Process Description | Applications of Biogas | Use of Slurry | Advantages of Biogas | Implementation of Biogas Programme | Technical Information | List of Approved Contractors

:: Process Description

Biogas generation is a process widely occurring in nature and can be described as the biological process in which biomass or organic matter, in the absence of oxygen, is converted into methane and carbon dioxide. It is characterized by low nutrient requirement and high degree of waste stabilization process, where biogas is one of the two useful end products, the other being, enriched organic manual in the form of digested slurry. It is essentially a three- stage process. The three stages are:

A) Hydrolysis
B) Acidification
C) Methane Generation

Stage I : Hydrolysis

The waste materials of plant and animal origins consist mainly of carbohydrates, lipids, proteins and inorganic materials. Large molecular complex substances are solubilized into simpler ones with the help of extracellular enzyme released by the bacteria. This stage is also known as polymer-breakdown-stage. For example, the cellulose consisting of polymerized glucose is broken down to dimeric, and then to monomeric sugar molecules (glucose) by cellulolytic bacteria.

Stage II : Acidification

The monomer such as glucose that is produced in stage 1 is fermented under anaerobic condition into various acids with the help of enzymes produced by the acid forming bacteria. At this stage, the acid-forming bacteria break down molecules of six atoms of carbon (glucose) into molecules of less atoms of carbon (acids) which are in a more reduced state than glucose. The principal acids produced in this process are acetic acid, propionic acid, butyric acid and ethanol.

Stage III : Methanization

The principal acids produced in stage 2 are processed by methanogenic bacteria to produce methane. The reaction that takes place in the process of methane production is called Methanization and is expressed by the following equations.

The above equations show that there are many products, by-products and intermediate products in the process of digestion of inputs in an anaerobic condition before the final product (methane) is produced.

Factors affecting generation of biogas

pH value
The optimum biogas production is achieved when the pH value of input mixture in the digester is between 6 and 7. The pH in a biogas digester is also a function of the retention time. In the initial period of fermentation, as large amounts of organic acids are produced by acid forming bacteria, the pH inside the digester can decrease to below 5. This inhibits or even stops the digestion or the fermentation process. Methanogenic bacteria are very sensitive to pH and do not thrive below a value of 6.5. Later, as the digestion process continues, concentration of NH4 increases due to digestion of nitrogen, which can increase the pH value to above 8. When the methane production level is stabilized, the pH range remains buffered between 7.2 to 8.2.

Temperature
The methanogens are inactive in extreme high and low temperatures. The optimum temperature is 350C. When the ambient temperature goes down to 100C, gas production virtually stops. Satisfactory gas production takes place in the mesophilic range, between 250C to 300C. Proper insulation of the digester helps to increase gas production in cold seasons. When the ambient temperature is 300C or less, the average temperature within the dome remains about 40C above it.

Loading rate
Loading rate is the amount of raw materials fed per unit volume of digester capacity per day. The optimum level for cattle dung slurry is in the range of 8-10% and any variation results in lower gas output. In Indian conditions, about 25 kg of dung per cum. gas generation is recommended. If the plant is overfed, acids will accumulate and methane production will be inhibited. Similarly, if the plant is underfed, the gas production will also be low.

Retention time
Retention time (also known as detention time) is the average period that a given quantity of input remains in the digester to be acted upon by the methanogens. In a cow dung plant, the retention time is calculated by dividing the total volume of the digester by the volume of inputs added daily. The retention time is also dependent on the temperature and up to 350C, higher the temperature, lower the retention time.

Toxicity
Mineral ions, heavy metals and detergents are some of the toxic materials that inhibit the normal growth of pathogens in the digester. A small quantity of mineral ions (e.g. sodium, potassium, calcium, magnesium, ammonium and sulphur) stimulates the growth of bacteria, while a very heavy concentration of these ions will have toxic effect. For example, presence of ammonia (NH4) from 50 to 200 mg/l stimulates the growth of microbes, whereas its concentration above 1,500 mg/l produces toxicity. Similarly, heavy metals such as copper, nickel, chromium, zinc, lead, etc. in small quantities are essential for the growth of bacteria but their higher concentration has toxic effects. Likewise, detergents including soap, antibiotics, organic solvents, etc. inhibit the activities of methane producing bacteria and the addition of these substances in the digester should be avoided.