Parallel BenchScale Digestion Studies

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Parallel Bench-Scale Digestion Studies

• Richard O. Mines, Jr.
• Laura W. Lackey
• Mercer University Environmental Engineering
• Mitchell Murchison
• Brett Northernor

2007 World Environmental Water Resources
Congress
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Acknowledgements

• Thank the Macon Water Authority for providing us
  with the ozonators.
• This project was performed by Mitchell Murchison
  and Brett Northenor as part of their senior
  design project at Mercer University.

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Presentation Outline

• Background
• Objectives of Study
• Materials Methods
• Results
• Summary Conclusions

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Background

• Sludge or biosolids are generated as a by-product
  of wastewater treatment
• Sludge treatment and disposal costs represents
  35-40 of the total cost of treating wastewater
  and they continue to increase
• Stringent effluent limits result in higher
  removals and higher sludge production rates

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Where is sludge produced? WWTP
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Aerobic Digestion

• Continuation of activated sludge process.
• Digesters operated in the endogenous phase.
• Microorganisms oxidize their own protoplasm into
  CO2,H2O, and NH3.
• Subsequently, ammonia is removed through
  nitrification.

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Ozonation Destruction Mechanism

• Scheminski et al. O3 attacks and destroys the
  cell wall releasing intracellular components.
• Cesbron et al. O3 solubilizes and converts
  slowly biodegradable particulate organics into
  low molecular weight, readily biodegradable
  compounds.

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Ozonation of Digested Sludge

• Scheminski et al. 60 of the digested sludge
  solid organic components can be transformed into
  soluble substances at an O3 dose of 0.5 g O3 per
  g of organic dry matter.
• Dissolved organic carbon (DOC) increased to 2300
  mg/L.

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Ozonation of WAS

• Park et al. achieved
• 70 mass reduction
• 85 volume reduction
• at an ozone dose of 0.5 g O3 consumed per g of
  dried solids compared to the control.

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Ozonation of RAS

• Yasui et al. reported elimination of excess
  sludge by ozonating 4Xtimes amount of waste
  sludge at 0.034 kg O3/kg SS.
• SVIs of ozonated sludge were 200-250 ml/g
  compared to 250-300 for AS.
• Sakai et al. eliminated excess sludge production
  by ozonating RAS at a dose of 34 mg O3 per gram
  of SS.

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Objectives of Study

• Evaluate the reduction of Total Solids and
  Volatile Solids in aerobic versus ozonated
  digesters.
• Evaluate the kinetics of Total Solids/Volatile
  Solids reduction in aerobic versus ozonated
  digesters.

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Objectives of Study

• Estimate quantity of oxygen required to destroy
  Total Volatile Solids.
• Determine quantity of ozone required to destroy
  Total Solids.

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Materials and Methods Both Phases

• COD was measured colorimetrically by HACH method
  8000.
• Solids analyses were conducted in accordance with
  Standard Methods.
• Ozone transfer rate measured by sparging O3 into
  potassium iodide solution.
• Ozone was measured by titration with 0.005N
  sodium thiosulfate (Standard Methods).

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Materials and Methods O3 Collection in Off-Gas
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Bench-Scale Aerated and Ozonated Digester
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Materials and Methods Phase I

• Two, 2-L batch digesters were operated in
  parallel for 30 days.
• Aerobic digester supplied with air _at_ 2.7 Lpm or
  810 mg O2/min 1.84 g O2/mgTS.
• Ozonated digester supplied with air ladened with
  O3 _at_ a rate of 6.5 Lpm or 0.88 mg O3/min 2.0 mg
  O3/mgTS.

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Materials and Methods Phase II

• Two, 2-L batch digesters were operated in
  parallel for 32 days.
• Aerobic digester supplied with air _at_ 4.0 Lpm or
  1200 mg O2/min 3.25 g O2/mg TS.
• Ozonated digester supplied with air ladened with
  O3 _at_ a rate of 3.25 Lpm or 0.44 mg O3/min 1.2 mg
  O3/mg TS.

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Results Aerobic DigestionIncreased O2 Loading
77
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Results Ozonation Increased O3 Loading 67
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Solids Degradation Rate Phase I
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Solids Degradation RatePhase II
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Solids Degradation Rates KD
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Oxygen Consumed Aerobic Digestion
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Oxygen Utilized per TVS Destroyed
EPA Manual 1.74 2.07 lb oxygen per lb of cell
mass oxidized.
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Ozone Consumed
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mg Ozone Utilized per mg TS Destroyed Increased
O3 67
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Total COD Removals
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Total COD Concentration Phases I and II
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Soluble COD ConcentrationPhases I and II
4.29.5 mg sCOD/g TS destroyed aerated
120146 mg sCOD/g TS destroyed ozonated
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pH Phases I and II
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SOUR Phases I and II
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Operating Cost Comparison

• 30 mgd WWTP
• 20/20 mg/L effluent limits for BOD/TSS
• SRT 10 days
• Y 0.6 g TSS/g BOD kd0.05 d-1
• 38 VS destruction
• 1.46 per lb ozone0.10 per kWH

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Operating Cost Ozonation

• 13,010 ppd TSS produced _at_68 VS
• 1.46 per lb ozone0.10 per kWH
• 38 VS destroyed resulting in 3362 ppd TS
  destroyed

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Operating Cost Aerobic Digestion

• 38 VS destroyed resulting in 3362 ppd TS
  destroyed
• 2.0 lb O2/lb VS destroyed0.10 per kWH
• STOR 2.5 lb O2/HP-hr 112 HP aerator

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Settling Characteristics
After 30 minutes of settling following 30 day
testing period
Aerobic
Ozonated
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Summary

• Two, 2-L batch digesters operated in parallel for
  30 and 32 days, respectively
• One, sparged with air and one with O3.
• Higher TS, VS, and TCOD removals were achieved in
  the ozonated digesters.
• Soluble COD concentrations increased during
  digestion for both the aerobic and ozonated
  digesters.

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Major Conclusions 1

• Ozone more effective at reducing TS and VS
• 50-56 for TS and 57-74 for VS ozone
• 23-35 for TS and 40-42 for VS aerobic
• Ozone degraded solids faster than air
• 0.067d-1 and 0.089d-1for aerobic digesters.
• 0.082d-1 and 0.12d-1for ozonated digesters.

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Major Conclusions 2

• Average oxygen required per mg of TVS destroyed
  was 1.89 for the aerobic digesters.
• Average ozone consumption
• 0.57 and 2.6 mg O3 consumed per mg of TS
  destroyed.

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Major Conclusions 3

• SOUR values were below 1.5 mg of O2/g of TS at
  the beginning of study and remained below this
  value.
• Ozonated sludge settled better than did aerated
  sludge however a cloudy supernatant produced.

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Future Work

• Pilot-scale studies (8-10 L)
• Biodegradability of supernatant
• N P characterization in supernatant
• Total fecal coliform reduction studies
• Investigate the effect of pH on degradation rates

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Questions?
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Pathogen Reduction Class A Six Alternatives

• Monitor
• Fecal coliform lt 1000 MPN per gm TS
• Salmonella sp. lt 3 MPN per 4 gm TS
• 1 Thermally Treated Sludge.
• 2 High pH-High Temperature.
• 3 Test for Viruses and Helminth Ova.
• 4 Unknown Sludge Treatment Process.
• 5 Use PFRP Process.
• 6 Use PFRP Equivalent Process

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Pathogen Reduction Class BThree Alternatives

• 1 geometric mean fecal coliform density of 7
  samples lt 2 million CFU or MPN per gm of TS.
• 2 Use PSRP Process.
• Aerobic Digestion
• Air Drying
• Anaerobic Digestion
• Composting
• Lime Stabilization
• 3Use PSRP Equivalent Process.

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Vector Attraction Reduction Eleven Options

• 1 38 VS reduction by aerobic or anaerobic
  digestion.
• 3 additional VS destruction lt 15 after 30 days
  further aerobic digestion.
• 4 SOUR for aerobically digested sludge ? 1.5 mg
  O2 per hr per gm TS.