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Parallel Bench-Scale Digestion Studies

Richard O. Mines, Jr.Laura W. LackeyMercer University Environmental EngineeringMitchell MurchisonBrett 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

GritRemoval

Bar Racks and

Screens

FlowMeter

AerationBasin

SecondaryClarifier

ChlorineContactBasin

EffluentInfluent

WasteActivatedSludge

Return Activated Sludge

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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

1. Evaluate the reduction of Total Solids and Volatile Solids in aerobic versus ozonated digesters.

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

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Objectives of Study

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

4. 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 @ 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.

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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 @ 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.

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Results: Aerobic DigestionIncreased O2 Loading 77%

OxygenApplied perMass of TS

TSRemoval

VSRemoval

1.84 g O2 /mg TS 23% 42%

3.25 g O2 /mg TS 35% 40%

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Results: Ozonation Increased O3 Loading 67%

OzoneApplied perMass of TS

TSRemoval

VSRemoval

1.20 mg O3/mg TS 50% 57%

2.00 mg O3/mg TS 56% 74%

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Solids Degradation Rate: Phase I

O3 KD= 0.12 days-1

y = -0.1106x + 9.0147

R2 = 0.9249

Air KD = 0.067 days-1

y = -0.0665x + 7.7547

R2 = 0.6548

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0 5 10 15 20 25 30

Digestion Time, days

LN

(Deg

rad

able

TS

)

Air O3 Linear (O3) Linear (Air)

Phase I

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Solids Degradation Rate:Phase II

O3 KD = 0.082 days-1

y = -0.0822x + 8.7081

R2 = 0.9381

Air KD = 0.089 days-1

y = -0.0898x + 8.2928

R2 = 0.9888

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

0 5 10 15 20 25 30

Digestion Time, days

LN

(Deg

rad

able

TS

)

Air O3

Phase II

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Solids Degradation Rates KD

Air 0.067 d-1 0.089 d-1

Increased O2

77%1.3 KD

Ozone 0.082 d-1 0.12 d-1

IncreasedOzone67%

1.5 KD

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Oxygen Consumed: Aerobic Digestion

LdigesterofVolumeV

LmgdigestionofendandstartsolidsvolatileTotalTVSTVS

LmgdigestionofendandstartCODTotalTCODTCOD

VTVSTVS

VTCODTCOD

TVS

ConsumedO

to

to

to

to

Destroyed

,

/,@&

,@&

2

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Oxygen Utilized per TVS Destroyed

DigesterOxygen Utilized

per TVSDestroyed

Aerobic: Phase IAerobic: Phase II

1.302.47

Average 1.89

EPA Manual 1.74 – 2.07 lb oxygen per lb of cell mass oxidized.

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Ozone Consumed:

LdigesterofVolumeV

lyrespectiveIIandIPhaseandRatenUtilizatioO

LmgdigestionofendandstartsolidsTotalTSTS

LmgdigestionofendandstartCODTotalTCODTCOD

VTSTS

RatenUtilizatioOAppliedO

TS

ConsumedO

to

to

toDestroyed

,

.,241.0303.0

/,@&

,@&

3

333

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

Phase Ozone Applied

Ozone Utilized per

TSDestroyed

II 1.20 mg O3 /mg TS

0.57 mg/mg

I 2.00 mg O3 /mg TS

2.6 mg/mg

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Total COD Removals

Air 30% 40%IncreasedApplied

O2

77%

Ozone 58% 57%IncreasedAppliedOzone67%

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Total COD Concentration: Phases I and II

0

2000

4000

6000

8000

10000

12000

14000

16000

0 5 10 15 20 25 30 35

Digestion Time, days

To

tal C

OD

, mg

/L

Air - I Air - II O3 - I O3 - II

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Soluble COD Concentration:Phases I and II

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35

Digestion Time, days

So

lub

le C

OD

, mg

/L

Air - I Air - II O3 - I O3 - II

4.2–9.5 mg sCOD/g TS destroyed aerated

120–146 mg sCOD/g TS destroyed ozonated

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pH: Phases I and II

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35

Digestion Time, days

pH

Air pH - I

Ozone pH - I

Air pH - II

Ozone pH - II

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SOUR: Phases I and II

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 5 10 15 20 25 30 35

Digestion Time, days

SO

UR

, mg

O2/h

r-g

of

TS

SOUR - I SOUR - 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 kd=0.05 d-1

38 % VS destruction $1.46 per lb ozone;$0.10 per kWH

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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

dayOlbday

TSlb

destroyedTSlb

Olb/2798$

46.1$336257.0

3

3

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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

daykWHday

hr

HP

kWHP /200$

10.0$24

112

746.0112

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Settling Characteristics

Aerobic Ozonated

After 30 minutes of settling following 30 day testing period

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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 < 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

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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.

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Vector Attraction Reduction: Eleven Options

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

3: additional VS destruction < 15% after 30 days further aerobic digestion.

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