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Biosolids Dewatering System Overview
Purpose
The dewatering system reduces the volume of biosolids while producing a stable product suitable for land application.
Theory of Operation
The digested sludge pump feeds sludge from the aerobic digesters to the flocculator unit.  Polymer solution is injected into the sludge at the flocculator.  A mixer in the flocculator establishes good contact between the solids and the polymer and allows the formation of flocculated sludge.  The conditioned sludge enters the press feed manifold and is directed to each rotary press channel.  The dewatering process occurs as the flocculated sludge travels through the press with water extracted through filtering elements.  The sludge becomes progressively denser as it advances in the channel.  A restrictor located at the channel outlet further removes moisture by compressing the sludge as it exits the press.  The sludge cake falls into a screw conveyor which transports the sludge to the storage pad. 
Equipment
The Biosolids Dewatering system includes the following equipment: 
  • Digested sludge pumps
  • Digested sludge flow meters
  • Polymer mixing system
  • Dewatering presses
  • Sludge conveyors
  • Dewatered sludge control valves 
  • Hydropneumatic tank
  • Compressed air system
  • Filtrate pump station
  • Filtrate flow meter 
Description
Aerobically digested biosolids are pumped by the digested sludge pumps to the rotary filter presses.  Two six-channel rotary presses are installed to handle the current loading conditions with a third unit available for redundancy.  Polymer solution is fed to the sludge to condition the sludge for dewatering.  The rotary presses include a flocculator unit to promote floc formation in the sludge.  The flocculated sludge enters the rotary press channel.  This channel is delimited by filtering elements on each side and by inner and outer spacers at the top and bottom.  The outer space contains two openings, one for the channel inlet and the second for the channel outlet where sludge cake is discharged.  A deflector separates the inlet from the discharge and prevents the feed sludge from mixing with the cake.  The discharge contains a restrictor to control the flow of sludge through the channel.
 
The rotary press consists of three dewatering zones:
  • The filtration zone where water leaves the channel through the filtering elements. Scrapers in continuous contact with the filtering elements remove the film of sludge particles captured during the filtering process.
  • The pressing zone where the channel's moving wheel drives the sludge forward while the restrictor at the discharge restricts the sludge flow. In this section of the channel the pressure that the cake exerts on itself increases as the friction between the sludge and the filtering elements increases.  The pressure increases exponentially as the cake moves toward the discharge outlet.
  • The restriction zone is the last dewatering area where the restrictor in the discharge outlet restricts the flow of sludge from the rotary press.
The dewatering system is designed to increase the solids content of the digested sludge from 2% to 16% total solids. 
 
A channel wash cycle utilizing nonpotable water keeps the filtering elements of the rotary press clean.  A hydropneumatic tank in the solids processing building provides NPW to the rotary presses.

Cake solids produced by the presses are discharged to a series of screw conveyors to transport the sludge cake to the solids storage pad.  A system of control gates on the conveyor system allows the sludge cake to be discharged to various areas on the solids storage pad or directly to a sludge truck for transport.  The conveyors outside the solids processing unit are heat traced to prevent freezing.
Filtrate generated in the dewatering operation is collected in the filtrate pump station and returned to the headworks building for treatment.

Rotary Filter Press Process Control

The rotary filter presses are designed to produce a thickened solids concentration of 16% at a maximum polymer feed rate of 18 pounds of active polymer per dry ton of solids with a minimum solids capture of 95%.  The manufacturer of the rotary presses has established procedures to optimize the performance of the presses which should be followed when making adjustments to the process.  Whenever the presses 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.  Dewatered sludge total solids (%)
                     3.  Filtrate TSS (mg/L)

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

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

                     Polymer Dosage (lbs/Ton)   =  (Volume of Polymer X Concentration of Polymer)   X   2000 lb
                                                              (Volume of Sludge X Concentration of Sludge)            Ton
                    
For example, if the polymer usage was 20 gallons with a concentration of 35% for feed sludge of 3% TS and a volume of 28,000 gallons, the dosage would be:

                     Polymer dosage (lbs/Ton)   =   (20 X .35)/(28000 X .03)      X      2000 lbs/Ton
                  
                                                          =   16.7 lbs/Ton

                                                         Polymer Dosage Live Formula

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 dewatering performance is the solids capture.  Solids capture is a measurement of the amount of feed sludge that ends up in the cake solids and is calculated as follows:

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

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

      Corrected feed sludge concentration =  Feed sludge concentration, %TS X Sludge feed flow, gpm
                                                              (Sludge feed 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 80 gpm, feed solids of 3% TS, polymer solution flow of 20 gpm, filtrate TSS of 2000 mg/L and a cake solids concentration of 15% TS,

                           % Filtrate solids    =   2000/10000
                                                      =   0.2%

     Corrected feed solids concentration  =   (3% X 80)/(80 + 20)
                                                      =   2.4%

                              Solids capture     =   .15 X (.024 - .002)/(.024 X (.15 - .002))   X 100%
                                                      =   92.9%                                                        


                                                         Solids Capture Live Formula
Digested Sludge Pumps6
 
Each filter press is fed by a dedicated digested sludge pump.  The pumps are set up to pull sludge from any of the four aerobic digesters.  The pumps are positive displacement and pump output is varied by adjusting the pump speed. Maintenance videos for the pumps are available on Vogelsang's website.
Digested Sludge Flow Meter 6
There is a flow meter on the discharge line of each digested sludge pump to monitor the flow to the dewatering presses.  The signal from the flow meter is sent to the dewatering PLC (29-PLC01) for use in modulating the digested sludge pump speed to meet the flow set point for dewatering.
Polymer Mixing System6
  Each rotary press utilizes a dedicated polymer mixing system to inject polymer solution into the sludge as it enters the flocculator.  The polymer system pumps neat polymer from a tote and mixes the polymer with service water to form a polymer solution. 
Dewatering Presses6
  The dewatering press consists of a flocculator unit, to condition the sludge with polymer, followed by the rotary press which forces the water from the sludge to produce a sludge cake. 
Sludge Conveyors6
The dewatered sludge screw conveyor system transports the sludge cake from the rotary presses to the sludge storage pad.
Dewatered Sludge Control Gates 6
Control gates in the conveyor system direct the sludge to the proper location on the sludge storage pad or to the truck loadout area.
Hydropneumatic Tank6
The hydropneumatic tank acts as a pressure reservoir for the nonpotable water system to prevent the unnecessary cycling of NPW pumps. 
Compressed Air System 6
The compressed air system provides high pressure air to the hydropneumatic tank and rotary filter presses.  The system includes two reciprocating air compressors, one air receiver, a refrigerated air dryer, particulate filters and a coalescing filter.  An adjustable pressure switch provides start/stop control.
Filtrate Pump Station6
The filtrate pump station collects filtrate generated in the dewatering operation and pumps the filtrate to the plant headworks for treatment. 
Filtrate Flow Meter 6
A flow meter on the discharge line from the filtrate pump station monitors the flow into the influent wet well in the Headworks Building.
 

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Last Updated: 9/20/2013 3:12:51 PM
Version 3.0.1