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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.