Abstract:
A wastewater aerating system that conditions air inlet to a blower of the system to adjust the condition of air discharged into the wastewater so as to increase the flocculation rate of suspended solids from the wastewater. The wastewater aeration system includes a conduit having an outlet submerged within a bath of wastewater to be treated, an air duct inlet in flow communication with a source of ambient air, a blower operable to cause air to pass through the air duct and through the conduit to be output through the conduit outlet into the bath of wastewater, and at least one heat exchanger installed in the air duct and adapted to condition, such as condense its vapor and sensibly cool, the air passing through the air duct. A method of treating a bath of wastewater is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/111,547, filed Dec. 9, 1998, and U.S. Provisional Application No. 60/147,116, filed Aug. 4, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention pertains to wastewater treatment, and, in particular, to an apparatus that introduces oxygen into the wastewater to increase the flocculation rate of suspended solids. 
     Sewage treatment by means of flocculation removes dissolved and suspended solids from wastewater. Such treatment frequently takes place at a wastewater treatment facility in a rectangular secondary tank downstream from the primary treated wastewater, which tank may be continuously seeded with bacteria-laden activated sludge to augment growth of the bacteria that oxidizes the organics and inorganics in the wastewater to achieve flocculation of the waste. Flocculate of activated sludge depends upon gravitational effects and settles on the tank bottom from where it is scraped off and pumped to another tank or digester for a subsequent anaerobic treatment or disposal. The flocculation rate depends upon the biological aspect of the solids being treated, and the environment and life supporting habitat within the wastewater for the bacteria that in effect devours the organic carbon and other suspended solids. 
     In particular, in order to increase the rate of flocculation of the activated sludge, wastewater treatment processes frequently involve aeration to provide oxygen for the growth of the bacteria seedings. The provided oxygen is diffused into the wastewater and in combination with the dissolved oxygen provides life support for the bacteria that produces the flocculation of the waste. The replenishment of suspended oxygen along with the dissolved oxygen fosters the respiration of the bacteria that impacts the retention time needed for flocculation, which time is based upon numerous other factors including the characteristics of the sewage being treated, hydrostatic head, compressed air discharge temperatures, and net diffuser discharge pressure. 
     One known wastewater aeration system is diagrammatically shown in FIG.  1 . Concrete reservoirs or baths  10 , each of which may be one of many in a series of similarly constructed baths, each holds a quantity of wastewater  12 , such as domestic sewage, being treated. Air supply pipes  18  include at their respective downstream ends a diffuser section  16  submerged within the wastewater  12 . The diffuser sections  16  include a multitude of small orifices through which air is output or bubbled into the wastewater. Although only one supply pipe and diffuser for each reservoir is shown in FIG. 1 for illustration purposes, multiple pipes and/or diffusers may be used to provide an adequate amount and distribution of aerating air. Each of the air supply pipes  18  is connected to a distribution manifold  20  connected to a large diameter duct  22  that is supplied with pressurized air from a blower or centrifugal compressor  24 . A filtered intake duct  26  that ports or opens to the outside where ambient air is connected to compressor  24 . 
     During operation, compressor  24  draws ambient air into duct  26 , which air then passes in sequence through compressor  24 , duct  22 , manifold  20 , and supply pipes  18 . The blower conveyed air continues out through diffuser sections  16  in the form of bubbles, shown at  30 , that bubble or percolate upward through wastewater  12  to provide oxygen for the respiration of the bacteria within the wastewater. 
     While the prior art aeration system shown in FIG. 1 does on occasion provide some benefit, its effectiveness at treating the wastewater is sometimes limited. Specifically, the respiration of the bacteria in the wastewater is at optimum in a habitat of around 68° F. (20° C.) to a high of around 140° F. (60° C.). 
     Except for some rare heat resistant strains of bacteria, higher temperatures may harm or kill the bacteria within a short period of time. However, in the prior art of FIG. 1, the heat of compression incidentally applied to the aerating air by the blower may, depending on ambient air conditions, increase the temperature of the air reaching the diffuser and the wastewater to undesirably high levels at which bacteria respiration is hindered, or the bacteria is killed, and the solubility of dissolved oxygen is diminished. For example, in some situations and blower configurations, air output from the diffuser will have a dry bulb temperature of in excess of 155° F. when ambient air has a dry bulb temperature of as low as 70° F. Although such diffuser output temperatures may not measurably change the overall temperature in the tank so as to adversely affect all the bacteria therein, bacteria passing near the diffusers will be subjected to thermal shock. 
     Another shortcoming of existing aeration systems is related to the net diffuser discharge pressure, which is the difference between the discharge pressure at the diffuser orifices and the wastewater hydrostatic pressure. The pressure delivered by a centrifugal blower is dependent on the intake ambient air mixture density and specific humidity, which density is a function of the temperature and humidity of the ambient air. Under some ambient conditions, such as high temperature with high humidity, pressure losses due to high temperatures and vapor condensation downstream of the blower result in the net diffuser discharge pressure being insufficient to produce bubbles or to prevent wastewater from clogging the air distribution pipes. Under certain other ambient conditions, the net diffuser discharge pressure is so high that the diameter of the diffused air bubbles are too large and the wastewater is made too turbulent for efficient absorption of oxygen into the wastewater. 
     Still another shortcoming of existing aeration systems is related to the fact that classical adiabatic blowers ingest air at constant volumes. Because the density of air varies with temperature, the mass of air conveyed by the blower varies with temperature. As the temperature and relative humidity of air inlet to the blower increases, the mass of air and therefore oxygen delivered to the diffuser for introduction into the wastewater for use by the bacteria undesirably decreases. And, although the horsepower required to compress this less dense mass flow is reduced, its rate of decline is much less than that of the mass flow. 
     Thus, it would be desirable to provide a wastewater aeration system that overcomes these and other shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a wastewater aeration system that improves the flocculation rate of suspended solids within the wastewater by conditioning an intake air flow prior to its introduction into the compressor of the aeration system. The intake air is conditioned by a vapor condensing heat exchanger within the air stream that de-hydrates and cools the air to a desired level. Another and optional heat exchanger positioned within the air stream more downstream of the vapor condensing heat exchanger will secondarily sensibly cool the air stream upstream from the bell-mouth of the compressor. The inventive heat exchangers may also be configured to heat the air stream when ambient conditions are cooler than desired. 
     In one form thereof, the present invention provides a wastewater treatment system for a bath of wastewater, including a conduit having an outlet opening into the bath of wastewater, an air duct having an inlet in flow communication with a source of air, at least one blower in flow communication with the air duct and the conduit and operable to cause air to pass through the air duct and through the conduit to be output through the conduit outlet into the bath of wastewater, and a heat exchanger installed in the air duct and adapted to condition air passing through the air duct. 
     In another form thereof, the present invention provides a wastewater aerating system including a conduit having at least one outlet submerged within a bath of wastewater to be treated, an air duct having an inlet in flow communication with a source of ambient air, at least one blower in flow communication with an air duct outlet and a conduit inlet and operable to cause air to pass through the air duct and through the conduit to be output through the at least one outlet of the conduit into the bath of wastewater, and at least one heat exchanger installed in the air duct and adapted to condition air passing through the air duct, whereby the temperature of the conditioned air reaching the at least one blower is modulated by the at least one heat exchanger to promote flocculation of suspended solids from the wastewater when the conditioned air is output into the bath of wastewater. 
     In still another form thereof, the present invention provides a method of treating a bath of wastewater including the steps of providing a conduit having an upstream end in communication with a source of ambient air and a downstream end opening into the bath of wastewater below the wastewater surface, inletting ambient air into the conduit through the upstream end, moving the inlet air through the conduit from the upstream end to the downstream end, outletting air from the conduit into the bath of wastewater through the downstream end, and conditioning the air moved through the conduit such that the air outlet from the conduit is at a temperature that promotes flocculation of suspended solids from the wastewater. 
     One advantage of the present invention is that it provides a water aeration system that is cost effective to operate. 
     Another advantage of the water aeration system of the present invention is that it either reduces compressor brake horsepower required to aerate a given tank, or results in an increased compressed air output at the diffuser in the tank for a given installed compressor horsepower, 
     Another advantage of the water aeration system of the present invention is that by increasing the amount of suspended oxygen that is introduced into the wastewater and thereby improving the habitat for the flora of bacteria therein, the flocculation rate of suspended solids is increased so as to essentially increase the capacity of the wastewater plant by decreasing the required retention time of the wastewater within the plant. 
     Another advantage of the present invention is that it provides a water aeration system that within a normal range of ambient air conditions will deliver air at a relatively constant net diffuser discharge pressure and temperature to a diffuser immersed in a bath of wastewater, thereby providing a suitable air quality for suspending oxygen into the wastewater, a reduction of thermal shock on the piping system, an even flow of air through the diffuser, a reduction of head loss due to air-side fouling, a reduction of power consumption due to liquid-side fouling, a suitable oxygen transfer efficiency to the wastewater, a suitable bubble formation with reduced vapor content at the diffuser, and a reduction in vapor condensation in the piping. 
     Another advantage of the water aeration system of the present invention is that it delivers air to the diffuser at temperatures less likely to create a thermal shock to the flora of bacteria. 
     Another advantage of the present invention is that it provides for an increased suspended oxygen supply to the flora of bacteria their respiration temperature. 
     Another advantage of the present invention is that it may lower the temperature of the air stream outlet at the diffuser, thereby lessening the turbulence of the water column and consequently increasing the settle rate of flocculate from the wastewater. 
     Still another advantage of the present invention is that it controls the diameter of the diffused air bubbles, thereby permitting a less violent action of bubble formation for a more efficient absorption rate of oxygen into the wastewater. 
     Still another advantage of the present invention is that it reduces turbulence and churning of the wastewater which diminishes the stillness required for sedimentation of the flocculate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other advantages and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following descriptions of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a diagrammatic front view, in partial cross-section, of a prior art aeration system for treating wastewater; 
     FIG. 2 is a diagrammatic front view, in partial cross-section, of portions of a wastewater aeration system of the present invention; and 
     FIG. 3 is diagrammatic view illustrating one embodiment of the components used to produce coolant for the wastewater aeration system of FIG.  2 . 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the invention, the drawings are not necessarily to scale and certain features may be exaggerated or omitted in order to better illustrate and explain the present invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 2, there is diagrammatically shown selected portions of one embodiment of a wastewater aeration system, generally designated  50 , of the present invention. In this embodiment, various aeration system components of the prior art shown in FIG. 1 have been supplemented with an air treating or conditioning system disposed upstream of the blower which functions to provide the aeration system with improved aerating capabilities even during adverse environmental operating conditions possibly experienced by the system. 
     In FIG. 2, wastewater aeration system  50  is shown being used to treat domestic sewage  52  contained in a series of similarly constructed secondary treatment cells or reservoirs  54  made of concrete. Reservoirs  54  depict secondary tanks that are downstream of the primary treated wastewater in a municipal wastewater treatment plant or facility. While only two cells are partially shown in FIG. 2 to facilitate illustration, such cells are representative of additional cells that may be provided for sewage retention. This shown and described configuration is intended to be illustrative and not limiting, however, as the present invention may be used in treating wastewater other than domestic sewage, such as industrial sewage and other polluted water that will become septic by nature, and further may be employed in other locations where wastewater is collected, such as in the primary tanks of the treatment plant or in lift stations. 
     Aeration system  50  includes an air diffuser  56  fitted to the downstream end of an air supply pipe  58  that extends into each reservoir  54 . Diffuser  56 , which is provided with orifices through which air bubbles into the wastewater, is shown submerged within a lower region of the wastewater  52  to allow a relatively large depth of the wastewater to be subjected to air percolation and suspended oxygen, but within the scope of the present invention may be positioned below the surface at depths of the wastewater other than as shown. The quantity, placement and configuration of each of pipe  58  and diffuser  56  may be selected by one of skill in the wastewater treatment art to achieve a suitable distribution of aerating air. 
     Air supply pipes  58  are connected to a distribution manifold  60  that is supported on a landing  62  between the adjacent cells  54 . A large diameter air duct  64  connects a blower or centrifugal compressor  66  to manifold  60 . Although only one blower is shown, multiple blowers may be provided if required to efficiently move the aerating air. Blower  66  is furnished with a supply of conditioned air through an ambient air conditioning assembly generally indicated at  70 . 
     Assembly  70  includes a louvered air inlet  72  at an upstream end of duct  74  which opens to the outdoors and its ambient air. Ambient air drawn through louvered inlet  72  by operation of blower  66  passes through a not shown air filter and over or through a heat exchanger abstractly shown at  76 . As for most applications it is envisioned that the conditioning of the ambient air will be in the form of a de-hydrating and a cooling of the air, heat exchanger  76  is further described herein as a vapor condensing heat exchanger. 
     Vapor condensing heat exchanger  76  may be of any conventional design known in the heat exchanger art that promotes dehydration and cooling of a passing air flow. Heat exchanger  76  is abstractly shown as a cooling coil over which air flows and which provides a circuitous path for relatively low temperature chillant or coolant being carried therethrough. Cooling coil  76  may be provided by cooling coil tube sections that are arranged in rows and columns in the air duct and oriented to be generally transverse to the flow of ambient air being drawn through inlet  72 . The cooling coil tube sections may be arranged, for example, to vertically or horizontally extend and with spacing between the cooling coil tube sections to provide a large surface area for contact with passing air. Cooling coil  76  is supplied with coolant through a thermally insulated inlet line  78  connected to a source of low temperature coolant abstractly indicated at  79 . A thermally insulated return line  80  routes coolant that is heated during its use in cooling coil  76  back to the source of coolant  79  for reuse. Although single inlet and outlet coolant lines for heat exchanger  76  are shown, multiple inlet lines and outlet lines may be used and separately circuited to the coil tube sections within the scope of the present invention. 
     Vapor condensing heat exchanger  76  is sized and arranged to condition the intake air flowing through assembly  70  to improve the aerating performance of the system. After passing heat exchanger  76 , the conditioned air is preferably at the conditions desired of air to be input to the suction side of blower  55  as described below. 
     The near arid conditioned air emerging from heat exchanger  76  then flows through a straight length of duct  74  of assembly  70 . Demisters generally indicated at  81  are arranged within duct  74  to further remove water vapor within the flowing air. Auxiliary insulation specification dampers, controls and backwashing cleaning components that are frequently parts of existing air duct systems in other arts may be employed within assembly  70 , but are not shown for purposes of illustration. As the air flow reaches the end of the straight initial portion of duct  74 , it encounters a ninety degree bend section  82  which is in air flow communication with a duct section  84  with a flow converging geometry. Although shown as having a ninety degree bend, bend section  82  may bend at other angles. In addition, the bend section may be eliminated such that the duct with the flow converging geometry is in line with duct  74 . Duct section  84  ports to a pipe  85  that inlets to the bell mouth or suction side of blower  66 , but in an alternate, not shown embodiment may port directly into the bell mouth of blower  66 . 
     Disposed within duct section  84  is a second sensible cooling heat exchanger  86 . Due to the exposure of the periphery of air duct  74  to ambient air, as well as the air flow being subject to impediments, the air being conveyed through air duct  74  downstream of heat exchanger  76  may rise in temperature. Heat exchanger  86 , which may include one or more cooling coils arranged in a similar fashion to the coil of the vapor condensing heat exchanger  76 , is configured to re-cool to the desired blower intake air temperature the air passing through the assembly  70  which has previously been conditioned by heat exchanger  76 . Heat exchanger  86  may be eliminated if heat exchanger  76  achieves satisfactory cooling of the air reaching blower  66 . Heat exchanger  86  is shown plumbed to coolant source  79  in parallel with heat exchanger  76  by way of insulated inlet line  78   a  and insulated outlet line  80   a , but may be plumbed in series in alternate embodiments. 
     Heat exchangers  76  and  86  are suitably sized and shaped, and controlled with a not shown controller, to condition the air such that for any ambient conditions, the air outlet through diffuser  56 , despite the air heating associated with compression by blower  66 , is at a controlled temperature that preferably is not unacceptably high, such as a temperature that might cause significant harm to the working bacteria in the wastewater  52 , and further is at a suitable discharge pressure at the diffuser. Although not shown, automatic controls with appropriate sensors, such as external to assembly  70  for monitoring ambient conditions or internal to duct  74  for monitoring the conditioned air, may be provided and circuited with the shown system to control the flow of coolant to the heat exchangers to insure that air inlet into blower  66  has the appropriate air properties at all times during blower operation. For example, sensors provided within air pipe  85  can be connected with a control system that automatically provides greater or lesser, depending upon the sensor readings, heat exchanging capabilities to the sensible cooling heat exchangers  76  and  86  in order to optimize the air intake to the blower  66  to the selected characteristics. 
     In a preferred embodiment, for any and varying ambient conditions, the conditioned air reaching blower  66  is modulated at all times to be at one or more temperatures selected or programmed by a user. When operating within higher ambient temperatures, suitable conditions of the air inlet to blower  66  include a temperature of preferably between approximately 45° F. and 55° F., and more preferably of about 50° F., and a relative humidity of one hundred percent. Temperatures outside this range may be used within the scope of the present invention. For such 50° F. and 100% RH conditions, in aeration systems employing a blower having a compression ratio of 1.69: 1, the temperature of air output at the diffusers is about 134° F., at which temperature bacteria typically will not be thermally shocked. Additionally, these air characteristics at the blower provide an increased quantity of oxygen output through diffuser  56  than for warmer air. For example, assuming a sea level pressure of 14.7 psia, a barometric pressure of 14.5 psia, a blower inlet suction pressure of 14.3 psia, a diffuser discharge pressure of 24.7 psia and a blower compression ratio of 1.69:1, inletting about seven thousand cubic feet per minute of air into blower  66  at 50° F. and 100% Relative Humidity instead of 104° F. and 85% RH results in over a sixteen percent increase in the amount of oxygen supplied to the wastewater through diffuser  56 . 
     Blower inlet air conditions of 50° F. and 100% RH advantageously result in diffuser air discharge temperatures even lower than 134° F. when the compression ratio of the blower is less than 1.69:1, thereby making thermal shock even more unlikely. For such lower compression ratio blowers, and preferably while ensuring the diffuser air will still be discharged at temperatures below about 135° F.-140° F. to reduce the likelihood of thermal shock to the bacteria, the temperature of the blower inlet air maintained by the system may within the scope of the invention be even greater than the preferred temperature range limit of 55° F. listed above. Even lower blower inlet air temperatures than 45° F. may be used if it is desired to further increase oxygen throughput to the diffuser for variously configured blowers. Although blowers having compression ratios greater than 1.69:1 that are fed intake air that has been cooled to 50° F. and 100% RH will deliver to the diffusers air that is at temperatures possibly above 140° F. which may cause thermal shock, for purposes of avoiding thermal shock such diffuser air temperatures are better than the temperatures of air discharged if the blower inlet had not been cooled by the present invention. 
     Coolant source  79  preferably provides inlet lines  78  and  78   a  with a cooled working fluid such as water at a temperature of typically between about 42° F. and 46° F. Coolant source  79  may be furnished in any manner that is known in the cooling arts. One suitable manner is in the form of an absorption refrigerator or chiller system, the general configuration and workings of which are further described in U.S. Pat. No. 4,936,109, the entire teachings of which are incorporated herein by reference. The absorption chiller, in addition to potentially being directly heated with standard fuel sources, may in an alternate embodiment be heated by the gas produced within a digester tank element that forms a part of many conventional wastewater treatment systems with which the aeration system of the present invention may find beneficial application. 
     Other suitable manners of furnishing coolant source  79  include vapor compression refrigeration equipment, as well as using natural sources of cooling fluid such as low temperature groundwater, lake water or potentially the wastewater being treated. For example, the operation of a pump could deliver cooler groundwater as a coolant to the heat exchangers. 
     Referring now to FIG. 3, there is shown one suitable and near non-parasitic way in which an absorption chiller is powered, as well as plumbed, to provide coolant to heat exchangers  76  and  86  of a water aeration system similar to that shown in FIG. 2, portions of which system are shown enclosed in a protective housing  89 . In particular, the coolant is created using the waste heat stream of a combustion turbine or air breathing heat engine of a power generation system, generally indicated at  88 , that is located on site at the wastewater treatment facility and operated to service the electric needs of the facility. 
     Although the overall configuration of certain relevant portions of the power generation system shown in FIG. 3 is more fully described in U.S. Pat. application Ser. No. 09/102,428, the entire teachings of which are incorporated herein by reference, power generation system  88  generally includes a combustion turbine abstractly shown at  92  that utilizes the heat of compression gases from its combustor to drive the shaft indicated at  93 . Shaft  93  is drivingly connected to a generator  94  from which generated electricity is drawn to operate the facility. Turbine  92  operates in the stationary mode and is fixedly mounted to a support surface or ground during use. Turbine  92  and generator  94  are housed within a compartmentalized enclosure  96 . 
     Ambient air introduced into enclosure  96  through inlet  97  passes through the enclosure compartment housing generator  94  so as to cool the enclosure and the enclosed generator, and is then exhausted to the atmosphere through stack  98 . Ambient air similarly introduced into enclosure  96  through inlet  99  passes through the enclosure compartment housing turbine  92  so as to cool the enclosure and the enclosed turbine, and is then exhausted to the atmosphere through stack  100 . Inlets  97  and  99  may deliver ambient air that has previously been cooled by contact with a heat exchanger positioned in the duct connecting the inlets to the source of ambient air. In an alternate embodiment not shown, ambient air cooled by a heat exchanger in an intake duct is routed through a non-compartmentalized or common enclosure to cool the generator and turbine in the manner described in U.S. Pat. application Ser. No. 09/102,426, the entire teachings of which are incorporated herein by reference. 
     Ambient air is also delivered to the turbine for operation by way of a duct system including a louvered air inlet  104 . Ambient air drawn through inlet  104  passes through a filter and over a first heat exchanger  106  that condenses vapor in and sensibly cools the air flow, past demisters  108 , and past a heat exchanger  110  that sensibly re-cools the air passing through the duct system. 
     The exhaust of turbine  92  is ported through enclosure  96  via conduit  112  to a waste heat recovery unit and stack exhaust system generally designated  114  that exhausts to the atmosphere. A pair of heat exchangers  115 ,  116  circuited in parallel with the absorption chiller are positioned within a flue of the exhaust stack and are used to draw heat from gases being exhausted in order to power the absorption chiller or refrigeration unit, abstractly shown at  120 , that is connected in a conventional fashion to a not shown cooling water tower. 
     The low temperature coolant supply line  122  from chiller  120  is plumbed via line  122   a  to supply coolant in series to heat exchangers  110  and  106  in the air duct of power generation system  88 , and via line  122   b  to supply coolant in series to heat exchangers  86  and  76  in the wastewater aeration system assembly  70 . Warmed coolant is returned from the various heat exchangers via return lines  124   a  and  124   b  that communicate with return line  124  to chiller  120 . 
     While this invention has been shown and described as having multiple designs, the present invention may be further modified within the spirit and scope of this disclosure. For example, in situations where inlet ambient air is actually below the desired temperature of air reaching blower  66 , heat exchangers  76  and  86  may be heating coils circuited with a source of high temperature fluid or an electrical power source if such coils use resistance heating. In addition, both a heating coil and a cooling coil may be employed within a single wastewater aeration system of the present invention such that the heating coil is selectively used when ambient conditions are cooler and the cooling coil is selectively used when ambient conditions are wanner. Still further, rather than using heat exchangers supplied with coolant in the manner described above to density the air inlet to the blower, other techniques to perform air densification may be employed despite their limitations. For example, adiabatic cooling in which water is sprayed in the ducted intake air stream, or the use of a cooling system in which vapor is first condensed and then a sensible cooling is accomplished by refrigeration means bringing the air mass to the design temperature, may be used to sensibly lower the temperature of air introduced to the blower. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.