Abstract:
The disclosed process and system are used for the denitrification of wastewater. The system comprises: an influent concentration analyzer for measuring an influent dissolved oxygen concentration, an influent nitrate concentration, and an influent nitrite concentration; and a feed chemical controller for providing a feed chemical at a controlled rate. The feed chemical controller is responsive to one or more output signals provided by an automated control loop that accepts input signals from the influent concentration analyzer, which input signals relate to at least two of the influent nitrate concentration, the influent nitrite concentration, and the influent dissolved oxygen concentration.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a Divisional of U.S. application Ser. No. 12/694,768, filed Jan. 27, 2010, now allowed, which is a Continuation of U.S. application Ser. No. 12/246,169, filed Oct. 6, 2008 (now U.S. Pat. No. 7,662,287), which is a Divisional of U.S. application Ser. No. 11/504,101, filed Aug. 15, 2006 (now U.S. Pat. No. 7,431,840), which claims priority to U.S. Provisional Application No. 60/710,612, filed Aug. 24, 2005, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a system and process for wastewater treatment and, in particular, to a system and process for denitrification of wastewater. 
     2. Related Art 
     According to a recent article, which appeared in the Washington Post (“Troubled Waters in the Shenandoah: Death of Smallmouth Bass Brings Attention But No Quick Answers on Improving Quality” By Michael Alison Chandler, Washington Post Staff Writer, Wednesday, Jul. 20, 2005; Page B01), questions are constantly being raised about the quality of the water that feeds into waterways in and around the Shenandoah Valley and, ultimately, the Chesapeake Bay. Among the factors cited in the article, the high nutrient (and nitrogen) content in the feed waters was noted as a significant culprit:
         The river is also known to have high nutrient levels. Nitrogen and phosphorus in high amounts cause excess plant or algae growth, which can reduce levels of dissolved oxygen. Fish struggle to breathe, and that can weaken their resistance to disease or bacteria.   The land along all three rivers affected by the fish kills is primarily agricultural. With more than 900 farms in the valley, the poultry industry dominates. High-nutrient waste from the farms is used as fertilizer and can wash into the river.
 
It is clear that better, more effective ways to control the nitrogen content of waste water are needed.
       

     It is known, that the control of feed chemicals used in the processing of liquids (e.g., waste water in a waste water treatment system) can be automated through the use of computerized control devices. Problems can occur during the automatic dosing of chemicals into the treatment system because of inaccuracies of measurements of a chemical present in the system and the variable ratio of chemical to liquid when the liquid flow rate is variable. 
     In the past, dosing was done by laboratory or bench testing the influent chemical concentration, in combination with influent flow rate measurements. Subsequently, dose calculations were performed and the dosing device, a chemical feed pump, for example, was manually adjusted according to the calculations. More recently, partial pacing of the dosing pumps was practiced using an influent water flow signal. Varying the dose rate to maintain a desired effluent chemical concentration test result was deemed a more direct approach. 
     In recent years, reliable automatic analyzers for chemical concentration have become available, thus enabling automation of the entire dosing procedure. Automatic analyzers can also be set up to detect several important chemicals in waste water treatment system, enabling the use of a variety of chemicals depending on the specific application, e.g., the addition of sodium bicarbonate into an aerated biological reactor or the addition of iron or aluminum salts before a clarifier to control phosphorus removal. 
     In U.S. Pat. No. 6,129,104 (the &#39;104 patent), the contents of which are incorporated herein by reference, a method for controlling the addition of liquid treatment chemicals by automatic dose control is provided. In this method, the calculation of the amount of chemical to be dosed into the system combines signals from a liquid flow meter, an influent chemical concentration analyzer, and an effluent chemical concentration analyzer. The signals are directed to a computerized chemical dose controller that analyzes and adjusts the data from the signals and generates an output signal that controls the chemical dosing mechanisms. According to the &#39;104 patent, this method may be used, for example, for denitrification of wastewater using methanol as the feed chemical. 
     Denitrification comprises the removal of nitrate and nitrite from a waste stream through the use of facultative heterotrophic bacteria. These facultative heterotrophic bacteria, in the presence of a carbon source (e.g., methanol), and in the absence of dissolved oxygen (DO), can strip the oxygen atoms from both nitrate and nitrite moieties, leaving nitrogen gas (N 2 ), which exits the waste stream and enters the atmosphere (air is about eighty percent nitrogen gas), hence “denitrifying” the waste stream. Thus, methanol consumption is dependent on influent nitrate and nitrite as well as influent DO, namely, enough methanol is required to first deplete the influent DO and subsequently to account for stripping all the oxygen atoms associated with nitrate and nitrite. 
     The &#39;104 patent, however, ignores DO and nitrites and further describes measuring influent and effluent concentrations of nitrates only in order to determine an amount of methanol to be fed into the system for denitrification. However, as discussed above, the measurement of the influent and effluent concentrations of nitrates is insufficient in determining the proper amount of methanol to be fed into the system for denitrification. Stated another way a methanol dosing system that strictly looks at influent and effluent nitrate would not account for methanol demands associated with varying levels of influent nitrite and DO, thus leading to possible overdosing or underdosing of methanol. For example, relying on influent nitrate measurements only can result in overdosing because of a drop in influent DO levels. By the same token, reliance on effluent nitrate measurements can result in underdosing because a low level of measured effluent nitrate misses altogether the fact that a reduction in nitrate levels can simply mean that all the nitrate has been converted to nitrite, which must still be reduced. That is, nitrate is first converted to nitrite (thus leading to an increase in the levels of nitrite) on the way to complete conversion to gaseous nitrogen. Instead, the &#39;104 patent relies on “fudge” factors, for instance, the use of an “adjustable factor [which] is determined by the operator” and/or the use of a “sensitivity factor [which] is selected by the operator” to compensate for the inaccuracy inherent in limiting measurements to nitrate concentrations. Initially these operator-controlled “factors” are no more than educated guesses and, at best, might be derived empirically. Implicit in such an operator-controlled technique is the necessity for an operator to “get up to speed” on system requirements, all of which represents a time consuming “learning curve.” 
     Therefore, there is a need for a more accurate, automated method of determining an amount of feed chemical (e.g., methanol) to be fed into a denitrification system without reliance on operator-controlled adjustable or sensitivity factors. In particular, there is a need for a method that takes influent concentrations of nitrogen-containing substances, in addition to nitrates (e.g., nitrites) (so called NO x ), and dissolved oxygen and effluent concentrations of these nitrogen-containing substances (i.e., NO x ) into account in calculating the proper dose of feed chemical. These measurements can be taken either from influent samples only or from both influent and effluent samples. 
     SUMMARY OF INVENTION 
     In one aspect, the invention provides an aqueous denitrification process for a wastewater treatment system. The wastewater treatment system has influent and effluent aqueous flows, a filtration bed harboring microbes, and feed forward and optional feed back control loops. The process comprises the steps of: a) determining influent flow (optionally expressed in millions of gallons per day), Q, influent dissolved oxygen concentration (optionally expressed in mg/liter), DO in , influent nitrate concentration (optionally expressed in mg/liter), NO 3 —N in , and influent nitrite concentration (optionally expressed in mg/liter), NO 2 —N in ; b) utilizing a feed forward control loop relationship (1) to determine a feed chemical requirement, FCR,
 
FCR= Q [( X*D O)+( Y *NO 3 —N in )+( Z *NO 2 —N in )]  (1)
 
in which X, Y, and Z are predetermined unitless factors ranging from about 0.7 to about 3.0; c) optionally determining effluent nitrate concentration (optionally expressed in mg/liter), NO 3 —N eff , and effluent nitrite concentration (optionally expressed in mg/liter), NO 2 —N eff ; and d) optionally utilizing feed back control loop relationships (2), (3) and (4) to determine an adjusted feed chemical requirement, AFCR, utilizing relationship (5),
 
                   AP   =     GAIN   *   ERR             (   2   )                 AI     (   new   )       =       AI     (   old   )       +     GAIN   *     dl   TI     *         ERR     (   new   )       +     ERR     (   old   )         2                 (   3   )               A   =     AP   +   AI             (   4   )                 A   ⁢           ⁢   F   ⁢           ⁢   C   ⁢           ⁢   R     =     F   ⁢           ⁢   C   ⁢           ⁢   R   *     (     1   +   A     )               (   5   )               
in which GAIN is a predetermined unitless action coefficient, ERR is the difference between measured NO x —N eff  (which is the sum of NO 3 —N eff  and NO 2 —N eff ) and set point NO x —N eff , dl is the sum of the microprocessor processing time (optionally expressed in seconds) and a time interval, TI (optionally expressed in seconds), between measurements, and references to new and old refer to a current measurement and a prior measurement. The value of FCR may be expressed in pounds per day by multiplying the value for FCR obtained via relationship (1) by a conversion factor, which is 8.34. The values for X, Y and Z are determined by the stoichiometric equation for the consumption of dissolved oxygen, nitrates and nitrites in the presence of methanol. A more detailed discussion of preferred embodiments is presented, below. However, in one embodiment of the invention, the value of X may be set, for example, at 0.81, the value of Y may be set, for example, at 2.25, and the value of Z may be set, for example, at 1.35. The value of GAIN may be set, for example, at 0.2. The value of TI may be set, for example, at 400 seconds. The value of set point NO x —N eff  may fall within the range of 0.25 mg/liter to 10.0 mg/liter. The quotient dl/TI may be approximately equal to 1. The values for AI (old)  and ERR (old)  may be set at the initial measurement to 0. It should be emphasized, however, that the invention is not limited to or by the equations provided above, which simply serve as illustrative of a preferred embodiment of the invention.
 
     In another aspect of the invention, a wastewater denitrification system is provided. The system comprises an influent flow meter for measuring an influent flow (optionally expressed in millions of gallons per day), Q; an influent concentration analyzer for measuring an influent dissolved oxygen concentration (optionally expressed in mg/liter), DO in , an influent nitrate concentration (optionally expressed in mg/liter), NO 3 —N in , and an influent nitrite concentration (optionally expressed in mg/liter), NO 2 —N in ; an optional effluent concentration analyzer for measuring an effluent nitrate concentration (optionally expressed in mg/liter), NO 3 —N eff , and an effluent nitrite concentration (optionally expressed in mg/liter), NO 2 —N eff ; and a feed chemical controller for providing a feed chemical at a controlled rate. The feed chemical controller is responsive to one or more output signals provided by an automated control loop that accepts input signals from the influent concentration analyzer and optionally the effluent concentration analyzer. The input signals relate to at least NO 3 —N in  and NO 2 —N in  and, optionally, NO 3 —N eff , and NO 2 —N eff . The automated control loop may also accept input signals relating to DO in . The automated control loop may also accept input signals relating to Q. The feed chemical may be provided as part of the influent flow. The feed chemical may include any source of carbon, including but not limited to acetic acid, sugars, methanol, ethanol, and the like. The automated control loop may be configured to accept input signals at a predetermined time interval, TI. 
     In yet another aspect of the invention, a method of automatically controlling a rate at which a feed chemical is provided to microbes harbored in a filtration bed is provided. The method comprises the steps of: (i) determining an influent flow rate (optionally expressed in millions of gallons per day), Q, an influent dissolved oxygen concentration (optionally expressed in mg/liter), DO in , an influent nitrate concentration (optionally expressed in mg/liter), NO 3 —N in , and an influent nitrite concentration (optionally expressed in mg/liter), NO 2 —N in ; (ii) determining a feed chemical requirement, FCR, based in part on the values for Q, DO in , NO 3 —N in , and NO 2 —N in , obtained from step (i); (iii) optionally determining an effluent nitrate concentration (optionally expressed in mg/liter), NO 3 —N eff , and an effluent nitrite concentration (optionally expressed in mg/liter), NO 2 —N eff ; (iv) optionally determining an adjusted feed chemical requirement, AFCR, based in part on the values for NO 3 —N eff  and NO 2 —N eff , obtained from step (iii) (that is, NO x —N eff ), and ERR, which is the difference between measured NO x —N eff  (which, in turn, is the sum of NO 3 —N eff  and NO 2 —N eff ) and set point NO x —N eff ; and (v) repeating steps (i), (ii), (iii), and (iv) at a predetermined time interval, TI. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a denitrification system according to a preferred embodiment of the invention. 
         FIG. 2  is a flow chart that illustrates a method for calculating a dosage of methanol to be provided to a denitrification system according to a preferred embodiment of the invention. 
         FIG. 3  is a color/shaded plot of the diurnal variations found for influent concentrations of influent nitrate, influent phosphate and influent dissolved oxygen. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a system and method for performing denitrification of a liquid flow, typically wastewater. Unlike conventional denitrification systems, which rely exclusively on the measurement of influent and effluent concentrations of nitrates to assist in a calculation of a dosage of a feed chemical, typically methanol, and requires an experienced operator to determine “adjustable” and “sensitivity” factors to “fudge” feed chemical dosage values, the present invention also includes the advantages of measuring influent and effluent concentrations of nitrites, as well as the influent concentration of dissolved oxygen, in order to determine a more accurate dosage of methanol to be used in the wastewater treatment process. Hence, no operator-controlled factors are required, as discussed further herein. The invention also encompasses a system in which effluent measurements are merely optional, thus reducing system complexity and cost. 
     Referring to  FIG. 1 , a block diagram of a denitrification system  100  according to a preferred embodiment of the invention is shown. The system  100  accepts an influent flow  105  of a liquid to be treated, typically wastewater. The influent flow  105  first encounters an influent flow meter  110 , which measures a flow rate of the liquid, and generates a flow rate signal corresponding to the measurement. The influent flow  105  then continues to an influent concentration analyzer  115 . The analyzer  115 , which may comprise one or more devices, measures concentrations of nitrates, nitrites, and dissolved oxygen within the influent flow, and generates signals corresponding to each of these influent concentrations. These signals can then be combined, along with the flow rate signal, to provide a control signal. Then, the influent flow  105  continues into the treatment process  120 . The treatment process  120  treats the wastewater. 
     The treatment process  120  may include a filter system. For example, a sand filter system may be used to filter out solid materials from the wastewater. In one preferred embodiment, one or multiple continuous backwash upflow sand filter systems may be used within the treatment process  120 . See, U.S. Pat. Nos. 4,126,546, 4,197,201, and 4,246,102 for examples of suitable sand filtration systems. The disclosures of these patents are incorporated by reference herein. 
     One aspect of the treatment process  120  is a denitrification of the wastewater. Denitrification is accomplished by providing a feed chemical, typically methanol, to the wastewater. As described, above, microbes harbored in the filtration bed consume the methanol along with the nitrates and nitrites that are present in the wastewater to produce gaseous nitrogen, which then diffuses naturally into the ambient atmosphere. In this manner, a substantial amount of the nitrogen content in the wastewater is eliminated, hence the term “denitrification.” 
     In a preferred embodiment of the invention, methanol is chosen as the feed chemical because of its availability and cost relative to other synthetic feed chemicals. Its presence in the downstream effluent must nevertheless be held to a minimum. Thus the present invention seeks to provide a more accurate determination of the amount of methanol required to accomplish the desired levels of denitrification, yet not provide an excess of methanol, whose presence in the effluent might prove problematic. 
     It is known that the methanol introduced into the denitrification system first consumes the dissolved oxygen present in the influent, according to the equation: 3O 2 +2CH 3 OH=2CO 2 +4H 2 O. Subsequently, reduction of nitrate and nitrite transpires. Each transformation can be described by the following stoichiometric equations: 6NO 3   − +5CH 3 OH=3N 2 +5CO 2 +7H 2 O+6OH −  and 2NO 2   − +CH 3 OH═N 2 +CO 2 +H 2 O+2OH − . The nitrogen gas produced diffuses into the atmosphere. Accordingly, stoichiometric amount of methanol required for complete denitrification is provided by the equation: CH 3 OH=0.7DO+2.0NO 3 —N+1.1NO 2 —N, in which methanol, influent dissolved oxygen, influent nitrate and influent nitrite are expressed in mg/L. Actual values for X, Y and Z can be chosen at the outset, but can be changed at a later time if desired. Actual values are likely to be a certain percentage in excess of stoichiometric needs. 
     After the treatment process  120  is complete, the treated wastewater then encounters an optional effluent concentration analyzer  125 . The analyzer  125  optionally measures concentrations of nitrates and nitrites present in the effluent flow  130 , which then exits the filtration system. The analyzer  125  can also generate signals that respectively correspond to the effluent nitrate and nitrite concentrations. 
     A critical aspect of the denitrification system  100  is the determination of the amount of the methanol dosage to be fed into the treatment process  120 . In order to optimize system efficiency, the calculated amount should be as accurate as possible. The present invention provides an improved accuracy by measuring multiple analytes, which provide a more complete picture of the amount of feed chemical required. 
     Referring to  FIG. 2 , a flow chart  200  illustrates a methodology for calculating an amount of methanol to be fed into the treatment process  120 . First, at step  205 , the influent flow rate is measured using the influent flow meter  110 . A flow rate signal is generated from this first measurement, and may be represented by the variable Q, and typically expressed in millions of gallons per day. In the second step  210 , measurements are taken for the influent concentrations of nitrates, nitrites, and dissolved oxygen, and corresponding signals are generated. These signals are typically expressed in units of milligrams per liter (mg/L), and may be represented by the following variables: Influent concentration of dissolved oxygen=DO in ; influent concentration of nitrates=NO 3 —N in ; and influent concentration of nitrites=NO 2 —N in . 
     Then, at step  215 , the generated signals are used to calculate a nominal value of the feed chemical requirement (FCR) according to Equation 1 below:
 
FCR= Q [( X*D O in )+( Y *NO 3 —N in )+( Z *NO 2 —N in )]  (1)
 
where X, Y, and Z are predetermined unitless factors that typically fall within the range of 0.7 to 3.0. As discussed above, the stoichiometric amount of methanol needed for complete denitrification requires that X=0.7, Y=2.0 and Z=1.1. These are the minimum values. However, one typically requires an excess of methanol to drive the stoichiometric reaction to completion. Hence, an excess of methanol, up to 50% in excess of stoichiometric requirements, may be desired. Typically, a 10-30% percent excess might be desired, preferably 15-20% excess. In one embodiment of the invention, therefore, X=0.9, Y=2.5 and Z=1.5. In yet another embodiment of the invention X=0.8, Y=2.3, and Z=1.4. The value of FCR may be converted to units of pounds per day by multiplying the initial value by the conversion factor, 8.34.
 
     At step  220 , optional measurements are taken from the effluent flow of the concentrations of nitrates and nitrites. Once again, signals are generated that correspond to these measurements. These signals may be represented by the following variables: Effluent concentration of nitrates=NO 3 —N eff ; and effluent concentration of nitrites=NO 2 —N eff ; and total effluent concentration of nitrates and nitrites=NO 3 —N eff +NO 2 —N eff ═NO x —N eff . These signals (namely, NO x —N eff ) are then used to calculate an optional adjustment to the nominal FCR value at step  225 . The entire method according to the flow chart  200  is then repeated continuously so that the methanol value is continuously updated in conjunction with the continuous influent flow. 
     In a preferred embodiment of the invention, the adjustment to the FCR value is determined through the use of an optional feedback process that uses a proportional-integral (PI) loop. The adjustment A includes a proportional component AP and an integral component AI; hence, A=AP+AI. The proportional component AP is defined by Equation 2 below:
 
 AP =GAIN*ERR  (2)
 
where GAIN=the desired magnitude of reaction as a function of the perceived error, and ERR=system deviation=the difference between the effluent set point and the process value. The effluent set point is the expected value of the total effluent concentration of nitrates and nitrites, and the process value is the actual, measured total value of the effluent concentration of nitrates and nitrites. Typically, the effluent set point falls within a range of 0.25 mg/L to 10.0 mg/L. So, for example, if at a given time, the effluent set point is 0.5 mg/L, the actual measured effluent concentration of nitrates is 0.5 mg/L, and the actual measured effluent concentration of nitrites is 0.4 mg/L, then the total effluent concentration of nitrates and nitrites is 0.5 mg/L+0.4 mg/L=0.9 mg/L, and thus ERR=0.9 mg/L−0.5 mg/L=0.4 mg/L. A typical value for GAIN could be 0.2. Thus, in this example, AP=0.2*0.4=0.08.
 
     The integral component of the adjustment, AI, is defined according to Equation 3 below: 
                     AI     (   new   )       =       AI     (   old   )       +     GAIN   *     dl   TI     *         ERR     (   new   )       +     ERR     (   old   )         2                 (   3   )               
TI=Time Interval, or Reset Time=a predetermined time interval between successive measurements. For example, TI may be set equal to 400 seconds. dl=Current Scan Time=an internal system function that tracks the time required to perform the function from the moment the system receives all of the process variables. The Current Scan Time dl is a sum of TI and the actual computation time, which is typically on the order of milliseconds, for example, approximately 20 ms. Thus, in this example, dl=400 s+20 ms=400.020 s. Therefore, the quotient dl/TI generally is approximately equal to 1, but it is always slightly greater than 1, never exactly equal to 1. The subscripts “new” and “old” refer to the present and previous calculations, respectively. Thus, if a value of the integral component AI is being calculated at present, i.e., AI (new) , then AI (old)  refers to the value of AI that was calculated 400 seconds ago.
 
     Initial values of AI (old)  and ERR (old)  are generally set to zero. So, if all example values above are inserted into Equation 3, the following result is obtained: AI (new) =0+0.2*(400.020/400)*[(0.5)*(0.4 mg/L+0)]=0.040002 mg/L=approximately 0.04 mg/L. Thus, the total adjustment A=AP+AI=0.08+0.04=0.12. 
     The total adjustment value A may be limited in order to ensure that adjustments do not exceed a predetermined maximum adjustment. For example, if a particular measurement deviates significantly from the trend of previous measurements, it could be deemed an outlier or an erroneous measurement. Limiting the maximum amount of any given adjustment accounts for such an outlier. In step  225 , utilizing equation 5, AFCR is calculated to be AFCR=FCR*(1+0.12) or a 12% increase over FCR. 
     Turning now to  FIG. 3 , the reader&#39;s attention is directed to a very surprising, unexpected result of measuring influent nitrates (red/gray blocks), influent phosphates (navy/black diamonds) and influent dissolved oxygen (light blue/light gray blocks) over a twenty-four hour period. As shown in the figure, the concentration (in mg/L) of influent nitrate decreases from an average initial value of about 5.0 mg/L to low average midday value of about 4.0 mg/L before rising again a few hours prior to midnight to a high average value of about 5.5 mg/L. Quite unexpectedly, influent dissolved oxygen steadily rises to reach a peak midday value of about 2.7 mg/L before falling off sharply over the next five hours. Thus the concentrations of influent nitrates and influent dissolved oxygen traveled in opposite directions. An operator measuring the concentration of only influent nitrates, even by resorting to “adjustable” or “sensitivity” factors, could not have accounted for an unexpected rise in influent dissolved oxygen and, thus, would have more than likely underestimated the proper amount of methanol required to achieve the desired level of denitrification. 
     While the present invention has been described with respect to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.