Abstract:
A method and apparatus for operating an activated sludge plant having a plurality of tandem aeration zones, each receiving mixed liquor from an upstream zone or an upstream source and discharging a mixed liquor to a downstream zone or a downstream process includes a control which determines a parameter at a downstream one of the zones. The parameter is representative of a concentration of ammonia in the mixed liquor in the downstream one of the zones and may be used to control at least one upstream zone. A value of airflow to one of the zones may be determined and used to determine a demand for dissolved oxygen in the mixed liquor in that zone as a function of airflow to that zone. An elevated level of demand may be used to indicate a dump of commercial waste having a high BOD demand. A depressed level of demand may be used to indicate the presence of chemicals that inhibit bacterial respiration.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is directed to an activated sludge plant and method for monitoring biological activity and controlling aeration in such a plant and, in particular, to such a plant being operated for ammonia removal. 
         [0002]    In the secondary process of a conventional activated sludge treatment plant, effluent from primary clarifiers is mixed with return activated sludge (RAS) to form mixed liquor. The mixed liquor consists of a suspension of flocs containing microbial species, which include heterotrophic and autotrophic bacteria. Both need oxygen in order to remove carbon and ammonia respectively from the surrounding solution. In the aeration section, high-volume low-pressure blowers are used to provide air to the aeration zones. Originally, blowers were turned on and a fixed volume of air was provided in an uncontrolled fashion. With the advent of dissolved oxygen (DO) sensors, instrument engineers recognized that the aeration system could be controlled. The blowers were operated to achieve a targeted header pressure. Each aeration zone had a DO sensor and an air control valve. PID logic was used to control the air valve in order to target a fixed DO set-point. 
         [0003]    DO has become the primary parameter monitored by plant operators. Most plants have several aeration zones usually each having a different DO set-point. DO is not an indicator of the rate at which ammonia is being converted into nitrate (nitrification rate). Operators become concerned when the actual DO value in a zone moves away from the set-point and remains away for an extended period of time—a daily occurrence in most plants. DO in mixed liquor is a complex parameter that is not well understood by operators and engineers. Hence, operating practices are often based upon misunderstandings and myths that result in energy being wasted and the risk of treatment being compromised. DO set-point control was designed by instrument engineers to control blowers. 
       SUMMARY OF THE INVENTION 
       [0004]    In order to control the rate at which carbon compounds and ammonia are being removed by microbes, there is a need for a parameter that relates the rate of biological activity to airflow in each aeration zone and in the aeration system as a whole. 
         [0005]    Both the flow of water and the concentration of compounds generated by humans vary significantly over a 24-hour period. Municipal wastewater treatment plants experience peak water flows and concentrations around noon with the low points being around sunrise. This diurnal effect is due to people waking up all about the same time each morning and using the toilet and the shower. Traditionally, plants that are run with fixed DO set-points will experience that, around sunrise, nitrification will be completed very early in the process while, around noon, the target ammonia discharge value may not be achieved before the mixed liquor exits the aeration system. DO values are set to ensure that the targeted discharge levels are usually achieved. For zones where the rate of ammonia removal cycles over a 24 hour period between being only marginally affected by ammonia concentration to being strongly affected, traditional DO set-point control using PID logic cannot operate in a stable fashion. Up to 70% of the aeration zones in a conventional plant can be so affected. 
         [0006]    Hence, while the accuracy and response of DO sensors has improved dramatically, stable DO control has remained elusive. In early zones where the ammonia concentration typically remains above 2.5, a conventional PID loop can be tuned so that DO remains close to the set-point throughout the day. In this situation, the rate of removal of ammonia is only marginally dependent upon ammonia concentration and mainly a function of airflow and DO. In aeration zones closer to the outlet ammonia concentrations will typically fall below 2.5 mg/L and the rate of ammonia removal will thus be increasingly governed by the ammonia concentration as shown in  FIG. 3 . For a fixed DO set-point, as the ammonia level falls, so too will the airflow required to maintain the DO set-point. 
         [0007]    In the range 0-3.0 mg/l, increasing DO increases the rate of nitrification. However, DO has an affect on the efficiency with which oxygen is transferred from the blower air into the mixed liquor. The lower the DO the more oxygen will be transferred from the same airflow. 
         [0008]    The present invention is directed to a method and apparatus for monitoring biological activity in an activated sludge plant controlled by conventional techniques to provide the operator with useful information on the biological activity in individual aeration zones. The present invention is further directed to a method and apparatus for controlling the aeration of the activated sludge plant in a manner that provides a stable process that is capable of reducing energy used in aeration. This is accomplished by changing the nitrification rate to fully utilize the time available for treatment. The technique endeavors to utilize minimum DO values in each aeration zone while achieving desired nitrification. This results in an efficient exchange of oxygen into the mixed liquor which minimizes air volume, thereby realizing energy savings. 
         [0009]    A method and apparatus for operating an activated sludge plant having a plurality of tandem aeration zones, each receiving mixed liquor from an upstream zone or an upstream source and discharging a mixed liquor to a downstream zone or a downstream process, according to an aspect of the invention, includes providing a control which determines a parameter at a downstream one of the zones. The parameter is representative of a concentration of ammonia in the mixed liquor in the downstream one of the zones. 
         [0010]    At least one upstream zone that is upstream of the downstream one of the zones may be controlled as a function of a value of the parameter. The downstream one of the zones may be the most downstream zone. The at least one upstream zone may be controlled in order to cause the concentration of ammonia in the downstream one of the zones to approach a particular level, such as less than approximately 2.5 mg/L. The at least one upstream zone may be controlled by controlling airflow to that zone. Airflow to the at least one upstream zone may be measured to the at least one upstream zone controlled as a function of airflow to the at least one upstream zone. 
         [0011]    The parameter may be representative of a demand for dissolved oxygen in the mixed liquor of the downstream one of said zones. The parameter may be proportional to airflow to the downstream one of the zones. The parameter may be proportional to the difference between a second parameter and dissolved oxygen in the mixed liquor. The second parameter may be a value of saturated concentration of oxygen in the mixed liquor. 
         [0012]    The at least one upstream zone may be controlled by establishing a set-point control for that zone and the set-point of that zone adjusted as a function of the value of the parameter at the downstream one of said zones. A value of the parameter at the at least one upstream zone may be calculated and utilized at the at least one upstream zone in the set-point control. A set-point value of the parameter may be established at the at least one upstream zone and adjusted as a function of the value of the parameter at the downstream one of the zones. Set-point values of the parameter may be established at a plurality of upstream zones and the sum of the set-point values at the plurality of upstream zones may be adjusted as a function of changes in the value of the parameter at the downstream one of the zones. The set-point control may adjust the dissolved oxygen set-point or the airflow set-point to at least one of the upstream zones. 
         [0013]    A method and apparatus for operating an activated sludge plant having a plurality of tandem aeration zones, each receiving mixed liquor from an upstream. zone or an upstream source and discharging mixed liquor to a downstream zone or a downstream process, according to another aspect of the invention, includes providing a control and determining a value of airflow to one of the zones with the control. A value of a parameter is determined for that zone as a function of airflow to that zone. The parameter is representative of a demand for dissolved oxygen in the mixed liquor in that zone. 
         [0014]    That zone may be controlled as a function of a value of the parameter. The parameter may be proportional to the difference between a second parameter and the level of dissolved oxygen in the mixed liquor in that zone. The second parameter may include a value of saturated concentration of oxygen in the mixed liquor in that zone. A feedback control may be established in that zone. The feedback loop adjusts airflow to that zone to cause the level of the parameter to approach a set-point level. The feedback loop may adjust a dissolved oxygen set-point level in the mixed liquor of that zone in order to cause the level of the parameter to approach the set-point level. 
         [0015]    The set-point level of the parameter may be established as a function of a condition in a downstream zone that is downstream of that zone. The condition in the downstream zone may be the concentration of ammonia in the mixed liquor in the downstream zone. A value of the parameter may be determined in a plurality of the zones and the airflow to the plurality of zones adjusted to cause the level of the parameter in the plurality of zones to approach set-point levels for those zones. A sum of the set-point levels for the plurality of zones may be adjusted as a function of changes of the condition in the downstream zone. 
         [0016]    A method and apparatus for operating an activated sludge plant having a plurality of tandem aeration zones, each receiving mixed liquor from an upstream zone or an upstream source and discharging mixed liquor to a downstream zone or a downstream process, according to another aspect of the invention, includes providing a control and determining a value of airflow to one of the zones with the control. A value of a parameter is determined for that zone as a function of airflow to that zone. The parameter is representative of a demand for dissolved oxygen in the mixed liquor in that zone. An elevated level of the parameter may be used to indicate a dump of commercial waste having a high BOD demand. A depressed level of the parameter may be used to indicate the presence of chemicals that inhibit bacterial respiration. 
         [0017]    These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of an activated sludge wastewater treatment plant, according to the invention; 
           [0019]      FIG. 2  is a schematic diagram of a control for an aeration zone; 
           [0020]      FIG. 3  is a diagram illustrating the relationship between metabolic rate and concentration of ammonia; 
           [0021]      FIG. 4  is a control algorithm diagram illustrating overall control of the aeration zones of the plant in  FIG. 1 ; 
           [0022]      FIG. 5  is a flow diagram of an iterative control process for controlling an aeration zone; 
           [0023]      FIG. 6  is the same view as  FIG. 5  of an alternative embodiment thereof; 
           [0024]      FIG. 7  is a schematic diagram of a conventional activated sludge wastewater treatment plant; 
           [0025]      FIG. 8  is a diagram illustrating hourly variation of dissolved oxygen in the mixed liquor stream in the aeration zones of the plant in  FIG. 7 ; 
           [0026]      FIG. 9  is a diagram illustrating hourly variation of biological activity index (BAI) in the mixed liquor stream in the aeration zones of the plant in  FIG. 7 ; 
           [0027]      FIG. 10  is a diagram illustrating, for multiple days, hourly variation of total biological activity index (MAI) in the mixed liquor stream in the aeration zones of the plant in  FIG. 7 ; 
           [0028]      FIG. 11  is a diagram illustrating variations of the daily average total biological activity index in the mixed liquor stream in the aeration zones in the plant in  FIG. 7 ; 
           [0029]      FIG. 12  is a diagram illustrating hourly variation of BAI during a dump of commercial waste to a plant similar to that in.  FIG. 7 ; 
           [0030]      FIG. 13  is a diagram illustrating hourly variation of TBAI during a dump of commercial waste to a plant similar to that in  FIG. 7 ; and 
           [0031]      FIG. 14  is a diagram illustrating hourly variations of TBAI/Q in the mixed liquor stream in the aeration zones of the plant in  FIG. 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    Referring now to the drawings and the illustrative embodiments depicted therein, an activated sludge wastewater treatment plant  10  is shown in  FIG. 1 . Waste is fed to an influent line  12  from an upstream supply, such as a primary clarifier effluent, and is supplied to a conventional anoxic zone  14 . The effluent of zone  14  is supplied to a tandem series of aeration zones  16 , which are designated zone  1 , zone  2 , . . . zone n in the direction of flow of the mixed liquor (primary effluent plus return activated sludge plus mixed liquor recycle). Each of the zones receives mixed liquor from an upstream zone and discharges mixed liquor to a downstream zone. In the aeration sections of an activated sludge process, air is bubbled through the mixed liquor. This provides the dissolved oxygen that certain species require in order to use the carbon compounds and ammonia present in the mixed liquor. The output  20  of final aeration zone  18  is recycled to influent line  12  in the form of mixed liquor recycled and to a secondary clarifier  22 . At least a portion of waste-activated sludge  24  from clarifier  22  is recycled to influent  12  as return-activated sludge providing flocs containing microbiological species to mix with the influent. The treated wastewater effluent is fed out of line  26 . 
         [0033]    Wastewater treatment plant  10  includes a control generally shown at  30  ( FIGS. 2 and 4 ). Control  30  includes a zone control  32  for controlling an aeration zone  16  having an air source  36 . Control  32  includes a conventional DO probe  34  for sensing dissolved oxygen in the mixed liquor in that zone. Control  32  includes a control device  38  for controlling airflow from air source  36 . While control device  38  may be a valve to modulate airflow to that zone from an air source  36  in the form of a blower that is common to more than one zone, it could also be a speed control for a separate variable speed fan, or the like. Zone control  32  additionally includes an airflow sensor  40  for determining a value of airflow to that zone. Devices  34 ,  38  and  40  connect with a controller  42 , which may be dedicated to that zone or shared across the zones  16 . 
         [0034]    Zone control  32  operates as follows. Zone control  32  controls air control device  38  in that zone so as to target an airflow set-point—AF sp . This ensures a stable flow of air to the zone. Zone control  32  has a controller  42  that monitors the DO value via a probe  34  and calculates the value of a parameter BAI (biological activity index). Parameter BAI is representative of a demand for dissolved oxygen in the mixed liquor in that zone. 
         [0035]    BAI, the Biological Activity Index for a zone, is defined as: 
         [0000]        BAI=AF *(β* C   sat   −DO )  (1)
 
         [0036]    where C sat  is the saturation concentration of oxygen in water and DO is the dissolved oxygen concentration measured in the mixed liquor. C sat  is a function of temperature. β is a constant that is between 0.5 and 1.0, but in the illustrated embodiment is approximately 0.95. 
         [0037]    BAI is proportional to the rate at which oxygen is being transferred into the mixed liquor in a zone. The BAI reflects the demand for dissolved oxygen which depends upon the needs of heterotrophic bacteria that have access to soluble carbon and autotrophic bacteria with access to ammonia. Under normal conditions, all soluble carbon is removed in the anoxic zone. Hence, oxygen being supplied to the aeration zones is principally being used by heterotrophic bacteria for nitrification. In early aeration zones, the rate at which ammonia is removed will be only slightly dependent upon the concentration of ammonia. This is due to the relationship between metabolic rate and substrate concentration shown in  FIG. 3 . The rate will depend upon the dissolved oxygen concentration (DO), the mixed liquor suspended solids (MLSS), the relative number of nitrifying bacteria in the mixed liquor, the geometry of the flocs, and the water temperature. Of these, the DO can change rapidly, whereas the other parameters change only slowly. When the DO is steady, the rate at which oxygen is being removed from the zone will equal the rate at which oxygen is being transferred into the zone. Hence, the BAI will generally be proportional to the rate at which oxygen is being consumed by the bacteria. 
         [0038]    Thus, it can be seen that a value of the parameter BAI can be used as a target in a feedback control algorithm  44  carried out by zone control  32  in the aeration zone ( FIG. 4 ). If the zone is controlled using traditional DO sp  control, the feedback loop  44  adjusts the DO set-point for the zone. This will cause the air flow to change. Alternatively, the zone could be controlled using an air flow set-point—AF sp . The feedback loop  44  adjusts the AF sp  which will cause the DO to change DO probes respond more slowly than air flow controls. Also, it takes time for the DO profile inside the floc to become stabilized. Hence, time must be allowed for the new DO and/or AF value to become stable before the value of the BAI corresponding to such changes is established. 
         [0039]    The BAI levels in zones  1  through n appear to be additive.  FIG. 10  shows plots of TBAI the sum of BAI for zones  1  through n, for several days 
         [0040]    The rate of nitrification depends upon the ammonia concentration according to the expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     % 
                      
                     
                         
                     
                      
                     maximum 
                      
                     
                         
                     
                      
                     rate 
                   
                   = 
                   
                     
                       [ 
                       
                         NH 
                          
                         
                             
                         
                          
                         4 
                       
                       ] 
                     
                     
                       ( 
                       
                         1.0 
                         + 
                         
                           [ 
                           
                             NH 
                              
                             
                                 
                             
                              
                             4 
                           
                           ] 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0041]    where 1.0 is the value used in the Activated Sludge Model for K s  for ammonia as shown in  FIG. 3 . When the ammonia concentration in a zone is around 1 mg/L the parameter BAI becomes a strong linear indicator of ammonia concentration. Zone n can be controlled using traditional DO set-point control, or with an appropriate fixed airflow, and BAI monitored. With traditional DO set-point control, a value of 1.0 mg/L or less can be chosen in order to minimize carryover of DO with mixed liquor recycling line  24  back to the anoxic zone  14 . Changes in the concentration of ammonia in zone n will be reflected in changes in BAI, Correlation can be established by taking samples and recording the BAI. Laboratory analysis can be used to establish ammonia concentrations. Hence, an ammonia target level in the discharge can be translated into a BAIT target (BAI t ). 
         [0042]    A change in BAI in the downstream zone n, can be used to change the target value for the TBAI for earlier zones  1  through n- 1 . This can be illustrated by reference to  FIG. 4  in which each upstream zone, upstream of zone n, has a feedback control loop  44  which receives input  46  from the condition of the associated zone and provides an output  48  to control that zone. All controllers  44  send the condition of their zone to controller  49 . Downstream zone n produces an output value  50  representative of the BAI of that zone, which is compared by controller  49  to BAI t  for zone n. A new TBAI target is calculated by controller  49  as well as new BAI targets for control loops  44 , for one or more of the upstream zones. In so doing, controller  49  will endeavor to maximize oxygen transfer by keeping the DO values in zones  1  through n- 1  at minimum values. Whenever changes are made to either the AF set-point or the DO set-point in a zone, time must be allowed for the system to stabilize. Until DO becomes stabilized, BAI cannot be taken as an indicator of the rate at which bacteria are using DO. This can take anywhere from 5-30 minutes, for example, but will mostly be achieved in less than 15 minutes. 
         [0043]    Each zone feedback loop  44  of the upstream zones may utilize various set-point parameters in order to change the BAI for that zone. One such set-point parameter may be the airflow for that zone. An iterative process involving incrementally changing AF set-points then waiting for the DO to stabilize will be described in more detail below. Alternatively, upstream zones may utilize DO as a set-point in an iterative process involving incrementally changing DO set-points then waiting for the airflows and DO to stabilize, as will be described in more detail below. The goal is to control the BAI in zones  1  through n- 1  so that the ammonia levels in zone n stay close to a target value, e.g., 0.5 mg/L throughout the day. Thus, if, in a chosen period of time (for example 15 minutes), the BAI in zone n increases by ΔBAI, the difference between the actual BAI and BAI t , the value of total biological activity index (TBAI) is increased by an amount proportional to ΔBAI. If, in a chosen period of time, the BAI in zone n decreases by ΔBAI, TBAI is decreased by an amount proportional to ΔBAI. 
         [0044]    While the ammonia concentration in the downstream zone n may be determined from the BAI level in that zone, it may, alternatively, be determined by other techniques, such as using an online ammonia analyzer. 
         [0045]    A goal is to operate with set-points in upstream zones  1  through n- 1  so that the rate of nitrification remains steady as evidenced by relatively stable DO and BAI values in these zones. Changing the value for the BAI in earlier zones may be used by feedback control loop  44  in an iterative process involving incrementally changing DO set-points then waiting for the airflows to stabilize, as illustrated in  FIG. 5 . In particular, a feedback control algorithm  52  may be carried out in which the BAI is determined in that zone from airflow and DO readings. Controller  49  has determined ΔBAI in downstream zone n and calculated a new BAI t  for the zone which is read at  54 . This is used at  56  to estimate a new DO set-point for the zone. The new DO set-point is adopted at  58 , and the airflow to the zone is automatically adjusted. Parameters in the zone are allowed to stabilize at  60  and a new BAI value is determined for the zone at  54 . If the new BAI is not sufficiently close to BAI t , the loop can be repeated. With experience, a relationship between DO set-point and the BAI may be established and used to speed up the process. 
         [0046]    Alternatively, upstream zones  1  through n- 1  can be operated with BAI targets calculated by controller  49  for each zone, as illustrated in  FIG. 6 . A feedback control algorithm  62  includes determining the BAI from stable DO levels and airflow at  64  and reading the value for BAI t  from controller  49 . A new air flow set-point is estimated at  66 . Change is made to the airflow set-point at  68 . Once the DO level has stabilized at  70 , the value for BAI is calculated and compared with the target at  64 . An iterative process is used to make further changes to the airflow in order to approach the BAI set-point 
         [0047]    As previously set forth, the goal is to operate upstream zones  1  through n- 1  so that nitrification is spread evenly across zones  1  through n- 1  as evidenced by relatively stable BAI values in these zones. This is an improvement over conventionally controlled activated sludge plants in which expected levels of DO and BAI vary to a great extent according to the time of day, especially for downstream zones. This is seen in  FIGS. 8 and 9 . For example, referring to  FIG. 8 , it can be seen that the DO level in aeration zone  3 AB has a major increase starting at about 6:00 a.m. then goes below the set-point of 2.0 close to noon and finally stabilizes around 3:00 p.m. The DO level in zone  4 A is only close to its set-point between 8:00 a.m. and noon. Calculating the parameter BAI for the conventional plant utilizing formula (1), it can be seen from  FIG. 9  that the value of the BAI for zone  2 C shows a drop between 7:00 a.m. and noon. The BAI for zone  3 AB starts falling at around 4:00 a.m. from around 9000 to 2300. It starts rising rapidly around noon and two hours later is over 9000. The BAI for zone  3 CD starts falling around 2:00 a.m. from 3800 to around 2000. It rises markedly around 2:00 p.m. when the BAI in  3 AB flattens out then falls off. This suggests that around 9:00 a.m. nitrification has been completed upstream of zone  3 AB. Around 4:00 p.m. nitrification is still occurring in zone  3 CD. It should be noted that zone  3 CD in the conventional system could correspond to zone n- 1 , according to the embodiment of the invention, and zone  4 AB corresponds to final zone n ( 18 ). 
         [0048]    By applying the techniques disclosed herein, the goal would be to adjust the BAI targets for zones  2 A,  2 B , 2 C,  3 AB and  3 CD so that nitrification is completed in Zone  4 AB throughout the day.  FIG. 10  shows a plot of TBAI over 24 hours for 6 consecutive days. This illustrates diurnal behavior similar to typical ammonia load variations entering the anoxic zone.  FIG. 11  shows the average TBAI for the same 5 days. 
         [0049]    The techniques carried out by control  30  may also he used to alert plant operators to a dump of commercial waste to wastewater treatment plant  10 . These occur in many commercial processing plants, such as food-processing plants, or the like, due to discharge of product or wash water from food-processing industries. Normally, readily available carbonaceous biochemical oxygen demand (CBOD) does not get past anoxic zone  14  where it is consumed by denitrifying bacteria. When a dump of commercial waste occurs, CBOD may break through anoxic zone  14  into the aeration zones  16  causing a high demand for DO by the heterotrophs that exist in much greater numbers than nitrifiers. This may cause DO levels to suddenly plummet. The air supply must be ramped up immediately to its maximum allowable value to maximize the rate oxygen is being transferred into the mixed liquor so that the dump can be processed in the shortest possible time. A sudden rise in the BAI, such as in the first aeration zone, can be used to alarm that a dump has occurred and airflows raised in all zones. By monitoring the TBAI during the dump, its magnitude can be established, as well as a clear indication as to when the dump is over and the plant can return to normal operation. Reference is made to  FIGS. 12 and 13  where it can be seen how BAI in the first aeration zone can be used to alarm that a dump has occurred and how TBAI can be used to establish the magnitude of the dump. Also, a sudden rise in the BAI may be used to trigger automatic samplers so that the compound can be established and the perpetrator identified. 
         [0050]    Also, the parameter BAI in one of the early upstream zones can be used to detect the presence of compounds that inhibit bacterial respiration. When this occurs on a large scale, the bacteria in the treatment plant can die, thus putting the plant out of action for months. The presence of compounds in the influent to the plant that inhibit bacterial respiration will cause a drop in BAI in the upstream zones below its normal pattern. This can be used to alarm so that action can be taken to protect the bacteria population from being destroyed. For example, the primary influent could be temporarily diverted around the aeration zones, or the like. 
         [0051]    BAI/Q, where Q is the flow rate of mixed liquor through the zones, will be proportional to the oxygen utilized per unit of mixed liquor while passing through the zone. Under normal conditions this will be used by nitrifying bacteria to convert ammonia into nitrate and for endogenous respiration. Because Q is the same for each aeration zone, TBAI/Q will be proportional to the oxygen used per unit volume of mixed liquor while passing through the aeration train. The hourly variation of TBAI/Q is shown in  FIG. 14 . When divided by the suspended solids in the mixed liquor it will track the specific rate of nitrification. Such trends can be used to increase or decrease sludge wasting. Data may be averaged over 24 hours or analyzed for daily peak or minimum values. 
         [0052]    Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.