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Waste Activated Sludge (WAS) Pumping and Thickening Overview
Purpose
Control of activated sludge wasting is one of the most important control parameters used to maintain the proper environment for the Biological Nutrient Removal (BNR) microorganisms.  Sludge wasting controls both the amount of sludge in the BNR system (sludge inventory) and the species of microoganisms in the BNR system.
Theory of Operation
The Biological Nutrient Removal system converts influent BOD and nitrogen to biological solids.  More solids are generated than are needed in the BNR system.  The surplus biological solids are referred to as waste activated sludge (WAS).  WAS pumping removes the surplus biological solids from the BNR system to maintain the solids inventory at the proper level.  Activated sludge process control strategies, primarily the solids retention time (SRT), determine the amount of waste activated sludge to be removed from the system.
 
The WAS thickening system removes excess water from the waste activated sludge, thus reducing the volume sent to the aerobic digestion system.  Rotary drum thickeners are used for dewatering with the assistance of polymer fed to the influent solids stream.  Thickening in a rotary drum thickener is achieved by separating the liquid from the flocculated solids through woven wire mesh.  Solids pass through four dewatering stages before being discharged from the unit.
Equipment
The WAS Pumping and Thickening system includes the following equipment:
  • WAS pumps
  • WAS flow meter
  • WAS control valves
  • Polymer feed system
  • Rotary drum thickeners
  • Nonpotable water (NPW) booster pumps
  • Thickened WAS (TWAS) pumps
  • TWAS flow meter
  • Digested sludge pumps
  • Digested sludge pump suction control valves

WAS and TWAS piping are painted Brown.

Description
Wasting of BNR system activated sludge solids is accomplished by pumping a portion of the return sludge flow from the secondary clarifiers to the WAS thickening and aerobic digestion system.  One of the most important parameters used to control the BNR process is controlling the solids inventory in the system with the solids wasting rate.  The wasting of activated sludge solids affects the process more than any other process control adjustment.  The rate that WAS is removed from the process affects:
  • Effluent quality
  • The growth rate of the microorganisms and the types present
  • Oxygen consumption
  • Mixed liquor settleability
  • The occurrence of foaming
  • BOD, nitrogen, and phosphorous removal efficiencies
The automated WAS flow control technique used for the BNR system is based on Sludge Retention Time (SRT).  Basically, SRT approximates the average time that a unit of activated sludge (microoganisms) will be retained in the BNR process.  The SRT for a particular system is determined by the minimum growth rate of organisms in the system.  The SRT must be high enough to prevent the desired organisms from being removed faster than they can reproduce.  Specifically, nitrifying bacteria reproduce slowly.  Thus, a relatively high SRT is required to maintain an adequate number of nitrifying bacteria in the BNR system aeration basins.
 
The design aerobic SRT for the Clinton BNR system is 12 days.  SRT for an existing set of conditions can be calculated as follows:
 
                                                               SRT = MLSS x B x 1.74
                                                                         RASSS x QWAS
                                    Where:
                                                               SRT = sludge retention time, days
                                                             MLSS = aeration basin mixed liquor total suspended solids, mg/L
                                                                  B  = number of BNR trains in service
                                                               1.74 = volume of each aeration basin, million gallons
                                                            RASSS = RAS total suspended solids, mg/L
                                                             QWAS = WAS flow rate, million gallons per day (MGD)
 
To illustrate how the SRT calculation can be applied to the BNR process, assume the following conditions:
 
                                          BNR trains in service = 2
                                                  average MLSS = 3,800 mg/L
                                                            RASSS = 6,000 mg/L
                                                             QWAS = 0.20 MGD
 
                                                               SRT  =  3800 x 2 x 1.74
                                                                             6000 x 0.2
 
                                                               SRT  = 11 days
 
The WAS flow rate required to maintain a desired SRT can be estimated by rearranging the above formula:
 
                                                             QWAS =  MLSS x B x 1.74
                                                                            SRT x RASSS
 
Using the previous example values, the WAS flow required to maintain the SRT at 12 days would be:
 
                                                              QWAS = 3800 x 2 x 1.74
                                                                            12 x 6000
 
                                                               QWAS = 0.18 MGD
 
When applying the result of SRT calculations to process control, the Clinton operation staff must be aware that:
  • The best SRT can only be determined through experience with the system and may differ somewhat from the design value.
  • Wastewater temperature will affect the SRT that must be maintained to produce the most efficient nitrification.  In general, as the wastewater temperature decreases the SRT and MLSS must be increased to maintain the necessary nitrification rate.  For each 18oF (10oC) decrease in wastewater temperature, the activity of the activated sludge organisms will normally decrease by one half.
  • When making WAS rate adjustments, the WAS rate changes should be no more than 10% at a time.  The operator should wait at least one (preferably two) SRT's to determine the effect of the change.
  • SRT's should be calculated and recorded daily.  Process control changes should be based on the 7-day moving average SRT rather than the individual daily values.
The WAS flow is directed to either the rotary drum thickeners or aerobic digesters 2803 and 2804.  This allows flexibility in WAS control and thickening operation.  Normally, WAS pumpage occurs every day which provides consistent control of SRT.  However, the thickening operation may only be scheduled for 5 days per week.  When the thickening operation is not occurring, WAS pumping can continue by directing WAS to aerobic digester 2803 (by opening control valve 28WAS-CV01) or 2804 (by opening 28WAS-CV02).
 
During thickening operations, the WAS pumps serve as the feed pumps for the rotary drum thickeners.  The WAS control valves (29WAS-CV01 and 29WAS-CV02) direct the WAS flow to each rotary drum thickener.  Polymer is fed to the WAS as it flows through the rotary drum thickener sludge feed lines by polymer mixing units 29PO-P01 and 29PO-PO2.  The sludge and polymer mixture first enters the flock tank of the rotary drum thickener where a mixer provides efficient contact between the sludge and polymer while promoting the growth of sludge floc.  The flocculated sludge flows from a tangential outlet on the flock tank into the rotary drum screen through a step-down header.  In the rotary drum screen the liquid separates from the flocculated solids through the woven wire mesh, is collected in the return water tank and exits through a drain in the bottom.  This centrate flows by gravity to the filtrate pump station.  The solids pass through four dewatering stages as the drum rotates before being discharged from the end of the unit into the hopper of the TWAS pump.  Mounted above the rotary drum is a self-cleaning wash water spray header which keeps the rotary drum screen openings clear of solids.  Non-potable water (NPW) is used for the spray header and is supplied by the two NPW booster pumps.  These pumps are controlled with the operation of the rotary drum thickeners.
 
Digested sludge pumps 28DS-P01 and 28-P05 can also serve as feed pumps for the rotary drum thickeners.
 
Each rotary drum thickener discharges to a TWAS pump. Operation of the TWAS pumps is controlled based on the level of thickened sludge in the pump feed bin.  The TWAS pumps convey the the thickened waste activated sludge to one of the four aerobic digesters depending on the position of the TWAS control valves.

Thickening Process Control

The rotary drum thickeners are designed to produce a thickened solids concentration of 5% at a maximum polymer feed rate of 7 pounds of active polymer per dry ton of solids with a minimum solids capture of 90%.  The manufacturer of the rotary drum thickeners has established procedures to optimize the performance of the thickeners which should be followed when making adjustments to the process.  Whenever the thickeners are in service, the following monitoring should be performed on each unit at least once each day based on grab samples:

                     1.  Feed sludge total solids (%)
                     2.  Thickened sludge total solids (%)
                     3.  Filtrate TSS (mg/L)

Additionally, the following data should be recorded at the time of sample collection for each rotary drum thickener:
               
                     1.  Polymer usage rate
                     2.  Washwater flowrate
                     3.  Polymer solution flowrate
                     4.  Polymer dilution water
                     5.  Sludge feed rate

The washwater flow is determined based on the pressure to the washwater nozzles.  Pressure/Flow Conversion

The following calculations can be used to measure the performance of a given polymer and the operation of the rotary drum thickeners:

                     Polymer Dosage (lbs/Ton)   =  (Volume of Polymer X Concentration of Polymer)   X   2000 lb
                                                              (Volume of Sludge X Concentration of Sludge)            Ton
                    
                                                            Polymer Dosage Live Formula


For example, if the polymer usage was 10 gallons with a concentration of 35% for feed sludge of 0.75% TS and a volume of 144,000 gallons, the dosage would be:

                     Polymer dosage (lbs/Ton)   =   (10 X .35)/(144000 X .0075)      X      2000 lbs/Ton
                  
                                                          =   6.5 lbs/Ton

The dosage can also be calculated based on flowrates for polymer and sludge with the same equation by substituting flows for volumes.  Another equation that is useful in evaluating the thickener performance is the solids capture.  Solids capture is a measurement of the amount of feed sludge that ends up in the thickened sludge and is calculated as follows:

                     Solids Capture (%)      =       C X (F-E)   X   100
                                                             F X (C-E)
                  
                     Where                C     =      Thickened sludge, %TS
                                              F     =      Corrected feed solids concentration, %TS
                                              E     =      Filtrate solids, %TS

The corrected feed solids concentration accounts for the additional volume of polymer solution and washwater leaving the thickener and is calculated as follows:                                                                                                           

      Corrected feed sludge concentration =  Feed sludge concentration, %TS X Sludge feed flow, gpm                        
                                                           (Sludge feed flow, gpm + Washwater flow, gpm + Polymer solution flow, gpm)

To convert the Filtrate TSS to total solids, the following equation can be used:

                           % Filtrate solids     =      mg/L TSS
                                                               10,000

For example, with a sludge flow of 300 gpm, feed solids of 0.75% TS, polymer solution flow of 5 gpm, washwater flow of 20 gpm, filtrate TSS of 1000 mg/L and a thickened sludge concentration of 5% TS,

                           % Filtrate solids    =   1000/10000
                                                      =   0.1%

     Corrected feed solids concentration  =   (.75% X 300)/(300 + 20 + 5)
                                                      =   0.69%

                              Solids capture     =   .05 X (.0069 - .001)/(.0069 X (.05 - .001))   X 100%
                                                      =   87.3%                                                        
                                      

                                                      Solids Capture Live Formula
WAS Pumps6
 
The two WAS pumps pull returned solids from the secondary treatment building wet well and convey the waste activated sludge to the rotary drum thickeners or to aerobic digesters 2803 and 2804.  Operation of the pumps is established for control of the solids retention time in the BNR system.
WAS Flow Meter 6
  Discharge flow form the WAS pumps is monitored with the WAS flow meter.  The flow from the pumps is used in the process control of SRT for the BNR system. 
WAS Control Valves6
  There are two sets of WAS control valves to establish the discharge location for the WAS pumps.  One set (28WAS-CV01 and 28WAS-CV02) directs the WAS to aerobic digester 2803 or 2804.  The other set (29WAS-CV01 and 29WAS-CV02) controls the flow to the two rotary drum thickeners.  The control valves have electric actuators that are normally controlled through SCADA.
Polymer Feed Systems6
Two polymer mixing units are dedicated to the rotary drum thickeners.  A polymer feed pump in the mixing unit pulls neat polymer from a tote container.  Plant service water is blended with the neat polymer to activate the polymer before the chemical is injected into the sludge feed line for the rotary drum thickener. 
Rotary Drum Thickeners6
The two rotary drum thickeners provide solid/liquid separation of the WAS to removed liquid and increase the total solids concentration from an estimated 0.75% to 3.5%.  Each thickener is designed to handle up to 400 gpm of WAS or digested sludge so that the units are capable of thickening the WAS during an 8-hour period.
Nonpotable Water Booster Pumps6
The NPW booster pumps are controlled with the operation of the rotary drum thickeners to provide adequate flow and pressure of wash water for the thickening units.  The wash water ensures the rotary drum screens are cleared of solids to promote efficient removal of liquid from the sludge solids.
Thickened WAS Pumps6
The TWAS pumps receive the thickened sludge from the rotary drum thickeners and convey the thickened solids to the four aerobic digesters.  Maintenance videos for the pumps are available on Vogelsang's website.
Thickened WAS Flow Meter 6
The TWAS flow meter monitors the discharge flow from the TWAS pumps to the aerobic digesters.  The flow data can be used to determine detention time in the aerobic digesters for compliance monitoring with biosolids regulations for pathogen reduction.
Digested Sludge Pumps6
Two of the five digested sludge pumps are designed to pump digested sludge to the rotary drum thickeners.  This allows the digested sludge to be thickened and returned to the aerobic digestion system.  Maintenance videos for the pumps are available on Vogelsang's website.
Digested Sludge Pump Suction Control Valves6
The suction control valves on the digested sludge pumps are motor operated valves to isolate the digested sludge pumps.  The valve is opened when the pump is in service and closed when the pump is not running.
 

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Last Updated: 5/9/2013 11:07:40 AM
Version 3.0.1