Biotechnological Conversion Processes

Hydroelectric, Wind power, Solar power, Biomass, Geothermal.

The Kyoto basket encompasses the following six greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the so-called F-gases(hydrofluorocarbons and perfluorocarbons) and sulfur hexafluoride (SF6). Each gas is weighted by its global warming potential and aggregated to give total greenhouse gas emissions in CO2 equivalents.

1. worries about energy security (oil crisis)

2. economic development, both in the developed world and developing countries, including the creation or sustaining of jobs in agriculture

3. the need to mitigate climate change and achieve lower greenhouse gas (GHG) emissions.

Two different fermentation processes:

Anaerobic (absence of oxygen): (Alcoholic) fermentation - mainly products produced

vs

Aerobic (presence of oxygen): Composting - mainly biomass produced (a lot of ATP)

Anaerobic metabolism: Alcoholic fermentation (digestion) from Saccharomyces -> not so much energy but we have a high yield of product (methane, H2, ethanol, CO2)

Aerobic metabolism: Composting -> A lot of energy (36 ATP), produce more cells and products (biomass, water, CO2)

> Substrate (energy crops or biowaste, slurry or manure)

> (pretreatment)

> Biogas Reactor

> Biogas (CHP cogeneration heat & power, NGG national gas grid) + Digestion Residue

> fertilizer

> acre

> Substrate

During the first two steps, polymers (lipids, proteins, carbohydrates, etc.) are converted to soluble monomers (long-chain fatty acids, glycerol, amino acids, sugars, etc.), which are subsequently further converted via various fermentation reactions to short-chain fatty acids, alcohols, H2 and CO2 . In the next step the acids and alcohols are degraded through anaerobic oxidation by proton-reducing syntrophic acetogens to form H2 , CO2 and acetate, which are used by the methanogens in the final step for the production of biogas (methane, H2).

The acetogenesis is combined with the methanogenesis because of the free energy change (ΔG, Gibbs standard: under standard conditions) -> energetically more feasible

• Ethanol fermentation (Acetate) ΔG+

• Methanogenesis (Methane production) ΔG-

• Syntrophic, coupled reaction ΔG-

ΔG- after the biological stage, it means ΔG+ for the bacteria.

-> The bacteria seem to be exchanging electrons during the coupled reaction

Both reactions occur simultaneously, nonetheless it is speculated that the energy transfer between methanogenic and acetogenic bacteria could take place via pili structures. Because of this, it is important to know that excessive stirring would increase the shear stress in the culture broth which could harm the overall methanation process and/ or drive them apart.

We can't cultivate these microbial cultures separately.

Acetogenesis-Methanogenesis. 

Acetogenesis is combined with methanogenesis because of the free energy change (ΔG, Gibbs standard: under standard conditions). In the methanogenesis, archaea produce methane using acetate from the acetogenesis step.

Excessive mixing and high shear rates have a negative effect on biogas production. This results in a lower rate of methanogenesis due to the reduced presence of methanogens and the dissipation of methanogenic centers in the vessel.
 

-> The bacteria seem to be exchanging electrons (maybe via pilli!? "Who knows…" ;>). This is also why one is not supposed to stir the enzyme mixture too much - one could stress the syntrophically functioning bacteria and make them drift apart.

A typical biogas community is dominated by members belonging to the bacterial phyla Firmicutes and Bacteroidetes (Clostridium, Bacteroides).

Hydrolysis: Cellulolytic enzymes 

• α-Amylase is produced by Bacillus for the degradation of starch   

• Lipase is produced by Pseudomonas for the degradation of lipids, fats

• Proteolytic enzymes are produced by Aspergillus niger for the degradation of proteins

Acidogenesis:

Bacteroides, Clostridia

Acetogenesis:  acetogenic bacteria

Syntrophobacter

Syntrophomonas

Methanogenesis: methanogenic bacteria and archaea

Methanobacteria

Methanosarcina

Methanococcus/Methanosaeta

> carbon chains containing a methyl group at one end and a carboxyl group at the other

> chemically distinct regions:
   1) a long hydrophobic hydrocarbon chain, which is not highly reactive; and
   2) a carboxyl (-COOH) group, which is hydrophilic and highly reactive.

> in the cell membrane fatty acids form covalent bonds with other molecules via the carboxylic acid groups.

Two different methanogenesis pathways:

CO2-type -> about 30% (most methane bacteria)

Acetate-type -> about 70% (some methanogens) 

It’s more difficult to use CO2-type because of H2 (needed a lot of energy to dissolve H2) -> technical problem). In the liquid phase, there is little solubility of H2. One solution -> overall pressure. There are more steps (& more enzymes) included in the use of C02-type.

It is cheaper than the CO2 type. This type doesn’t need many steps and many enzymes like the CO2 type. More efficient process because they also use H2 as a source of electrons.

Generally organic substrates (carbohydrates - e.g. cellulose; proteins; fats).

Types of substrates

-Organic Residues: food waste, agricultural byproducts, starch, beer, dairy, wine, waste water etc.

-Residues of Livestock: manure, sewage

-Renewables: corn, sugar cane, rape seed

Methanol CH3OH

Glucose C6H12O6

C6H12O6 + (6 – (12/4) – 6/2)H20 -> (6/2 – (12/8) + 6/4)CO2 + (6/2 +(12/8)-6/4)CH4

C6H12O6 -> (3)CO2 + (3)CH4

50% CO2 + 50% CH4

Inhibition by Hydrogen Sulfide

• Toxic to bacterial and human beings at low concentrations (100 ppm)

• chemical precipitation of heavy metals (micronutrients)

• odor emission

• corrosive

-> can lead to problems in CHP

H2S forms in the biogas reactor from the degradation of some amino acids (e.g.cystein from rape).

Technical options to prevent high concentrations of H2S in biogas plants:
1. Injection of air (biological) 2. Addition of iron salts (precipitation) 3. Activated carbon (adsorption) 4. Gas scrubbing (biological)

Inhibition by Heavy Metals

 The source of heavy metals

• Cu, Pb Disinfection in agriculture, Pipes

• Zn veterinary medicine

Why undesirable

• Cu, Ag, As, Pb, Hg, Cd are toxic to bacterial and human beings at low concentrations (e.g. chelate formation, binding to enzymes)

Technical prevention: Precipitation as carbonate or sulfide

The advantages of using manure as a cosubstrate are that it has high N content and helps the C:N:P ratio and that there are additional microorganisms for the degradation.

Hydrogen sulfide is a chemical compound with the formula H2S. It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs. It is poisonous, corrosive, and flammable. 

This chemical compound reacts as an inhibitor in the biogas reactor because it affects the microorganism’s growth.

• Toxic to bacterial and human beings at low concentrations (100 ppm)

• chemical precipitation of heavy metals (micronutrients)

• odor emission

• corrosive

But when purified it can actually be useful and is used for other purposes (e.g. production of sulfur and sulfuric acid).

C:N:P ratio: 100:5:1 – 200:5:1

The mean oxidation state of the feedstock determines the stoichiometry of the end products. Carbohydrate substrates yield 50% methane and 50% carbon dioxide, while more-reduced feedstocks (e.g., lipids) yield higher proportions of methane

In order to balance the C:N:P ratio in our preferable values (100:5:1 – 200:5:1) and to increase the microorganisms in the reactor for the degradation. 

E.g. Manure has an influence on the nitrogen fixation. And wastewater from dairy farms needs to be enriched in nitrogen, as the CNP-ratio would otherwise not be met.

For storage and conservation process. It takes mainly 1 year. Production of lactic acid takes place by microbial process as a consequence of the reduction of the pH-level in the substrate. Silaging is an important asset in order to make substrate available over all seasons and in order to conserve the substrates.

#AcidPretreatment

Macronutrient: what microorganisms need from the substrate

Carbon: Main component of the microorganism -> Skeletal

Nitrogen: Protein Biosynthesis -> Enzymes

Sulfur: Amino Acids (Cysteine , Histidin) -> Enzymes

Phosphate: ATP, NADH -> Currency of Energy

Sodium: Sodium Pumps -> Generating chemical bound energy especially for methanogenesis

Micronutrient: cofactor for the enzyme production

E.g.: Ni, Co, Se, Fe, Na+
In detail:
Production of ATP: Na+, Ni , Co, Se
H2 Uptake: Ni , Se, Zn
CO2 Uptake: Mo (W), Zn , Se and Fe
Acetate Metabolism: Ni , Fe-S, Co and Zn
Uptake of Methylgroups: Co and Zn

The beginning of biogas plants - in the 1930s they identified anaerobic bacteria and the conditions that promote methane production.

AND

Gas calculation equation :)

-> The equation that predicts the Biogas yield: C{a}H{b}O{c}+(a-b/4-c/2)H2o -> (a/2-b/8+c/4)CO2+(a/2+b/8-c/4)CH4

e.g. glucose (C6H12O6):

C6H12O6 + [6-3-3] H2O →[3-1.5+1.5]CO2 + [3+1.5 - 1.5]CH4

C6H12O6 →3 CO2 + 3 CH4

therefore will produce 3 mol of methane for every mol of glucose.

An increase in acetate is correlated to the accumulation of acetate as a result of methanogenic bacteria having died off due to the addition of too much ammonium (/ammonia). <3

High conc. of ammonia can be fatal to the biogas reaction as it is a toxin to some of the microorganisms.

Rape can induce hydrogen sulfide to the mix (e.g. through high cysteine concentrations) that act as an inhibitor to biogas. This acid is undesirable because of:

- toxic to bacterial and human beings at low concentrations (100 ppm)

- chemical precipitation of heavy metals (micronutrients)

- odor emission

- corrosive  

If there is H2S present in the tank (during the burning process) we get a lower yield of heat and electricity! :<

Improved kinetic of biogas production

Improved biogas yield

Improved conservation 

Improved technical characteristics, lower viscosity

In the case of corn, we have a lot of starch (which is cut down into monomers by the a-amylases) hence, the lactic acid producers have a lot of easy food, and silaging is faster.