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


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.


Presentation Outline

  • Background

  • Objectives of Study

  • Materials & Methods

  • Results

  • Summary & Conclusions


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


Bar Racks

and

Screens

Aeration

Basin

Flow

Meter

Secondary

Clarifier

Chlorine

Contact

Basin

Influent

Grit

Removal

Effluent

Return Activated Sludge

Waste

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

Aerobic

Ozonated


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.


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


Questions?


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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