dissolved in the water. Clay particles have a negative charge
associated with it, while the counter ions, typically cat-ions (or positively charged ions)
are dispersed in the water phase.
In the case of most naturally occurring substances, the larger ion, when in suspension, has
a negative net charge (anionic). The smaller, counter ion is positive (cationic). The
residual charge of the larger particles is negative, which causes them to repel each other,
preventing them from forming agglomerates. The size of the particles never becomes large
enough to settle out, so they remain dispersed in suspension.
This phenomenon creates an energetically stable system. In order to cause the suspended
particles to agglomerate and settle out, the energy of the system must be upset. There are
numerous mechanical means to accomplish this, but the addition of chemical flocking
agents to the suspension, drastically reduces the time and is far more efficient.
Chemical additives perform two functions, charge neutralization & bridging. Both of these
techniques allow the small particles to floc and grow sufficiently that Brownian motion
can no longer support them. Due to the high density of the particle, flocs will form and
settle as fine sludge.
