Session 1

Anaerobic digestion of sewage sludge

Jaime L. Garca de las Heras

2A - FUNDAMENTALS

3The anaerobic process

4The anaerobic process

A complex system Simultaneous biochemical transformations by bacteria Set of chemical compounds degraded by different groups

of microorganisms Reactants => Products / metabolites

Substrates for other microorganisms Some of them are inhibitors

=> Pool of microorganisms living in their substrates

A liquid environment where physico-chemical transformations occur physico-chemical variables affect the biochemical processes

5Bacterial growth in a substrate

Substrate

Biomass

Products

GROWTH

DECAY

Substrate : S Biomass (bacterial population) : X

6Insoluble organics

Soluble organics

CH4CO2

Volatile acidsH2

Hydrolysis

Acid formation

Methanogenesis

7Stoichiometry

8Mass balanceChemical reactions* (100% mass)

Reactants => Products + Energyaerobic Glucose + O2 => CO2 + Energy anaerobicGlucose => Butirate + Propionate + Acetate + H2

Mass distributionSubstrate depleted=> Biomass growth + Energy*

9Aerobic processExternal electron acceptor (O2)

10

Anaerobic processNo external electron acceptor

CH3CH2COOH => CH3COOH + H2

Electrons

Scheme chemical reaction Process

Electron acceptor

H+ in VFA H+ + e- ==> H2

+1 0 Anaerobic oxidation of VFA

Electron donor

C in VFA and LCFA C ==> CH3COOH + e-

-2n/(2+n) 0 Anaerobic oxidation of VFA

11

Oxidation state of Carbon

Entalpy of reactionRespiration quotientEnergy produced/Oxigen consumedTheoretical CODCOD/VS ratioSpecific methane potentialMethane content (%) in biogas

12

Estados de oxidacin (1) El estado de oxidacin (EO) de un elemento en un compuesto se mide

mediante el n de valencia o n de oxidacin (NO): NO: n de electrones ganados por el elemento (elemento se reduce: EO

negativo) o perdidos (elemento se oxida: EO positivo) al pasar a formar parte del compuesto /Babor pg 456/

En compuestos hidrogenados el EO del H es+1. En compuestos oxigenados el EO del O es -2

N => NH3 N + 3 e- => N-3 , EON = -3N => NO3 N => N+5 + 5 e- EON = +5

En los compuestos orgnicos ... el EO de los elementos es fijo, salvo para el C H+1,, O-2,, N-3,, P+5

Conocidos los EO de los elementos del compuesto, el estado de oxidacin del C se obtiene por restaPropionato C3H6O2 ,, EOC = - (6*1 2*2)/3 = - 2/3

13

Estados de oxidacin (2)

Theoretical Oxigen Demand (ThOD) es similar pero en peso de Oxgeno.

Ejemplo: Hidrgeno

Oxidado H0 => H+ + 1 e-Oxidante O 0 + 2 e- => O -2--------------------------------------

2 H0 + O 0 => H2O

Cada electrn cedido equivale a 8 g de ThOD demandados

ThOD del H0 = +8 g de ThOD / mol H

14

Theoretical Oxigen Demand

/Ref ASM3 IWA 2000/

Element or charge Z State of reference Equivalent ThOD C Carbon CO2 + 32 g ThOD (mol C) -1 H Hydrogen H2O +8 g ThOD (mol H) -1 O Oxygen O2 -16 g ThOD (mol O) -1 N Nitrogen NH4+ -24 g ThOD (mol N) -1 P Phosphorous PO43- +40 g ThOD (mol P) -1 - Negative charge Zero charge +8 g ThOD (mol (-)) -1 + Positive charge Zero charge -8 g ThOD (mol (+)) -1

H2O

S Sulfur SO4-2 +48 g ThOD/mol S

15

Calculation of Chemical Oxygen Demand ratio (1)ThOD/VS

ThODcompuesto = (Mi ThODi)

Mi moles del elemento i en el compuestoThOD i ThOD del elemento i MW Peso molecular del compuesto

COD/VS = ThODcompuesto/MW

Exercise

Calculation of Chemical Oxygen Demand ratio(ThOD/VS) for Glucose (C6H12O6)

16

17

Calculation of Chemical Oxygen Demand ratio (2)COD/VS

18

Calculation of Chemical Oxygen Demand ratio (3)COD/VS

CnHaOb + (...)O2 => (...)CO2 + (...) H2OCOD/VS = 8 (4n+a-2b) / (12n+a+16b)

CnHaObNd + (...)O2 => (...)CO2 + (...) H2OCOD/VS = 8 (4n+a-2b-3d) / (12n+a+16b+14d)

Cn Ha Ob Nd + x H2O => y CH4 + z CO2 + d NO3

19

Calculation of Specific Biological Methane Potential (1) Theoretical BMP (Nl CH4/ gVS)

/Buswell/

a + 2x = 4yb + x = 2zn = y + z

Cn Ha Ob + x H2O => y CH4 + z CO2

20

Calculation of Specific Biological Methane Potential (2) Theoretical BMP

CnHaObNd + [n-(a/4)-(b/2)+(3d/4)] H2O =>=> [n+(a/4)-(b/2)-(3d/4)]/2 CH4 + [n-(a/4)+(b/2)+(3d/4)]/2 CO2

%CH4 = 100 {[n+(a/4)-(b/2)-(3d/4)]/2 }/n

%CO2 = 100 {[n-(a/4)+(b/2)+(3d/4)]/2 }/n

/Buswell/

Cn Ha Ob Nd + x H2O => y CH4 + z CO2 + d NH4

21

Stoichiometric 0.35 Nm3CH4 / kg CODremoved

Buswell

C oxidation stateThODCH4 = 132 + 48 = 64 gCOD/molCH41 mol 22,4 liters of gas22,4 l gas / 64 g = 0,35 l CH4/g CODremoved

22

CnHaObNd

Specific Biological Methane PotentialTh SBMP (Nl CH4/g VSrem) = 0.35COD/VS*

%CH4 Buswell => 100 {[n+(a/4)-(b/2)-(3d/4)]/2 }/ n

from ThOD* =>COD/VS MW/64 1/n 100 ??

Exercise

23

24

Name Sugars Amnoacids LCFatty acidsFormula C6H12O6 C4H6.1O1.2N

x C16O2H32COD / VS 1,07 1,53 2,88C ox. st. 0,00 -0,18 -1,75ThSBMP 0.37 0.53 1.01

Nm3/kg VS

=0.35 COD/VS

25

Calculation of biogas potential %CH4

26

More oxidation degree (state) Less COD

y = -1,064x + 1,1867R2 = 0,8996-2,0

-1,5

-1,0

-0,5

0,0

0,5

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

COD / VS

C

o

x

i

d

a

t

i

o

n

s

t

a

t

e

Carbohidrates

Proteins

Fats

27

Mayor grado (estado) de oxidacin del C Less methane content in biogas

Porcentaje de metano en el biogas

Sugars

Acetate

Carbohydrates

Biomass

Proteins/Aminoacids

PropionateWaste*

ButyrateValerate

Lipids

Fatty Acids

0

10

20

30

40

50

60

70

80

-2 -1.5 -1 -0.5 0

Estado de Oxidacin del Carbono

%

C

H

4

28

% CH4 en el biogs vs relacin DQO/SV

Porcentaje de metano en el biogas

F

a

t

t

y

A

c

i

d

s

L

i

p

i

d

s

V

a

l

e

r

a

t

e

B

u

t

y

r

a

t

e

W

a

s

t

e

*

P

r

o

p

i

o

n

a

t

e

P

r

o

t

e

i

n

s

/

A

m

i

n

o

a

c

i

d

s

B

i

o

m

a

s

s

C

a

r

b

o

h

y

d

r

a

t

e

s

A

c

e

t

a

t

e

S

u

g

a

r

s

0

10

20

30

40

50

60

70

80

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

ThOD/SV

%

C

H

4

Produccin de metano vs relacin DQO/SV

29

30

Reaction kinetics

31

Kinetic expressions

Disintegration First order in Xcomposites Hydrolysys First order in particulate polymers The others:

Km Monod Activation Inhibition Xbiomass Monod: S/(Ks+S) Activation: So/(Ko+So) Inhibition: Ki/(Ki+Si)

(Valerate, butirate Km Monod Competition)

Decay First order in Biomass

32

Factors affecting kinetics

Temperature pH Inhibition

33

experimental

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50

Temperature ( C)

(

r

a

t

T

)

/

(

O

p

t

i

m

u

m

r

)

34

Influence of Temperature on Km, Ks, Kdecay

/van Lier 1997/

35

Arrhenius + decrease

)TT(Krr opopT = for TTop (2.22)

)TT(CoT oerr

=

Tmax: Temperature at which there is no microbial growth Top: Temperature of optimum growth K: Coefficient rop: Optimum reaction rate (at Top)

Traditional Arrhenius

rT mesfila (To=308)rT=rTo*e**C(T-T0)

0

2

4

6

8

10

0 10 20 30 40 50

T-273

r

T

rT vs T (TTop)

36

20 25 30 35 40 45 50 55 60 65 70

Temperatura (C)

k

m

(

T

)

k m (35C)

k m (55C)

22 )15.328(01.0)C55(

)15.308(01.0)C35( )(

+= TmTmm ekekTk

(van Lier et al., 1997)

37

Influence of pH

Factor to correct reaction rate

0

0,2

0,4

0,6

0,8

1

1,2

5 5,5 6 6,5 7 7,5 8 8,5 9

pHUL = 7,5pHLL = 6,5

)pHpH()pHpH(

)pHpH(5.0

LLUL

ULLL

101011021I

+++=

38

Influence of inhibition

Non Competitive - Constant concentration of Inhibitor

Substrate concentration

Spec

ific

grow

th r

ate

( )

Inhibitor concentration = I1)

KI1()

SK

1(

1

is

max++

=

39

22,

22

ooA

oo SK

SA += Activacin por oxgeno

44,

44

nhnhA

nhnh SK

SA += Activacin por nitrgeno inorgnico

44,

44

hpohpoA

hpohpo SK

SA += Activacin por fsforo inorgnico

22,

22

cocoA

coco SK

SA += Activacin por carbono inorgnico

40

22,

2,2

ooI

oIo SK

KI += Inhibicin por oxgeno

2,2,

,2,,2

hfahI

fahIfah SK

KI += Inhibicin por hidrgeno de la acetognesis de los cidos grasos

24,2,

4,2,4,2

hchI

chIch SK

KI += Inhibicin por hidrgeno de la acetognesis de los cidos valrico y butrico

2,2,

,2,,2

hprohI

prohIproh SK

KI += Inhibicin por hidrgeno de la acetognesis del cido propinico

33,

3,3

nhnhI

nhInh SK

KI += Inhibicin por amoniaco

22,,

2,,

,++

=haahI

aahIaapH SK

KI Inhibicin por pH de las transformaciones

de acidognesis y acetognesis

22,,

2,,

,++

=hachI

achIacpH SK

KI Inhibicin por pH de la metanognesis

acetoclstica

222,,

22,,

2,++

=hhhI

hhIhpH SK

KI Inhibicin por pH de la metanognesis

hidrogenfila

41

Physicochemical processes

42

Gas absorption/desorption.Acid/Base

Precipitation

43

Importance

The acid-base systems Buffer capacity => process stability Inhibition due to

pH (prediction of H+) free form of acids (VFA) = f(pH) soluble gas in liquid phase: soluble free NH3 = f(pH)

Transfer from gas state to liquid state Gas flow (CO2) Partial Alkalinity (HCO3)

44

Most common acid / base pairs in A.D.

Acid Base + H+

CO2 HCO3- + H+NH4 NH3 + H+HAc Ac- + H+HPro Pro- + H+HBu Bu- + H+HVa Va- + H+

45

Acidity constant

Three main acid-base pairs (buffer systems) NH4+ / NH3 pKa = 9.3 CO2 / HCO3- pKa = 6.3 HVFA / VFA- pKa = 4.8

(SCO2 liq >> SH2CO3)

Algebraic expressions as a function of pH

Ka = Sbase SH+ / Sacid

Sacid + Sbase = Stotal __________________________________

Sacid = Stotal SH+ / (Ka + SH+)

Sbase = Stotal Ka / (Ka + SH+)

46

Acid Base + H+

CO2 HCO3- + H+ NH4 NH3 + H+ HAc Ac- + H+ HPro Pro- + H+ HBu Bu- + H+ HVa Va- + H+

47

% NH3 & % NH4 VS pH

0

20

40

60

80

100

6 6,5 7 7,5 8 8,5 9 9,5

pH

%

% NH3 % NH4

48

% HCO3 & % CO2 VS pH

0

20

40

60

80

100

4 4,5 5 5,5 6 6,5 7 7,5 8

pH

%

%HCO3 %CO2

49

% ac - & % HAC VS pH

0

20

40

60

80

100

3 3,5 4 4,5 5 5,5 6 6,5 7

pH

%

% ac - HAC

50

0

20

40

60

80

100

6 6,5 7 7,5 8 8,5 9 9,5

%

pH

% NH3 & % NH4 VS pH

% NH3 % NH4% HCO3 & % CO2 VS pH

0

20

40

60

80

100

4 4,5 5 5,5 6 6,5 7 7,5 8

pH

%

%HCO3 %CO2

% ac - & % HAC VS pH

0

20

40

60

80

100

3 3,5 4 4,5 5 5,5 6 6,5 7

pH

%

% ac - HAC

51

Acid/Base dynamic equilibrium

NH4 + NH3 + H+

dSNH4+/dt=R2-R1 R1=K1SNH4+ R2=K2SNH3SH

R1

R2

52

Liquid to Gas transfer

If dilute soluble gas concentration (Sliq,i), Henry law applies for equilibrium

Liquid => gas transfer : two film theory

: Specific mass transfer rate of gas i (Kg or Kmole/m3d) KLa = (D/d) * (A/V) :Volumetric mass transfer coefficient (d-1)

Overall mass transfer coefficient (D/d) Specific transfer area (A/V)

Sliq,i : Soluble gas i concentration in the bulk liquid KH : Henry constant (Mbar-1) pgas,i : Partial pressure of gas i

GAS ) pK - a(Sk igas,Hiliq,LiT, =

LIQUID

GASiT,

53

B REACTOR DESING

TYPE OF REACTORS

54

55

Reactor types

CSTR Batch

Plug flow Contact

Completely (Continuous) Stirred Tank Reactor - CSTR Contact Reactor

Continuous Complete mix

Concentration change instantly Uniform concentration

56

Anaerobic Filter Upflow sludge reactor

57

Upflow Anaerobic Sludge Bed Reactor - UASB Anaerobic Hybrid Reactor

58

CSTR Anaerobic Digester

59

60

DESING CRITERIA

61

NomenclatureHRT Hydraulic retention time Tiempo de retencin hidrulico Volumetric loading HRT = VR/Q0 (days) OLR BV,COD Organic loading rate Carga orgnica volumtrica Velocidad de carga orgnica Organic loading Organic volumetric loading OLR = Q0S0/VR (Kg Substrate/m3day)

Qo: Influent flow rate (m3/d) So: Influent substrate concentration

(kgS/m3) VR: Reactor volume (m3) XBH: Biomass concentration

(kgBH/m3)

62

Nomenclature F/M BX,COD Food to microorganism ratio Carga msica (fangos activos) Sludge loading F/M = Q0S0/VRXBH (Kg Substrate/Kg Biomassday) SRT Sludge retention time Cell retention time Tiempo de retencin celular Sludge age SRT = VRXBH/QwXBw (days)

Qo: Influent flow rate (m3/d) So: Influent substrate concentration

(kgS/m3) VR: Reactor volume (m3) XBH: Biomass concentration

(kgBH/m3)

63

Relationships

OLR (ds/dt)utilizatoin

FM K = Km S/(ks+S)

64

Desing criteria for CSTR

HRT criteria

HRTdesign = VR/Qo Qo

65

Insoluble organics

Soluble organics

CH4CO2

Volatile acidsH2

Hydrolysis

Acid formation

Methanogenesis

Simple model

K1

K2

K3

66

A) Hydrolysis as limiting step

if K1 Efficiency hydrolysis = f(HRT)=> Total methanification of solubilized matter

Mass balance in Xc:Xc = Xco/(K1 HRT + 1)Efficiency = K1 HRT / (K1 HRT + 1)HRT = f(efficiency )

67

B) Bacterial wash-out at low HRT

and HRT < HRTcritical=> Bacterial wash-out

Mass balance in XBH:HRTcritical = 1/(m Kdecay)

68

C REACTOR OPERATION

69

Performance parameters

Organic matter removal

Solubilization efficiency Removal of particulate organic matter (%)

VSS Particulate COD

(Total) Organic matter removal efficiency (%) VS Total COD (COD)*

Methanification* = %COD transformed in CH4 Specific Methane production

Nm3/kg VS fed (related to OM removal efficiency) Stoichiometry Nm3CH4/kg COD removed

(Acidification = CH4 + VFA produced in %)

70

Performance parameters

Effluent quality

Dissolved COD VFA Soluble hydrolized organics (su, aa, lcfa)

(Dissolved TOC) Ammonia Phosphorous

71

Performance ratios

COD/VS ratio %CH4 in the biogas

Alkalilnity AI/AP,, AI/AT

Carbon oxidation stateBuswelCOD-SV.ppt

Acid/base systems Buffer capacity

72

Parameters of digester stability

Parameters of stability

Acetate Propionate VFA Intermediate Alcalinity Partial Alcalinity AI/AT AI/AP.................. %CH4 pH

Early warningparameters

73

Total Alk

Volatile acids

%CH4

pH

VFA/Total alkalinity

%CO2

ppm CO ??

ppm H2

74