Abstract:
A method and apparatus for controlling water flow in a wastewater treatment plant is provided. The method comprises the steps of providing a trickling filter effluent water basin having filter media provided over a storage space for water. Water outlet flow from the basin is controlled via a flow weir or other suitable structure, thereby enabling the trickling filter effluent water basin to fill with water and store water when desired. Such storage is particularly useful during wet weather and/or overflow events. A trickling filter effluent basin modified so as to accommodate water storage and methods of modifying existing trickling filter effluent basins are also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention provides a method and apparatus for in-plant control, containment and management of wet weather overflow in a wastewater treatment plant (WWTP). Specifically, modified trickling filter tanks are used as holding tanks during wet weather events.  
       BACKGROUND INFORMATION  
       [0002]     Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a wastewater treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. Thus, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other waterways.  
         [0003]     The overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated human and industrial waste, toxic materials and debris. Overflows are a major source of water pollution in communities with combined sewer systems.  
         [0004]     The problems related to overflow and infiltration of untreated water into sewer systems also occur in closed (non-CSO or separate) wastewater systems. Due to deteriorating collection systems and illegal cross-connections, discharge of untreated wastewater is a common occurrence in closed systems as well. Excess flows in either system can take the form of infiltration into the system due to cracked pipes, offset pipe joints, and the like, or direct inflow into the system, such as with direct surface water connections, foundation drains, and the like.  
         [0005]     The costs of remedying the wet weather overflow problem are not insignificant. In Pennsylvania alone, it was estimated in 2001 that over $4 billion would be needed to bring wastewater discharge into compliance with state and national water quality standards. Estimates of the repair costs for wastewater and drinking water systems statewide in Pennsylvania exceed $11 billion dollars.  
         [0006]     One method of managing wet weather overflows is to build overflow tanks positioned outside the wastewater treatment plant, before the wastewater point of entry into the plant. When the flow rate into the plant is heavy, wastewater is diverted into the overflow tank(s), and the water is stored in the tank until levels within the plant subside and the plant can accommodate the additional flow. A problem with this method is that overflow tanks are very expensive to build. For example, the cost of building a 700,000 gallon capacity tank in 2005 is about $3.1 million dollars. Additionally, if the capacity of the overflow tank is exceeded, wastewater is diverted to surface waterways, bypassing the treatment process and degrading the water quality of the waterway. Most methods of control aimed at increasing the capacity of the plant are very expensive and are land intensive.  
         [0007]     There remains a need for a cost-effective solution to the problem of reducing and managing wet weather overflow in the wastewater treatment process so that water discharged to surface waters meets the applicable state and national water quality standards.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, in one aspect, the present invention provides a method of controlling water flow in a wastewater treatment plant, the method comprising the steps of: (a) providing a trickling filter effluent water basin having filter media provided over a storage space for water and a flow control assembly for controlling outlet flow from the basin; and (b) controlling outlet flow from the basin, thereby enabling the trickling filter effluent water basin to fill with water and store water. The water basin is used to store water during overflow events. In step (b), the water basin is permitted to fill with water by reducing the flow of water out of the basin after influent water has contacted filter media. In one embodiment, the flow control assembly is a flow weir in combination with a flow restriction valve. In another embodiment, the flow control assembly is a weir gate.  
         [0009]     The basin further includes a support platform containing filter media, the support platform positioned at a height above a base of the basin. The water basin is permitted to fill with water to a predetermined level below the height of the support platform containing the filter media. When desired, the stored water is released from the water basin and further processed within the wastewater treatment plant, such as by any one or more of the steps of clarification, filtration and/or disinfection.  
         [0010]     In an additional aspect, the present invention provides a trickling filter effluent water basin comprising: a) a retaining wall; b) a support platform to support filter media at a height to provide storage space for water below the platform; c) an outlet pipe in fluid communication with the storage space; d) a flow control box in fluid communication with the outlet pipe and the storage space, the flow control box having an elevation extending above the outlet pipe; and e) a flow control assembly for controlling flow, received within the flow control box, which permits water to be contained in the storage space upstream of the flow control assembly. The outlet pipe positioned downstream of the flow control box.  
         [0011]     In one embodiment, the flow control assembly is a flow weir in combination with a flow restriction valve. In another embodiment, the flow control assembly is a weir gate.  
         [0012]     Optionally, the water basin can further comprise one or more ventilation ports defined in the retaining wall, and optionally include a compressor in fluid communication with the storage space to provide additional ventilation to the basin.  
         [0013]     In yet an additional aspect, the present invention provides a method for modifying a trickling filter water effluent basin having a retaining wall, filter media and a support platform to support the filter media, the method comprising the steps of: a) providing an extension to the retaining wall to increase a height of the wall; b) elevating the support platform to provide a storage space for water under the platform; c) providing a flow control assembly to control water height in the storage space upstream of the means for controlling flow; and d) providing an elevated flow control box to receive the flow control assembly.  
         [0014]     The method can optionally further include the step of providing a ventilation port or a forced air blower in the retaining wall, the port or blower in fluid communication with the storage space.  
         [0015]     In an additional aspect, the present invention provides a trickling filter effluent basin for use in filtering water comprising: (a) a primary retaining wall defining a receptacle area having a floor; (b) filter media provided within the receptacle area on a support platform at a height above the floor; (c) a secondary retaining wall positioned adjacent to the primary retaining wall; (d) a plurality of passageways defined in the primary retaining wall; (e) an underdrain channel defined between the primary retaining wall and the secondary retaining wall; (f) a flow control assembly for controlling flow received in the underdrain channel; and (g) an outlet pipe positioned downstream of the flow control assembly; whereby when water is pumped into the basin, the water flows over the media to the floor of the receptacle area, through the passageways into the underdrain channel and through the outlet pipe out of the basin.  
         [0016]     In a further aspect, the present invention provides a trickling filter effluent basin for use in filtering water comprising: (a) a primary retaining wall defining a receptacle area having a floor; (b) filter media provided within the receptacle area on a support platform at a height above the floor; (c) a secondary retaining wall positioned adjacent to the primary retaining wall; (d) an interior underdrain channel in the floor of the receptacle; (e) an exterior underdrain channel in fluid communication with the interior underdrain channel and defined between the primary retaining wall and the secondary retaining wall; (f) a flow control assembly received in the underdrain channel; and (g) an outlet pipe positioned downstream of the flow control assembly; whereby when water is pumped into the basin, the water flows over the media to the floor of the receptacle area into the interior underdrain channel, out of the basin into the exterior underdrain channel and through the outlet pipe out of the basin.  
         [0017]     The present invention provides a cost-effective solution to the above problem. By modifying existing trickling filter tanks or building new tanks with a dual purpose, the cost of building expensive overflow tanks is avoided. Additionally, overflow water is contained within the plant and is processed and treated prior to release to surface waters. The tanks can also be used to store water for plant operation and maintenance activities. As will be understood by one skilled in the art, the present invention can be used for both municipal and industrial wastewater treatment systems.  
         [0018]     The present invention offers additional advantages. No major changes in piping are required to implement the dual-purpose trickling filter tank, and it is easy to operate. Aeration can be added to the primary trickling filter basin to aid in reduction of biological oxygen demand (BOD) during seasonal weather changes, and the basin of a secondary trickling filter can be used for flocculation prior to final clarification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention is further illustrated by the following drawings in which:  
         [0020]      FIG. 1  is a prior art schematic diagram of water flow through a wastewater treatment plant;  
         [0021]      FIG. 2  is a top perspective, partially in section, view of a typical prior art trickling filter effluent water basin with rock media;  
         [0022]      FIG. 3  is a top perspective, partially in section, view of another prior art trickling filter effluent water basin with newer plastic cross-flow block media;  
         [0023]      FIG. 4  is a top perspective, partially in section, view of the modified trickling filter effluent basin made in accordance with the present invention, during normal plant flow;  
         [0024]      FIG. 5  is a top perspective, partially in section, view of the modified trickling filter effluent basin shown in  FIG. 4  as used for wet weather storage;  
         [0025]      FIG. 6  is a top perspective, partially in section, view of another embodiment of the trickling filter effluent basin made in accordance with present invention having a peripheral wall surrounding the trickling filter tank for storage of water;  
         [0026]      FIG. 7  is a top plan sectional view of another embodiment of the present invention, in which a weir and restriction valve or weir gate is placed on either side of an underdrain channel;  
         [0027]      FIG. 8  depicts operation of the weir gate (illustrating the closed position) shown in  FIG. 7 ;  
         [0028]      FIG. 9  depicts operation of the weir gate (illustrating the open position) shown in  FIG. 7 ;  
         [0029]      FIG. 10  is a top plan view of a portion of the embodiment shown in  FIGS. 4 and 5  without filter media;  
         [0030]      FIG. 11  is a cross sectional view of the embodiment shown in  FIGS. 4 and 5  without filter media;  
         [0031]      FIG. 12  is an end elevation view of a flow control box shown in  FIGS. 4 and 5 ;  
         [0032]     FIGS.  13 ( a ) and  13 ( b ) are end elevation views of a gate valve shown in  FIG. 12 , wherein  FIG. 13 ( a ) shows the open position and  FIG. 13 ( b ) shows the closed position; and  
         [0033]     FIGS.  14  ( a ) and  14 ( b ) are end elevation views of a flow control box shown in  FIGS. 4 and 5  in which the gate valve and weir are combined in one structure, a weir gate, wherein in  FIG. 14 ( a ) the weir gate is in the open position and in  FIG. 14 ( b ) the weir gate is in the closed position. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     As used herein, the term “overflow event” refers to any event which causes the inlet pipes to the wastewater treatment plant to surcharge and/or overflow, such as during heavy rains, snowmelt or other wet weather fluctuations in water. Overflow events include those events which cause the flow entering the WWTP to exceed permitted design averages and/or peak permitted flows. Such events may cause excess flows to be discharged from the WWTP untreated and/or cause hydraulic and/or organic overloads of existing treatment facilities.  
         [0035]     In all figures, like numerals refer to like features having the same described function.  
         [0036]     The terms “trickling filter effluent water basin”, “trickling filter tank”, “trickling filter biotower” and “trickling filter” are used herein interchangeably and refer to a wastewater treatment tank in which water flows downward over biological media to remove soluble organic contaminants in the water, as is known in the art. Such tanks are used in both municipal and industrial wastewater treatment systems. As will be understood by one skilled in the art, any of the embodiments described herein can be used in either setting.  
         [0037]      FIG. 1  shows a schematic diagram of the flow of wastewater through a typical prior art wastewater treatment plant. Wastewater  8  enters the influent manhole and gate valve  10  and flows to a raw sewage wet well  12 . The wastewater is then pumped to a primary clarifier distribution box  14  and flows into primary clarifiers  16  for separation of solids from the flow. The water is pumped to a primary trickling filter wet well  18  and then pumped to a primary trickling filter  19 , for removal of biological material from the water. Effluent from the primary trickling filter  19  then flows to a secondary trickling filter wet well  20  and is pumped to a secondary trickling filter  22 , after which it enters a secondary clarifier distribution box  24  and then a secondary or final clarifier  26 . Following secondary clarification, the water flows to a disinfection facility  32  where the water is disinfected through the use of chlorine, hypochlorite, ultraviolet light or other processes, after which the outflow is directed to surface waters  34 . As will be understood by one skilled in the art, numerous modifications to the above process are often made, depending on the type of wastewater being treated and other considerations. For example, some plants may have tertiary treatment after the secondary clarifiers, in which water would either flow or be pumped onto sand filters for removal of any extra fine suspended solids.  
         [0038]     Referring now to  FIG. 2 , a prior art trickling filter tank  36  with retaining wall  38 , receptacle area  39  holding the rock filter media  40  is shown. The filter media provides a large surface area upon which the biological slime growth develops. An inlet pipe  42  conveys wastewater to be treated to a trickling filter distributor  44  having a distributor base  46  to support rotating distributor arms  48  and distributor bearings  52  which allow the distributor arms  48  to rotate. The distributor  44  contains a center well  54  which provides for higher water head to maintain equal flow to distributor arms  48 .  
         [0039]     Stay rods  56  support the distributor arms  48  and turnbuckles  58  on the stay rods  56  permit adjusting and leveling of the distributor arms  48  to produce an even distribution of wastewater over the filter media  40 . The speed of the distributor arms  48  is controlled with the speed retarder orifice  62  or with other means such as a mechanical driver.  
         [0040]     Wastewater flows through the inlet pipe  42  and is pumped up through the center well  54  of the distributor  44 , through the distributor arms  48  and over the filter media  40  via outlet orifices  64  which control the flow of water to the filter media  40 . Outlet orifices  64  are adjustable to provide an even distribution of wastewater to each square foot of filter media  40 . Splash plates  60  on the distributor arms  48  distribute the flow from the outlet orifices  64  evenly over the filter media  40 .  
         [0041]     An arm dump gate  72  drains the distributor arms  48  and controls filter flies along the filter retaining wall  38 . The dump gate  72  is also used for flushing the distributor arms  48  to remove accumulated debris that could block the outlet orifices  64 .  
         [0042]     In trickling filter systems with rock media, the media rests on a bottom  73  of the tank on a support grill  74  which lays directly on top of the underdrainage system  76 . The support grill  74  holds the filter media  40  in place and keeps it out of the underdrainage system  76 . The underdrainage system  76  is a network of pipes made of clay directly beneath the media support grill  74 . As will be understood by one skilled in the art, the underdrainage system in a trickling filter tank with rock media cannot store water, nor can it be adapted to do so. After the wastewater flows over the filter media  40 , it falls to the underdrainage system  76  where it is conveyed to an underdrain channel  78  via a sloped floor  80 . The underdrain channel  78  drains filter effluent to an outlet box  82 , where it is collected before it flows to the next step in the wastewater treatment process. An outlet valve  84  in outlet box  82  is used for maintenance and is placed at the outlet pipe  86 . In rock media systems, the outlet valve is normally in the open position but is closed when it becomes necessary to backwash the filter, such as when it becomes clogged.  
         [0043]      FIG. 3  illustrates a prior art modern trickling filter tank in which old rock media has been replaced with plastic filter media  90  such as plastic cross flow block media. The modern tank functions as described above, with the following modifications as described below.  
         [0044]     After wastewater flows over the plastic filter media  90 , it falls to the floor where it is conveyed to an underdrain channel  78  via the sloped floor  80 . The underdrain channel  78  drains filter effluent to an outlet box  82 , where it may be collected before it flows to the next step in the wastewater treatment process. An outlet valve  84  at the outlet pipe  86  is used for maintenance purposes.  
         [0045]     The height of the original retaining wall  38  may be extended with the addition of an extension wall  92  made of steel or other suitable material, for the purpose of adding additional media or providing a wind break to the media. A standard hydraulic flow-driven distributor (not shown) or a mechanical drive type of distributor  44  is used to control the speed of the distributor arms  48 . Piers  94  made of concrete or other suitable material support cross-beams  96  made of concrete or other suitable material, which in turn support the plastic media  90  at a height of about one to three feet above the floor of the tank. The retaining wall  38  may contain one or more ventilation ports (not shown) or forced air blowers  108  below the level of the filter media  90 , to allow air to flow to the filter media  90 . In both the old rock media tank shown in  FIG. 2  and the modern trickling filter tank shown in  FIG. 3 , the underdrain channel  78  and outlet pipe  86  are positioned below the bottom (floor level) of the tank, such that the top of the outlet box  82  is at ground level. Also in both prior art tanks shown in  FIGS. 2 and 3 , the outlet valve  84  is positioned at the downstream end of the outlet box  82  and directly on the outlet pipe  86 .  
         [0046]      FIG. 4  shows an embodiment of the trickling filter made in accordance with the present invention. Water is distributed through the tank and over the media as generally described above. In one embodiment, an existing older rock media tank or an existing newer plastic media tank has been modified according to the present invention. The height of the original retaining wall  38  is extended with the addition of an extension wall  92  made of steel or other suitable material, to provide additional storage space for overflow water. The media support platform such as piers  94  made of concrete or other suitable material are extended in height above the floor  80  of the tank. The piers  94  and cross beams  96 , also made of concrete or other suitable material, support the plastic media  90  at a height  91  of greater than about three feet above the floor  80  of the tank, preferably at least four feet or more above the floor  80  of the tank, more preferably about at least six or seven feet above the height of the floor  80 . The extension wall  92  optionally contains one or more ventilation ports  98  below the level of the filter media  90  but above the high water level to allow air to flow through the filter media  90 . Optionally, an air compressor or blower  108  in fluid communication with the storage area inside the tank may be used, also placed above the high water level during storage. During times of peak water storage, it may be necessary to use a blower to provide proper aeration of the filter media. Air can also be provided to the media by other means, such as ventilation pipes through the media from the top of the tank.  
         [0047]     The trickling filter tank of the present invention is further modified with the addition of a flow control box  100 . The flow control box  100  extends the height of the prior art outlet box  82  to a height near the height of the support platform cross beams  96  containing the media  90 . The flow control box  100  contains a flow control assembly to set the elevation of water retained in the tank when storage is needed, such as a flow weir  126  or other suitable structure in combination with a flow restriction valve or gate  104 , or a weir gate (described herein below in reference to  FIGS. 8, 9  and  14 ), as will be understood by one skilled in the art. The terms “flow restriction valve” and “gate” are used herein interchangeably and each term includes the other. Referring to  FIG. 5 , the flow weir  126  is adjustably sized at the maximum depth of the tank that will be used for storage (provided in interior storage space  102 ), such that when full, the tank still contains sufficient air space  103  below the media  90  for ventilation. The size of the air space can be anywhere from a few inches to a foot or more, depending on the type of media used and the amount of aeration required, as will be determined by one skilled in the art. Aeration of the media can also be accomplished by other methods, such as aeration pipes through the media from the top of the tank (not shown). The flow restriction valve  104  is disposed adjacent to the flow weir  126 . During normal operation of the tank, water flows over the filter media  90  and falls to the floor  80  where it is conveyed to an underdrain channel  78  via a sloped floor  80 . The flow restriction valve  104 , which regulates flow of water out of the tank, is in the open position and water flows through the underdrain channel  78 , through the flow control box  100 , into the outlet pipe  86  and to the next stage of processing in the wastewater treatment plant.  
         [0048]     In another embodiment, the trickling filter tank of the present invention is built as a new tank, incorporating all of the modifications described above, including elevating the support platform for the filter media, increasing the height of the flow control box and providing a flow control assembly such as a flow weir in combination with a flow restriction valve.  
         [0049]      FIG. 5  illustrates an embodiment of a trickling filter tank of the present invention as used for storage. When storage is desired, such as during wet weather events, the flow restriction valve  104  is partially or substantially closed to restrict the flow of water through the outlet pipe  86 . Filling up the interior storage space  102  can be achieved by temporarily increasing the flow at the head of the plant when the flow restriction valve  104  is being used. It is usually necessary to maintain a minimum level of water flow out of the tank to downstream processing steps. When flow is restricted, water accumulates in the interior storage space  102  of the tank to the level of the weir  126  and can be stored until further processing is desired. If the level of water exceeds the capacity of the interior storage space  102  it will flow over the weir  126  into the flow control box  100  and out of the tank through the outlet pipe  86 . Water is released from storage by opening the flow control assembly, such as opening the flow restriction valve  104 , as will be understood by one skilled in the art.  
         [0050]      FIG. 6  illustrates an additional embodiment of the present invention, in which a peripheral wall  110  is built around a trickling filter tank  134  having a receptacle area  39  for the plastic filter media  90 . Such a system would be desirable, for example, when the trickling filter tank is a newer model of tank such as a biotower, which is taller than standard tanks but of smaller diameter. Such a tank  134  would have less storage space for water in the interior storage space  102  under the media support platform as described above. In this situation, a peripheral wall  110  can be built around the tank  134  and provide additional external storage area  124  for water in the area between the retaining wall  38  of the tank  134  and the peripheral wall  110 . The height of the peripheral wall  110  is a height lower than the support platform holding the media inside the tank  134 . The underdrain channel  78  is extended past the outer wall of the tank to the new peripheral wall  110 , and the flow control box  100  is placed at the outer peripheral wall  110 . The flow control box  100  will have the flow control assembly as described above. In operation during a wet weather event, the flow restriction valve  104  in the flow control box  100  is closed, and both the interior storage space  102  under the support platform and the external storage area  124  fill with water to the desired level as set by the flow weir  126 .  
         [0051]      FIG. 7  illustrates a variant on the embodiment shown in  FIG. 6 , in which water is released from the tank through flow outlet holes  122  rather than the underdrain channel described above. The peripheral wall  110  provides an external storage area  124  for water, and the floor  128  (shown in  FIG. 9  described below) of the external storage space  124  is sloped to direct water flow to the effluent drop zone  120 , where it flows out through the outlet pipe  86 . In this embodiment, the flow weir  126  and flow restriction valve  104  are placed on either side of the effluent drop zone  120 .  
         [0052]     As will be understood by one skilled in the art, flow out of the tank or the external storage area in any of the embodiments described herein, including embodiments having a peripheral retaining wall, can be restricted up to the desired level by using, for example, a flow restriction valve or gate in combination with a flow weir of the desired height, or a weir gate, as that term is understood in the art. The weir gate functions to both set the level of water stored in the tank and control the flow of water out of the outlet pipe. Other suitable structures or combinations of structures can also be used, provided that they provide the desired functions of setting the storage water level and controlling flow out of the tank. A weir gate is shown in  FIGS. 8 and 9  as used in the embodiment shown in  FIG. 7 . In  FIG. 8 , the weir gate  130  is in the closed position, and water is held in the external storage space  124  shown in  FIG. 7 . In  FIG. 9  the weir gate  130  is in the open position and water flows from the external storage space  124  into the effluent drop zone  120  and out through the outlet pipe  86 .  
         [0053]      FIG. 10  is a top plan view of the embodiment shown in  FIGS. 4 and 5  without filter media. The retaining wall  38  of the tank is shown, with the underdrain channel  78 , the distributor  44 , the media support piers  94  and the cross-beams  96 . Also shown is the flow control box  100  having a flow restriction valve  104  and a flow weir  126 .  FIG. 11  is a cross-sectional view of the embodiment shown in  FIGS. 4 and 5 , also without filter media, illustrating the retaining wall  38 , underdrain channel  78 , distributor  44 , the media support piers  94  and cross-beam  96 .  
         [0054]      FIG. 12  is a view of the flow control box  100  as described above and shown in  FIGS. 4, 5  and  6 , illustrating the flow restriction valve  104 , valve handle  106 , drive shaft  107 , flow weir  126 , underdrain channel  78  and the outlet pipe  86 . The handle  106  and drive shaft  107  are threadably mounted on the flow restriction valve  104  and rotate to permit raising and lowering of the valve.  
         [0055]      FIG. 13 ( a ) shows the flow restriction valve  104  in the open position, allowing water from the underdrain channel  78  to flow through the outlet pipe  86 .  
         [0056]      FIG. 13 ( b ) shows the flow restriction valve in the closed position. Water will be retained in the storage space (either interior or external) up to the height of the flow weir  126  as shown in  FIG. 13 ( b ). FIGS.  14 ( a ) and  14 ( b ) depict an alternate embodiment, a weir gate  130  in the open position ( FIG. 14 ( a ) permitting water to flow from the underdrain channel  78  through the outlet pipe  86 .  FIG. 14 ( b ) shows the closed position of the weir gate  130 . Water will accumulate behind the gate to the height of the gate.  
         [0057]     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.