Abstract:
In a wastewater treatment system comprised of an anoxic tank outputting to a biological reactor and outputting from the reactor to a clear well, a process for controlling the rate and timing of flow through the reactor by means of a floating pump within the anoxic tank. Further enhancement of denitrification is achieved by sending internal recycles into the sludge zone of the anoxic tank via a baffle positioned within the sludge layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicants claim the priority benefits of U.S. Provisional Patent Application No. 61/592,663, filed Jan. 31, 2012. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to waste treatment systems, and in particular, to a method for removing nitrogen from wastewater. 
         [0003]    Biological removal of nitrogen from wastewater is a multi-step process and is fairly complex. Nitrogen (in different forms) is a component of many of the molecules present in wastewater. Some examples of nitrogen-based molecules include ammonium, urea and proteins. Removal of nitrogen occurs through biochemical transformations mediated by multiple types of microorganisms, for example ammonification occurs during the oxidation of organic material containing nitrogenous compounds. As soluble organic matter is oxidized nitrogen bound compounds, such as amines (NH 2   − ) are released and bond with H +  ions forming ammonia or ammonium. 
         [0004]    Removal of the ammonia and ammonium forms of nitrogen from wastewater is a two step process. In the first step the oxidation of ammonium to nitrate (nitrification) is accomplished by the aerobic growth of chemolithotrophic, autotropic bacteria in an aerobic environment. Nitrification occurs only when the quantity of organic carbonaceous matter has been reduced according to the well-established criterion for the transition from oxidation or organics to nitrification, within the biofloc. (See Williamson and McCarty 1976; Owen and Williamson 1976; Riemer 1977; Harremoes 1982; Harremoes and Gonenc 1985). 
         [0005]    In the second step organic carbonaceous matter (organics) is oxidized by the growth of heterotrophic bacteria utilizing nitrate as the terminal electron accepter, i.e., denitrification. The nitrate is converted to nitrogen gas (N 2 ) and released to the atmosphere. The equation describing the biochemical transformation depends on the organic carbon source utilized. The following is the mass based stoichiometric equation, normalized with respect to nitrate, with the influent waste stream as the organic carbon source (Water Environment Federation 1998). 
         [0000]      NO 3   − +0.324C 10 H 19 O 3 N&gt;0.226 N 2 +0.710CO 2 +0.087H 2 O+0.027NH 3 +0.274OH −   
         [0000]    This results in the removal of nitrogen from the wastewater stream by releasing gaseous nitrogen. 
         [0006]    One of the complexities in this process is that, some of the organic matter must be removed before nitrification can occur; however, organic matter is required for denitrification. The inventors have countered this problem in their prior art processes by providing a biological reactor, which grows a biomass on media within the reactor. The reactor is intermittently aerated to create an environment that alternates from aerobic to anoxic. The term, anoxic, means an environment in which respiration with nitrate as the terminal elector is available. The prior art biological reactor processes do not regulate air flow to the reactor. Also, prior art biological reactor processes do not control the flow through the reactor. Flow has been by gravity with a varying pressure head that as a function of the quantity of the internal recycle. 
       SUMMARY OF THE INVENTION 
       [0007]    Prior art wastewater treatment systems basically comprised of an anoxic tank outputting to a biological reactor and outputting from the reactor to a clear well are known. The present invention provides a major improvement to prior art wastewater treatment systems by controlling the rate and timing of flow through the biological reactor using a programmable logic controller (PLC) controlled valve or pump. This permits operators to insure that all liquid passing through the reactor is subjected to both aerobic and anoxic periods before passing through the reactor to a clear well. This allows for the residence time within the anoxic tank containing carbon from sludge digestion, to be controlled so as to improve denitrification and flow to the reactor at the appropriate time. By controlling the flow to the reactor and manipulating the addition of air and wastewater, applicants have greatly improved nitrogen removal when compared to the uncontrolled gravity flow. 
         [0008]    Currently, biological reactor recycle streams are returned to the settle zone of the anoxic tank. Recycling brings nitrified wastewater into contact with carbonaceous wastewater in a low dissolved oxygen environment. Further enhancement of denitrification is achieved by sending the internal recycles into the sludge zone of the anoxic tank where there is more carbon and less oxygen than in the settled zone. Agitation and suspension of the sludge is reduced by the use of a baffle which distributes the recycle flow within the sludge layer. Any undesirable suspension is mitigated by control of the flow which allows the sludge to resettle before flow is allowed to flow to the reactor. Thus, only settled wastewater flows to the reactor. 
         [0009]    Two different environments are required for nitrification and denitrification: an aerobic environment for nitrification and an anoxic environment for denitrification. These environments can be maintained within the present invention process. However, the wastewater must be delivered to the invention process so that optimal conditions exist to maximize the removal of nitrogen and eliminate or reduce the need for a supplemental electron donor. 
         [0010]    These together with other objects of the invention, along with various features of novelty, which characterize the invention, are pointed out with particularity in this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of the present invention biological nitrogen removal feed process. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0012]    Referring to the drawings in detail wherein like elements are indicated by like numerals, there are shown a process flow for a typical wastewater treatment system  1 , and the invention process installed in the anoxic tank portion of the wastewater treatment system  1 . 
         [0013]    The wastewater treatment system  1  has an anoxic tank  10  outputting to a biological reactor  30  and outputting from the reactor to a clear well  50 . The anoxic tank  10 , often a septic tank, typically provides primary treatment for wastewater. The anoxic tank  10  has a bottom  21 , top  22 , an input wall  23 , an output wall  24  and two side walls  25  interconnecting the input and out put walls, said bottom, top, input wall, output wall and side walls defining an anoxic tank interior  20 . The anoxic tank input  23  has a raw wastewater input pipe  26  connecting an external raw wastewater source with the anoxic tank interior  20 . The input pipe  26  is located near to the anoxic tank top  22 . The anoxic tank output wall has an output pipe  29  for anoxic tank effluent  12 . Unlike prior art wastewater treatment systems, the present invention does not operate from anoxic tank to biological reactor via gravity flow. Rather, a “floating” pump  15  is placed within the anoxic tank interior whereby flow from the anoxic tank  10  to the biological reactor  30  is controlled. 
         [0014]    The biological reactor  30  has a top  31 , a bottom  32 , receiving side  33 , discharge side  34 , two opposite side walls  35  interconnecting the receiving and discharge sides, said top, bottom, receiving side, discharge side and side walls defining a reactor interior  36 . The reactor interior  36  has a filter  37 , an open head-space  38  above the filter, and a sump  39  formed beneath the filter and reactor bottom  32 . The anoxic tank output pipe  29  connects to the biological reactor  30  and the anoxic tank effluent  12  is pumped by the floating pump  15  to the biological reactor interior head-space  38  just above the reactor filter  37 . The biological reactor  30  has a backwash/recycle pipe  40  interconnecting the biological reactor interior head space  38  with a hollow vertical recycle pipe  27  located in the anoxic tank interior near to the anoxic tank input wall  23 . The anoxic tank vertical recycle pipe  27  is fluidly connected to an elongated baffle  28  comprised of perforated horizontal pipe  28  positioned adjacent the anoxic tank interior bottom  21  extending centrally from the vertical recycle pipe  27  toward the anoxic tank output wall  24 . 
         [0015]    The biological reactor  30  has a discharge pipe  42  interconnecting the biological reactor sump  39  with a clear well interior  56 . The clear well  50  is comprised of a bottom  51 , a top  52 , a receiving side  53 , a discharge side  54 , and two side walls interconnecting said receiving and discharge sides  55 , said bottom, top, receiving side, discharge side and side walls defining a clear well interior  56 . The biological reactor discharge pipe  42  connects to the clear well inlet pipe  57  in the clear well receiving side  53  providing a channel for the biological reactor treated wastewater into the clear well interior  56 . The clear well interior  56  contains a first pump  58  on the clear well bottom  51 , said first pump being connected to said clear well inlet pipe  57 , and being adapted to provide reverse flow from the clear well interior  56  back through the biological reactor interior  36 . The clear well interior  56  also contains a second pump  59  on the clear well bottom  51 , said second pump being interconnected to a clear well discharge outlet  60  in the clear well discharge side  54 , said second pump  60  adapted to discharge the contents of said clear well interior  56  out through said discharge outlet  60 . 
         [0016]    Raw untreated sewage wastewater having a significant concentration of waste solids is introduced into the anoxic tank interior  20  through the anoxic tank input pipe  26 . Solids having a higher density than liquid sink to the tank bottom  21  to form a sludge layer  11 . The elongated baffle  28  is in the sludge layer  11 . The liquid portion of the wastewater, which exits the anoxic tank discharge end  24  by means of gravity, a pump, or a siphon, is the anoxic tank effluent  12 . The anoxic tank effluent  12  is brought into the biological reactor  30  for treatment in an aerobic environment causing bacteria to oxidize the ammonia nitrogen to nitrate nitrogen, a process known as nitrification. By then treating the effluent in an anoxic environment, the nitrified wastewater is denitrified and the nitrogen gas formed is released to the atmosphere while the treated wastewater, with a reduced level of nitrogen compounds, is returned to the receiving stream or to the clear well  50 . 
         [0017]    The invention process enhances a typical prior art biological nitrogen removal feed process through the use of two mechanisms: a return of nitrified wastewater into the sludge layer  11  of an anoxic tank  10 , and through the use of the present invention feed process. 
         [0018]    The return of nitrified wastewater via the reactor recycle pipe  40 , the anoxic tank vertical recycle pipe and baffle  28 , into the sludge layer  11  is significant because it allows the system to release unused carbon for denitrification. The return to the sludge layer consists of returning liquid from the reactor head-space via the recycle pipe  40  into the anoxic tank vertical pipe  27  and baffle  28  for discharge into the sludge layer  11 . In order to avoid carryover to the biological reactor  30  of sludge particulate matter which is disturbed during the return, the wastewater is held in the anoxic tank interior  20  for a fixed time. The floating pump  15  is turned off for a period of time to allow the sludge to settle and prevent sludge from moving forward into the biological reactor  30  before it settles in the anoxic tank interior  20 . This allows sludge settling before the floating pump  15  is reactivated moving the anoxic tank effluent forward. In an alternate embodiment a remotely controlled check valve  13  may be placed in the anoxic tank output pipe  29  to prevent sludge-filled effluent from moving toward the biological reactor for a period of time. 
         [0019]    The present invention feed process is comprised of the following sub-cycles: (a) dose cycle, (b) rest cycle, (c) bump cycle, (d) rest cycle, (e) aeration cycle, and (f) rest cycle. 
         [0020]    During the dose cycle, a measured volume of wastewater (Dose) is delivered from the anoxic tank interior  20  to the biological reactor  30  via the anoxic tank output pipe  29 . The Dose is calculated as the volume of liquid required to displace approximately half of the liquid in the reactor filter media  37 . Delivery may be controlled by a PLC  5  using the floating pump  15  or check valve  13 . The PLC controls how long the feed pump  15  runs or how long the valve  13  is opened. The Dose enters the biological reactor interior  36  and displaces treated wastewater to the clear well  50 . 
         [0021]    A Dose rest period is then entered. The rest period allows time for the delivered Dose to be denitrified. The Dose liquid received by the biological reactor is ideal for denitrification because it contains nitrates, carbon and a very low dissolved oxygen. The time period of the Dose rest period is a defined value. 
         [0022]    After the Dose rest period, a Bump cycle is entered. During the Bump cycle, nitrified and oxygenated wastewater is returned to the biological reactor  30  from the clear well  50  using the clear well first pump  58 , through the clear well inlet pipe  57 , back through the biological reactor discharge pipe  42 , into the biological reactor sump  39 , up into the filter media  37 , and mixed with the Dose that was put into the reactor during the dose cycle. Following the Bump cycle, a Bump rest period is entered. The Bump rest period allows time for the removal of carbon introduced by the Dose cycle. 
         [0023]    After the Bump rest period, an aeration cycle is entered. During the aeration cycle, oxygen is provided to the biological reactor interior  36  via an air pipe  43  inserted into the biological reactor interior  36  near to the reactor top  31 , through the filter media  37 . The air pipe  43  is connected to an air source, such as an air pump and/or blower (not shown), on the ground surface  2 . During the aeration cycle, air is provided into the air pipe  43  to the reactor bottom  37  and  39 . Oxygenation is provided to the biological reactor interior  36  while air is being blown into the air pipe  43 . The blower “on” time is recomputed at regular intervals by the PLC and is based on the volume and constituent load of influent wastewater sensed by the system during the previous time interval. 
         [0024]    Following the aeration cycle, the air blower is shut off and an aeration rest period is entered. The aeration rest period allows time for nitrification. This rest period is a value that is variable and may be modified to optimize treatment of wastewater. At the end of the aeration rest period, a Dose cycle will begin. 
         [0025]    It is understood that the above-described embodiment is merely illustrative of the application. Other embodiments may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.