when it is tested dry in a room maintained at a
temperature of 21 degrees C (70 degrees F) and a
relative humidity between 30 and 50 percent. To
date, this is the accepted method of measuring the
ability of porous diffuser media to diffuse air and
provide a desirable rate of oxygen absorption.
Lower permeability should produce smaller bubbles,
which should result in higher rates of oxygen
absorption. Lower permeability rating, however,
requires higher air pressure and results in more
rapid clogging and higher pressure losses.
Consequently, any benefits obtained initially by
specifying a permeability rating may be offset by
the higher maintenance and operating costs. The
best balance between desired oxygen absorption and
operating and maintenance cost is afforded by a
permeability rating of from 0.203 to 0.406 (40 to
80). Non-porous diffusers do not have a
permeability rating.
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Diffusers shall have a mean permeability rating of [_____] and shall be one
of the following types:
2.5.2.1
Porous Ceramic Discs
Porous ceramic plate diffusers shall be silica sand bonded together with a
synthetic silicate, fused alumina, or an organic bond; grains of
crystalline aluminum oxide bonded with high alumina glass; aluminum
silicate grains ceramically bonded at high temperature; crushed porcelain
grains bonded together with alumina glass or electrically fused aluminum
grains bonded together with alumina glass. Diffuser plates shall each
provide [_____] square mm square inches of horizontal diffuser area.
2.5.2.2
Porous Membrane Tubes with Supports
Porous membrane media shall be a tubular flexible synthetic membrane sheath
that is open at one end and closed at the other. The tubular sheath shall
fit over a tubular air duct/air plenum frame or be supported by a one-piece
semicircular corrosion resistant support rod. The sheath shall be clamped
to a nozzle with a corrosion resistant removable clamp. The sheath shall
be capable of flexing from its unexpanded shape to its expanded inflated
convex hollow cylindrical shape when air is diffused through it to slough
foulants. The apertures of the sheath shall close whenever the air flow is
shut off and purge themselves when air is restarted. Each diffuser shall
have a check valve for wastewater backflow prevention. The nozzle shall be
compatible with the diffuser connector on the air header.
2.5.2.3
Porous Cloth Media with Plastic Tube Liner
Porous cloth media shall be a tubular flexible synthetic fiber cloth sheath
that is open at one end and closed at the other. The media shall fit over
a plastic tube liner and shall be clamped to a cast iron nozzle with a
stainless steel clamp. The cast iron nozzle, except for the threads for
attaching to the header, shall be vinyl coated. The diffuser media shall
be [_____] mm inches in length and a diameter of [_____] mm inches.
SECTION 11375A
Page 27
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