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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

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Parallel bench scale digestion studies

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

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

Where is sludge produced wwtp

Bar Racks
















Return Activated Sludge




Where is sludge produced? WWTP

Aerobic digestion

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

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

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

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

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

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 study1

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 both phases

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 o 3 collection in off gas

Materials and Methods: O3 Collection in Off-Gas

Bench scale aerated and ozonated digester

Bench-Scale Aerated and Ozonated Digester

Materials and methods phase i

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

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 digestion increased o 2 loading 77

Results: Aerobic DigestionIncreased O2 Loading 77%

Results ozonation increased o 3 loading 67

Results: OzonationIncreased O3 Loading 67%

Solids degradation rate phase i

Solids Degradation Rate: Phase I

Solids degradation rate phase ii

Solids Degradation Rate:Phase II

Solids degradation rates k d

Solids Degradation Rates KD

Oxygen consumed aerobic digestion

Oxygen Consumed: Aerobic Digestion

Oxygen utilized per tvs destroyed

Oxygen Utilized per TVS Destroyed

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

Ozone consumed

Ozone Consumed:

Mg ozone utilized per mg ts destroyed increased o 3 67

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

Total cod removals

Total COD Removals

Total cod concentration phases i and ii

Total COD Concentration: Phases I and II

Soluble 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

pH: Phases I and II

Sour phases i and ii

SOUR: Phases I and II

Operating cost comparison

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

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

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

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

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

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

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

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 a six alternatives

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 b three alternatives

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

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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