Abstract:
An apparatus and method for removing contaminants from contaminated water. One embodiment of the apparatus comprises a separation tank in which a first portion of the contaminants float to the top of the water, the tank including a water inlet and a water outlet, a coalescing device, an air injection system in which water from the tank is pressurized, passed through an air eductor so as to receive air bubbles, and returned to the tank as a gas-containing stream, a circulating mop that contacts the floating contaminants and removes them from the water, the circulating mop comprising a continuous loop of contaminant-absorbing material, a mop-cleaning member that removes contaminants from the circulating mop, and a filter system through which water leaving the separation tank via the water outlet is passed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         FIELD OF THE INVENTION  
         [0003]    The present invention relates generally to removing pollutants from wastewater and more particularly to the separation and removal of total suspended solids (TSS) such as fats, oil and grease (FOG), and non-grease solids (e.g. food scraps), which produce an undesirable biochemical oxygen demand (BOD). Still more particularly, the present invention relates to an apparatus and method for pre-treating wastewater from restaurants, food processing facilities and other industrial operations to reduce the concentration of total suspended solids (TSS) and biochemical oxygen demand (BOD) contaminants, which place a burden on sewage piping and sewage treatment facilities.  
         BACKGROUND OF THE INVENTION  
         [0004]    Typical food preparation and clean up operations generate wastewater with high contents of foodstuff, grease and animal fat. A particular problem occurs when small molecules of grease unite with food particles and other waste and accumulate on the inside of pipelines, clogging drainage lines from sinks, dishwashers and the like. As this wastewater is discharged into the sewage system, liquid FOG cools and solidifies with other solids, accumulating on the inside walls of the wastewater pipe. As this deposited layer grows, it obstructs the flow of wastewater to the treatment facility.  
           [0005]    The removal of these obstructions is a costly and time-consuming process. In addition, elevated levels of FOG can disrupt the treatment of waste-water at municipal facilities. Consequently, applicable governmental regulations now require removal of fats, oil and grease (FOG) from wastewater in many instances. In particular, the Environmental Protection Agency (EPA) has promulgated regulations establishing standards for acceptable total suspended solids (TSS), including fats, oil and grease (FOG), and biochemical oxygen demand (BOD) content in wastewater prior to discharge.  
           [0006]    Because of these regulations, sewage treatment authorities have sought ways to cope with the burdens placed on their facilities by the total suspended solids (TSS), including fats, oil and grease (FOG), being discharged into the sewers. They have focused on commercial kitchen operations, such as restaurants, cafeterias, hotels, etc., to attempt to reduce the amount of such materials received or at least equitably spread the cost of treating their wastewater. One of the methods authorities have used is to apply a surcharge (fine) to sewage bills of commercial kitchen operations to reflect the added demands put upon the sewage treatment facility by sewage emanating from them. These surcharges have been levied by authorities in several ways; some inspect and levy based upon just fats, oil and grease (FOG), others by total suspended solids (TSS) and biochemical oxygen demand (BOD). In some situations, treatment authorities have required that commercial kitchens simply stop discharging into the sewers. These remedies have imposed a financial burden upon commercial kitchens but do not provide a solution in most cases and do not reduce the actual burden of pipeline clean up or treatment facilities operations.  
           [0007]    In order to avoid surcharges and reduce the burden of pipeline clean up, it has become desirable for commercial kitchens and similar operations to remove substantially all of the total suspended solids (TSS) from their wastewater before it is discharged into a public sewage system. For example, many conventional FOG removal or recovery systems make use of the differences in specific gravity of FOG and water to permit gravitational separation of the FOG and water. Such systems typically rely on settling or other passive separator techniques.  
           [0008]    It is also known to use grease traps, particularly inground grease traps, to separate floatable grease components from wastewater. A typical FOG-water separator, grease recovery unit (GRU) or grease trap includes a settling tank or compartment in which the wastewater is collected for gravitational separation and removal of FOG from the water. While the wastewater is within the settling container, the wastewater gradually separates into FOG and water, with the less dense FOG floating to the upper surface of the water. This upper portion, being heavily laden with FOG, is then intermittently removed from the settling container by manual removal, in the case of grease traps, or a skimming operation.  
           [0009]    It is also known to pass the effluent through settling tanks and strainers in order to remove large or heavy food particles.  
           [0010]    The major difficulty with all the above devices is that they operate on the premise that all FOG has a lower specific gravity than the wastewater in which it is carried. They also do not provide the residence time that is typically necessary to allow the FOG to rise to the surface. For example, while some of the very small droplets of FOG (e.g. less than 10 microns) have a specific gravity lower than the water, their slow rise rate may not allow them to reach the surface during the time the waste is in the separation compartment. Additionally, some of the FOG may adhere to foodstuff, thereby becoming heavier than the water and sinking, or worse yet, attain the same specific gravity as the water and pass through into the sewage system.  
           [0011]    Although solids can be successfully treated in conventional sewage treatment facilities, those that contain large amounts of fats, oils and grease (FOG) require a very long residence time in the facility, which increases the expense of the treatment process. Accordingly, there is a need in the art for a new apparatus/method that achieves separation of all the TSS, thereby eliminating the FOG and at the same time reducing BOD.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention is an apparatus and method for pre-treating wastewater from restaurants, food processing facilities and other industrial operations to reduce the concentration of total suspended solids (TSS) and biochemical oxygen demand (BOD) contaminants which place a burden on sewage piping and sewage treatment facilities. A preferred embodiment of the present apparatus for removing contaminants from contaminated water comprises a separation tank in which a first portion of the contaminants float to the top of the water, the tank having a water inlet and a water outlet, a coalescing device in the separation tank, the coalescing device including means for causing a portion of the contaminants to coalesce, a circulating mop that contacts the floating contaminants and removes them from the water, a mop-cleaning member that removes contaminants from the circulating mop, and a filter system through which water leaving the separation tank via the water outlet is passed. In still more particular embodiments, the coalescing device comprises a plurality of corrugated plates that define a tortuous fluid flow path therethrough, the circulating mop comprises a continuous loop of contaminant-absorbing material, the mop-cleaning member comprises a pair of rollers, and/or the tank inlet is configured to feed water into the tank in a direction that is substantially parallel to the wall of the tank. The preferred apparatus further includes a pressurization loop that receives water from the tank, pressurizes it, and returns it into the tank in a manner that causes fluid in the separation tank to flow around the perimeter of the tank and thus produces a toroidal fluid flow pattern in the tank, an air injection system that receives water from the tank, provides it with air bubbles, and returns it into the tank, and/or a receiving device for recovering contaminants removed from the mop. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]    A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings:  
         [0014]    [0014]FIG. 1 is a schematic diagram of a system constructed in accordance with one embodiment of the invention;  
         [0015]    FIGS.  2 A-C are enlargements of portions of FIG. 1; and  
         [0016]    [0016]FIG. 3 is a schematic diagram of a adsorbent belt constructed in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The present invention relates to an apparatus/method for removing nearly all FOG particles from wastewater and for reducing BOD therein. To better understand the removal mechanism, the invention may be divided into four main steps: (1) straining, (2) frothing (3) separating, and (4) filtering. These steps are discussed in detail below and are performed in the present apparatus, which is preferably configured as follows.  
         [0018]    Apparatus  
         [0019]    Referring initially to FIG. 1, the present invention comprises a strainer system  101 , frothing system  201 , and filter system  301 .  
         [0020]    Referring now also to FIG. 2C, strainer system  101  includes an inlet  10  and a non-pressurized vessel  20  containing a perforated basket  30  and having a cover  60 . Basket  30  is preferably large enough to contain all the solids larger than its openings (holes)  25  that will be discharged within a day. In some embodiments, openings  25  may be in the range of {fraction (1/16)} inch to ¼ inch in diameter depending on surface area and loading. Preferably openings  25  may be {fraction (1/16)} or ⅛ of an inch in diameter. Basket  30  can be automatically cleaned or removed and dumped daily. Additionally, vessel  20  has an overflow outlet  40  that allows dirty water to bypass strainer basket  30  if it becomes clogged, full, or not in service.  
         [0021]    Referring now also to FIG. 2B, frothing system  201  includes a round or oblong tank  70  containing a corrugated plate separator  190  and an outlet pipe  460 . A dirty water line  50  carries strained water from strainer system  101  to inlet  55  of tank  70 . The volume of tank  70  is preferably at least three times the inlet flow rate per minute. For example, if the flow rate into the tank is 25 gallons per minute (gpm), the tank should hold at least 75 gallons. In some embodiments, tank  70  may include a lid  110  and hold-downs (not shown) that make it air tight, but it is preferred that, if present, these components are easily removed for maintenance and service.  
         [0022]    In some instances, it may be desirable to provide one or more immersion heaters  100  within the volume of tank  70 . Heaters  100  may be similar to those commonly found in large water heaters. Preferably, a heater  100  will be located directly below tank inlet  55  and below the height of the pump return fluid. The heater may operate at  110  or  220  volts, and is preferably designed for continuous duty. Heaters  100  may always remain on and should be appropriate size and number to maintain a constant temperature in the tank, preferably between 120° F. and 130° F. at all times. In some embodiments, the temperature in tank  70  is controlled using feedback from a temperature sensor in the tank.  
         [0023]    Inlet  55  is preferably tangentially oriented, so that in-flowing water flows along the side of tank  70  and the liquid moves around separator tank  70  in toroidal flow. It is known that, for this type of flow pattern, the liquid velocity is smaller near the center of tank  70 . Liquid that has been in tank  70  the longest will be in this central region. For this reason, outlet pipe  460  is preferably provided with an intake  450  that is positioned so that it receives water from the center vortex.  
         [0024]    Still more preferably, the intake  450  is disposed such that liquid from tank  70  must pass through corrugated plate separator  190  before flowing out through intake  450 . Separator  190  removes the very small droplets of free oil and mechanically emulsified oil by capturing oil droplets when they make contact with its surface. As more and more droplets attach, they coalesce until they form large enough drops to rise at a rate sufficient to overcome the downward velocity of the liquid. Separator  190  preferably includes a plurality of plates  195 , which are configured so as to cause flow through the separator to be tortuous. The flow path through the separator is preferably tortuous so that droplets have an increased chance of contacting a separator surface and being captured. While a corrugated plate separator is used in this example, an incline plate separator or a coalescing pack, or other suitable device may also be used. In some embodiments, it may be preferable to include a cylindrical separator ring  200  that surrounds separator  190  and keeps water from by-passing separator  190 .  
         [0025]    In one preferred embodiment, separator plates  195  are spaced one and one-half inches (1½″) apart and compartmentalize every three and one-half inches (3½″). Preferably, each plate  195  has five flat surfaces, three of which are at 90-degree angles, and two of which are at 45-degree angles. Typically, three chevrons are pressed into the 45-degree surfaces. When a number of the plates are arranged in a pack, the liquid travels though plates in a tortuous flow pattern. For example, the water may enter separator  190  and travel vertically for one inch. It then turns and travels at a 45-degree angle for three inches. The liquid may then travel vertically for one inch before turning back and traveling at 45 degrees in the opposite direction for three inches. Finally, the liquid resumes vertical travel before exiting.  
         [0026]    After entering outlet  460  via intake  450 , the clean liquid flows into water leg  470 , which has an inverted U-shape and includes ascending leg  472 , lateral leg  474 , and descending leg  476 . Water leg  470  ensures that the liquid in tank  70  is maintained at a constant level, as indicated at  480 . As discussed below, maintaining a constant liquid level in tank  70  helps ensure adequate residence time in tank  70 , so as to allow natural gravitational separation to develop and coalescing time for air flotation. In come cases, the liquid may exceed constant water level  480 . Water leg  470  preferably includes a siphon breaker line  80  to prevent siphoning. Preferably, tank  70  also includes a vent  90  located at or near the top of tank  70 .  
         [0027]    Referring now to FIG. 3, the upper portion of tank  70  includes a mopping system  150 , which preferably includes a, an oleophilic continuous loop belt  155 , preferably fabricated from a durable material such as nylon, with long fibers  158  sewn into it. Fibers  155  are preferably affixed to both sides of belt  155 . The fibers  158  are preferably fabricated from a hydrophobic material such as polypropylene, polyethylene or any other polymerized resin to which FOG can adhere.  
         [0028]    Referring now to FIGS. 1, 2B and  3 , mopping system  150  is preferably configured such that a first portion of the path of belt  155  lies above the surface of the liquid in tank  70  and a second portion of the path of belt  155  lies at or near the surface. Between the first and second portions, belt  155  passes between a pair of opposed rollers  140 . Rollers  140  are preferably mounted such that the clearance between them is only slightly larger than the thickness of belt  155 .  
         [0029]    Material that is squeezed out of fibers  158  as shown at  145  drops into a trough  148 , from which it flows via pipe  170  (FIG. 2B). It will be understood an variety of devices could be used to remove material from belt  155 . Suitable alternatives to rollers  140  include but are not limited to one or more combs, blades, scrapers, vacuum equipment and combinations thereof.  
         [0030]    To support belt  155  in the preferred configuration, mopping system  150  further includes at least two, preferably three or more drive/support rods  157  positioned in a spaced-apart arrangement as shown in FIG. 3. Specifically, at least one rod, lower rod  157   a , maintains a portion of the belt path near the surface of the liquid. A second rod, upper rod  157   b , maintains a portion of the belt path above the surface of the liquid. A third rod, end rod  157   c  is preferably laterally spaced apart from lower rod  157   a . If the diameter of at least one of the rods is large enough to lift a sufficient portion of belt  155  out of the liquid, lower rod  157   a  or upper rod  157   b  may be eliminated.  
         [0031]    In one embodiment, one or both rollers  140  drive belt  155  and are driven in turn by a motor  130 . In another embodiment (not shown), one of rods  157  is driven by a motor and thus drives belt  155 , while the remainder of rods  157  rotate passively. Belt  155  and the drive rod(s)  157  can be provided with corresponding sets of teeth or the like, which allow the drive rod to more effectively engage belt  157 , if desired.  
         [0032]    Referring again to FIG. 2B, heavy solids tend to settle to the bottom of tank  70 . A suction inlet  315  is positioned at the bottom of tank  70  and connected to a suction pump  240  via filtration line  230 , allowing water and heavy solids to be drawn off simultaneously. In an alternative embodiment, the bottom of tank  70  can be slightly conical, as shown in phantom at  72  ins FIG. 2B. In this embodiment, an additional outlet  74  can be provided to allow settled solids to be removed from tank  70 . The shape and utility of outlet  74  will depend in part on the nature of the solid wastes entering tank  70 . A valve  76  may be provided to control the flow through outlet  74 .  
         [0033]    Referring now also to FIG. 2A, after passing through pump  240 , liquid from filtration line  230  is preferably fed into filter system  301  via pressurized feed line  280 . Filter system  301  preferably comprises a vessel  300 , first and second filters  310 ,  320 , a clean water outlet line  350 , and a drain line  330 . A valve  340  controls the flow through drain line  330  and a valve  360  controls the flow through outlet line  350 .  
         [0034]    Filtration unit  325  is a duplex system, using two filters  310 ,  320 , having different mesh sizes. In some embodiments, a triplex filter may be used for very fine filtration. Depending upon the discharge specifications, filters may have various mesh sizes. Preferably, filter  310  is relatively coarse (from about 75 microns to about 40 microns), while, filter  320  is fine (between about 20 microns and about 10 microns). One or both of these filters may be constructed of woven wire mesh, sintered metal, woven fiber mesh, felt socks or the like. In a preferred embodiment, both filters are made of woven wire mesh.  
         [0035]    When selecting a filter material, filter vessel  300  should be designed to insure that the filter elements can be easily removed and easily cleaned. It is important that the filters are designed with enough surface area to allow for an acceptable run time between cleaning and possess a low clean differential pressure drop.  
         [0036]    Filtered water that has passed through first and second filters  310 ,  320  can leave vessel  300  via either line  330  or line  350 . Liquid leaving vessel  300  via line  350  enters either a gas-entraining line  352  or a liquid injection line  354 . Gas-entraining line  352  preferably includes an eductor  390 . As is known in the art, as liquid passes through eductor  390 , air is entrained into the liquid stream. Air enters eductor  390  via a gas entrainment line  430 , which includes an air intake  180  located either in the head space  85  of tank  70 , as shown, or outside tank  70 . The rate of air flow through line  430  is controlled by a valve  440 . When the air-flow through is restricted by valve  440  to a point very close to causing cavitation within eductor  390 , very small bubbles (&lt;5 microns) are generated in the fluid flowing in line  352 .  
         [0037]    The contents of line  352  are injected into tank  70  via a tangential inlet  220 , which is preferably located at about one-half the depth of the liquid in tank  70 . Because line  352  contains entrained air, this results in a flow of air bubbles into and up through the contents of tank  70 . In tank  70 , these small bubbles contribute to successful operation of the present air flotation process.  
         [0038]    Once the bubbles pass through the liquid in separation tank  70 , they break back out into the head space  85  of tank  70 .  
         [0039]    Similarly, liquid injection line  354  feeds a liquid stream into tank  70  via a tangential inlet  210 . Since this liquid stream contains no entrained air, it is not as preferred that the stream be injected at about half the liquid depth. A valve  420  controls the flow of fluid in line  354 .  
         [0040]    It is preferred that the fluid pressure in lines  352 ,  354  be substantially greater than the fluid pressure in tank. In addition, as mentioned above, both inlet  210  and inlet  220  are configured such that liquid entering tank  70  has a velocity vector that is horiztonal and parallel to the tank wall at the point of entry. The momentum of this entering fluid is imparted to the fluid in the tank, with the result that circular flow is generated in tank  70 . Because of friction with the walls and floor, this in turn results in a toroidal flow pattern, in which liquid flows down along the walls, inward across the floor, upward in the center of the tank and outward at the liquid surface. The toroidal flow tends to sweep particles that have fallen to the tank floor toward the center of tank  70 , where suction inlet  315  is located.  
         [0041]    A bypass line  270  preferably connects pressurized feed line  280  to gas-entraining line  352  and liquid injection line  354 . Valve  250  preferably controls the flow of liquid through line  280 . When valve  250  is closed, liquid is shunted around filter system  301  via bypass line  270 . A control valve  290 , or check valve (not shown), or both, is preferably included on bypass line  270 .  
         [0042]    It should be noted that not all water exiting tank  70  via pump discharge outlet  315  enters pump  240 . Rather, in some cases, such as when it is determined that the water at the tank bottom is sufficiently clean, the water may be allowed to flow through separation drain outlet  500  and drain valve  510 . Typically, the drain valves are not used during operation and are used onlyu to drain the vessels for maintenance.  
         [0043]    Operation  
         [0044]    Initially, FOG-containing liquid is poured into a sink or other drain. The liquid flows into a strainer, where the large particles (herein defined as &gt;2 mm) are removed. This strainer can be of the basket type, which requires manual removal and dumping, or an automatic type that continuously dumps the collected solids.  
         [0045]    Frothing  
         [0046]    After the large solids are removed, the liquid and remaining solids are introduced into tank  70  as a toroidal stream. In one embodiment, pump draws liquid from the center of the tank and injects it tangentially to the tank wall, thereby enhancing toroidal flow. This in turn has the effect of lengthening the fluid flow path and increasing residence time. Chamber residence time can be estimated by measuring the distance the liquid must travel between the inlet and the outlet, determining the linear velocity at the inlet or outlet, then dividing the distance by the velocity. A long flow path allows more participation of the liquid within the tank; flow patterns in tank  70  are preferably maintained such that the last liquid entering the tank is the last liquid out. Hence, a long flow path increases the residence time that the liquid stays within tank  70 . By increasing the residence time, more time is allowed for gravitational separation to occur. This means that those particles that have a difference in specific gravity will either rise or sink within the liquid.  
         [0047]    It is preferable for small bubbles to be educted into the liquid while it is circulating in tank  70 . This is achieved by using a portion of pressurized liquid from pump  240  as a motive for eductor  390 , which induces small air bubbles into the stream. The air is derived from vapor space in the top of the tank and is recycled from the tank to the eductor. This liquid/air mixture is then fed into tank  70 , where it is circulated through the liquid in the tank (due to toroidal flow). All of the liquid will pass through the rising air bubbles many times before reaching the exit. After passing through the liquid, the air rises back to the vapor space in the top of the tank. As the tiny air bubbles rise through the liquid, they contact and adhere to some of the small solid particles.  
         [0048]    Even though these small particles may be heavier than the liquid, they may attach to the bubbles and be carried to the top of the liquid. Oil separation is further enhanced because rising bubbles (1) cause small oil droplets to attach to them or (2) push the small oil droplets together, coalescing them into larger droplets. As these oil droplets continue to increase in size and density, their buoyancy or velocity rise rate will increase according to Stokes&#39; Law, shown by Equation 1.  
         F=6πrηv  (1)  
         [0049]    where F is the force acting in resistance to the fall, r is the radius of the sphere, η is the viscosity of the liquid, and v is the velocity of fall.  
         [0050]    As the bubbles attach to solids and the small oil droplets coalesce into larger droplets, a froth composed of oil, solid particles and air forms and collects on the surface of the liquid. Once the froth has accumulated on the surface of the liquid, it is removed by the mop system  150 .  
         [0051]    As described above, by making the liquid flow through a torturous path and at low velocity (herein defined as &lt;6 feet per second) before leaving tank  70 , almost all of the small FOG droplets come in contact with corrugated separator plate  190 . Once contact is made the droplets will attach, migrate upward into chevrons, coalesce and form into large enough drops to rise to the surface.  
         [0052]    Because the smallest droplets of oil are often not effectively removed by gravity separation, even when the separation is enhanced by gas sparging, the FOG content of the liquid leaving tank  70  via pipe  460  often exceeds environmental standards. To reduce the small droplets of oil, and the overall FOG content, a corrugated plate separator is used to collect the oil by attaching to the oil when the oil makes contact with its surface. As more and more of the droplets attach they will coalesce and form large enough drops to rise at a rate to overcome the downward velocity of the liquid.  
         [0053]    Separating  
         [0054]    As mop  150  passes through the froth at the surface of the liquid, bubbles and solid and liquid particles of FOG and the like cling to its fibers and are lifted out of the liquid. This material is removed from the belt fibers by opposed rollers  140 . As discussed above, the material is squeezed out of the fibers  158  as shown at  145  and drops into a trough  148 , from which it flows via pipe or trough  170  into a collection container (not shown). It is preferred to remove the froth soon after it forms because over time the air will break out of the froth, allowing heavy solids to sink. If desired, a blade or other mechanical device (not shown) can be used to facilitate the collection of the recovered material after it is squeezed out of the belt.  
         [0055]    If desired, heat may be used to aid the separation process. Because heat is lost as the wastewater passes through piping and the skimmer basket into the separation tank, it is preferable to add heat to maintain a constant process temperature. This prevents solids and FOG from forming into large globules. Heat can be added by the use of one or more electric immersion heater elements placed in the liquid, such as element heating  100 , a heat exchange system, or other suitable heating mechanisms. By increasing or maintaining the temperature between 115° F. and 130° F., FOG will separate freely from those solids and rise to the top. At the same time, the solids will sink to the bottom. In order to prevent temperature loss and reduce the electric heating cost, piping and tanks should be insulated. This is important when the restaurant is not discharging hot water into the system.  
         [0056]    Filtering  
         [0057]    As discussed above, by increasing the time the liquid has between the inlet and the outlet, most of the particles will rise to the top or sink to the bottom. The toroidal stream creates a vortex that causes heavy particles that have sunk to the bottom of the tank to move to the center of the tank, where they can be removed by suction inlet  315  or outlet drain  74 .  
         [0058]    In order to remove the solids that have collected on or near the bottom of the separation tank, they are withdrawn and passed through a specially designed filter system for removal. This is done by placing the suction of the eductor pump near the center of the separation tank and placing the filter between the pump discharge and the eductor or tank return. Like the inlet strainer, this filter system needs to be emptied and cleaned periodically. The filter preferably removes particles down to the 10 micron range, thereby insuring that the total liquid quality exiting the separation tank will be within discharge specifications.  
         [0059]    Pump  240  pressurizes the liquid to a predetermined pressure that allows filtration, air eduction and force for producing the toroidal flow. Typical pressure is between 20 and 50 psig. The flow rate through pump  240  is preferably at least two times the flow rate through inlet  55 . For example, if the system has an inlet flow rate of 10 gpm, then the pump discharge should be at least 20 gpm. The discharge rate of pump  240  is preferably greater than the flow through inlet  55  because the passage of water through filters  310 ,  320  does nor result in complete removal of solids. Therefore, it is preferred to remove and clean at least two tank volumes for each new wastewater volume added.  
         [0060]    In a preferred embodiment, filtration unit  301  includes two inline pressure indicators  282 ,  412 , which indicate fluid pressure upstream and downstream of the filters, respectively. When valve  290  is closed, indicator  282  will indicate the pressure in pressurized feed line  280 . When valve  420  is closed, indicator  412  will indicate the pressure in gas-entraining line  352 . By comparing the pressures at the two indicators, it is possible to determine when the pressure differential across the filters reaches a point where the filter elements need to be cleaned. Filter bypass line  270  may be used during the time the filters are being cleaned. Because this is such a short time and dilution would not be greatly affected, filtration system  301  does not need to be shut down. The filters are preferably cleaned when no wastewater is entering the system (e.g. when the restaurant is closed).  
       EXAMPLE  
       [0061]    All tests were conducted in an operating restaurant using actual wastewater. On average, the restaurant serves over 14,000 meals per month and has a total water usage of 183,000 gallons per month.  
         [0062]    Testing was performed on approximately 25% of the total water from the kitchen. The water used was from the sink where the pots, pans and all dishes were rinsed before the dishes went into the dishwasher. This water had the highest overall levels of FOG and TSS tested.  
         [0063]    Using the present invention, TSS was significantly reduced. Results are shown below in table 1.  
                                           TABLE 1                           TSS Results                Time   TSS content           (hours)   (mg/L)                             0*   1679           1   368           2   250           24    108            1-4**   367                                              
 
         [0064]    Samples were collected during the heaviest loading times (e.g. midday and early evening). The city collects samples over the entire time the restaurant is open. Typically, it collects a composite sample every 15 minutes from the time the restaurant opens until it closes. This is similar to the composite sample collected in table 1.  
         [0065]    Because good results were obtained using the dirtiest water, it is believed that the present invention will yield even better results when all the restaurant discharge water is used.  
         [0066]    While the present invention has been disclosed and described in terms of a preferred embodiment, the invention is not limited to the preferred embodiment. For example, while the present invention has been described for treatment of waste water streams such as are generated in restaurants and food preparation facilities, it should be understood that the present apparatus is useful for treating a variety of waste streams. In addition, various modifications to the apparatus, including the arrangement and size of the components, among others, can be made without departing from the scope of the invention. For example, valves recited herein can be any suitable flow control mechanism, including gate valves, ball valves, and the like, pumps recited herein can be any suitable pressure-increasing mechanism, including impellers, positive displacement pumps and the like, and filters recited herein can comprise any suitable particulate removal mechanism, including mesh, screen, porous materials, and the like. In the claims that follow, any recitation of steps is not intended as a requirement that the steps be performed sequentially, or that one step be completed before another step is begun, unless explicitly so stated.