© 2002 by CRC Press LLC Diffused Aeration 3.1 INTRODUCTION Diffused aeration is defined as the injection of air or oxygen enriched air under pressure below a liquid surface. All of the equipment discussed in this chapter meets this definition. However, certain hybrid equipment that combines gas injection with mechanical pumping or mixing is also covered under this topic. These hybrid devices include jet aerators and U-tube devices. Other devices, such as sparged turbine aerators and aspirating impeller pumps, are covered under mechanical aeration systems. Although the aeration of wastewater began in England as early as 1882 (Martin, 1927), major advances in aeration technology awaited the development of the acti- vated sludge process by Arden and Lockett in 1914. A review of the history of aeration technology is most interesting and instructive. Early investigators were aware of the importance of bubble size, diffuser placement, tank circulation and gas flow rate on oxygen transfer efficiency. Perforated tubes and pipes provided the material framework for early aeration methods. One of the earliest patents for a diffuser was granted in 1904 in Great Britain for a perforated metal plate diffuser (Martin, 1927). In Great Britain, porous tubes, perforated pipes, double perforated tubes with fibrous material in the annular space and nozzles were used in early methods (Federation of Sewage and Industrial Wastes Associations, 1950). Investi- gators sought more efficient aeration through the development of finer bubbles. In England, experiments were conducted with sandstone, firebrick, mixtures of sand and glass and pumice. Most of these early materials were dense, creating high head losses. A secret process employing concrete was used to cast porous plates that were placed in cast iron boxes by Jones and Atwood, Ltd. around 1914. This system was used for many years by Great Britain and its colonies. Meanwhile, in the U.S., porous plates produced by Filtros were widely used in newly constructed activated sludge plants. In Milwaukee, research was conducted using grids of perforated black iron pipes, basswood plates, Filtros plates and air jets. The Filtros plates were selected for the plant placed in operation in 1925 (Ernest, 1994). The Filtros plates, patented in 1914, were constructed from bonded silica sand and had permeabilities (see Section 3.4.1) in the range of 14.1 to 20.4 m 3 N /h (9 to 13 scfm) at 5 cm (2 in) water gage. Similar plates were installed in the Houston North-Side plant in 1917, as well as at Indianapolis; Chicago; Pasadena, CA; Lodi, CA; and Gastonia, NC (Babbitt, 1925). Ernest (1994) provides an excellent history of the development of the aeration system at Milwaukee where siliceous plates from Ferro Corporation (Filtros) are still used. Over time, aluminum oxide that was bonded with a variety of bonding agents, as well as silica became the major media of choice. Permeabilities continued to rise as well, up to as high as 188 m 3 N /h (120 scfm). In addition, new shapes were introduced, including domes and tubes and more recently, discs. 3 © 2002 by CRC Press LLC In Great Britain, the sand-cement plates were predominately used until approx- imately 1932. In 1932, Norton introduced porous plates bolted at either end. Norton introduced the first domes in 1946 with permeabilities in the range of 62.8 to 78.5 m 3 N /h (40 to 50 scfm). In Germany, early aeration designs (commencing about 1929) incorporated the Brandol plate diffusers produced by Schumacher Fabrik. Later they developed a tube design, and the material was modified as silica sand bonded by a phenol formaldehyde resin (Schmidt-Holthausen and Bievers, 1980). Diffuser configuration was considered to be an important factor in activated sludge performance even as early as 1915. The Houston and Milwaukee plants were designed with a ridge and furrow configuration. In 1923, Hurd proposed the “cir- culatory flow” or spiral roll configuration for the Indianapolis plant. The Chicago North-Side plant also employed this diffuser configuration (Hurd, 1923). The design was promoted on the belief that the spiral roll would provide a longer contact time between wastewater and air than the full floor coverage. One set of basins at Milwaukee was converted to spiral roll in 1933, but even the 1935 database suggested that the spiral roll configuration required more air per unit volume of wastewater treated. The spiral roll configuration was abandoned at Milwaukee in 1961 after extensive oxygen transfer studies (Ernest, 1994). It is also interesting to note that the early plants employed a range of diffuser densities (percent of floor surface area covered by diffusers, A d / A t × 100) ranging from about 25 percent at Milwaukee and Lodi, CA to 7 to 10 percent at the spiral roll plants (Babbitt, 1925). Clogging of diffusers appears to have been a problem in some cases according to the earliest studies. Generally speaking, the porous diffusers produced the greatest concern but examples of clogging of perforated pipes can be found (Martin, 1927; Ernest, 1994). Early work by Bushee and Zack (1924) at the Sanitary District of Chicago prompted the use of coarser media to avoid fouling. Later, Roe (1934) outlined in detail numerous diffuser clogging causes. Ernest (1994) detailed cleaning methods adopted by Milwaukee in maintaining porous diffusers at their installations. Nonetheless, by the 1950s, many plants were using the large orifice type of diffuser. The newer designs improved upon their earlier counterparts and were designed for easy maintenance and accessibility. In general, these devices produced a coarser bubble, thereby sacrificing substantial transfer efficiency. The Air Diffusion in Sewage Works manual (Committee on Sewage and Industrial Wastes Practice, 1952) provides an excellent summary of air diffusion devices proposed and tested between 1893 and 1950. It should be emphasized that the trend toward coarser diffuser media was followed in the U.S. but not in Europe, where the porous diffusers continued to predominate in many designs. An alternative to the diffused aeration systems was the mechanical aeration designs, which had been introduced in the early 1900s. These, too, began to replace some of the older diffused aeration systems where fouling was considered to be a problem. A more detailed discussion of the mechanical aeration systems is presented in Chapter 5. With the emphasis on more energy-efficient aeration in the 1970s, porous diffuser technology received greater attention in the U.S. Since about 1970, the wastewater treatment industry has witnessed the introduction of a wide variety of new diffuser © 2002 by CRC Press LLC materials and designs. Many of the lessons learned with this technology in the early part of the century were revisited. Improvements in materials of construction, blower designs, and measurement technology have resulted in a new generation of highly efficient diffuser systems and the methodologies for maintenance of these systems. This chapter addresses the current state of technology for diffused aeration. Although diffused aeration devices are often referred to as fine, medium and coarse bubble based on the perceived or measured bubble size, such classifications are often confusing and differentiation between devices is difficult. Therefore, in this chapter, diffused aeration devices are discussed based on the physical characteristics of the diffuser device. Two general categories are used, porous and nonporous devices. The reader is cautioned, however, to avoid drawing generalities about equipment perfor- mance based on these labels alone. These classifications are intended more as a guide for organization than as a categorical statement of performance. 3.2 DESCRIPTION OF DIFFUSED AERATION SYSTEMS 3.2.1 P OROUS D IFFUSER D EVICES Porous diffuser devices are defined in this text based on the current high efficiency devices now on the market as diffusers that will produce a head loss due to surface tension in clean water of greater than about 5 cm (2 in) water gauge. These devices are often referred to as fine pore diffusers and typically produce bubbles in the range of 2–5 mm (0.08–0.20 in) when new. An excellent reference on fine pore aeration technology is the USEPA’s Design Manual, Fine Pore Aeration Systems (1989). 3.2.1.1 Types of Porous Media Although several materials are capable of serving as effective porous media, few are being used in the wastewater treatment field because of cost, specific charac- teristics, market size, or other factors. Porous media used today may be divided into the following three general categories: ceramics, porous plastics and perfo- rated membranes. 3.2.1.1.1 Ceramics Ceramics are the oldest and currently the most common porous media on the wastewater market. Ceramic media consist of irregular or spherically shaped mineral particles that are sized, blended together with bonding materials, compressed into various shapes, and fired at elevated temperatures to form a ceramic bond between the particles. The result is a network of interconnecting passageways through which air flows. As air emerges from the surface pores, the pore size, surface tension, and airflow rate interact to produce a characteristic bubble size. Ceramic materials most often used include alumina, aluminum silicate and silica. Alumina is refined from naturally occurring bauxite and subsequently crushed and screened to provide the appropriate size. Synthetic or naturally occurring aluminum silicates may also be used and are often referred as mullite when consisting of three parts alumina and two parts silica. The alumina and aluminum silicate particles are © 2002 by CRC Press LLC ceramically bonded to form the appropriate diffuser material. Silica is typically a mined material although crushed glass may be used. It is less angular and available in somewhat more limited particle sizes than the aluminum minerals. Silica minerals are normally vitreous-silicate bonded although resin bonding of pure silica is also practiced. It has been claimed that silica materials may be more resistant to fouling and more easily cleaned (Schmidt-Holthausen and Bievers, 1980), but no scientifi- cally controlled experiments have been conducted to support this claim. No studies have been published that suggest there is a difference in process performance between diffusers made with different materials. Performance would be more a function of grain size, binding agent, shape of the unit, and other factors. Alumina may be the most abrasion resistant, but actual strength and abrasion resistance depends on the ceramic bond. Silica porous media are generally considered to have the lowest overall strength, thereby requiring greater thickness. Sources of ceramic diffuser media include companies supplying industrial abra- sives or refractories. They may provide diffusers to aeration equipment manufactur- ers who specify the characteristics of the media, or they may market finished diffuser assemblies. Ceramic diffusers have been used since the turn of the century, as described above, and their advantages and operational characteristics are well documented. As a result, they have become the standard for comparison. Each new generation of porous diffusers reportedly offers some advantages in cost or operation over ceramics. However, as in the past, the new diffusers have not always met expectations. As a result, ceramic diffusers continue to capture a significant share of the porous diffuser market. 3.2.1.1.2 Rigid Porous Plastics Rigid porous plastics are made from several thermoplastic polymers, including polyethylene, polypropylene, polyvinylidene fluoride, ethylene-vinyl acetate, styrene-acrylonitrile (SAN), and polytetra-fluoroethylene (EPA, 1989). The two most common types of plastic media used in wastewater aeration are high-density poly- ethylene (HDPE) and SAN. Relatively inexpensive and easy to process, HDPE diffusers are typically made from a straight nonpolar homopolymer in a proprietary extrusion process. SAN diffusers have been made from small copolymer spheres fused together under pressure. The material is brittle, however. SAN diffusers have been used for more than 20 years in U.S. wastewater treatment plants. Although plastics have advantages of lighter weight and lower costs as compared with ceramic materials, their use has fallen out of favor in the U.S. due to lack of quality control and the emerging cost competitiveness of other fine pore diffuser devices. 3.2.1.1.3 Perforated Membranes Membrane diffusers differ from the first two groups of diffuser materials in that the diffusion material does not contain interconnecting passageways for transmitting gas. Instead, mechanical means are used to create preselected small orifices in a membrane material that allows passage of air through the material. The earliest of this type diffuser was introduced in the 1960s and was referred to as a sock diffuser. Made from plastics, synthetic fabric cord, or woven cloth, a woven sheath of this material was supported by a metallic or plastic core. The diffuser design allowed easy removal from retrievable aeration piping for cleaning or replacement. These socks were © 2002 by CRC Press LLC capable of high transfer efficiencies but readily fouled and were often removed by operators and not replaced. There is virtually no market for these socks today. In the late 1970s, a new generation of perforated membranes was introduced. They consisted of a thin flexible thermoplastic, polyvinyl chloride (PVC). The membrane was perforated with a pattern of small slits. The plastic PVC membrane was found to undergo dramatic changes while in service, which significantly affected oxygen transfer. Consequently, the material was found to have relatively short operating life in many wastewaters. A new type of membrane material was introduced in the mid 1980’s identified as an elastomer. The predominant elastomers used in perforated membrane diffusers today are ethylene-propylene dimers (EPDMs). These new copolymers promise to address many of the material deterioration problems of the earlier plasticized PVC membranes. Different rubber fabricators have developed EPDM elastomers indepen- dently, and the manufacturing process, ternomer, and catalyst systems employed can vary significantly. These factors can affect molecular weight distribution, chain branching and cure rate. Furthermore, EPDM master batch formulas can contain varying amounts of EPDM, carbon black, silica, clay, talc, oils, and various curing and processing agents. By varying these components and their method of manufac- ture, it is possible to obtain a product for a specific application. This engineering of EPDM (and other membrane materials) has resulted in significant improvement of product performance and resistance to environmental attack. As a result, membranes have been engineered for several industrial applications including pulp and paper, textile, food and dairy and petrochemical wastewater. Today, several equipment manufacturers are actively engaged in engineering new and improved perforated membrane materials. Polyurethane that provides high modulus of elasticity and contains no oils has been used in wastewater applications (Messner in Europe and marketed in the U.S. by Parkson as panels). Although no chemical changes are observed with this material, the thinner membrane is sensitive to creep under stress of air pressure. The hydrophobic silicones, which also contain no oils, are claimed to be chemically resistant to a number of wastewater chemicals. Yet, once perforated, early designs exhibit little tear resistance. With more experi- ence, these materials and others will be improved and may serve important niches in the wastewater treatment business. An important feature of the new perforated membranes is the perforation number, size and pattern. Perforations are produced by slicing, punching, or drilling small holes or slits in the membrane. Each hole acts as a variable aperture opening. The slit or hole size will effect bubble size (and therefore, oxygen transfer efficiency) and back pressure; smaller slits will generate smaller bubbles at a sacrifice of some head loss. Typical slit or hole size is 1 mm, although manufacturers continue to experiment with opening size and pattern to optimize performance. The current panel system marketed in the U.S. employs a very fine perforation. Several manufacturers offer both a fine and coarse perforation in their membrane diffuser offerings. Most perforated membrane devices are designed so that when air is off, the membrane relaxes down against a support base, and a seal is formed between membrane and support plate. This closing action will reportedly eliminate or at least minimize the backflow of liquid into the aeration system. © 2002 by CRC Press LLC 3.2.1.2 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 m 3 N /h/m 2 of diffuser surface area (0.6 to 1.2 scfm/ft 2 ). 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/m 2 (0.05 to FIGURE 3.1 Typical plate diffuser (courtesy of EDI, Columbia, MO). © 2002 by CRC Press LLC 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 m 3 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). © 2002 by CRC Press LLC 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]. © 2002 by CRC Press LLC FIGURE 3.4 (continued) FIGURE 3.5 Membrane tube design (courtesy of OTT Systems, Inc., Duluth, GA). © 2002 by CRC Press LLC 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 m 3 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). [...]... Density (% ) Airflow Rate 3 (m N /h/unit) 3. 0 m 4.6 m 6.1 m Reference 10 7.5 11. 7 15.1 6.0–6 .3 6.9–7.7 8.9–10.2 35 .6–84.7 1.4–4.7 1 .3 4.6 1.1–4.1 2 .3 5.0 0.9 3. 9 0.9–5 .3 — 20–22 21–24 22–25 — — — — 27 33 30 34 31 34 25–29 25 30 27 34 30 –40 34 37 35 –41 38 –41 32 38 33 –40 31 –40 12.0–12.8 16.4–21.6 Type 0.6–4.4 1.1–4.9 — — 25 36 27 38 34 39 31 38 12.0 4.8 6.1–6 .3 8.1–8.4 10.7–12.1 17 .3 1.9 0.8 3. 9 0.8 3. 9 0.8 3. 9... Ceramic disc 18-cm grid 22-cm grid 24-cm grid Ceramic dome 18-cm grid Perforated membrane discb 51-cm 30 -cm 2 3- cm Panelb Airflow Rate 3 (m N /h/diffuser) SAE a (kg/kWh) 3. 8–6 .3 3.1 11. 0 17.6–18.8 4.7 11. 0 4.5–5.2 2.4 3. 2 1.6–2.7 2.6 3. 9 0.8–10 0.8–18.8 4.2–5.7 3. 4–5.8 0.6–5.5 0.6–5.0 1.1–4.7 3. 6–5.2 4.1–6.1 4.4–5.5 0.8 3. 9 3. 4–6.0 24–172 13 237 13 280 4–74 2.7–4.6 2.7–6.1 2.4–7.1 3. 1–6.9 3 1.0 m N /h... 1.9 0.8 3. 9 0.8 3. 9 0.8 3. 9 0.8 3. 9 0.8 3. 9 — — 16– 23 20–24 17– 23 18–26 32 33 23 31 25 32 27 37 27 35 27 34 — 28–40 30 –41 31 –44 33 –47 — 3. 9 5.8 6.8 9.2 0.9–5.5 0.9–5.5 0.8 3. 6 0.6–2 .3 15–18 16–21 — 19–22 22–27 24–28 25 31 26 32 — — — — Johnson, 19 93 EPA, 1989 EPA, 1989 EPA, 1989 EPA, 1989 EPA, 1989 EPA, 1989; Johnson, 19 93 EPA, 1989 EPA, 1989; Johnson, 19 93 Johnson, 19 93 EPA, 1989 EPA, 1989 EPA, 1989... 1988 1988 3 1 m = 3. 28 ft; 1.0 m N /h = 0.64 scfm; 1.0 kg/kWh = 1.644 lb/hp-h G = Grid; S = Spiral roll; MW = Mid-width TABLE 3. 3 Clean Water Oxygen Transfer Efficiency — Aspirators and Jets Type and Placement Jets Aspirator tube Airflow Rate 3 (m N /h/unit) Dir Clu Clu 5.5 kw 15 kw Submergence (m) SOTE (% ) SAE (kg/kWh) Reference 21.1 119 7.1–86 .3 7.7–50.5 — — 4.6 3. 0 6.1 2.0 2.5 15–24 8–14 21 33 — — 1.7–2.0... TABLE 3. 2 Clean Water Oxygen Transfer Efficiency — Nonporous Diffusers Type and Placement Airflow Rate 3 (m N /h/unit) Fixed orifice S perforated tube S S S S G G MW Sparger S MW MW MW Static tube G G G G ? Submergence (m) SOTE (% ) SAE (kg/kWh) 9 .3 32 .8 8.6–40.0 9 .3 64 .3 16.0 39 .6 8.9 31 .5 7.5– 23. 2 8 .3 24.4 6.6–18.8 12.9–51 .3 18.7–57.0 19.8–60.2 19.8–59.2 15.7–60.2 15.7–65.6 15.7–66.4 24.4–51.0 37 .0–68 .3. .. LLC TABLE 3. 4 Clean Water Oxygen Transfer Efficiency — Porous Tubes SOTE (% ) at Depth Type Placement Airflow Rate 3 (m N /h/unit) Porous plastic G DS DS S S G G S 3. 8–6 .3 4.7 11. 0 14.1–17 .3 3.1 11. 0 12.6–18.8 4.7 3. 0–10.0 0.8–10.0 — — — — — — — 13 19 — 10–16 10–14 12–15 10–15 — 27–28 17–21 28 32 16–24 15–17 15–20 10–17 — — 26 35 — 22 32 21–26 22–25 22 45 — — DS 0.8–18.8 10–20 15–21 21 36 27 36 Perforated... 3. 7 Ceramic disc (courtesy of Sanitaire, Brown Deer, WI) © 2002 by CRC Press LLC A FIGURE 3. 8 Several membrane disc configurations [(A) courtesy of Nopon Oy, Helsinki, Finland; (B) courtesy of Sanitaire, Brown Deer, WI] © 2002 by CRC Press LLC A FIGURE 3. 9 Several membrane disc configurations [(A) courtesy of Wilfey Weber, Inc., Denver, CO; (B) courtesy of EDI, Columbia, MO] 15.7 m3N/h (1 to 10 scfm)... diffusers may be installed along one side (single band) or both sides (wide band) of the pipe © 2002 by CRC Press LLC FIGURE 3. 16 Fine pore grid layout (courtesy of Sanitaire, Brown Deer, WI) © 2002 by CRC Press LLC FIGURE 3. 17 Fine pore grid layout (courtesy of Nopon Oy, Helsinki, Finland) © 2002 by CRC Press LLC FIGURE 3. 18 Tube grid layout (courtesy of EDI, Columbia, MO) For full floor grid arrangements,... 6.6–18.8 12.9–51 .3 18.7–57.0 19.8–60.2 19.8–59.2 15.7–60.2 15.7–65.6 15.7–66.4 24.4–51.0 37 .0–68 .3 7 .3 5.2–5.6 4.1–4.8 3. 0 3. 8 2.7 3. 0 6.1 4.6 4.1–4.8 3. 0 4.6 6.1 3. 0 4.2 6.1 4.2–4.6 5.2 21–25 11 18 5–17 6–14 6–7 7–8 17–20 11 13 9– 13 6–7 10 11 15–17 6–8 11 15 13 20 8–12 12–15 — — — — — 1 .3 1.5 2.0–2.2 1.5–1.6 — 1 .3 1.5 1.5–1.6 1.8–1.9 1.1–1.5 1.5–1.8 1.7–1.9 — — Reference Johnson, 1992 Johnson, 1992 Johnson,... mixing device, and the diffusers are fixed to the bottom of the basin (Figure 3. 20 and 3. 2 1) Results of testing of these configurations appear in the Performance section of this chapter 3. 3.7 DEEP TANKS Deep tank aeration is being practiced on a limited scale in the U.S and abroad Limited land availability and the need for increased plant capacity have led to the © 2002 by CRC Press LLC FIGURE 3. 20 Mixer–diffuser . may range from 1 to 2 m (3 9 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. acetate, styrene-acrylonitrile (SAN), and polytetra-fluoroethylene (EPA, 198 9). The two most common types of plastic media used in wastewater aeration are high-density poly- ethylene (HDPE) and SAN Helsinki, Finland; (B) courtesy of Sanitaire, Brown Deer, WI]. A © 2002 by CRC Press LLC 15.7 m 3 N /h (1 to 10 scfm) per diffuser for the discs up to 30 cm (1 2 in) in diameter and 4.7 to 31 .4