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Biological Nutrient Removal Overview
The biological nutrient removal (BNR) processis designed for total nitrogen removal (nitrification and denitrification) and phosphorus removal. Total nitrogen and phosphorus limits are a part of the City of Clinton's discharge requirements as shown in thecurrent NPDESpermit. The Iowa Nutrient Reduction Strategy uses guidelines written into the strategy to establish permit limits. Limits are calculated based on establishing annual average effluent limitations. The calculation procedure evaluates effluent total nitrogen and total phosphorus discharge data to find a 99th percentile concentration. The 99th percentile concentration is then evaluated as an average annual mass load limit using the design average wet weather flow.

Biological nutrient removal provides additional benefits beyond enabling discharge requirements to be met. Biological phosphorus removal selects for organisms that improve mixed liquor quality and settleability. The incorporation of denitrification for total nitrogen removal provides reduced aeration demand and alkalinity recovery.
Theory of Operation
There are three zones in the biological nutrient removal system: (1) anaerobic, (2) anoxic, and (3) aerobic.  Wastewater enters the first anaerobic cell and is mixed with return activated sludge (RAS) from the secondary clarifiers. 

Within the anaerobic zone of each BNR Basin, the following process reactions occur:
  • Influent volatile fatty acids (VFAs) are critical in the anaerobic zones as the food source for phosphorus accumulating organisms (PAOs). The anaerobic zone acts as a selector zone for PAOs, whereby PAOs are able to use VFAs to build-up internal energy storage as poly beta hydroxybutyrate (PHB). The PAOs use internal energy stored as polyphosphate granules, which release orthophosphate in the anaerobic zone to facilitate uptake of VFA and the build-up of PHB.
  • Under some conditions, particularly low oxidation reduction potential (ORP) conditions, fermentation converts organics (BOD5) to VFAs in the anaerobic zone further improving the selection of PAOs.
  • The ORP for PAO selection ranges from -250 mV to -100 mV (standard hydrogen electrode). Fermentation occurs under “deep” anaerobic conditions in the range from -300 mV to -200 mV.
After the anaerob ic zone, mixed liquor travels to the anoxic zone where it is mixed with mixed liquor recycle (MLR) from the aeration basin.  In the anoxic zone, the following process reactions occur:
  • Organisms that have the ability to utilize chemically bound oxygen in place of dissolved oxygen reduce the nitrates to nitrogen gas (denitrification).  The reaction requires the presence of nitrates, absence of dissolved oxygen, and a source of biodegradable organics.  The organics are treated (oxidized) and nitrates are converted (reduced) to nitrogen gas , which is released to the atmosphere thus removing nitrogen from the waste stream.
  • Denitrification can be monitored using ORP as a surrogate for nitrate. The ORP range for denitrification varies from -150 to -75 mV but may occur with lower values as well.
  • Treatment of the organics with nitrates instead of dissolved oxygen results in savings for aeration. For every pound of nitrate removed, 2.86 pounds of BOD5 are removed. Additional BOD5 is used for the growth of denitrifiers. In all, between four and six pounds of BOD5 oxygen demand may be saved for every pound of nitrate removed in the anoxic zone.
  • The reduction of nitrate-nitrogen via biological denitrification results in the recovery of a portion of the alkalinity consumed during the nitrification process.  For every pound of nitrate-nitrogen reduced, 3.6 pounds of alkalinity (as calcium carbonate) are recovered.  Recovery of alkalinity by denitrification reduces the need for addition of alkalinity to maintain the desired pH in the aeration basins.
Mixed liquor from the anoxic zone flows to the aerobic zone (aeration basin) where dissolved oxygen is added via aeration through fine bubble diffusers.  The following biological reactions occur in the aerobic zone:
  • Aerobic organisms convert organic nitrogen to ammonia-nitrogen.  The ammonia nitrogen is then oxidized first to nitrite-nitrogen and finally to nitrate-nitrogen by nitrifying bacteria (nitrification).
  • Ammonia oxidation (nitrification) rates are highest with dissolved oxygen concentrations greater than 1 mg/L and ORP measurements greater than 100 mV.
  • Aerobic organisms oxidize the BOD5 remaining in the anoxic zone effluent.
  • PAO's use the internally stored PHB as a carbon and energy source. Phosphorus is taken up to renew stored polyphosphate.  The PAO's take up all phosphate released in the anaerobic zone and additional phosphate present in the aeration basin influent (luxury uptake).  Net phosphorus removal is achieved by wasting these organisms from the process as part of the waste activated sludge (WAS).
  • Aerobic PAO activity is reported to be adequate at ORP measurements above 25 mV.
The biological nutrient removal system includes the following equipment: 
  • Splitter box water control gates
  • Basin mixers
  • Mixed liquor recycle pumps
  • Aeration basin diffusers
  • Aeration blowers
  • Aeration control valves
  • Air flow meters
  • Aeration blower inlet air filters
  • Aeration basin scum pumps
  • Basin drain pump
  • Anaerobic zone ORP probes (option)
  • Anoxic zone ORP probes
  • Aeration basin dissolved oxygen analyzers
Influent wastewater is blended with return activated sludge at the effluent end of the elongated flume in the headworks building.  This combined aeration basin influent is conveyed to the influent splitter box by a 42" pipe.  Three control gates in the splitter box distribute flow to each of the three biological nutrient removal basins through 36" influent lines.  Each basin consists of three separate zones.  The first zone is an anaerobic cell containing a submerged mixer.  The anaerobic zone effluent enters the first of two anoxic cells through a submerged weir.  Nitrate rich mixed liquor from the aeration basin is added to the flow, via the mixed liquor recycle pump, at the anoxic basin entry point.  A submerged mixer in the first anoxic cell provides the energy to mix the contents of the anoxic cell.  Flow continues through another submerged weir to the second anoxic cell which also contains a submerged mixer.  The effluent from the second anoxic cell flows over a weir to the aeration basin. 
The aeration zone in each basin is a “racetrack” design. As the waste stream enters the basin, it is circulated around the basin. By circulating the flow, the oxygen transfer efficiency is improved and complete mix conditions are achieved.  Dissolved oxygen is added to the aeration basin with fine bubble diffusers, distributed throughout the aeration basin.  Twosubmerged mixers in each aeration basin work in conjunction with the diffused air system to promote mixing in the basins. Air is supplied by three high speed turbo blowers located in the secondary treatment building.  A dissolved oxygen (DO) probe in each aeration basin provides data to the plant PLC for automatic control of the air supply.  A motorized flow control valve on the supply air to each aeration basin modulates to maintain the desired DO setpoint.  The air supply is automatically adjusted by starting/stopping blowers and varying the speed of the blower motors as needed.  When multiple blowers are in service, the control program will select the speed(s) that provide the required air with the most efficient use of electrical power. 
Drain valves on each aeration basin allow the tankage to be drained to the basin drain pump station.  This pump station also handles the draining of the secondary clarifiers.  The pump discharges to the aeration basin effluent so solids in the system are retained.
Process Control
The biological nutrient removal system consists of three separate zones each with its own process control and performance evaluation.  The first cell of each BNR train provides the basin's anaerobic zone. 
                           Anaerobic Zone Process Control                                                             
  1. Mixed Liquor Suspended Solids (MLSS)
               Sample Point:                     Anaerobic Cell
               Sample Type:                     Grab
               Sample Frequency:              Daily
               Normal Range:                     2,000-4,000 mg/L
               Controlled By:                     Wasting/Solids Retention Time (SRT)
               Process Control:                  The MLSS concentration in each BNR basin is a function of the basin influent load and WAS
                                                       flow rate.
   2.    Oxidation Reduction Potential  (ORP)
               Sample Point:                     Anaerobic Cell
               Sample Type:                     Grab
               Frequency:                        Daily
               Normal Range:                     <-100 mV 
               Controlled By:                     RAS Flow Rate, Nitrate Concentration, and DO Concentration
               Process Control:                  Residual DO normally results from high recycle loads of nitrates and DO from the aeration
                                                       basins and reduces phosphorus removal efficiency when carrying over to the anaerobic
   3.   Nitrate (NO3)
               Sample Point:                     Anaerobic Cell
               Sample Type:                     Grab
               Sample Frequency:              As needed
               Normal Range:                     0.0 mg/L
               Controlled By:                     Denitrification in the BNR anoxic zone
               Process Control:                  If excessive NO3 is measured in the anaerobic zone, check operation of the anoxic zone
                                                       to verify proper operation.  Similar to dissolved oxygen, nitrate in the anaerobic cell
                                                       reduces phosphorus removal efficiency.
   4.   Orthophosphate (ortho-P)
               Sample Point:                     Influent wastewater and each anaerobic cell
               Sample Type:                     Grab
               Sample Frequency:              Daily
               Normal Range:                     Increase through anaerobic cells
               Controlled By:                     Anaerobic conditions, ORP
               Process Control:                  If proper anaerobic conditions exist, an increase in ortho-P should be measured from the
                                                       influent to the anaerobic zones.  Ortho-P increases as the PAOs release phosphorus.  If no
                                                       increase or only a small increase is measured, check for DO or nitrates in the anaerobic 
   5.   Detention Time
               Normal Range:                     1.0 - 2.0 hours
               Process Control:                  The design anaerobic zone detention time is a function of the number of basins in service
                                                        and the influent flow to each basin.  Anaerobic zone detention time can be calculated
                                                        using the following formula:
                                                         T = .15 x B    x 24        LIVE FORMULA                      
                                                        T = Detention time, hours
                                                        B = BNR trains in service (0.15 MG is the volume of each anaerobic cell)
                                                        Q = Plant influent flow, MGD
                                                         The minimum detention time should not be less than 0.5 hours.  If all BNR trains are not in
                                                         service, bring additional basins on line to increase detention time.
The second two mechanically mixed cells of each BNR train provide the basin's anoxic zone.  Anaerobic zone effluent flows to the first anoxic cell where mixed liquor recycle is pumped from the aeration basin by a submersible pump.  Each anoxic cell is mixed by a mechanical mixer and baffle walls between the cells provide "plug-flow" of the mixed liquor through the anoxic zone.
                                                Anoxic Zone Process Control
   1.  Oxidation Reduction Potential (ORP)
               Sample Point:                         First anoxic cell
               Sample Type:                         Online Meter/Portable ORP Meter
               Sample Frequency:                  Continuous with ORP sensor/Daily as needed by handheld ORP probe
               Normal Denitrification Range:     -150 mV to 0 mV
               Suggested Operational Range:   -150 mV to -100 mV
               Controlled By:                        Aeration basin DO and nitrates, availability of readily biodegradable organics in anaerobic
                                                          zone effluent.
               Process Control:                     The presence of DO in the recycled mixed liquor reduces denitrification efficiency.
                                                          Reduce the mixed liquor recycle rate to reduce DO carryover. Apply aeration basin DO
                                                          control and air distribution as appropriate to maintain 1.0 to 2.0 mg/L DO in the aerated
                                                          portion of the basin while minimizing DO in the mixed liquor recycled to the first anoxic
                                                          cell and maintaining an ORP near -100 mV in the anoxic zone. 
               Manual operation:                   The MLR flow rate should be reduced if the target aeration basin DO concentration is
                                                          between 1.0 and 2.0 mg/L and ORP in the anoxic zone ORP reads too high. The MLR flow
                                                          rate may be increased if the ORP drops below the suggested operational range.
               Automatic operation:               Set the MLR flow rate to adjust automatically in order to maintain the target ORP
                                                          setpoint in the anoxic zone. A default ORP setpoint of -150 mV is suggested. Adjust
                                                          accordingly until a small residual nitrate concentration (≤ 0.1 mg-N/L is measured).
   2.   Nitrate (NO3)
               Sample Point:                        Each anoxic cell
               Sample Type:                        Grab
               Sample Frequency:                 As needed
               Normal Range:                       Reduction to near zero across the anoxic cells
               Controlled By:                        Full anoxic conditions, nitrate in recycled mixed liquor, biodegradable organics (BOD5) in
                                                          anaerobic zone effluent.
               Process Control:                     Results of nitrate measurements should show a decrease to near zero across the anoxic
                                                          cells.  Check process parameters discussed above and make adjustments as needed.
   3.   Detention Time
               Normal Range:                        2.0 - 4.0 hours
               Process Control:                     The design anoxic zone detention time is a function of the number of BNR trains in
                                                          service and the influent flow to each basin.  Anoxic zone detention time can be
                                                          calculated using the following formula:
                                                                        T = 0.36 x B   x 24
                                                           T = Detention time, hours
                                                           B = BNR trains in service (0.36 MG is the volume of each anoxic zone)
                                                           Q = Plant influent flow, MGD
                                                           The minimum detention time should not be less than 2.0 hours.  If all BNR trains are not
                                                            in service, additional trains should be put into service to increase the detention time.
Mixed liquor from each anoxic zone enters the associated aeration basin over a weir at the connection of the two basins.  Within each aeration basin, mixing is provided by fine bubble diffused aeration and submersible mixers.
                                                Aeration Basin Process Control
   1.   Dissolved Oxygen (DO)
               Sample Point:                           DO sensor within each basin.  Other points as needed to adjust air distribution.
               Sample Type:                           DO sensor/portable DO meter
               Sample Frequency:                    Continuous with DO sensor/Daily with portable DO meter
               Normal Range:                           1.0 - 2.0 mg/L
               Controlled By:                           Aeration air flow rates vs. aeration BOD5 and nitrogen loading.  Normally, air flow rate 
                                                             to each aeration basin is automatically controlled based on DO setpoint.
               Process Control:                        The recommended DO concentration for the aeration basins is between 1.0 and 2.0
                                                             mg/L.  Air flow to the aerobic cells should be adjusted as necessary to maintain the
                                                             required DO levels.
   2.   pH
               Sample Point:                           Aeration basin effluent
               Sample Type:                           Grab
               Sample Frequency                     Each shift
               Normal Range:                           6.8 - 8.0
               Controlled By:                           Nitrification vs. available alkalinity.  For each pound of ammonia oxidized, 7.1 pounds
                                                            of calcium carbonate alkalinity is destroyed.  A portion of the destroyed alkalinity is
                                                            recovered during denitrification.
               Process Control:                        If pH drops below 6.0, nitrification rate will rapidly decrease.  Verify proper anoxic zone
                                                             operation to insure maximum alkalinity recovery.
   3.   Mixed Liquor Suspended Solids
               Sample Point:                           Each aeration basin
               Sample Type:                           Grab
               Sample Frequency:                    Daily
               Normal Level:                            2,000 - 4,000 mg/L
               Controlled By:                           Influent load and SRT. An increase in influent loading increases MLSS.
               Process Control:                        WAS Rate/SRT. Increasing the WAS rate reduces the MLSS and SRT. Reducing the
                                                             WAS rate increases the MLSS and SRT.
   4.   Sludge Retention Time (SRT)
               Sample Point:                           Aeration basin MLSS and WAS
               Sample Type:                           Grab
               Sample Frequency:                    Daily
               Normal Range:                           Summer: 0-14 days
                                                             Winter: 12-16 days
               Controlled By:                           WAS flow
               Process Control:                        The SRT is calculated from the following formula:
                                                                     SRT = B x 1.73 x MLSS
                                                                              WAS x WAS Q
                                                            SRT = sludge retention time, days
                                                              B   = number of aeration basins in service
                                                          MLSS  = mixed liquor concentration, mg/L
                                                            WAS = WAS concentration, mg/L
                                                         WAS Q = WAS flow, MGD
                                                    SRT LIVE FORMULA

   5.   MLSS Settleability
               Sample Point:                              Aeration basin MLSS
               Sample Type:                              Grab
               Sample Frequency:                       Daily
               Normal Range:                              Based on operating experience
               Controlled By:                              Aeration basin loadings, SRT, MLSS inventory, general MLSS "health"
               Process Control:                           The settleability test is used as an indicator of the ability of the MLSS to settle
                                                                in the secondary clearifiers.  The test also provides an indirect indication of
                                                                biomass quality.
   6.   Microscopic Evaluation
               Sample Point:                              Aeration Basin MLSS
               Sample Type:                              Grab
               Sample Frequency                        Weekly
               Normal Range:                              Based on operating experience
               Controlled By:                              Aeration basin loadings, SRT, MLSS inventory, biomass quality
               Process Control:                           Microscopic evaluation of MLSS is used to determine the relative health of the
                                                                biomass based on the types and predominance of certain indicator organisms.
                                                                Microscopic evaluation can give an indication of changes in the activated sludge
                                                                environment and can be used to troubleshoot MLSS settleability problems resulting
                                                                from filamentous organism growth. Microscopic examination can also help spot
                                                                toxicity issues when inactive multicellular organisms (daphnia and tardigrades) are
   7.   Detention Time
               Normal Range:                              10 - 16 hours
               Controlled By:                               Influent flow vs. number of BNR trains in service
               Process Control:                           Detention time in the aeration basins must be adequate for the BOD removal,
                                                                nitrification and phosphorus uptake reactions to occur.  Detention time can be
                                                                calculated using the following formula:
                                                                              T = 1.73 x B      x 24
                                                                 T = detention time, hours
                                                                 B = number of aeration basins in service
                                                                 Q = Plant influent flow, MGD
Water Control Gates6
Slide gates in the aeration basin splitter structure control flow to each BNR basin train.  Each gate is downward opening.
Basin Mixers6


Submersible mixers in the anaerobic and anoxic cells provide all mixing in those zones.  The mixers ensure the mixed liquor is completely exposed to the influent wastewater and prevent solids from settling in the basin.  There is one mixer installed in each anaerobic and anoxic cell.  Two submersible mixers are also installed in each aeration basin to provide mixing in basin areas that don't contain aeration diffusers.
Mixed Liquor Recycle Pumps6
Recycling mixed liquor from the aeration basin to the anoxic zone enhances biological denitrification. A mixed liquor recycle pump installed between the first anoxic cell and the aeration basin pumps mixed liquor from the aeration basin to the anoxic zone. The pumps are located approximately 6' from the bottom of the aeration basin.  A flap gate is located at the discharge end of the 24" mixed liquor discharge line in the anoxic cell to prevent anoxic cell contents from backflowing into the aeration basin.  The pumps are variable speed with a maximum discharge rate of 5.2 MGD.
Aeration Basin Diffusers6
Air is supplied to the aeration basins by a network of fine bubble diffusers.  The diffusers provide oxygen for the aerobic biological reactions and input mixing energy into the basin contents. Each aeration basin consists of four aeration zones, each with a specific number of diffusers.  There are a total of 1144 diffusers in each aeration basin.  The fine bubble diffusers are a circular membrane type that produce air bubbles with an average diameter of 2 - 4 millimeters (mm).
Aeration Blowers6
The three aeration blowers provide air to the aeration basins.  The blowers are high speed turbo units with variable speed drives.  The blowers are designed for a wide range of air demands, 2,700 cfm at startup to 11,500 cfm at 2030 peak hour conditions.  The blowers are controlled by the oxygen demands in the aeration basins as measured by the dissolved oxygen probes. 
Aeration Control Valves6
The air flow from the blowers to each aeration basin is controlled by the aeration control valves.  These 16" butterfly valves modulate with electric operators to direct the appropriate air flow to each aeration basin. 
Process Air Flow Meters 6
Air flow meters are installed on each main air header to the aeration basins.  The air flow meters work in conjunction with the dissolved oxygen sensors to provide automatic control of the air flow to each aeration basin.
Aeration Blower Inlet Air Filters6
Each aeration blower is equipped with an inlet air filter to protect the blower from dust and other air particles that could damage the blower.  The air filters also remove materials that could plug or foul the fine bubble diffusers.
Aeration Basin Scum Pumps6
Foam and scum can form in the aeration basins.  This floating material flows into the scum box as the aeration basin contents flow to the secondary clarifiers.  The scum is directed to the aeration basin lift station that contains two submersible pumps.  These aeration basin scum pumps convey the foam and scum to the aerobic digesters. 
Basin Drain Pump6
The drain from each aeration basin is connected to the basin drain wet well.  The wet well contains the basin drain pump that discharges to the aeration basin effluent channel leading to the secondary clarifiers.  Normal operation would occur when an aeration basin is removed from service.  The basin drain pump also serves the secondary clarifiers.  The aeration basin wet well is also connected to the subdrain sump to remove groundwater in case the subdrain pumps are out of service.
Aeration Basin Dissolved Oxygen Analyzers 6
Each aeration basin is equipped with a dissolved oxygen probe to monitor the dissolved oxygen levels in the aeration basins.  The dissolved oxygen probe works in conjunction with the air flow meters to control the air sent to each aeration basin.
Anoxic Basin ORP Probes  6


The Anoxic zone of each aeration basin is equipped with an ORP probe to monitor ORP within the basin. The ORP probe works in conjunction with the MLR pumps to maintain an ORP setpoint.

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Last Updated: 8/13/2020 9:00:18 AM
Version 3.0.1