Page 134 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.2 CHAPTER THREE

GENERAL

Feedwater, raw water, and source water are various ways of referring to a solution whose
components are intended to be separated. Filtration is the process used for separation and
retention of suspended and colloidal particles by mechanical capture and adsorption from
fluids by passage through a porous medium. Mechanical capture physically prevents a
contaminant particle from passing through a barrier with openings (pores). Adsorption is
the attraction to and adhering of a particle to the surface of the filter medium. Adsorption
can occur even if the pore is larger than the particle. This attraction is due to a variety of
surface chemical forces between the particle and filter medium.
The mechanical properties of the particles suspended in the water stream must also be
considered. At one extreme are solid, undeformable particles such as sand or quartz, and
at the other extreme are gelatinous or deformable materials such as synthetic colloids and
bacteria. Because they can deform, they are more likely than hard particles of the same size
to pass through a filter.

FILTER CATEGORIES AND DEFINITIONS

Screen, surface, and depth filtration are the three broad categories of the filtering process.
A screen filter is best thought of as a single, thin layer of a material that has a sym-
metrical arrangement of openings or passages called pores. These pores trap all particles
larger than the pore size on the surface of the filter. This process is called sieving, or size
exclusion, and is the classic filtration method. Sieving can also be referred to as screening
or straining. Screen filtration is essentially absolute because any particle larger than the
pore size cannot pass through. Another mechanical capture mechanism, called bridging,
occurs as particles captured by direct interception form a particle mat, or bridge, across the
filter medium. By partially blocking the filter pores, this bridge or filter cake may produce
a smaller filter pore structure that will aid in particle capture. Examples of screen filters
are woven metal, nylon, and dacron mesh. Cast polymeric membranes are used where the
smallest size pores are required for submicronic and macromolecular separations.
A surface filter is thicker than the screen filter and constructed from thick or multiple
layers of filter media, often glass or polymeric fibers. When the water passes through a
surface filter, particles larger than the spaces within the fiber matrix are retained, primarily
on the surface. Smaller particles are trapped within the matrix, giving this type of filter the
properties of both a screen and depth filter.
A depth filter relies on the density and thickness of the layers to mechanically trap the
particles, and it will retain relatively large quantities of them. Depth filtration occurs on the
surface and throughout all or part of the filter medium as the water passes through a complex
network of flow channels. The particles are retained by random adsorption and mechanical
entrapment. Depth filters can be of two types, granular and preformed. Preformed depth filters
are composed of fibrous or sintered materials that have a random pore structure. Granular depth
filters have either a graded or consistent density of granular media and typically are long in
length. Graded granular filters have layers of media that become progressively denser through
the matrix as water flows through them. Constant density granular filters’ have the same size
filter media or openings throughout the matrix.
An oil skimmer removes oil from water by means of a moving medium, such as a belt
or disk, that causes oil to be retained on the medium by surface tension. Oil and grease are
removed from the skimmer by wiper blades or pinch rollers. The skimmers must be operated at
an elevated temperature to keep the oils in a fluid state. For more demanding situations, skim-
mers could be an effective and cost-effective pretreatment.
A filter that is hydrophilic is one that has an affinity for water; it can be wetted with
almost any liquid. A hydrophobic filter is one that cannot be wetted by an aqueous solution.

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