Heat exchanger for aeration tank

The invention relates to a tube bank heat exchanger for use in reducing the temperature of air delivered under pressure to an air diffuser system within a wastewater treatment facility. The tube bank heat exchanger, submerged within the wastewater, is of a configuration which extends the air flow path from the air source to the diffuser system. By such configuration the temperature of the air delivered to the diffuser system is reduced to a temperature of no more than about 140 degrees F.

BACKGROUND OF THE INVENTION 
This invention is directed to an improved wastewater treatment aeration 
tank, which includes a unique air distribution system to reduce the air 
temperature delivered to an array of air diffusers within the tank, where 
such diffusers are a vulnerable component of the aeration tank. 
A typical aeration tank, illustrated in FIG. 1, is shown disposed between a 
primary treatment tank and a secondary clarifier. By way of background, 
for the primary treatment, preliminary sedimentation is the first process 
where removal of substantial quantities of suspended solids and materials 
causing biochemical oxygen demand in wastewater flow occurs. They are 
relatively quiescence in operation so as to permit suspended solids to 
settle if the specific gravity is greater than that of water, or float if 
the specific gravity is less. With the removal of the heavier sludge and 
the lighter skimmings being facilitated by the above noted separation, the 
remaining clarifier liquor may be transferred to the aeration tank. 
FIG. 1 illustrates schematically the treatment of activated sludge in an 
aeration tank. This is an aerobic suspended-growth process in which 
biodegradable organics in wastewater are intimately mixed with a 
concentrated mass of biota and oxygen within an aeration tank. New 
microorganisms grow and flocculate as the biotic mass adsorbs, oxidizes, 
and reduces the organic wastes. As the mixed liquor leaves the aeration 
tank following several hours of aeration, the biotic mass with the newly 
formed floc is separated within one or more final settling tanks. A 
portion of the settled floc, the activated sludge, is returned to the 
aeration tank to maintain the required concentrations of biota, while 
excess sludge is removed for solids handling and ultimate disposal. Air, 
required for utilization of dissolved oxygen in metabolism and 
respiration, is a critical element to the process to maintain and prevent 
sedimentation in the aeration tank. 
A commercial system that is currently in operation is one that uses a 
flexible membrane diffuser assembly, where an array of diffusers are 
disposed along the bottom of an aeration tank to transmit air under 
pressure from an air blower into the liquid medium--see U.S. Pat. No. 
5,330,688, granted Jul. 19, 1994, to the inventor hereof, where the 
contents of said patent are incorporated herein in their entirety. It was 
discovered that the use of such flexible membrane diffuser assemblies, 
including a flexible membrane, typically formed of a polymer, known 
commercially as EPDM, are subject to a premature hardening at elevated 
temperatures. This has been particularly demonstrated at temperatures of 
140 degrees F., and higher. Evidence has shown that some wastewater plants 
operating in warm climates using EPDM membrane diffusers, for example, 
have had premature membrane failures in as few as six months. This failure 
mode indicates that the membranes shrink and become hard. It is believed 
that the hardening of the membrane is generally a result of the contact 
between the membrane and the high process air temperatures, causing the 
membrane to polymerize. In warm climates, for example, the blower 
discharge temperature may be as high as 250 degrees F. The elevated 
temperature is the result of high ambient air temperature and the heat of 
compression in the air blower (approximately 15 degrees per psig of 
compression). It was thus critical find a means to lower the incoming air 
temperature to ensure sufficient diffuser life. 
Cooling systems, to reduce the temperature of a transmitted medium, have 
been known for years, as exemplified by U.S. Pat. No. 2,071,509, to 
Dudley. The patent is directed to a regenerative cooling system comprising 
a helically wound coil as a heat exchanger, including means for effecting 
a continuous recirculation of a cooling liquid through the coil. In such a 
system, incoming air and recirculated air is passed over and around the 
coil, and cooled thereby. 
U.S. Pat. No. 4,486,310, to Thornton, directed to the field of wastewater 
treatment facilities, relates to the operation of a wastewater trickling 
filter treatment system housing a medium with a dome sealingly attached. 
Means are provided in the dome to recirculate air through the filtering 
medium to change the temperature of the filtering medium, particularly to 
raise same during cold or winter temperature operations. The problem, as 
defined therein, is that the biological life within the filtering medium 
is the active agent for the treatment of the wastewater, however, its 
efficiency is temperature dependent. Thus, a means had to be devised to 
raise the temperature of the medium during cold temperature periods. 
While the patent to Thorton relates generally to the area of waste-water 
treatment, and to the temperature control of the operation, it fails to 
address or even recognize the problem of deteriorating flexible polymeric 
membrane, diffuser assemblies. And, the latter is just one of the types of 
diffused aeration units in use today, including a membrane cap diffuser 
(coarse bubble membrane), a diffuser tube (medium and fine bubbles), and a 
porous membrane (fine bubbles). The present invention relates to polymer 
type membranes of all types. Aeration systems relying on the fine bubbles 
of a flexible porous membrane diffuser have distinct advantages. However, 
these advantages can be lost with the premature failure of the membrane 
and the time lost in frequent maintenance. The manner by which the 
problems herein noted are overcome will become apparent in the 
specification which follows, particularly when read in conjunction with 
the accompanying drawings. 
SUMMARY OF THE INVENTION 
The invention relates to the combination of an aeration system for a 
wastewater or water treatment facility consisting of a treatment tank of 
predetermined depth, a plurality of air diffusers disposed along the 
bottom of the tank, an air blower, and an air distribution piping system 
to deliver air under pressure from the air blower to the air diff-users, 
and a tube bank heat exchanger located in the wastewater within the tank. 
The tube bank heat exchanger, arranged in line with the air distribution 
piping system within the tank, comprises a piping system, through which 
air from said air blower passes, of an equivalent length that exceeds 
three times the predetermined depth of the treatment tank. A preferred 
tube bank heat exchanger system comprises a multi-tube manifold type 
system, and an air diffuser including an essentially planar flexible, 
porous diffuser membrane secured to a diffuser housing by a threadably 
engaging retaining ring.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention relates to an improved wastewater treatment system, 
particularly in the design and operation of an aeration tank, where an 
array of air diffuser assemblies, having a polymeric type porous membrane, 
may be exposed to temperatures which can have a deleterious effect on the 
performance of such porous membranes. 
A typical liquid wastewater treatment system. as known in the art, may 
comprise (a) pretreatment to neutralize, alter, or remove nonbiologically 
degradable, toxic, hazardous, and highly corrosive materials, (b) primary 
treatment to achieve primary sedimentation or clarification of substantial 
quantities of suspended solids and materials causing biochemical oxygen 
demand in wastewater flow, and (c) secondary or biological treatment. This 
may include an aeration tank and secondary clarifier, by the use of 
biological organisms, predominantly aerobic bacteria, to convert and 
metabolize dissolved and colloidal matter remaining in wastewater to new 
cellular material, carbon dioxide, and water. 
It is rather obvious from the above that proper and efficient waste-water 
treatment requires the efficient operation of a number of treatment 
facilities. Many such facilities use mechanical devices which can be 
readily removed for maintenance or repair, without significant loss in 
down-time. However, the aeration tank, where the design and operation 
thereof is the thrust of this invention, with its array of diffuser 
assemblies disposed along the bottom of the tank, is not a system that 
lends itself to easy maintenance. If repairs or maintenance are required, 
the entire system or wastewater flow must be stopped and the aeration tank 
emptied to expose the diffuser assemblies for repair or maintenance. 
The use of porous membrane diffuser assemblies has now become popular due 
to their low cost and efficiency. However, porous, flexible membranes, 
when manufactured from polymeric materials, such as EPDM, for use in such 
assemblies, may exhibit a limited life when exposed to temperatures above 
its polymerization temperature, typically about 140 degrees F. That is, 
the EPDM type flexible membranes begin to heat age which causes hardening, 
and subsequent failure. This is particularly true in warm climates, or 
where aeration tanks are exceptionally deep, as will be explained 
hereinafter. 
It was discovered that the useful life of the diffuser membrane is critical 
to the further success of the flexible, porous membrane type diffuser in 
the wastewater treatment industry. Thus, one way to increase the life of 
the diffuser membrane is to reduce the temperature of the process air, see 
FIG. 1, coming in contact with the membranes. In doing so, the amount of 
premature hardening taking place in the membrane material will be reduced, 
thereby preventing the shrinking and hardening of the membrane, known as 
heat aging. Through experimentation, it was found that maintaining a 
maximum air temperature of 140 degrees F. at the membranes will 
sufficiently prevent the degradation of the membranes due to temperature. 
This experimentation led to the development of a unique heat exchanger 
tube bank, to effect the desired temperature reduction, as more clearly 
described in the accompanying drawings. 
Turning now to the several Figures, FIG. 1 illustrates schematically the 
operation of the aeration tank 10 within the broader wastewater treatment 
operation. Wastewater 12, from the primary treatment, along with 
recirculated activated sludge 14, enters the aeration tank 10 where the 
biodegradable organics in the wastewater are mixed with a concentrated 
mass of biota and oxygen, where the oxygen comes from air under pressure 
through a standard drop pipe 16 and diffuser assemblies 18. Typically, 
compressed air, such as by means of an air blower 20, is delivered by the 
straight or standard drop pipe 16 to introduce and maintain dissolved 
oxygen and mixing. In warm climates, for example, the air blower 
temperature may be as high as 250 degrees F. The elevated temperature is 
the result of high ambient air temperature and the heat of compression in 
the air blower 20 (approximately 12 to 15 degrees F. per psig of 
compression). Significant positive pressure is required to force the air 
through the diffuser assemblies 22 (FIG. 2), which are typically submerged 
beneath about 18 feet of wastewater to be treated. The greater the depth, 
the greater the pressure required for operation of the system. 
FIG. 2 illustrates more clearly the contributions of this invention. There 
are shown the major components of the improved aeration system, including 
the tank 24, into which the wastewater is received for treatment, a piping 
system including a manifold 26 for delivering air to the array of flexible 
diffuser assemblies, and a tube bank heat exchanger 28. The tube bank heat 
exchanger is submerged within the wastewater being treated, and positioned 
to receive air under pressure from the air blower 20 for distribution of 
such air to the manifold 26, thence to the porous membranes 30 of the 
diffuser assemblies to be released therethrough as fine bubbles. One 
advantage of the tube bank heat exchanger 28, to be described shortly, is 
the ease with which it may be retrofitted to existing systems. 
Specifically, the heat exchanger may be positioned in-line with the single 
drop pipe 32 between the air blower 20 and manifold 26. 
The preferred tube bank heat exchanger 28 is best illustrated in FIG. 3. 
When submerged within he wastewater, the heat exchanger acts as an 
air-to-water heat exchanger by breaking the single drop pipes 32 down into 
several smaller drop pipes. The preferred heat exchanger 28 comprises a 
pair of horizontally disposed closed-ended pipes or chambers 34 and a 
plurality of smaller pipes or conduits 36 extending therebetween. As a 
preferred embodiment, with a drop pipe having an I.D. of six inches, the 
pair of horizontal pipes will have an I.D. of six inches, spaced about 18 
feet apart, and the plural vertical pipes, preferably eight in number, 
will have an I.D. of two inches, all of which are formed of a heat 
conductive metal, such as stainless steel. By this design arrangement, 
there is an increased surface contact area between the heat conducting 
pipes and the wastewater, and an increased contact time by reducing the 
air velocity in the smaller plural vertical pipes 36. With this design, it 
was possible to reduce the temperature of the air being delivered to the 
diffuser assemblies 18 to a maximum of 140 degrees F. The aeration of the 
wastewater, for example, causes the wastewater to flow around the tube 
bank heat exchanger, extracting or removing heat therefrom, to provide 
improved heat dissipation or heat exchanging. To supplement the cooling of 
the air being delivered to the diffuser assemblies 18, evaporative cooling 
may be used. Specifically, a 90 degree angled evaporative cooling pipe may 
be placed in the drop pipe 32, with the opening directed down stream, 
where a cooling mist is injected therethrough to further cool the incoming 
air. 
FIG. 4 represents an alternate embodiment to the heat exchanger of FIG. 3. 
In the embodiment, the drop pipe 40 has been changed from a vertically 
arranged single piece to a single pipe that is formed in a snake-like 
fashion 42, or a sinusoidal configuration, where the length of the curved 
pipe is at least three times the depth of the wastewater. By this simple 
arrangement, the contact time between the heat conducting pipe surface and 
the wastewater has been significantly increased to the point that the 
temperature of the air passing through the pipe section 42 is reduced. 
Since the introduction of the flexible diffuser assembly, as exemplified by 
U.S. Pat. Nos. 3,997,634 and 5,330,688, to the inventor hereof, 
improvements have been made to the construction and design of the diffuser 
assembly. FIGS. 5 to 8 represent two preferred embodiments for a diffuser 
assembly, where such new diffuser assemblies are distinguished by a 
retaining ring which overrides the porous membrane for threading 
engagement to a base member. 
FIGS. 5 and 6 illustrate a first preferred embodiment for a diffuser 
assembly 50, which incorporates the retaining ring 52 and porous membrane 
54. In addition, for this embodiment, there are incorporated a membrane 
support plate 55, and a base 56, where the latter is secured to and in 
communication with a header pipe 58, as hereinafter described. The 
retaining ring 52, preferably formed of PVC, as known in the art, 
comprises an annular wall 60, internally threaded 62 for engagement with 
external complementary threads on said base. Additionally, the retaining 
ring 52 is provided with an inwardly directed shoulder 64, along the top 
edge 66 of annular wall 60. As best seen in FIG. 6, the shoulder 64 
overrides and retains the porous membrane 54 in position. 
The porous membrane 54 is distinguishable from the inventor's prior art 
membranes, in that it is essentially planar in design. Rather than 
incorporate a wrap around edge, as with earlier designs, the periphery of 
the membrane 54 is characterized by an upstanding annular rib 66 and a 
slightly larger downwardly projecting rib 68 having an annular groove 70 
therein. The latter rib 68, when the membrane is positioned over the 
support plate 55, fits snugly onto a shoulder, as later described. The 
upstanding rib 66 provides some structural stability to the assembly and 
bears against the retaining ring 52. Further, as with the earlier membrane 
designs, a thicker, upraised center portion 72 is provided to give 
additional structural stability, particularly over the opening 67 or air 
passageway. Finally, the membrane includes a plurality of slots 69, 
arranged in a series of concentric circles, to allow air to pass 
therethrough. 
Supporting and underlying the membrane 54 is the support plate 55. Such 
plate comprises a slightly domed support member 74 having an L-shaped 
peripheral shoulder 76 to snugly receive a complementary annular rib 68 on 
the membrane 54, as noted above. Centrally positioned along the domed 
support member 74 is an opening 69, which, as noted hereafter, allows air 
under pressure to pass from the base 56 to beneath the porous membrane and 
exit through the slots 69 as fine bubbles of air. 
The base 56 is designed to be secured to the top-to-side quadrant of a 
header pipe 58. Such design is particularly suited where the header pipe 
may be in close proximity to the tank wall, where positioned, and space is 
at a premium. In any case, the base 56 includes a chamber 80, a peripheral 
side wall 82, having external threads 84 thereabout for engagement with 
the complementary threads 62. Internally, the chamber 80 is provided with 
an annular shoulder 86 upon which the support plate 55 is seated. 
Externally, the chamber includes a curved wall 88, where the shape of such 
curved wall is comparable to the shape of the header pipe 58, so as to be 
secured snugly thereagainst. Such wall 88 also includes an opening 90 
aligned with a comparable opening 92 in the wall of header pipe 58. 
The alternate preferred embodiment illustrated in FIGS. 7 and 8 is of the 
type for centrally positioning the diffuser assembly 94 along a header 
pipe 58. Such assembly 94 incorporates a similar retaining ring 52 and 
porous membrane 54. The base 95 is different and consists of a single 
piece, which is preferably formed or molded from polypropylene. The base 
comprises an upper support member 96, for supporting the porous membrane 
54, where the periphery thereof includes a groove 97 defined by a 
peripheral wall 98 and shoulder 99 for snugly receiving the downwardly 
projecting rib 68 of the membrane 54. Externally the peripheral wall 98 is 
threaded 100 to engage the complementary internal threads 62 of the 
retaining ring 52. 
Centrally of the upper support member 96 there is provided a through hole 
102 within concentric, downwardly extending projections 104, 106, 108. The 
uppermost projection 104 defines a larger cavity 110 with a circular 
extension 112 through which hole 102 passes. The extension 112 may include 
one or more notches 114. The intermediate projection 106 may be 
multi-sided, such as hexagonal, to accommodate a tool for securely 
engaging the retaining ring 52 and the base 95. Finally, the lowermost 
projection 108 is threaded 116 to engage a complementary hole 117 in the 
header pipe 58.