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Aerobic Digestion Overview

The aerobic digestion system provides sludge stabilization for compliance with biosolids regulations.


Theory of Operation
Aerobic digestion is a biological treatment process that utilizes long-term aeration to stabilize and reduce the total mass of sludge by destroying volatile solids.  The process extends decomposition of solids and regrowth of organisms to a point where available energy in active cells and storage of waste materials are sufficiently low to permit the waste solids to be stable enough for discharge to the dewatering system.  The waste solids fed to the aerobic digester consist of suspended solids that are generated by biomass growth and suspended solids removal during the BNR treatment process.  The aerobic digestion process renders the digested solids more dewaterable, less likely to generate odors on disposal, and reduces bacteriological hazards.
The Aerobic Digestion system includes the following equipment: 
  • Coarse bubble diffusers
  • Aeration blowers
  • Air flow control valves
  • Air flow meters 
  • Thickened waste activated sludge control valves
  • Aerobic digester level transmitters
  • WAS control valves
  • Aerobic Digester dissolved oxygen sensors
The four aerobic digesters receive thickened waste activated sludge and provide the biological environment for the stabilization of the solids.  Aerobic digesters 2803 and 2804 are also set up to receive scum from the secondary clarifiers and aeration basins as well as unthickened waste activated sludge.  Each digester is 82.5' x 82.5' with a 15' SWD. The floor of each digester is sloped to a sump in the corner where the suction lines for the digested sludge pumps are located.  There is a 3' x 6" opening cast in the wall between digesters 2801/2802 and digesters 2803/2804.  This opening allows the overflow from one digester to flow to the adjacent digester.
Air is supplied to the aerobic digestion system by four positive displacement blowers located in the blower room of the Solids Processing Building.  Each blower discharges into a common 36" air header that runs to the digestion system.  An air control valve on the air line to each digester controls the flow of air based on an operator adjustable setpoint established in the control system.  There are two droplegs to each digester feeding air to the coarse bubble diffusers covering the bottom of each digester.  The air system is designed for minimum mixing air flow of 30 standard cubic feet per minute (scfm) per 1000 cubic feet of tank volume and airflow to the  basin of 3060 scfm. 
The blowers and air control valves work together through the control system utilizing a Most Open Valve (MOV) control logic.  This assures the most efficient delivery of air to the digestion system at a minimum of blower energy.  The digester with the highest air demand will have the control valve completely open.  The other air control valves will modulate to maintain the required air flow to the other digesters.
The aerobic digesters are covered with two access hatches per digester (NOTE: the aerobic digesters are confined spaces and entry must be based of the requirements of the Clinton Confined Space Policy).  An adjustable weir box in each digester allows for the collection of decant which is directed to the filtrate pump station.  The weir box has a range of operation of 5 feet from the high water elevation in the digesters.  Pressure type level transmitters are installed to monitor the level in each digester.  Each foot of depth in the digester is equivalent to approximately 51,000 gallons.
Sludge is removed from the digesters by the digested sludge pumps.  Two of the digested sludge pumps are associated with the rotary drum thickeners which allows sludge to be removed from the digesters, thickened, and returned to the digesters.  The other three digested sludge pumps convey the solids to the dewatering presses.  Only aerobic digesters 2801 and 2802 have the necessary piping arrangements for dewatering.
The ferric chloride system is designed to control the soluble phosphorus concentration in the aerobic digesters.
Aerobic Digester Process Control and Performance Evaluation
Oxygen Requirements
The characteristics of the waste activated sludge pumped to the aerobic digesters will determine the oxygen requirements for the process.  The most critical parameter in the aerobic digestion process is dissolved oxygen (DO).  Maintaining adequate oxygen levels allows the biological oxidation process to take place and prevents the formation of objectionable odors.  A typical DO level ranges from 0.5 to 2.0 mg/L.  Inadequate DO levels result in incomplete digestion and odor problems.
The rate of oxygen used by the microorganisms depends on the rate of biological oxidation.  The oxygen uptake rate is expressed in terms of milligrams of oxygen per hour per gram of volatile suspended solids (VSS) (mg O2/hr/g VSS).  The oxygen uptake rate is used to determine the level of biological activity and the resulting solids destruction occurring in the digester.  During active aerobic digestion, a typical rate of oxygen uptake is from 5 to 10 mg O2/hr/g VSS, compared with a range of 10 to 30 mg O2/hr/g VSS in the active phase of the activated sludge process. 
Based on the 503 Biosolids Regulations, a well digested sludge will have a specific oxygen uptake rate of not greater than 1.5 mg O2/hr/g Total Solids at 20oC.  Total solids are used to provide consistency with the rest of the 503 regulations where all loadings are expressed on a total solids basis. 
Two products of aerobic digestion that tend to lower the digester pH are carbon dioxide and hydrogen ions.  Proper venting of the digester will prevent the buildup of carbon dioxide.  A pH drop can occur when ammonia is oxidized to nitrate if the alkalinity of the wastewater is insufficient to buffer the solution.  Whether the decline in pH will require pH adjustment depends on the stability of the bacteria and the buffering capacity of the carriage water at the WWTP.  As a biological system, the process performs better at either neutral or slightly higher pH.
The liquid temperature in the aerobic digesters significantly affects the rate of volatile solids reduction and pathogen destruction.  The temperature in the digester is a function of the wastewater temperature, weather conditions and the heat generated during the digestion process.  The aerobic digesters are covered to reduce heat losses from the system during winter weather.  Typical temperature range for the aerobic digesters is from 15o to 35oC.
The 503 Biosolids Regulations require a specific SRT based on temperature to meet the requirements for Processes to Significantly Reduce Pathogens (PSRPs) for Class B Biosolids.  Required SRTs range from 40 days at 20oC to 60 days at 15oC.  Between 15o and 20oC, the following equation can be used to determine the required SRT:
                                                SRT = 40 x (1.08(20-T))
                                                SRT = required solids retention time, days
                                                  T  =  digester temperature, oC
Mixing is essential to proper performance of the aerobic digester.  A well-mixed biological system ensures adequate contact between the organisms and their food supply (undigested biosolids) and ensures uniform distribution of oxygen throughout the digester.  Mixing of the digester is provided by the aeration system which is sized to provide 30 scfm of air per 1000 cubic feet of digester volume.
Solids Retention Time (SRT)
Solids retention time is a significant factor in the effective operation of the aerobic digestion system.  SRT is the total mass of biosolids in the reactor divided by the mass of solids removed from the reactor on a daily average.  Typically, increased SRT results in an increase in the degree of solids reduction.  The aerobic digestion system is designed to provide a 60 day SRT.
Solids Concentration in the Digester
The aerobic digestion system is designed to receive thickened waste activated sludge.  While the rotary drum thickeners can produce a thickened sludge with a solids concentration of 5 - 7% TS, oxygen transfer limitations can occur in the aerobic digester if the feed sludge concentration exceeds 3.5% TS.  Because of this, the aerobic digestion system was designed for a feed sludge concentration of 3.5% TS.  As volatile solids are destroyed in the digestion process, the total solids concentration in the digester will drop.
Process Performance
The performance of the aerobic digester should be measured in terms of the ability to stabilize biosolids into a product that is suitable for subsequent processing.  Parameters used to evaluate performance are:
  • Volatile solids reduction,
  • Oxygen uptake rate,
  • supernatant quality,
  • dewatering characteristics of the sludge,
  • Reduction in ammonia concentrations,
  • Reduction in pathogenic organisms, and 
  • Aesthetic character of the residual solids.
 Process Control Monitoring 
There is a 3' x 1'-10" opening in the top of the wall between the digesters and pipe gallery centered on the valve operator for each adjustable weir box to allow sample collection.  The recommended aerobic digester monitoring is as follows:
                     Monitoring parameter               Sample type                     Frequency           
                     Temperature, oC                        Grab                              Daily
                     pH                                           Grab                             Daily
                     Dissolved oxygen, mg/L               Grab                              Daily
                     Alkalinity, mg/L as CaCO3               Grab                              Weekly
                     Total solids, %                          Grab                              Weekly
                     Total volatile solids, %                Grab                              Weekly
                     Sludge settleability, mL/L             Grab                              As necessary
                     Ammonia-nitrogen, mg/L              Grab                              As necessary
                     Nitrate-nitrogen, mg/L                 Grab                              As necessary
                     Phosphorus, mg/L                       Grab                              As necessary
Additional monitoring may be required for compliance with the local Biosolids Management Plan.
Operation of Aerobic Digesters
Operational flexibility has been designed into the aerobic digestion system to allow different modes of operation.  The operational modes are:
  1. The digesters can be operated in batch mode where thickened waste activated sludge is added to one digester until the digester is full.  Feed is then switched to another digester while digestion proceeds in the full digester.  After sufficient digestion time in the digester, digested sludge is removed and dewatered.  When the digested sludge has been removed, the digester becomes available for filling again.
  2. Two adjacent digesters (2801/2802 and 2803/2804) can be operated as a continuous flow system with two digesters in series.  The opening between the adjacent digesters allows the overflow from the first digester to flow to the second digester.  After sufficient digestion time has been achieved, digested sludge is removed from the second digester on a semi-continuous basis and dewatered to provide room for the overflow from the first digester.
Additional operational flexibility exists within each operation mode.  The rotary drum thickeners are sized so that the daily volume of waste activated sludge can be thickened in an 8-hour period.  If thickening is not performed on weekends, unthickened waste activated sludge can be directed to digesters 2803 and 2804.  These digesters are also set up to receive scum from the aeration basins and secondary clarifiers.

The contents of each digester can be thickened by pumping the sludge from the digester and through the rotary drum thickener.  After thickening, the sludge can be pumped to any digester.  Each digester is also equipped with a decant structure consisting of an adjustable weir box.  Aeration is stopped in the digester and the solids are allowed to settle leaving the supernatant at the surface.  A sludge settleability test of the digester contents will provide an indication of the supernatant quality during this operation.
Coarse Bubble Diffusers6

Coarse bubble diffusers are installed in each aerobic digester to supply aeration for the biological process and mixing to keep the solids in suspension.  
Aeration Blowers6
The four positive displacement blowers, located in the Solids Processing Building, provide air to the aerobic digesters.  The blowers supply air based on air flow requirements for each digester. 
Air Flow Control Valves6


 Air to each aerobic digester is controlled by an air flow control valve.  The valve modulates to maintain a set air flow rate to the digester.
Air Flow Meters 6
The air flow meters monitor the air flow to each digester.  The signal from the flow meters is used to control the the air flow to each digester based on an operator set point.  The air flow signal is also used by the PLC to adjust air flow based on the level in the digester. 
Thickened Waste Activated Sludge Control Valves6
 Each aerobic digester is equipped with a thickened waste activated sludge control valve to control the feed to each digester.  The valves are manually controlled, either locally or through SCADA, to allow the discharge of the thickened waste activated sludge pumps to discharge to the appropriate digester.
Aerobic Digester Level Transmitters6
Each aerobic digester is equipped with a level transmitter to monitor the level of the contents of the digester.  The level measurement is provided to the SCADA system for air flow and sludge feed control. 
Waste Activated Sludge Control Valves6
 Aerobic digesters 2803 and 2804 are equipped with waste activated sludge control valves to allow unthickened WAS to be fed to these digesters.  This provides flexibility in operating thickening systems while still maintaining appropriate SRT controls in the BNR system.
Aerobic Digester Dissolved Oxygen Sensors 6
Each aerobic digester is equipped with a dissolved oxygen sensor to monitor the dissolved oxygen levels in the digester. The dissolved oxygen sensor works in conjunction with the air flow control valve to control the air sent to each digester.

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Last Updated: 6/30/2020 2:18:32 PM
Version 3.0.1