Abstract:
A system for anaerobically and aerobically treating wastewater comprises a source of activated sludge for promoting anaerobic and aerobic digestion of wastewater, a compressor for supplying a volume of pressurized air to the system for promoting aerobic digestion of wastewater, a pump for circulating wastewater through the system, at least one anaerobic reactor for anaerobically treating wastewater with the activated sludge, at least one aerobic reactor for aerobically treating wastewater with the activated sludge and pressurized air, a pressure control system for regulating pressure in the system, a discharge system for removing byproducts of the system, and a ported injector for increasing a surface area of the volume of pressurized air from the compressor as it injects the pressurized air into the circulating wastewater.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present application claims the benefit of provisional Application No. 60/710,357, filed Aug. 23, 2005 by Craig Brase, entitled “System and Method for Introducing High Pressure Air into a Wastewater Treatment System” according to 35 U.S.C. § 119(e), which is incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Wastewater treatment has been and continues to be a matter of great environmental importance, including issues relating to municipal sewage or animal waste streams. Traditionally, organic matter containing excessive biological oxygen demand (BOD) has been treated using microbial action in a manner that separates the organic matter, to form a mass of solids in the form of an activated sludge, from the remainder or water fraction. The treatment problem is decidedly two-fold because the water and the sludge fractions both must be treated to be safely returned to the environment, and the two fractions may contain different impurities to be removed.  
         [0003]     Treatment typically involves digestion of the organic material through fermentation of the sludge involving aerobic or anaerobic bacterial action or some combination thereof. These processes are used to reduce or consume the chemical oxygen demand (COD) and biological oxygen demand (BOD) of the material and reduce them to an environmentally safe level in the organic materials. It is also necessary to remove undesirable inorganic materials from the water fraction, which typically contains undesirable quantities of phosphorus and nitrogen compounds including phosphates and nitrates.  
         [0004]     Wastewater treated by conventional wastewater treatment systems contains soluble, partially soluble and insoluble material as well as contaminates. Materials in the wastewater may be decomposable, partially decomposable or not decomposable. Wastewater treatment systems are designed to provide controlled decomposition of wastes to reduce pollution, health hazards and offensive odors.  
         [0005]     Decomposable and partially decomposable materials are referred to as biodegradable; that is, the material may be biologically broken down, or stabilized by bacterial action. Decomposable material is stabilized in wastewater treatment systems by bacteria, protozoa, and other microorganisms. Bacterial consumption of material, creating energy and reproducing bacterial cells, is the foundation of activated sludge wastewater treatment.  
         [0006]     Conventional wastewater treatment systems may include pretreatment, primary treatment, secondary treatment, and advanced treatment. Pretreatment includes screening, comminuting (mechanical cleaning of screens by shredding solids to a size which can pass through screen openings), degritting, and grease and scum removal. Primary treatment includes removal of suspended solids from wastewater by clarification and skimming. This typically involves a tank or channel and the following steps: reducing flow velocity, settling heavier solids, and skimming relatively light solids. Primary treatment may include anaerobic digestion processes, aerobic digestion processes, or a combination thereof. Primary treatment systems typically include sludge collection mechanisms, sludge suction devices, grit removal devices, and sludge dewatering devices to reduce the volume of sludge to be disposed. Secondary treatment systems are typically aerobic systems including an aeration phase and a clarification phase. Secondary treatment systems typically include an aeration tank, an air distribution system, a clarifier, sludge collection mechanisms, and sludge removing devices. Advanced treatment includes further removal of suspended and dissolved organic solids by means including filtration and removal of pathogens and chloroforms by oxidation, chlorination or heating, precipitation of minerals, adsorption, or other methods. In a further process in advanced treatment, the purified liquor from the clarifier is typically filtered and refined through chlorination, oxidation, or heating.  
         [0007]     In the activated sludge process of primary or secondary treatment, microorganisms are contained in an activated sludge and mixed with incoming wastewater; the wastewater providing food for the microorganisms whereby more activated sludge is produced. Such mixing is accomplished in an aeration tank or channel. In the aerobic activated sludge process, oxygen is intrinsically mixed with the activated sludge and the wastewater. The microorganisms convert suspended organic solids into energy, carbon dioxide, water, and additional microorganism cells. The aerobic activated sludge process therefore typically includes mixing of wastewater, activated sludge, and oxygen in an aeration tank; consumption of suspended organic solids by bacteria; settling of activated sludge in a clarifier; returning the activated sludge to the aeration tank for further treatment; removing purified liquor from the clarifier; and removing and disposing of the final, inert sludge.  
         [0008]     Existing processes and installations for the treatment of such residential and community wastes have generally been large scale operations having installation costs measured in terms of millions of dollars such as associated with typical municipal treatment plants. There exists a need to provide a compact, low-cost system and method for treating wastewater from residential and community sources. There also exists a need for a portable, modular wastewater system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     A system for anaerobically and aerobically treating wastewater comprises a source of activated sludge for promoting anaerobic and aerobic digestion of wastewater, a compressor for supplying a volume of pressurized air to the system for promoting aerobic digestion of wastewater, a pump for circulating wastewater through the system, at least one anaerobic reactor for anaerobically treating wastewater with the activated sludge, at least one aerobic reactor for aerobically treating wastewater with the activated sludge and pressurized air, a pressure control system for regulating pressure in the system, a discharge system for removing byproducts of the system, and a ported injector for increasing a surface area of the volume of pressurized air from the compressor as it injects the pressurized air into the circulating wastewater. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a diagram of one embodiment of the wastewater treatment system of the present invention.  
         [0011]      FIG. 2  is a diagram showing the charge cycle of the wastewater treatment system.  
         [0012]      FIG. 3  is a diagram showing the fill cycle of the wastewater treatment system.  
         [0013]      FIG. 4A  is a diagram showing the run cycle of the wastewater treatment system.  
         [0014]      FIG. 4B  shows a cross sectional view of one embodiment of an anaerobic reactor of the present invention.  
         [0015]      FIG. 4C  shows a cross sectional view of one embodiment of an aerobic reactor of the present invention.  
         [0016]      FIG. 5  is a diagram showing the bypass cycle of the wastewater treatment system.  
         [0017]      FIG. 6  is a diagram showing the de-gassing cycle of the wastewater treatment system.  
         [0018]      FIG. 7  is a diagram showing the system discharge portion of the discharge cycle of the wastewater treatment system.  
         [0019]      FIG. 8  is a diagram showing the sludge discharge portion of the discharge cycle of the wastewater treatment system.  
         [0020]      FIG. 9  is a diagram showing the clarifier elements of the wastewater treatment system of the present invention.  
         [0021]      FIG. 10  shows one embodiment of the wastewater treatment system of the present invention as operable in a mobile transport vehicle.  
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  shows one embodiment of the system of the present invention.  FIG. 1  is intended to be representative of a system that can be used to perform the process but is presented by way of example and by no means with any intention to limit the scope of either the process or the types of systems capable of performing it. Placement and use of components such as valves are discernible by those skilled in the art.  FIG. 1  shows a schematic of system  10  having supply line  12 , fill pump  14 , anaerobic reactor  16 , first aerobic reactor  18 , second aerobic reactor  20 , compressor  22 , vapor tank  24 , clarifier  26 , discharge pump  27 , circulation pump  28 , sludge tank  30 , surge tank  32 , chlorinator  34 , dechlorinator  36 , ventilation systems  38 A and  38 B, filter  40 , programmable logic controller (PLC)  41 , limit switch  42 , exit  43  and valves  44 A through  44 R. Also shown is injector section  46  of anaerobic reactor  16 . System  10  can also be equipped with various sensors that are used in conjunction with PLC  41  for controlling the flow of wastewater through the system. Such sensors might include pressure sensors, flow meters and oxygen demand sensors.  
         [0023]     The present invention provides a process together with a compact, low-cost wastewater treatment system. The process is particularly suited to treat wastewater material from sources such as residences and businesses. Influent wastewater obtained from a direct connected flushing system for one or more sources of wastewater such as residences or businesses is connected to system  10  at inlet  12 .  
         [0024]     The system and method remove up to about 98% of the BOD and up to about 60% of the phosphates from the wastewater. The method for treating wastewater includes an aerobic step in which BOD is metabolized in aerobic reactors  18  and  20 , and an anaerobic step in which phosphorous is metabolized or resorbed in anaerobic reactor  16 . The present invention utilizes a continuous source of active biological cultures from sludge tank  30  for processing the wastewater. This system uses a bubble flow technique in which a high surface area of high oxygen content air is infused at injector section  46  into a stream of circulated wastewater. The entire biomass is mixed and circulated through anaerobic reactor  16 , aerobic reactors  18  and  20  and vapor tank  24 . The system is maintained at an elevated pressure, and gas is vented from system  10  through vapor tank  24  as needed to maintain pressure. Granules of phosphates, nitrates and ammonia that formed in the sludge during the anaerobic and aerobic reaction processes are screened out by filter  40 . Treated liquid and solid materials are separated at clarifier  26  where liquids are further processed for removal from system  10  and activated sludge solids are recycled into sludge tank  30 .  
         [0025]     In one embodiment, the system is operable in six cycles: charge, fill, run, bypass, degassing and discharge. In the charge cycle, system  10  is charged with activated sludge from sludge tank  30 . This ensures there is enough active bacteria in the system for reacting the wastewater. In the fill cycle, influent wastewater from an outside source is introduced to the system  10  through supply line  12 . In the run cycle, the influent wastewater and the sludge is cycled through anaerobic reactor  16 , first aerobic reactor  18 , second aerobic reactor  20  and vapor tank  24  by circulation pump  28  in order to allow bacteria to digest high BOD organic material. In the bypass cycle, the influent wastewater and sludge is continually circulated through first aerobic reactor  18 , second aerobic reactor  20  and vapor tank  24 , bypassing anaerobic reactor  16 . Anaerobic reactor  16  is closed off from the circulating wastewater in order to allow bacteria to digest phosphorus present in the wastewater. In the de-gassing cycle, excess pressure from the system is vented through vapor tank  24  and ventilation systems  38 A and  38 B. In the discharge cycle, properly treated wastewater is removed from system  10  in solid and liquid form through surge tank  32  and clarifier  26 , returning newly created activated sludge to sludge tank  30 . After a discharge cycle, additional influent wastewater can be added to the system through supply line  12  and new activated sludge can be added to anaerobic reactor  16  from sludge tank  30 .  
         [0026]      FIG. 2  shows the charge cycle of system  10 . Before influent wastewater is added to the system, activated sludge from sludge tank  30  is pumped into anaerobic reactor  16  using circulator pump  28 . In one embodiment, pump  28  is activated for about 5 to about 10 seconds. The inflow of activated sludge is shown in  FIG. 2  with arrows. Valves  44 K,  44 M,  44 I and  44 J are closed to allow circulator pump  28  to draw in activated sludge through open valve  44 L. Sludge is then pumped through valve  44 F into anaerobic reactor  16 . The presence of activated sludge in the system before circulation begins, ensures that there is sufficient biological activity in the system to decompose the organic matter containing excessive BOD. Both the anaerobic and aerobic steps are performed utilizing the same naturally occurring heterotrophic bacteria which become conditioned to withstand high pressures and temperatures up to about 150° F. (65° C.). In one embodiment, a ratio of activated sludge to influent wastewater ranges between 1:1 and 1:15. In one embodiment, the activated sludge that is added to anaerobic reactor  16  has an age of at least about five days to sustain the nitrobacteria and nitrosomonas that enable system  10  to convert unincorporated ammonia (NH 3 ) into nitrites (NO 2 ). The nitrite is later converted to nitrate (NO 3 ). Nitrogen (N 2 ) is stripped and released by the continuous alternating anaerobic and aerobic action that occurs during processing of the wastewater.  
         [0027]      FIG. 3  shows the fill cycle of system  10 . After activated sludge is added to system  10 , raw wastewater is added to system  10  through inlet  12  using fill pump  14 . Influent wastewater from a community or neighborhood sewage system discharge pipe or other source of wastewater is connected to inlet  12 . Influent wastewater is shown in  FIG. 3  with arrows. Influent wastewater is pumped through open valves  44 A and  44 C by fill pump  14  into anaerobic reactor  16 . Valves  44 B,  44 D,  44 E and  44 F are closed to direct wastewater into anaerobic reactor  16 . Valve  44 G is also closed to direct wastewater into aerobic reactor  18 , aerobic reactor  20  and vapor tank  24 . During the fill cycle, system  10  is not open to the atmosphere and valves  44 O,  44 P and  44 Q of ventilation systems  38 A and  38 B are closed. Typically, the total volume in aerobic tanks  18  and  20  is large compared to the volume in anaerobic tank  16 . In one embodiment, the filling of reactors  16 ,  18  and  20  is indicated by level indicators. Vapor tank  24  contains limit switch  42  which controls fill pump  14 . Once vapor tank  24  becomes approximately half full, limit switch  42  is tripped and fill pump  14  shuts off. The system is now ready to be cycled for treatment of the wastewater.  
         [0028]     In one embodiment, the influent wastewater contains up to about 10% solid organic waste and is optionally conditioned in a pretreatment step as by emaciating or pulverizing. In one embodiment, devices such as emaciators and screens (not shown) are provided to chop or otherwise divide up and filter the solids in the material to be carried from the community or neighborhood sewage system prior to entry into inlet  12 , such that only pulverized entrained solids are contained in the influent stream moved by fill pump  14  or circulation pump  28 . This prevents the build up of solids on screens or filters and prevents line blockage from hulls and fibers and other non-digestible material contained in the solids. Continued circulation of the wastewater further breaks up solid particles.  
         [0029]     In one embodiment, fill pump  14  has a relatively high flow volume to expedite the filling or charging of system  10 . In one embodiment, pump  14  is a chopper pump and has associated emaciating capabilities to divide solid material. The material pumped by pump  14  is in the form of a sludge containing finely divided active solids plus extraneous solid material carried along in the flow. One embodiment of the invention also includes a flow controller and reverse flow-preventing shut-off valve, which is typically a solenoid valve. The flow controller and reverse flow-preventing shut-off valve cooperate to produce a controlled pressurized feed stream. In one embodiment, fill pump  14  operates at about 1750 rpm.  
         [0030]      FIG. 4A  shows the run cycle of system  10 . In the run cycle, wastewater is continually cycled through anaerobic reactor  16 , first aerobic reactor  18 , second aerobic reactor  20  and vapor tank  24  by circulation pump  28 . Circulated wastewater is shown with arrows in  FIG. 4 . The circulation ensures mixing of the influent wastewater with activated sludge provided to anaerobic reactor  16  during the charge cycle. Valves  44 K,  44 L,  44 G,  44 I and  44 M are closed to allow cycling through anaerobic reactor  16 , aerobic reactors  18  and  20  and vapor tank  24 . As wastewater enters aerobic reactor  18 , valve  44 N is opened to allow high oxygen content air supplied by compressor  22  to be infused with the flow of wastewater through injector section  46 , thus creating an aerated flow of wastewater. This is indicated in  FIG. 4  with double arrows. The aerated wastewater is charged with a supply of oxygen and is ready for aerobic treatment in aerobic reactors  18  and  20 . During the run cycle, valves  44 O,  44 P and  44 Q of ventilation systems  38 A and  388 B and valve  44 R of vapor tank  24  remain closed to allow the pressure in system  10  to rise to the desired level. Valves  44 C,  44 D and  44 F remain closed in order to circulate wastewater through anaerobic reactor  16  and aerobic reactors  18  and  20 . Throughout circulation, bacteria of the activated sludge in first aerobic reactor  18  and second aerobic reactor  20  progressively digest the high BOD organic material in the circulated wastewater utilizing the oxygen from the infused air. While two aerobic reactors  18  and  20  are shown, additional reactors can be provided in other embodiments to handle additional material or further reduce BOD. During the run cycle, anaerobic reactor  16  also aerobically treats the wastewater.  
         [0031]     Vapor tank  24  further includes a vent valve  44 R which can be operated to vent in a pulsing degassing manner that maintains a desired operating pressure during the run and bypass cycles or can be opened to atmospheric pressure, such as during the charging and discharging portions of the cycle. In this embodiment, excess gas is discharged from system  10  via vapor tank  24 , which releases the necessary amount of gas to atmosphere and maintains the desired system pressure. When system pressure reaches about 87-89 psi, vapor tank  24  opens the vent valve for about 3-5 min in order to let off about 15 to about 20 psi of pressure. Then, compressor  22  is run to resupply the vented air. The compressed air utilized for an approximately 3 gpm wastewater feed sized system is nominally about 40 scfm incoming air. The volumetric content of air in the circulating stream of system  10  is generally between about 1% and about 15%. In one embodiment, vapor tank  24  includes a deflector in order to direct circulated wastewater away from release valve  44 R.  
         [0032]     In one embodiment, anaerobic reactor  16  utilizes ported pipe  48  (shown in  FIG. 4B ) in air injector section  46  to maximize the amount of compressed air that is dissolved into the wastewater stream, which helps maximize digestion of the absorbed BOD. Also, in one embodiment, aerobic reactors  18  and  20  use a mixer, such as standpipe  50  (shown in  FIG. 4C ) or a draft tube, which also aids in dissolving the oxygen.  
         [0033]      FIG. 4B  shows anaerobic reactor  16  and air injector section  46 . The flow of wastewater through anaerobic reactor  16  is shown in  FIG. 4B  with arrows. For simplicity, ventilation systems  38 A and  38 B have been omitted. Compressor  22  supplies high oxygen content air into system  10  through valve  44 N connected to anaerobic reactor  16 . The flow of air into the stream of wastewater exiting anaerobic reactor  16  is shown in  FIG. 4B  with double arrows. Air is introduced into system  10  through injector section  46 . In one embodiment, injector  48  is a ported pipe with holes having a diameter of about 0.125 inch. Injector  48  causes air to enter the stream of wastewater in the form of small bubbles. The high number of small bubbles caused by injector  48  increases the total surface area of the bubbles entering system  10 . The increased surface area of the small bubbles leads to increased efficiency in aerobic reactors  18  and  20  as the air mixes with and aerates the output wastewater material of anaerobic reactor  16 . The typical residence time of material in air injection section  46  is approximately about one-half minute to about two minutes, after which, as the plug flow progresses to the top of the anaerobic reactor  16 , it is transferred to aerobic reactor  18 . The oxygen level is raised based on air dissolved at the pressure under which the system is operating. In one embodiment, a pressure indicating sensor is provided to maintain the pressure within system  10 . The air rate is adjusted to maintain a high dissolved oxygen level for maximum oxygen uptake upon mixing and during exit from anaerobic reactor  16 . The required amount is proportional to the specific oxygen uptake rate; the system is designed to maintain about 10 ppm dissolved oxygen at an uptake rate of about 100 mg of oxygen per gm of biomass per hour.  
         [0034]     Owing to the small remaining head space in each reactor  16 ,  18  and  20 , compressor  22  quickly pressurizes system  10 . In one embodiment, air compressor  22  includes an accumulator, flow metering air control input valve, shutoff valve and water hammer prevention valve. The elevated pressure and continuous flow insure that the bubbles flowing in system  10  will remain small and that the amount of available oxygen will remain in a supersaturated condition throughout aerobic processing. In one embodiment, system  10  is maintained under a pressure preferably between about 3 and 10 atmospheres (atm) (44-147 psi); more preferably between about 5 and 7 atmospheres (70-100 psi); and most preferably at about 5.85 atmospheres (86 psi). During the run cycle, anaerobic reactor  16  is filled with the circulated mixture of partially aerobically digested influent wastewater and activated sludge in preparation for the bypass cycle.  
         [0035]      FIG. 4C  shows the flow of wastewater through aerobic reactor  18 . The flow of wastewater through aerobic reactor  18  is shown in  FIG. 4C  with arrows. For simplicity, ventilation systems  38 A and  38 B have been omitted. Anaerobically treated wastewater enters aerobic reactor  18  from anaerobic reactor  20 . Bacteria received from the anoxic or anaerobic reaction in anaerobic reactor  16  are especially primed to vigorously take part in the BOD metabolism under aerobic conditions in aerobic reactors  18  and  20 .  
         [0036]     Anaerobically treated wastewater material is released close to the bottom of aerobic reactor  18  through standpipe  50 . In this manner, the wastewater material containing the greatest amount of oxygen travels toward the bottom of aerobic reactor  18  and the O 2  content diminishes as the material moves either to the top or the bottom of aerobic reactor  18 . The high rate of circulation through standpipe  50  ensures continuous churning of aerobic reactors  18  and  20 . It will be appreciated that the high flow volume through standpipe  50  or a draft tube, together with the high rate of circulation and replenishment under pressure, keeps the dissolved oxygen content at or above 5 mg/l (ppm) so that a high rate of aerobic reaction may be maintained.  
         [0037]     The infused high oxygen content air maintains relatively high dissolved oxygen content in aerobic reactors  18  and  20 . A high level of dissolved oxygen is maintained so that the high phosphorus content (hpc) bacteria can digest preabsorbed fat created in the anaerobic reaction at a high respiration rate. The net amount of air used is generally about 40 scfm for a 3 gal/minute process. Moreover, the oxygen is not diffusion-limited through the cell walls of the bacteria. The small portion of the reactor fluid recycled to the anaerobic reactor  16  assures a continuous supply of sufficient bacteria to anaerobically resorb and metabolize a major portion of the phosphorus content of the feed.  
         [0038]      FIG. 5  shows the bypass cycle with anaerobic reactor  16  bypassed. In this configuration, processed wastewater is continuously cycled through first aerobic reactor  18  and second aerobic reactor  20 , while anaerobic reactor  16  is closed off from the circulated wastewater in order to allow bacteria to anaerobically digest phosphorous present in the wastewater inside anaerobic reactor  16 . In the bypass cycle, the valves are configured in the same way as the run cycle, except valve  44 G is opened and valve  44 F is closed. Circulated wastewater is shown in  FIG. 5  with arrows. During the bypass cycle, compressor  22  continues to supply high oxygen content air to the wastewater entering aerobic reactors  18  and  20  through injector section  46 . Infused air is shown in  FIG. 5  with double arrows.  
         [0039]     In one embodiment, anaerobic reactor  16  is closed off from the run cycle for about 10 minutes at a time so that circulated wastewater is able to anaerobically decompose within anaerobic reactor  16 . After the oxygen is depleted in anaerobic reactor  16 , the microbes absorb and metabolize the phosphorous and develop a higher concentration of adenosine triphosphate (ATP) in the cells. This allows the bacteria to thereafter absorb large amounts of BOD and convert it directly into cell fat. The conversion to fat is an exothermic reaction that evolves approximately 20 KCAL per kg of COD which compares with the release of 480 KCAL per kg of COD for the complete metabolism of the BOD to CO 2  and H 2 O. The reactor conditions of the invention favor these microbes and they tend to actually dominate the species found in the process of the invention, washing out methane formers and other undesirable organisms that produce odors. Moreover, any air bubbles in anaerobic reactor  16  have time to float to the top of anaerobic reactor  16  and are positioned to thereby flow into aerobic reactor  18  during the run cycle. The bacteria absorbs and metabolizes the majority of the phosphorus in the circulated wastewater. Recirculation further conditions the species of bacteria to develop and adapt to carrying a high phosphorus content (hpc). This enables the effective removal of a large quantity of phosphorus from the circulated wastewater. Additionally, these hpc bacteria have increased energy available to absorb BOD constituents in the biomass later in the process. Cellular energy converts BOD to fat in anaerobic reactor  16  during the absorption and metabolism of phosphorus and this later stored energy is regenerated or made available when the bacteria enter aerobic reactors  18  and  20  where the fat is metabolized.  
         [0040]     After anaerobic reactor  16  is closed off for about 10 minutes, valve  44 G closes and valve  44 F opens and the system is returned to the run cycle configuration. This process is repeated every ten minutes to increase the anaerobic decomposition rate of system  10 . During the intermittent run cycles, the anaerobically treated wastewater is reintroduced into the circulating wastewater. This helps enhance the overall efficiency of system  10 . After about 5 to about 50 passes through anaerobic reactor  16 , the process of the present invention removes about 80% to about 90% of the combined nitrogen in the wastewater.  
         [0041]      FIG. 6  shows the de-gassing cycle of system  10 . In one embodiment, excess gas is system  10 , such as methane gas, is released through ventilation systems  38 A and  38 B and through vapor tank  24  during the de-gassing cycle. Exiting gas is shown in  FIG. 6  with arrows. The de-gassing cycle slowly brings the system pressure down to atmospheric pressure. In one embodiment, pressure in system  10  is reduced slowly after a batch of wastewater has been processed. Once a batch of wastewater has been processed to the desired discharge BOD level, pressure is reduced over a period of time. In one embodiment, the air supply from compressor  22  is reduced as ventilation systems  38 A and  38 B and valve  44 R are utilized to ramp down or slowly reduce the system  10  pressure. For example, pressure reduction can occur at a rate of about 1 to about 2 atmospheres of pressure per minute, until the pressure reaches an ambient level. Such pressure reduction further enhances nitrogen stripping. In one embodiment, complete processing of a batch occurs during each full process cycle, which lasts from about 8 hours to about 14 hours and preferably lasts about 10 hours.  
         [0042]      FIG. 7  shows the system discharge portion of the discharge cycle of system  10 . During the system discharge step, newly produced activated sludge that has collected in anaerobic reactor  16  and aerobic reactors  18  and  20  is emptied into surge tank  32 . Discharged sludge is shown with arrows in  FIG. 7 . Valves  44 C,  44 D and  44 E are opened and sludge is gravity fed into surge tank  32  through valve  44 B. In one embodiment, valve  44 C is opened for five seconds, followed in sequence by valves  44 D and  44 E. As the sludge moves into surge tank  32 , it passes through filter  40 . In one embodiment, filter  40  is a bar screen filter. Phosphorous and nitrogen compounds are predominantly contained in the solid fraction of the treated wastewater. Filter  40  screens out granules of phosphates, nitrates and ammonia that formed in the sludge during the anaerobic and aerobic reaction processes. The granules are removed at this point before entry into clarifier  26  and can be collected for other external applications, such as in fertilizers.  
         [0043]     Sludge collected in surge tank  32  is allowed to settle. In one embodiment sludge settles for one hour. While sludge settles in surge tank  32 , further anaerobic and aerobic reactions take place which further treat the wastewater.  
         [0044]      FIG. 8  shows the sludge discharge step of the discharge cycle of system  10 . Discharged sludge is shown with arrows in  FIG. 8 . During the sludge discharge step, sludge is pumped by pump  27  from surge tank  32  into clarifier  26 . In one embodiment, pump  27  is a chopper pump which more finely divides any solids in the liquid effluent. Pump  27  has a low output rate and is activated alternately one hundred seconds on and four hundred seconds off. Suitable pumps can be obtained from Moyno, Inc., Springfield, Ohio.  
         [0045]      FIG. 9  shows clarifier  26  of system  10 . In one embodiment, clarifier  26  includes weir ring  52  that separates solid and liquid portions of the treated wastewater. In one embodiment, weir ring  52  has a 6 inch diameter and ⅛ inch clearance between plates. In one embodiment, after the primarily liquid portion passes through the clarifier, it is further passed through a bar screen filter to screen out any solid particles larger than the 0.25 inch screen openings. Liquid portions are passed on to chlorinator  34  and dechlorinator  36  where the liquid is sanitized to be safely returned to the environment at exit  43 . Suitable chlorination and dechlorination units can be obtained from PPG Industries, Inc., Pittsburgh, Pa.  
         [0046]     Solid sludge portions are pumped from clarifier  26  to sludge tank  30  where the activated sludge can be reused to charge system  10  for later batch processing.  
         [0047]     The operation and timing of valves  44 A- 44 R, fill pump  26 , discharge pump  27 , circulation pump  28 , vapor tank  24 , air compressor  22  and other components of system  10  can be controlled using programmable logic controls, a microprocessor-based control system or other such systems. The timing of the programming depends on the initial BOD level in the incoming wastewater. Higher BOD wastewater requires longer cycle times, as can be controlled with the PLC programming. In one embodiment, an oxygen demand sensor can be used in conjunction with PLC  41  to control the cycle times of system  10  based on the sensed BOD of the sensor. In one embodiment, system  10  operates for about ten hours. In a preferred embodiment, system  10  operates for about three to four hours.  
         [0048]      FIG. 10  shows one embodiment of system  10  as operable in a mobile transport vehicle, such as truck  54 . System  10  takes up very little space and reduces the land area needed for sewage or wastewater treatment. Influent wastewater can be introduced into system  10  at inlet  12 . Clean wastewater leaves system  10  at exit  43 . Also, by requiring only the net feed stream of fill pump  14  from inlet  12  to be pressurized and maintaining system pressure using air compressor  22 , the power requirements of system  10  are minimized. In one embodiment, system  10  fits into a three-dimensional rectangular space having dimensions of about 18 feet long by about 8 feet wide by about 8 feet high and has a batch capacity of about 350 gallons. Thus, system  10  fits easily into portable truck  54  for high mobility. The modularity of system  10  allows more than one system  10  to service the needs of a particular community or application. In one embodiment, increased capacity is obtained by using a plurality of systems  10  in parallel; alternatively, system  10  is scaled up for such applications. In another embodiment, increased effectiveness is obtained by using a plurality of systems  10  in series. In an exemplary embodiment, the processing capacity of system  10  is sized so that it can accommodate expected flow input from the source to be treated. In one embodiment, increased capacity can be obtained for a particular system  10  by decreasing reaction or residence time, thereby slightly decreasing treatment effectiveness.  
         [0049]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.