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Secondary Clarifiers and RAS Pumping Overview
The secondary clarifiers provide solid/liquid separation of the mixed liquor from the aeration basins.  The settled solids are returned to the BNR basins by the return activated sludge (RAS) pumping system.
Theory of Operation
The secondary clarifiers have two basic functions:
  • To clarify the BNR effluent through solids/liquid separation prior to discharge.
  • To thicken and rapidly collect the settled activated sludge solids for return to the BNR basins or wasting to the waste activated sludge system.

To properly operate the activated sludge BNR process, a mixed liquor with good settling characteristics must be developed and maintained.  The mixed liquor solids settle in the secondary clarifiers and most are returned to the BNR influent stream as RAS.  This provides a sufficient concentration of activated sludge organisms in the BNR basins to accomplish BOD and nutrient removal within the detention time provided.  The return of activated sludge from the secondary clarifiers to the BNR basins is a key process control parameter.

Mixed liquor from the BNR aeration basins flows to the secondary clarifier influent splitter structure.  Control gates in the splitter structure allow the flow to be directed to the three secondary clarifiers.  The secondary clarifiers are center feed units with an effluent periphery weir trough.  The activated sludge solids are removed in the secondary clarifiers and returned by gravity to the wet well of the Secondary Treament Building.  The sludge line from each secondary clarifier contains a flow meter and control valve to allow adjustment of return sludge flow from each clarifier. 
The four RAS pumps remove the settled solids from the wet well and convey the RAS to the influent line of the BNR basins.  This provides a thoroughly mixed solution of RAS and influent wastewater that enters the anaerobic zone of the BNR basins.
Scum formed in the secondary clarifiers is collected by a skimmer on each clarifier drive mechanism and discharged into the clarifier scum trough. The scum flows by gravity through a 6" scum line to the secondary clarifier scum lift station.  The two submersible pumps in the lift station pump the scum to the aerobic digesters.  The clarifiers are drained to the aeration basin drain pump station.

A drain sump in the lower level of the Secondary Treatment Building contains a level switch to generate a high level alarm at the HMI if the level in the sump activates the switch.  The HVAC system in the Secondary Treatment Building is monitored with HMI alarms generated if there is a failure of the makeup air unit (24-MAU01) or exhaust fan (24-EF01) as detected by discharge differential pressure switches.
Process Control
Final clarifier performance should be evaluated and process control adjustments performed based on the effluent quality and clarifier operating parameters.
  1. Effluent Total Suspended Solids (TSS)
         Sample Point:                           Secondary clarifier effluent
         Sample Type:                           24-hour composite
         Sample Frequency:                    Daily
         Normal Range:                          10 mg/L or less
         Controlled By:                           MLSS settleability and clarifier loading rates
         If the secondary clarifier TSS exceeds the design performance value, the following process parameters should be reviewed. 
   2.  Hydraulic Loading Rate (Surface Settling Rate or SSR)
        The design BNR secondary clarifier SSR is 501 gpd per square feet of clarifier surface area.  This value is based on the peak
        design flow.  Surface settling rate is an indication of the upward velocity of wastewater in the clarifier.  If the SSR is too
        high, the ability of the activated sludge to settle in the clarifiers is reduced.  In extreme cases, solids may be washed out.
         For the Clinton WWTP, secondary clarifier SSR can be calculated as follows:
                                          SSR     =                 Q                     
                                                             11,310 x C
                        Where:        SSR      =      Surface settling rate, gpd/ft2
                                           Q        =      Influent wastewater flow, gpd
                                      11,3120    =      Surface area of each clarifier, ft2 
                                            C       =      Number of secondary clarifiers in service

                                                LIVE SSR FORMULA
         If the calculated SSR exceeds the design value, or if it appears that the hydraulic loading is contributing to high effluent
         TSS, the SSR can be reduced by increasing the number of secondary clarifiers in service if all units are not operating.
   3.  Solids Loading Rate (SLR)   
         The design BNR secondary clarifier solids loading rate is 34 lbs/day/ft2 at the maximum MLSS of 4,000 mg/L.  Secondary
         clarifier solids loading is calculated as follows:
                                          SLR      =      (Q + QRAS) x MLSS x 8.34
                                                                       11,310 x C
                        Where:         SLR      =      Clarifier solids loading, lbs/day/ft2
                                             Q      =       Influent wastewater flow, MGD
                                          QRAS    =       RAS flow, MGD
                                             C       =      Number of clarifiers in service

                                             LIVE SLR FORMULA
         Values above the design limit can indicate solids overloading which can result in poor settling and loss of solids in the
         clarifier effluent.  Secondary clarifier solids loading can be reduced by:
                                       1. Utilizing additional secondary clarifiers if available.
                                       2. Reducing the RAS flow setpoint.
                                       3. Reducing the BNR basin MLSS concentration.
         The use of all available BNR basins and secondary clarifiers makes it possible to reduce the MLSS concentration without
         changing solids inventory due to the extra volume of the additional BNR basins.  This effectively reduces the solids loading
         rate on each secondary clarifier.
         If no additional process trains are available, the second approach should be used.  The RAS flow should be reduced by 10
         to 20 percent.  The depth of sludge in the secondary clarifiers should be measured periodically to ensure the sludge blanket
         does not build to an excessive depth.  Observe the clarifiers to see if the poor settling characteristics are improved.  If the
         settling does not improve by reducing the RAS flow, the MLSS should be decreased by slightly increasing the wasting rate.
         A practical limit for MLSS redcuction would be a 10 percent gradual change in one week.  If no improvement occurs with
         these adjustments, the high effluent TSS concentrations are most likely not due to solids overloading.
The rate at which the settled activated sludge solids are returned from the secondary clarifiers to the BNR basins affects the solids balance between these process units.  The solids must first settle to form a sludge blanket at the bottom of the clarifiers.  Monitoring and control of the clarifier sludge blankets is critical to overall process performance and effeciency.
Auto mode of RAS flow control involves pacing the RAS flow with influent flow rates on a ratio setpoint.  The ratio setpoint establishes the RAS flow rate as a percent of the influent wastewater flow.  RAS flow can also be run in a manual mode with a constant RAS flow to the BNR basins.  Process conditions can differ because of diurnal flow variations and sludge settling characteristics.  Therefore, it is critical for the BNR system operator to monitor and understand the interrelationships among the RAS flow control and the BNR basin MLSS, sludge blanket level, and diurnal flow variation.
The auto mode of RAS control paces the RAS flow rate in proportion to the BNR basin influent flows.  This control mode keeps the MLSS concentration in the BNR basins and secondary clarifier sludge blankets more constant during high and low flow periods.  Becasue the BNR basin MLSS and clarifier sludge blankets are maintained at relatively constant levels, the food to microorganism (F:M) ratio and SRT will be more constant and easier to control.
Constant RAS flow results in continuously varying the BNR basin MLSS concentration.  The MLSS concentration will be at a minimum during peak influent flows and a maximum during minimum influent flows.  The BNR basins and secondary clarifiers function as a system that stores MLSS in the BNR basins during minimum flows and then transfers the MLSS to the clarifiers as the wastewater flow initially increases.  The clarifiers act as a storage reservoir for the MLSS during periods of high flow.  Therefore, the depth of the sludge blankets in the individual clarifiers constantly changes as the MLSS moves from the BNR basins to the clarifiers and vice versa.  The thicker blanket at high flows tends to produce a more concentrated RAS that tends to return more pounds of solids per gallon of RAS.
Several techniques can be used to determine the proper RAS flow rate.  Basically, the RAS flow rate will help maintain a BNR basin MLSS concentration that is set by the operating process loadings, it will maintain a given sludge blanket depth within the secondary clarifiers, or it will optimize the return sludge cocnentration for returning and wasting.  The primary objectives of RAS flow control should be to maintain sludge blanket and RAS concentrations within a desired range.  The most commonly used techniques include:
  • Direct sludge blanket level control
  • Settleability
  • Secondary clarifier mass balance
  • BNR basin mass balance
Of these techniques, all but the first are fundamentally similar; they differ primarily in how accurately they control solids inventory during abnormal conditions, the critical times when accurate process control is most needed.  Major adjustments in the return rate should only be needed occasionally if the BNR process is operating properly.

Direct Sludge Blanket Level Control
With this method of RAS control, an optimum sludge blanket level is maintained in the secondary clarifiers.  This level is determined by experience and is one that is low enough to permit efficient settling and high enough to store a sufficient quantity of return sludge and provide a properly thickened RAS concentration.  Generally, the sludge blanket level should be maintained between 1 and 3 feet in the clarifiers.  Also the blanket should not be allowed to rise beyond 25% of the sidewater depth of the clarifiers unless dictated by other control parameters.
High sludge blanket levels (thick blanket) normally indicates that sludge is not being removed rapidly enough from the clarifiers.  Under this condition, the RAS solids concentration will normally be high.  Low sludge blanket levels (thin blanket) indicate that sludge is being removed too rapidly.  Under this condition, the RAS solids concentration will usually be low.  High blankets combined with a low sludge solids concnetration is normally caused by a dispersed or poorly settling sludge.  This condition is serious and requires prompt corrective actions.
When using blanket level to control the RAS flow rate, the operator must daily review the blanket levels and return rates from the previous several days.  The operator then keeps the return rate unchanged or adjusts it to maintain a level within the target range.  Generally, changes should be small; no more than 10 percent per day.  For example, if the blanket is trending upward at a 50% average return rate and may soon exceed the desired maximum value, then the return rate may be increased to 55%.  In determining the magnitude of changes to make, the operator is guided only by previous experience with such changes and the knowledge that increases in the RAS rate will tend to lower the sludge blanket and RAS solids concentration, while decreases in the return rate will tend to raise the blanket and the RAS solids concentration.  However, an increase in the return rate with poor settling sludge may cause the blanket to rise.  Increased return rates will also increase the flow into the secondary clarifiers, causing more turbulence.  Caution should be used when applying the technique.
Final clarifier sludge blanket levels can vary considerably during the day because of varying wastewater flow and characteristics.  The levels are best measured during the maximum daily flow because the clarifiers are operating under the highest loading rate.  The position of the clarifier collector arms should be 90o from the bridge during the measurements.
As with any sampling or monitoring program, consistency is critical.  The location, time of day, and method of sludge blanket level measurement must be consistent from day to day.
The settleability test for estimating the desired RAS flow rate uses the settled sludge volume in a settleometer at the end of a 30-minute settling period to represent the clarifier underflow the supernatant volume to represent the clarifier overflow, according to the following equation:
                                                   QRAS      =            SSV    
                                                      Q                  1000-SSV
                           Where:                QRAS     =         RAS flow, MGD
                                                         Q      =         Influent wastewater flow, MGD
                                                      SSV      =         Volume of settled sludge after 30 minutes, mL/L
To illustrate the use of the above equation, assume the 30-minute settled sludge volume is 350 mL/L.  The ratio of RAS flow to BNR influent flow calculation would be:
                                                   QRAS        =              350     
                                                      Q                     1000 - 350
                                                                  =         0.54
The result of the calculation is multiplied by the BNR influent flow to determine the RAS flow rate.  If the influent flow is 5 MGD, the RAS flow should be:
                                                 5.0 x 0.54   =         2.7 MGD
Secondary Clarifier Mass Balance        
The secondary clarifier mass balance approach is a useful tool for calculating the return rate; however, it assumes that the sludge blanket level in the clarifiers is constant.  The calculations are based on a mass balance of the suspended solids that enter and leave the secondary clarifiers.  The solids entering the clarifiers must equal the solids leaving, assuming the sludge blankets are not changing.  If the effluent suspended solids are low, the resulting equation is:
                                                 QRAS         =         (MLSS x Q) - (WASSS x QWAS)
                                                                                     RASSS - MLSS
                              Where:         MLSS          =         Mixed liquor suspended solids, mg/L
                                                WASSS        =         Waste activated sludge suspended solids, mg/L
                                                 RASSS        =         Return activated sludge suspended solids, mg/L
                                                 QWAS         =         Waste activated sludge flow, MGD
The mass balance should be calculated daily either by actually measuring the mixed liquor and RAS suspended solids, or by estimating them with a centrifuge once a correlation is developed between the analytical suspended solids and the respective centrifuged volumes.  Knowledge of the solids balance allow the operator to make the necessary RAS adjustments during the day.    
BNR Basin Mass Balance       
The desired RAS flow can also be estimated by a BNR basin mass balance.  Neglecting the influent suspended solids, the RAS flow calculation is:
                                                QRAS            =                  MLSS     
                                                   Q                           RASSS - MLSS
For example, assume the desired areation basin MLSS is 2800 mg/L and that the RAS suspended solids concentration is 7500 mg/L.  The ratio of RAS to BNR influent flow required to maintain the desired MLSS would be:
                                                QRAS            =                2800      
                                                   Q                            7500 - 2800
                                                                    =             0.59
Multiplying the results of this calculation by the BNR basin influent flow gives the required RAS flow rate.  For example, if the influent flow is 5 MGD, the RAS flow required to maintain 2800 mg/L MLSS would be:
                                                   5.0 x .59      =         2.95 MGD  
The Secondary Clarifier and RAS Pumping system includes the following equipment: 
  • Water control gates
  • Secondary clarifier mechanisms
  • Secondary clarifier scum lift station
  • RAS control valves
  • RAS flow meters
  • RAS pumps
  • RAS wet well level monitoring

RAS piping is painted Brown.  Scum piping is painted Dark Brown.

Water Control Gates6
The three water control gates are located in the secondary clarifier splitter structure and control the flow to each secondary clarifier.  The gates are manually operated and are downward opening.
Secondary Clarifier Mechanisms6
  The secondary clarifier mechanisms collect the settled solids and pull the solids to the sludge hopper in the clarifier.  A 24" RAS line runs from the sludge hopper to the wet well of the secondary treatment building. 
Secondary Clarifier Scum Lift Station 6
Scum formed in the secondary clarifiers is collected by a skimmer on each clarifier drive mechanism and discharged into the clarifier scum trough. The scum flows by gravity through a 6" scum line to the secondary clarifier scum lift station.  The two submersible pumps in the lift station pump the scum to the aerobic digesters.
RAS Control Valves6
  The RAS control valves work in conjuction with the RAS flow meters to control the RAS flow from each clarifier.  The control of the RAS control valves is described in the Controls section.
RAS Flow Meters 6
Each clarifier sludge return line contains a magnetic flow meter to monitor the RAS flow from each clarifier.  The flow signals are used by SCADA to adjust the position of the RAS control valves as described in the Controls section.
RAS Pumps 6
The four RAS pumps remove the settled solids from the wet well and convey the RAS to the influent line of the BNR basins.  This provides a thoroughly mixed solution of RAS and influent wastewater that enters the anaerobic zone of the BNR basins.  The pumps have adjustable speed drives to allow the pump discharge to be controlled based on process requirements.
RAS Wet Well Level Monitoring6
The wet well in the secondary treatment building contains level monitoring equipment to provide wet well level measurements for RAS pump control.

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Last Updated: 9/20/2013 2:53:28 PM
Version 3.0.1