Anaerobic Co Digestion of Municipal Sludge and High ... · has led to adoption of co-digestion of municipal sludge and high-strength ... Retain solids handling capacity (i.e., operate

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Anaerobic Co‐Digestion of Municipal Sludge and High‐Strength Waste

A Path to Net Bioenergy Production

Spyros G. Pavlostathis, PhD, BCEEM, F.IWA, F.WEFSchool of Civil & Environmental Engineering

Georgia Institute of TechnologyAtlanta, GA 30332‐0512

[email protected]

Georgia ASCE Environmental and Water Resources GroupAtlanta, GA

September 18, 2015

S. G. Pavlostathis/GIT 2

Anaerobic Digestion of Complex Organic Material

COMPLEX ORGANIC POLYMERS(Polysaccharides, Proteins, Lipids)

MONO AND OLIGOMERS(Sugars, Amino acids, Peptides, Long-chain Fatty Acids)

PROPIONATE, BUTYRATE, ETC.

1

1

CH4, CO2

H2 + CO2 ACETATE3

4 5

2

1

2

1

Physiological Groups of Microorganisms:1. Hydrolytic, fermentative, acidogenic bacteria2. Hydrogen-producing acetogenic bacteria3. Hydrogen-consuming (= hydrogenotrophic) acetogenic bacteria4. Carbon dioxide-reducing (= hydrogenotrophic) methanogens5. Acetoclastic (= acetotrophic) methanogens


Anaerobic Digestion Model 1(ADM 1)

International Water Association -- Task Group for Mathematical Modelling of Anaerobic Digestion Processes; IWA Publishing, 2002

Death

Complex particulate waste and Inactive biomass

Inert soluble

Sugars Amino acids LCFA

Inert particulate

Carbohydr. Proteins Lipids

CH4

Propionate HVa, HBu

Acetate H2

1 3

5

6 7

4

2

Processes(1) Acidogenesis from sugars(2) Acidogenesis from amino acids(3) Acetogenesis from LCFA(4) Acetogenesis from propionate(5) Acetogenesis from butyrate and valerate(6) Acetotrophic methanogenesis(7) Hydrogenotrophic methanogenesis


Advantages of Anaerobic Treatment

Energy production (CH4)

Low energy consumption (O2 is not required; thus, no O2 transfer limitations; Carbon credit!)

Low biomass (i.e., sludge) production

Low nutrient requirements (e.g., N, P; important for industrial wastes)

Detoxification of xenobiotics


Primary vs. Waste Activated Sludge

Methane Production(m3 @ STP/tonne COD fed):

- PS: 175 – 263- WAS: 123 – 158

Thus, on average:PS/WAS = 1.6

Primary Sludge WAS

ULT

IMAT

E D

IGES

TIB

ILIT

Y (F

ract

ion)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

- Extent of Digestibility & Methane Production


Primary vs. Waste Activated Sludge

Incubation Time (Days)

0 20 40 60 80 100 120 140 160 180

Deg

rada

ble

CO

D D

estr

uctio

n(fr

actio

n)

0.0

0.2

0.4

0.6

0.8

1.0

Primary Sludge Waste Activated Sludge

- Kinetics


Co-Digestion: Rationale & Motivation Domestic wastewater is increasingly viewed as a resource for water,

energy, and nutrients. To increase efficiency and improve the sustainability of municipal

wastewater treatment plants (WWTPs), efforts have focused on reducing energy consumption and/or increasing onsite renewable energy production.

Anaerobic digestion is a major source of energy production at municipal WWTPs via the conversion of sludge (and wastewater) to methane.

Use of methane as a fuel along with combined heat and power (CHP) systems are employed in several municipal WWTPs in the USA (below 10% of those with an influent flow rate large enough to justify CHP systems).

The realization that most municipal anaerobic digesters are underloadedhas led to adoption of co-digestion of municipal sludge and high-strength waste to increase both energy production and digester efficiency.


Gas Composition vs. Mean Oxidation State of Substrate Carbon

(Complete Mineralization; Ignores Microbial Growth)

Mean Oxidation State of Carbon (OS) = 4 - 1.5(COD/TOC)

CH4 (%) = 100 - 12.5 (OS + 4)

Gujer and Zehnder, 1983


Substrate vs. Biomass Yield, Gas Composition

Waste Component

Molecular Formula

ThOD g O2/g

BiomassYield

g VSS/g COD consumed

Gas Composition

CH4, % CO2, %

Carbohydrates C6H12O6 1.067 0.138 48 52

Proteins C16H24O5N4 1.500 0.040 69 31

Fat (Fatty Acids) C16H32O2 2.875 0.030 72 28

Municipal Sludge C10H19O3N 1.990 0.054 70 30


Co-Digestion w/ High-Strength WasteBenefits Bioenergy production (methane) Increased digester efficiency (?)

Constraints Retain solids handling capacity (i.e., operate above a

certain, lower solids retention time) Acceptable lower organic content/strength

Concerns & Attributes w/ High-Strength Waste Toxicity Pathogen type and load Biodegradability (COD and solids destruction) Ease of handling


F. Wayne Hill Water Resources Center; Gwinnett County, Georgia, USA

Plant capacity: 50 MGD (190,000 m3/d) 5 Egg-shaped, mesophilic digesters fed with

primary + WAS (20:80 VS weight ratio) Goal: Co-digestion of municipal sludge-mix and

high-strength waste streams to increase methane production up to a limit based on plant CO2emissions

Bench-scale study performed in conjunction with a 2.1 MWatt combined heat and power project (American Resource Recovery Act, 2009)

Case Study: Municipal Sludge Co-Digestion


F. Wayne Hill WR Center


Selection of High-Strength Wastes1. LES FOG: Gravity dewatered fat-oil-

grease (FOG)2. WriW: Industrial wastewater (mostly

sugars)3. Sludge-mix: Primary:Thickened

Waste Activated Sludge (TWAS), 40:60 on TS basis

4. Publix DAF: Supermarket dissolved air flotation (DAF) skimmings (not used due to toxicity and low strength)

5. Grease trap: Restaurant grease trap waste without dewatering (not used due to low strength)

Total OLR (kg COD/m3-d)

4 5 6 7 8 9 10 11 12

HR

T (d

)

4

6

8

10

12

14

16

18

20

22HSW

Vol

umet

ric L

oadi

ng R

ate

(L/m

3 -d)

0

20

40

60

80

100

120

140

160

1 LES FOG2 WriW3 Sludge-mix4 Publix DAF5 Grease Trap

3

4

5

1

2

345

1

2


Sample Analysis

a Mean ± standard deviation (η = 3); b BDL, below detection limit;c Expressed as glucose; d PS:TWAS, 40:60 on TS basis.

Parameter PS TWASSludge-

mixd FOGIndustrial

Waste

pH 5.6±0.1a 6.4±0.1 5.9±0.1 5.2±0.1 4.5±0.1TS (g/L) 30.5±0.5 59±0.3 42.9±0.4 155±9 120±1VS (g/L) 23.4±0.4 47.2±0.3 33.8±0.4 126±6 119±1VS/TS (%) 77 80 79 81 99Total COD (g/L) 56.9±1.0 79.3±1 66.7±1 254±4 146±4Soluble COD (g/L) 3.1±0.1 0.3±0 1.8±0.1 7±0.1 132±2Total COD/VS 2.4 1.7 2.0 2.0 1.2VFAs (g COD/L) 1.9±0.1 BDLb 1.1±0.1 3.6±0.1 18.3±1Ammonia (mg N/L) 61±3 190±3 117±3 97±2 BDLTKN (mg N/L) 1,878±56 4,892±32 3,194±46 4,357±31 2,145±12Crude protein (g/L) 11.4±0.1 29.4±0.1 19.2±0.1 26±0.1 13.4±0.1Carbohydrates (g/L)c 4.9±0.1 8.7±0.1 6.6±0.1 9.7±0.1 100±1Lipids (g/L) 7.5±0.1 5.7±0.1 6.7±0.1 82.1±0.5 5.9±0.1Phosphorus (mg P/L) 857±22 2,378±23 1,521±22 1,142±3 818±8


Batch Ultimate Degradability Test (103 d)

Parameter PS TWASSludge-

mixFOG

IndustrialWaste

TS destruction (%) 25.0 21.2 24.1 77.8 68.4

VS destruction (%) 46.9 37.0 39.1 79.5 72.8

Total COD destruction (%) 50.4 29.6 36.0 85 64.0

Crude protein destruction (%) 24.7 31.4 47.5 100.0 90.9

Methane (%) 73.4 70.1 69.9 69.9 57.9

Methane-COD/Initial COD (%) 49.2 35.2 40.3 80.7 72.7

TIME (Days)0 20 40 60 80 100 120m

g M

ETH

ANE-

CO

D/m

g In

itial

CO

D

0.0

0.2

0.4

0.6

0.8

1.0PS TWAS Sludge-mix

IndWFOG

Initial loading- PS, TWAS, Sludge-mix = 4.5 g COD/L- Industrial Waste = 2.25 g COD/L

Tandukar and Pavlostathis; Water Research (in press)


Anaerobic Degradation of Lipids

Processes(1) Acidogenesis from glycerol(2) Acetogenesis from LCFA(3) Acetogenesis from propionate(4) Acetogenesis from butyrate

and valerate(5) Acetoclastic methanogenesis(6) Hydrogenotrophic methanogenesis

Glycerol LCFAs

CH4 + CO2

Propionate HVa, HBu

Acetate H2

1 2

4

5 6

3

Lipids

Hydrolysis


Methane Production vs. FOG/Sludge Loading

COD LOADING (mg/L)

0 2000 4000 6000 8000 10000 12000

MET

HAN

E (m

g/L)

0

2000

4000

6000

8000

10000

12000

Slope = 0.439 ± 0.012 (R2 = 0.997)

Slope = 0.977 ± 0.0001(R2 = 1.000)

Total COD

Sludge-Mix CODFOG COD

Slope = 1.192 ± 0.013(R2 = 0.999)

EnhancementM

ETH

AN

E (m

g C

OD

/L)


Enhanced Sludge COD & VS Destruction

FOG COD RATIO (%)

0 10 20 30 40 50 60 70 80

ENH

ANC

ED C

OD

DES

TRU

CTI

ON

(%)

0

2

4

6

8

10

12

FOG VS RATIO (%)

0 10 20 30 40 50 60 70 80

ENH

ANC

ED V

S D

ESTR

UC

TIO

N (%

)

0

2

4

6

8

10

12

SOLIDS RETENTION TIME (Days)

0 2 4 6 8 10 12 14 16 18 20 22

CO

D D

ESTR

UC

TIO

N (%

)

0

5

10

15

20

25

30

35

40

45

50

25%

56%

40%

Activated SludgeUltimate Biodegradability


Continuous-Flow Testing ConditionsParameter

Digester B(Control)

Digester C Digester DDigester E

Run 1 Run 2 Run 1 Run 2 Feed type SMa SM+IndW SM+IndW SM+FOG SM+FOG SM+IndW+FOG

HRT/SRT (d) 15.0 14.1 15.0 15.0 15.0 15.0OLR (g COD/L·d)b

Sludge-mixIndWFOGTotal

4.45----

4.45

4.450.58--

5.03

4.390.12--

4.51

4.12--

1.275.39

4.36--

0.364.72

4.300.120.364.78

OLR (%)Sludge-mixIndWFOG

100----

88.511.5--

97.32.7--

76.4--

23.6

92.4--7.6

90.02.57.5

a SM, sludge-mix;b OLR, organic loading rate


Reactors Performance



Reactors Performance

ParameterDigester B(Control)

Digester C Digester DDigester E

Run 1 Run 2 Run 1 Run 2

Feed type SMa SM+IndW SM+IndW SM+FOG SM+FOGSM+IndW

+FOGBiogas production (mL/L-d) 891±95b 1,269±41 946±51 1,830±163 1,123±39 1,166±27Methane content (%) 65.3±2 65.2±2 66.1±2 68.9±2 67.6±2 66.8±3Methane production (mL @23oC/L-d) 582±1.9 827±0.1 625±0.8 1,261±3.1 759±0.6 779±0.7COD to methane conversion (%) 34.5 43.4 36.5 61.8 42.5 42.9Biogas needed to fulfill the plant energy deficit (mL@23oC/L-d)

1,233

Methane needed to fulfill the plant energy deficit (mL@23oC/L-d)

801

Fraction of biogas deficit fulfilled (%) 72.3 102.9 76.7 148.4 91.1 94.6

Fraction of methane deficit fulfilled (%) 72.7 103.2 78.0 157.4 94.8 97.3

a SM, sludge-mix; b Mean ± standard deviation (n = 3)



Enhanced COD Destruction/CH4 Production


Enhancement


Filtrate Quality

SampleSoluble

COD(mg/L)

Ammonia(mg N/L)

Total P(mg/L)

Sludge-mix (feed)a 1,800±100b 131±4 53±2

Digester B effluent 447±69 1,067±23 46±4

Digester C effluent; Run 2 502±46 1,005±36 38±1

Digester D effluent; Run 2 469±11 1,112±41 41±2

Digester E effluent 437±91 1,010±21 39±1

a40:60 mixture of PS and TWAS on a TS basis;bMean ± standard deviation (n = 2) Tandukar and Pavlostathis;

Water Research (in press)


Time-to-Filter – Influent vs. EffluentConditions:

Whatman #1 filterSample volume = 10 mL60 to 64 cm Hg vacuumRoom temperature (22 to 24oC)

TIME (Min)

0 10 20 30 40 50

FILT

RAT

E VO

LUM

E (m

L)

0

1

2

3

4

5

6

BC

DE

Sludge-mix (Feed )

Digester Effluent

Filtrate volume =50% of Sample Volume



Case Study – Conclusions Co-digestion of sludge-mix with Industrial Wastewater, FOG or

both: Increased methane production by at least 2X and was able

to generate biogas and methane very close to target values necessary to fulfill the plant energy deficit.

Enhanced the destruction of sludge COD and volatile solids (VS) by as much as 10.9%.

Resulted in an effluent quality very similar to that of the control digester fed only with the municipal sludge-mix.

Did not affect the digested sludge dewaterability (based on time-to-filter results).

Net annual cost savings estimated to be $300,000 at start-up flow (30 MGD; 114,000 m3/d) and $700,000 at full design capacity (60 MGD; 228,000 m3/d)(2012 basis).


Overall Conclusions High methane production can be achieved by the co-

digestion of municipal sludge with increased high-strength waste loading as long as the digester retention time, as well as the COD and VS destruction do not drop below acceptable lower values.

Overall digestion system performance, along with plant-wide materials and permit constraints, should be considered in order to benefit from the trade-off between energy production and solids destruction to meet the energy needs of a wastewater treatment facility.

Pre-screening and evaluation of potential toxicity, ultimate biodegradability, and methane production potential of high-strength wastes is necessary to avoid unnecessary complications and realize the benefits of co-digestion for net-zero energy treatment plants or even net bioenergy production.


Other Success Stories…..

M. BufeWater Environ. & Technol. January 2012, 19-21

East Bay MUD Study: 120 m3 methane/ton of sludge at 15 d SRT; 367 m3 methane/ton of food waste at 10 d SRT. Anaerobic digestion of 100 tons of food waste per day, five days a week, provides sufficient power for approximately 1,000 homes.http://www.epa.gov/region9/waste/features/foodtoenergy/ebmud-study.html


Acknowledgements This study was financially supported by the Department of

Water Resources, Gwinnett County, GA, through a federal grant based on the American Resource Recovery Act, administered by the Georgia Environmental Facilities Authority's “Green Project Reserve” program.

Special thanks to Dr. M. Tandukar (Research Associate), R. Porter, H. Elmendorf, S. Jalla, T. Richards (Gwinnett County) and S. Hardy, R. Latimer (Hazen and Sawyer, PC; Atlanta, GA).

For further information contact: S. G. PavlostathisE-mail: [email protected] Website: http://www.ce.gatech.edu/people/faculty/961/overviewPersonal Website: http://people.ce.gatech.edu/~spavlost/

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Anaerobic Co Digestion of Municipal Sludge and High ... · has led to adoption of co-digestion of municipal sludge and high-strength ... Retain solids handling capacity (i.e., operate