3. Biology of anaerobic digestion - ETH Z · 2016-01-27 · UMWEKO GmbH, Dr Konrad Schleiss 3....

UMWEKO GmbH, Dr Konrad Schleiss

3. Biology of anaerobic digestion (after Peter Weiland, Institute of Technology and Biosystems Engineering Federal Agricultural Research Centre, Braunschweig, Germany)

• Microbiology and biochemistry of anaerobic digestion

• Physical and chemical parameters

• Substrates

• Setting up and managing the process

• Synthesis and outlook


Microbiology and biochemistry of AD

1st stage Hydrolysis

2nd stage Acido-

genesis

3rd stage Aceto-

genesis

4th stage Methano-

genesis

Biomass

Polysaccharides

Proteins

Fats

Sugars

Amino acids

Fatty acids

H2/CO2

Carbonic acids: Acetate Propionate Butyrate Succinate Alcohols

Acetate H2, CO2

BiogasCH /CO4 2

Acetogens

Methanogens

Fermentative bacteria

Hydrolytic bacteria



ca. 70% ca. 30%



In anaerobic digestion, the microorganisms can only make use of a small part of the chemical energy bound in the substrates they digest. Most of the energy is lost to them as “useless” biogas.

A direct consequence of this low energy yield is the small amount of bacterial biomass which is formed.

Another consequence is the small amount of waste heat released: AD is an isothermic process.


Physical and chemical parameters

Comparative formation of bacterial biomass during aerobic degradation (with oxygen) and anaerobic degradation (digestion)

aerobic degradation anaerobic degradation

C fraction

(100%)

C fraction

(100%)

biomass biomass



Generation times of different microorganisms

Anaerobic microorganisms

Acidogenic bacteria

Bacterioides < 24 hours

Clostridium 24 - 36 hours

Acetogens 80 - 90 hours

Methanogens

Methanosarcina barkeri 5 – 15 days

Methanococcus ca. 10 days

Aerobic microorganisms

Escherichia coli 20 minutes

Belebtschlamm 2 hours



Medium requirements for the anaerobic digestion of input materials and waste

Parameter Hydrolysis / Acidogenesis Methanogenesis

Temperature 25 - 35°C mesophilic: 32 - 42°C

thermophilic: 50 - 58°C

pH 5.2 – 6.3 6.7 – 7.5

C:N ratio 10 - 45 20 - 30

Total solids < 40 %TS < 30 % TS

Redox potential +400 – -300 mV < -250 mV

Nutrients: C:N:P:S ratio 500 : 15 : 5 : 3 600 : 15 : 5 : 3

Trace elements no special requirements essential: Ni, Co, Mo, Se



Comparison between mesophilic and thermophilic digestion

Range of application

Range of application

Mesophilic

methanogenesis

Thermophilic

methanogenesis



Inhibiting and toxic factors

Inhibitor Inhibiting concentration Comment

Oxygen > 0,1 mg/l O2 Inhition of the methanogens (obligatory anaerobes).

Hydrogen sulphide > 50 mg/l H2S Inhibitory effect increases as pH decreases.

Volatile fatty acids > 2000 mg/l HAc (pH = 7,0) Inhibitory effect increases as pH decreases. The bacteria exhibit high adaptability to this factor.

Ammonium-N > 3500 mg/l NH (pH = 7,0) Inhibitory effect increases as pH decreases and temperature rises. The bacteria exhibit high adaptability to this factor.

Heavy metals Cu > 50 mg/l Zn > 150 mg/l Cr > 100 mg/l

Only dissolved metals are inhibitory. Deztoxification by precipitation as sulphides.

Desinfectants Antibiotics

n.d. Inhibitory effect is product spezific.



Inhibition by ammonia

Inhibition of methanogenesis by NH3

(methane formation from acetic acid)

Inhibition [%]



Inhibition by sulphides

Inhibition of methanogenesis

(methane formation from acetic acid)

Inhibition [%]



Inhibition by carbonic acids

Inhibition of methanogenesis

Inhibition [%]

mg/l acids (undiss.)

Propionic acid

Acetic acid


Substrates

Theoretical gas yield of various classes of substances

Fats Proteins Starch

Gas yield Nl/kg org. TS


Substrates

Composition of the biogas according to the digestion substrate

Class of substances Biogas yield [l/kg oDM]

Methane content [Vol.-%]

Calorific value [kWh/m³]

Carbohydrates 700 - 830 50 - 55 5.0 – 5.5

Proteins 700 - 900 70 - 75 7.0 – 7.5

Fats 1.000 - 1.400 68 - 73 6.8 – 7.3

Biowaste 350 - 500 55 - 68 5.5 – 6.8

Renewable raw materials 500 - 700 50 - 62 5.0 – 6.2


Substrates

Composition of the biogas according to the average oxidation level of

the carbon

oxalic acid

formic acid

citric acid

carbohydrates, acetic acid

propionic acid

butyric acid

proteins

fats methanol

Mean oxidation level of the C


Substrates

Biogas yields of various digestion substrates

Agricultural waste Agricultural raw materials Non-agricultural waste

Co

w m

an

ure

25

Pig

ma

nu

re 3

6

Fo

dd

er

be

et 9

5

Sila

ge

ma

ize

19

0

Ra

y g

rass 1

10

Bio

wa

ste

12

0

Fo

od

wa

ste

24

0

Gre

ase

se

pa

rato

r re

sid

ue

40

0

Waste

fa

t 8

00

Biogas yield [m3 gas/t substrate]


Synthesis and outlook

• Knowledge of the microbial metabolic processes is

indispensable to operate a biogas plant.

• Operational stability is only possible if account is taken of the

microbial requirements and chemical parameters.

• Sustainable biogas production is only economically feasible in

the long term if the full value can be extrated from the biomass

by optimising the digestion process and using all the energy.

3. Biology of anaerobic digestion - ETH Z · 2016-01-27 · UMWEKO GmbH, Dr Konrad Schleiss 3....

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