Parallel bench scale 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. Acknowledgements.

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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


  • 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.

Presentation Outline

  • Background

  • Objectives of Study

  • Materials & Methods

  • Results

  • Summary & Conclusions


  • 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

Bar Racks
















Return Activated Sludge




Where is sludge produced? WWTP

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.

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.

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.

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.

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.

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.

Objectives of Study

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

  • Determine quantity of ozone required to destroy Total Solids.

Materials and Methods: BothPhases

  • 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).

Materials and Methods: O3 Collection in Off-Gas

Bench-Scale Aerated and Ozonated Digester

Materials and Methods: Phase I

  • Two, 2-L batch digesters were operated in parallel for 30 days.

  • Aerobic digester supplied with air @ 2.7 Lpm or 810 mg O2/min: 1.84 g O2/mgTS.

  • Ozonated digester supplied with air ladened with O3 @ a rate of 6.5 Lpm or 0.88 mg O3/min: 2.0 mg O3/mgTS.

Materials and Methods: Phase II

  • Two, 2-L batch digesters were operated in parallel for 32 days.

  • Aerobic digester supplied with air @ 4.0 Lpm or 1200 mg O2/min: 3.25 g O2/mg TS.

  • Ozonated digester supplied with air ladened with O3 @ a rate of 3.25 Lpm or 0.44 mg O3/min: 1.2 mg O3/mg TS.

Results: Aerobic DigestionIncreased O2 Loading 77%

Results: OzonationIncreased O3 Loading 67%

Solids Degradation Rate: Phase I

Solids Degradation Rate:Phase II

Solids Degradation Rates KD

Oxygen Consumed: Aerobic Digestion

Oxygen Utilized per TVS Destroyed

EPA Manual1.74 – 2.07 lb oxygen per lb of cell mass oxidized.

Ozone Consumed:

mg Ozone Utilized per mg TS Destroyed: Increased O3 67%

Total COD Removals

Total COD Concentration: Phases I and II

Soluble COD Concentration:Phases I and II

4.2–9.5 mg sCOD/g TS destroyed aerated

120–146 mg sCOD/g TS destroyed ozonated

pH: Phases I and II

SOUR: Phases I and II

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 kd=0.05 d-1

  • 38 % VS destruction

  • $1.46 per lb ozone;$0.10 per kWH

Operating Cost Ozonation

  • 13,010 ppd TSS produced @68% VS

  • $1.46 per lb ozone;$0.10 per kWH

  • 38% VS destroyed resulting in 3362 ppd TS destroyed

Operating Cost Aerobic Digestion

  • 38% VS destroyed resulting in 3362 ppd TS destroyed

  • 2.0 lb O2/lb VS destroyed;$0.10 per kWH

  • STOR = 2.5 lb O2/HP-hr; 112 HP aerator

Settling Characteristics

After 30 minutes of settling following 30 day testing period




  • 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.

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.

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.

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.

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


Pathogen Reduction: Class ASixAlternatives

  • Monitor:

    • Fecal coliform < 1000 MPN per gm TS

    • Salmonella sp. < 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

Pathogen Reduction: Class BThree Alternatives

  • 1: geometric mean fecal coliform density of 7 samples < 2 million CFU or MPN per gm of TS.

  • 2: Use PSRP Process.

    • Aerobic Digestion

    • Air Drying

    • Anaerobic Digestion

    • Composting

    • Lime Stabilization

  • 3:Use PSRP Equivalent Process.

Vector Attraction Reduction: Eleven Options

  • 1: 38% VS reduction by aerobic or anaerobic digestion.

  • 3:additionalVS destruction < 15% after 30 days further aerobic digestion.

  • 4:SOUR for aerobically digested sludge  1.5 mg O2 per hr per gm TS.

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