Types of Porous Media Diffusers

There are five general shapes of porous diffusers on the market: plates, panels, tubes, domes and discs. Each is briefly described below.

3.2.1.2.1 Plate Diffusers

One of the original designs for porous diffusers was the plate as described above.

These plates were usually 30 cm (12 in) square and 25–38 mm (1–1.5 in) thick. Most were constructed of ceramic media. Installation was completed by grouting the plates into recesses in the basin floor or cementing them into prefabricated holders. Air was introduced below the plates through a plenum. Typically, no airflow control orifices were used in these designs. Although their use has declined since 1970, these ceramic plates are still used in Milwaukee and Chicago. A newer plate design was introduced in the late 1980s that employs either a ceramic or porous plastic media. They are marketed in sizes of 30 cm × 61 cm (12 × 24 in) and 30 cm × 122 cm (12 × 48 in).

These units are typically mounted on ABS plastic plenums and subsequently placed on the basin floor. Air is introduced to each module by means of rubber tubing, and individual orifices control airflow. (See Figure 3.1.) Depending upon the layout, plate diffusers are typically operated at flux rates ranging from 0.09 to 0.18 m³_N/h/m² of diffuser surface area (0.6 to 1.2 scfm/ft²).

3.2.1.2.2 Panel Diffusers

Currently, the only panel marketed in the U.S. uses the perforated polyurethane membrane. The membrane is stretched over a 122 cm (48 in) wide base plate of variable length ranging from 183–366 cm (6–12 ft) in 61 cm (24 in) increments.

The base plate may be constructed of reinforced cement compound, fiber-reinforced plastic, or Type 304 stainless steel. Air is introduced via tubing and an airflow control orifice attached at one end. The panels are placed on the flat bottom surface of the aeration basin and fastened with anchor bolts (Figure 3.2). These plates are designed to operate over a range of airflows from 0.007 to 0.111 m ³_N/h/m² (0.05 to FIGURE 3.1 Typical plate diffuser (courtesy of EDI, Columbia, MO).

0.76 scfm/ft) of membrane surface. Pressure loss across the panels ranges from 50 to 100 cm (20 to 40 in) water gauge (4.8 to 9.6 kPa [0.7 to 1.4 psi]).

3.2.1.2.3 Tube Diffusers

Like plates, tube diffusers have been used for many years in wastewater applications.

The early tubes, Saran wound or aluminum oxide ceramic, have now been followed by SAN copolymer, porous HDPE and more recently, by perforated membranes.

Most tubes on the market are of the same general shape, typically 51 to 61 cm (20–24 in) long with a diameter of 6.4 to 7.7 cm (2.5 to 3.0 in). The “magnum”

tubes may range from 1 to 2 m (39 to 78 in) in length with diameters ranging from 6.4 to 9.4 cm (3.0 to 3.7 in). Diffusers may be placed on one (single band) or both (wide band) sides of the lateral header, which delivers the air to the units. An orifice inserted in the inlet nipple to aid in distribution typically controls airflow.

Whereas ceramic and porous plastic tubes are strong enough to be self-supported with aid of end caps and a connecting rod (Figure 3.3), perforated membranes require an internal support structure (Figure 3.4). The support is usually constructed from plastic (PVC or polypropylene) and has a tubular shape. The tube provides support either around the entire circumference or only the bottom half. Holes in the inlet connector, specially designed slots, or openings in the tube itself allow air distribution to the membrane surface. The membrane is usually not perforated at the air inlet points, so when airflow is off, the membrane collapses and seals against the support structure.

Most components of the tube assemblies are made of either stainless steel or a durable plastic. The gaskets are usually of a soft rubber material. Tubes are normally designed to operate at airflows ranging from 1.6 to 15.7 m³_N/h (1–10 scfm) per diffuser, although most are operated at the lower end for optimum efficiency. It should be noted that because of the shape, it is difficult to design tubular diffusers to discharge around the entire circumference of the unit. The air distribution is a function of airflow rate and head loss across the media, usually improving with increased head loss. Fouling may occur in those regions where airflow is low or zero. New designs have developed internal air distribution networks that provide more uniform distribution of air around the entire circumference (Figure 3.5).

FIGURE 3.2 Typical panel diffuser (courtesy of Parkson Corp., Fort Lauderdale, FL).

FIGURE 3.3 Ceramic tube diffuser (courtesy of Sanitaire, Brown Deer, WI).

FIGURE 3.4 Membrane tubes [(A) courtesy of Sanitaire, Brown Deer, WI; (B) courtesy of EDI, Columbia, MO].

FIGURE 3.4 (continued)

FIGURE 3.5 Membrane tube design (courtesy of OTT Systems, Inc., Duluth, GA).

3.2.1.2.4 Dome Diffusers

As described above, the porous dome diffuser was introduced in the U.K. in 1946 and was widely used in Europe prior to its introduction in the U.S. in the 1970s.

The dome diffuser is a circular disc with a downturned edge. Today, these diffusers are 18 cm (7 in) in diameter and 38 mm (1.5 in) high. The media is ceramic, usually aluminum oxide.

The diffuser is normally mounted on a PVC or mild steel saddle-type baseplate and attached to the baseplate by a bolt through the center of the dome (Figure 3.6).

The bolt is constructed from a number of materials including brass, plastics, or stainless steel. A soft rubber gasket is placed between the baseplate and the dome, and a washer and gasket are also used between the bolt head and the top of the diffuser. These gaskets are critical to the integrity of the diffuser as overtightening can lead to permanent compression set and eventual air leakage. Note that air pressure will force the dome upward off the baseplate. To distribute the air properly through the system, control orifices are located in the hollowed-out center bolt or drilled into the baseplate. Various means are used to fix the dome to the air distribution header.

The baseplate may be solvent welded to the header in the shop or may be fastened to the header at the plant site by drilling a hole with an expansion plug.

Dome diffusers are normally designed to operate over a range of airflow rates from 0.8 to 3.9 m³_N/h (0.5 to 2.5 scfm) per diffuser. Diffuser fouling and airflow distribution normally set the lower airflow rate and efficiency. Back pressure con-siderations normally dictate the higher rates.

3.2.1.2.5 Disc Diffusers

Disc diffusers, being relatively flat, are a newer innovation of the dome diffuser.

Whereas dome diffusers are relatively standard in size and shape, available disc diffusers differ in size, shape, method of attachment, and type of diffuser material.

Disc diffusers are available in diameters of 18 to 51 cm (7 to 20 in). The shape of porous plastic or ceramic media is normally two flat parallel surfaces with at least one exception whereby the manufacturer produces a raised ring sloping slightly FIGURE 3.6 Ceramic dome (courtesy of Sanitaire, Brown Deer, WI).

downward toward both the periphery and the center of the disc. A step on the outer periphery is often built into the disc to improve uniformity of air flux and effective-ness of the seal at the diffuser edge (Figure 3.7).

As with the dome diffusers, porous plastic and ceramic disc diffusers are mounted on a plastic, saddle-type base plate. Two methods are used to secure disc media to the holder: a center bolt or a peripheral clamping ring. The center bolt and gasket arrangement is similar to that used for domes. Use of a screw-on retainer ring is more commonly the method of attachment. A number of different gasket arrangements may be employed, including a flat gasket below the disc, a U-shaped gasket that covers a small portion of the top and bottom and the entire edge of the disc, and an O-ring gasket placed between the top of the outer periphery of the disc and the retainer ring.

Two methods are used to attach the porous plastic or ceramic disc to the air header. The first method is to solvent cement the base plate to the header in the shop. The second type of attachment is completed through mechanical means using either a bayonet-type holder or a wedge section placed around the pipe. These mechanical attachments are performed in the field. Holes are drilled in the header and the disc assemblies are subsequently attached. Future expansion of the system is accommodated by predrilling and plugging holes or by drilling the required holes at the needed time. Individual control orifices in each diffuser unit are used to provide uniform air flux in the system. For bolted systems, the bolt may be hollowed and an orifice drilled in its side. Other designs incorporate either an orifice drilled in the base plate or a threaded inlet in the base where a small plug containing the desired orifice can be inserted.

Perforated membrane discs are designed to lie over a support plate containing apertures that allow air to enter between the membrane and the plate. The membrane is normally not perforated over the apertures and when the air is off, the membrane will seal against mixed liquor intrusion. The membrane may be secured to the base around the periphery by a clamping a ring, wire or a screw-on retaining ring. When the air is on, the membrane will flex upward approximately 6 to 64 mm (0.24 to 2.6 in). Flexing beyond the manufacturer’s recommendations could lead to maldis-tribution of air. Therefore, some designs include additional means of support at the center to prevent overflexing. The base of the membrane support frame is usually threaded. A saddle that is also threaded is glued or clamped to the air header and receives the base plate. Several manufacturers utilize holders identical to that used for a ceramic or porous plastic disc. Such a design allows interchanging of mem-branes and porous diffuser discs. Several configurations of perforated membrane discs are shown in Figure 3.8a and b and 3.9.

Ceramic and porous plastic diffusers typically have design airflow rates ranging from 0.8 to 4.7 m³_N/h (0.5 to 3 scfm) per diffuser. The optimum airflow depends on disc surface area but continuous operation at airflows below about 0.8 m³_N/h (0.5 scfm) per diffuser may lead to poor airflow distribution over the entire disc surface. In applications above 3.1 m³_N/h (2 scfm) per diffuser, the control orifice must be properly sized so that the head loss produced does not adversely affect the economics of the system. For perforated discs, design airflows range from 1.6 to

FIGURE 3.7 Ceramic disc (courtesy of Sanitaire, Brown Deer, WI).

© 2002 by CRC Press LLC

FIGURE 3.8 Several membrane disc configurations [(A) courtesy of Nopon Oy, Helsinki, Finland; (B) courtesy of Sanitaire, Brown Deer, WI].

A

15.7 m³_N/h (1 to 10 scfm) per diffuser for the discs up to 30 cm (12 in) in diameter and 4.7 to 31.4 m³_N/h (3–20 scfm) per diffuser for the larger discs.

3.2.2 NONPOROUS DIFFUSER SYSTEMS

Nonporous diffusers differ from porous diffusers in that they use larger orifices or holes to discharge air. Introduced as early as 1893 these diffusers are available in a variety of shapes and materials. This section will describe these diffusers under the categories of fixed orifice, valved orifice, static tubes, perforated tubes, and other units.

FIGURE 3.9 Several membrane disc configurations [(A) courtesy of Wilfey Weber, Inc., Denver, CO; (B) courtesy of EDI, Columbia, MO].

A