Coagulation
is defined as the process by which colloidal particles are destabilized,
mainly by neutralizing their electric charge. Normally,
Colloids are negatively charged. Destabilization is achieved by
means of chemical addition of coagulants. Typical coagulants
are aluminum or iron salts e.g. Al2SO4 and Fe2(SO4)3. Flocculation
is the process of massing together (agglomeration) the coagulated
particles, forming larger flocs that can be easily separated either by
sedimentation or filtration. Flocculation is achieved by means of
chemical addition of flocculent or coagulant aids. Flocculants (sometimes called coagulant aids) can be inorganic or organic in
nature. Activated silica and polyelectrolyte are typical coagulant
aids that can be used in the flocculation process.
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Settling & Clarification is a process that brings about separation of of
colloids and suspended solids at a satisfactory rate utilizing gravitational forces, where
the suspended particles sink to the bottom while the clarified water overflows at the
top. The Clarification process normally follows the Coagulation/ Flocculation
process, and effectively removes particles larger than 25 microns.
Clarification is generally a multi-step process used to reduce
turbidity and suspended matter. Steps include the addition of chemical
Coagulants or pH adjustment chemicals that react to form Floc. The
floc settles by gravity in settling tanks or is removed as the water
percolates through a gravity filter. The Clarification process
is also used to reduce naturally occurring iron and to remove colors, taste,
and odor by adding strong oxidizing agents, such as chlorine. Where
gravity filters are used, carbon slurries are sometimes added to aid in
color and odor removal.
Flotation process utilizes the difference in specific mass between solids or
liquid droplets and the liquid in which they are suspended. Typically,
air is introduced to form a gaseous phase in the form of bubbles, to which
the solid or liquid particles link up, forming "particle-gas
composites". Having lower density, the formed composites
float to the surface and removed by means of skimmers.
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Filtration is a
process that consists of passing a solid-liquid mixture through porous material (filter
media) which retains the solids and allows the liquid (filtrate) to pass through. If
the suspended solids are larger than those of the pores of the filter media, the particles
are retained on the filter surface. This type of filtration is known as Surface
Filtration. When the solids are retained within the porous mass, the
process is known as In-Depth Filtration. Media filters and micron
filters are typical conventional filtration equipment. Suspended solids can also be
removed by using a more advanced Membrane Separation technology such as Microfiltration
(MF). Membrane separation is a Crossflow Filtration
process, which is covered under the Dissolved
Solids Removal Technology.
