Biogas Part 1 - Microorganisms and Organic Matter Digestion

Normally, clean water bodies i.e., non-contaminated by organic matter, keep a certain amount of dissolved oxygen. This oxygen is utilized by fishes and other aquatic species for their own breathing, being directly responsible for the species survival. The quantity of dissolved oxygen present in the water bodies is directly proportional to the atmospheric pressure and inversely proportional to the temperature e.g., at 2,300 feet of altitude and at 68 degrees Fahrenheit, the maximum quantity of dissolved oxygen in some water bodies like a river for instance, would be at approximately 8.4 mg/L.?The existing organic matter in a regular sanitary sewage consumes as an average 300 mg/L of dissolved oxygen to be degraded (solid waste is much higher depending on the type of waste), therefore the consumption is higher than the available dissolved oxygen, hence this is the one of the reasons to treat the sewage so we can keep the life in the water bodies.

Other reasons to control these emissions is the fact, organic matter has itself microorganisms (bacteria, protozoa) some of them pathogenic to the human health.

The presence of organic matter in any water stream or solid waste, establishes the necessary conditions for the organic matter aerobic decomposers microorganisms to grow and consequently when feeding from this substrate, they consume oxygen by their own breathing.

It is the amount of dissolved oxygen present in certain organic matter which determines the growth and the quantity of the aerobic microorganisms (Bacteria).

In some circumstances the available dissolved oxygen is extinguished, creating conditions for the growth of other microorganisms. These facultative microorganisms also feed from the organic matter underneath the presence or absence of the dissolved oxygen.

There are also the strictly anaerobic microorganisms which feed themselves under the absence of oxygen only.

Saprophyte Heterotrophs Organisms

These organisms get their energy from dead and decaying organic matter. They are Bacteria, Protozoa, fungi.

·????????They utilize water, simple and complex substances in their nutrition generating new cells from synthesis.

·????????They are capable to process complex substances (proteins) into simpler substances (amino acids) through exogenous enzymatic action.

·????????They get energy through the glucose oxidation.

Heterotrophs Microorganisms Breathing

a)???????The aerobic microorganisms utilize the molecular free ?or the dissolved Oxygen (DO).

·????????Inside the microbiologic cell the following reaction occurs?

As above indicated in the chemical reaction, the aerobic microorganisms, oxidize the glucose, breathing Oxygen and generating carbon dioxide and energy (approximately 673 Kcal).

b)?????The anaerobic microorganisms live within environments without free oxygen. The process to digest the organic matter happens in 03 phases.

·????????Phase 1 – Fermentation – Hydrolytic and fermentative microorganisms

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·????????Phase 2 – Hydrogen synthesis – acetogenic bacteria

·????????Phase 3 – Methane synthesis – methanogenic bacteria

o???????By the CO2?reduction

o??By the decarboxylation of acetates?

The sulphates reductor bacteria generate acetates, Hydrogen and sulphides which are used by the methanogenic bacteria. Depending on the sulphate’s concentration, they can act as acetogenic bacteria, favouring the process, or competing with the methanogenic bacteria, in this case inhibiting the methanogenesis.

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c)??????The facultative microorganisms can live with or without Oxygen.

As a summary of the organic matter degradation process, while Oxygen is present, the aerobic microorganisms act. When the oxygen is almost no longer available, the facultative microorganisms take place and finally, when there is no more the presence of Oxygen, the anaerobic microorganisms play the role in processing the organic matter.

Important to mention the facultative microorganisms act in two phases, where initially they breath the remaining Oxygen present in the medium, and in the sequence, they operate under anoxic conditions (denitrification). The denitrification is the process where these microorganisms’ breath the Oxygen associated to the nitrates.

Biogas production takes place at the final phase of the organic matter digestion process.

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