Introduction into Digester

Biology

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Workshop for international experts

Oberschleißheim, 12 October 2015

Katrin Kayser

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Conversion process…

… dependent on efficiency factors!

- Efficiency factor of the digesterProductivity [m³ CH4/(m³RV d)]

- Efficiency factor of the inputsYield [m³ CH4/t oTS]

- Electrical efficiency factor [%]

- Thermal efficiency factor [%]

2. physical1. biochemical

CHP

biogasOrganic

matter

Digester

Electricity

Heat

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Principles of the biogas process

1 The biogas process

2 Environmental conditions

3 Engineering process parameter

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The 4 stages of the fermentation gas formation

Acidification Biogas formation

Hydrogen H2

Methane CH4

Carbon dioxide

CO2

1. Stage

Hydrolysis

2. Stage

Acidogenesis

3. Stage

Acetogenesis

4. Stage

Methanogenesis

acetic acid H2

Hydrolytic

bacteria

Acidogenic

bacteria

Acetogenic

bacteria

Methanogenic

bacteria

Biomass

Polysaccharides

Proteins, Fats

Acetic acid

H2

Sugar,

Amino acids,

fatty acids

H2/CO2

Biogas

CH4/CO2

Fatty acids

(propionic acid)‏

alcohols

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Generation time of different bacteria

Anaerobe Microorganisms

Acid producing bacteria

Methanogenic bacteria

Bacterioides < 24 h.

Clostridien 24 - 36 h.

Acetogenic bacteria 80 - 90 h.

Methanosarcina barkeri 5 - 15 d

Methanococcus ca. 10 d

Aerobe microorganisms Escherichia coli 20 Min.

Activated sludge bacteria 2 h.

!

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The different degradation processes…

• … occur at the same time simultaneously In agricultural biogas plants the separation of the degradation stages plays a

minor role

• … strongly depend on each otherIntermediate products are needed for following processes

• … can cause mutual inhibition ▪ Intermediate products may not accumulate ▪ Product inhibition

• … develop slowly in advanced stages Hydrolysis hydrolysis is the fastest, methane formation the slowest

one-stage process

biogas digestate

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Environment conditions of anaerobic degradation

W0104124CDREnvironmental requirements for the fermentation of raw – and residual

substrates

FAL – TB

Essential: Ni, Co, Mo, SeNo specific requirementsTrace elements

600 : 15 : 5 : 3500 : 15 : 5 : 3Nutrient demand

C:N:P:S

< -250 mV+400 – 300 mVRedox - potential

< 30 % DM< 40 % DMSolid content

20 - 3010 - 45C:N-Relation

6,7 – 7,55,2 – 6,3pH value

Mesophilic: 32 – 42 ºC

Thermophilic: 50 – 58 ºC

25 – 35º CTemperature

Methane formationHydrolysis/acidificationMeasured variable

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The most important requirements

for the biogas process

• Temperature

• pH-value

• Salt content

• Trace elements

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Temperature ranges

• Psychrophil (< 25 °C)

low growth rate long retention times

inefficient for biogas production no longer in use

• Mesophil (32 - 45 °C)

stable biocoenosis satisfying gas yield with acceptable retention time

common, particularly in wet fermentation processes

• Thermophil (50 - 60 °C)

high gas yield after short retention time

sensitive biocoenosis caution with rapid degradable substrates,

(hydrolysis develops too fast)

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pH-Value and CO2-solubility

sampling

approx. 40 Vol-% CO2

0,03 Vol-% CO2

insideoutside

Immediately after

taking a sample,

outside the digester

CO2 begins to escape.

pH-value rises

pH-value in the

digester is lower than

measured outside the

digester!

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pH-value

• Metabolites would reduce the pH-value, but:

– Plants with manure have normally good buffering systems:

– Carbonate buffer, Ammonium buffer, ...

• pH-value as sole parameter is not suited to evaluate the

process

Attention: H+ H2‏≠

Hydrogen ion (H+)- concentration

It‘s a question of buffer!

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pH-stability through buffer systems

Source: Novatech GmbH Support program

Säureanalyse

0

1

2

3

4

5

6

7

8

9

10

KW 5

1/05

KW 0

1 / 0

6

KW 0

6 / 0

6

KW 0

7 / 0

6

Säu

re in

[g

/ l F

MI]

n-Valeriansäure

n-Buttersäure

Propionsäure

Essigsäure

pH 7,7 pH 7,6 pH 7,6 pH 7,8

Acidity analysis

Ac

idit

y i

n [

g/l

FM

]

Acidification

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Salt content or electric conductivity

• Simple measurement

– Should be always included in case of inexplicable changes

• High salt content dry up the bacteria

– Osmotic pressure

• Unit: mS/cm

– Values > 60 mS/cm are critical

– Correction on a temperature of 25°C

• If value is critical: add water

Usually no problem during

fermentation of energy crops,

BUT

You have to analyse during

fermentation of food-waste (e.g.

canteens, grease separator,

salted matter)

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Principles of the biogas process

1 The biogas process

2 Environmental conditions

3 Engineering process parameter

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Functional principle

Overflow system

communicating vessels• Through hydrostatic pressure, liquid level is at

the same height in the connecting pipe

• Liquid levels balance themselves

• Overflow works

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Fundamental process engineering parameters

• Hydraulic retention time; T (HTR)

• Dry Matter; DM (oDM)

• Organic loading rate; BR

Input

Output

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Solid matter contentW

et/

Fre

sh

mass (

FM

)

Mineral solids = ash (minerals)

Water content

So

lid

s

Solid content

= Dry Matter [DM]

= organic Dry Matter or

Volatile solids [oDM;

VS]

(FM = fresh matter)

Organic solids = Volatile solids [from % FM or % DM ]

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(Hydraulic) Retention time T, HRT

Digester volume = Work volume (gross volume without gas storage space)

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(Hydraulic) Retention time T, HRT

Digester volume = Work volume (gross volume without gas storage space)

• central parameter in the case of liquid manure plants

• less important in the case of plants operating with energy crops

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Organic loading rate BR

• Solids (DM, oDM) – load per m³ work volume and day

• Tendency for higher load ‏

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Example for calculation: Organic loading rate BR

Work volume = 800 m³

Daily feeding of organic DM = 1926 kg

Process sensitivity increases with a larger organic loading rate

Substrate Substrate [t/a] oDM [%FM] oDM [t/a] oDM [kg/d]

Cattle manure 2200 9,0% 198 542

Leftovers 700 17,0% 119 326

Chicken dry manure 500 34,0% 170 466

Grease waste 800 27,0% 216 592

Total 4200 703 1926

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Critical organic loading rate and retention time

Organic loading rate [kg oDM/(m³*d)]

Hydraulic retention time [d]G

as p

rod

uctivity

[m³

Gas/(

m³ F

er.*

d)]

Ga

s y

ield

[m³

Gas /k

g o

DM

]

1

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2

Critical hydraulic retention

time and organic loading rate

Be curious!

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Any questions?

Thank you for your attention!

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Speaker:

Dipl.-Ing. Katrin Kayser

IBBK Fachgruppe Biogas GmbH,

Am Feuersee 6

74592 Kirchberg / Jagst, Germany

k.kayser@biogas-zentrum.de

Credentials: Birgit Pfeifer, bioreact GmbH

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