BIOGAS Energy

The byproduct of anaerobic digestion of organic materials are commonly referred to as "biogas" because of the biological nature of gas production. Biogas technology refers to the production of a combustible gas (called biogas) and a value added fertilizer (called slurry or sludge) by the anaerobic fermentation of organic material under certain controlled conditions. Biogas produced by microbial activities and can be used only at the place where it is produced.

PROCESS:-

The different pathways of the bio-methanation process are suggested by several investigators. Macro level energy conversion of organic waste into bio-menthanation. In this process of conversion of energy from organic mass is mainly utilized in cell synthesis and formation of methane and carbon dioxide besides some part of it remains in the effluent.

A simple flow chart may describe the three step process of bio-methanation.

Hydrolysis:- Saprophytic bacteria convert complex organic compounds into less complex organic compounds, which are water soluble.

Acid Formation:- Acid forming bacteria degrades organic compounds to volatile fatty acids and ammonia.

Methane  Formation:- Methane forms bacteria utilize these acids to form methane.

                                                     Cell

                    ↑ →→→→            Synthesis

                    ↑

                    ↑

Energy in   ↑ →→→→            Effluent

Organic      ↓                             Substrate

Matter        ↓

                    ↓

                    ↓ →→→→         Methane and 

                                                Carbon Dioxide

                                                Production

           Energy Conversion in Process

Each stage is being carried out by different types of bacteria with different environmental requirements.

Hydrolysis:- 

It is the first step in anaerobic degradation and also the rate limiting step. The hydrolysis of organic polymers such as polysaccharides, fats and proteins convert these polymers into smaller units such as sugars, long chain fatty acids and amino acids. This group of bacteria called as facultative anaerobes/microbes.

Acid forming Microbes or Acitogenesis:-

The sugars, long chain fatty acids and amino acids resulting from hydrolysis are used as substrates by a wide variety of bacterial generation of different fermentative organisms or by anaerobic oxidizers. The complex organic matter in liquid phase digestion converts the small water soluble molecules by fermentation into acetate, carbon dioxide and hydrogen. 

Three Stage Process of Bio-methanation

Methane forms Microbes or Methanogenesis:-

Acetate, carbon dioxide and molecular hydrogen can be directly utilized as a substrate by the group of anaerobic micro-organisms called methane genesis or methane forming bacteria. Methane can be synthesized via two different pathways, of which one involves acetate and the other molecular hydrogen. The estimations indicated that about 70% of the methane is produced from acetate and 30% comes from hydrogen. The Volatile Fatty Acids (VFA) accumulation is to be avoided in the digester to produce the high gas content.

Stoichiometric Calculation of the Biogas Yield and Composition:-

The bio - methanation process is carried out by the symbiotic action of methane and acid producing bacteria. The interrelated behaviors of all classes of bacteria create the environment for each other to survive and grow simultaneously, The estimation of biogas production form the organic waste materials may be done on the basis of Stoichiometric equation of the conversion process.

The simple equation is considered by Ghaly and Ram Kumar (1999) for calculation of the theoretical yield of methane and carbon dioxide i.e. biogas, produced by anaerobic digestion using the Buswell"s formula:

                          CnHaOb + (4n-a-2b) H 2 O → (4n + a-2b) / 8 CH 4+ (4n-a + 2b)

Veziroglu, (1991) has given the formula for the biogas production from organic substances which are not containing sulfur materials and can be written as:-

(CaHbOcNd + (4a-b-2c + 3d) / 4H2O → (4a + b-2c-3d) / 8CH4 + (4a-b + 2c + 3d) / 8CO2 + dNH3

The formula for the conversion of organic substance in an aqueous environment into CH4 & CO2 and into ammonia and hydrogen sulphide, (if N & S are contained in the substrates) is Boyle (1996).

CnHaObNcSd + (4n-a + 2b + 3c + 2d) / 8H2O → (4n-a + 2b + 3c + 2d / 8CO2 + (4n + a-2b-3c-2d) / 8 CH4 + cNH3 + d H2S

It is interesting to note that any organic matter will generate biogas they will create.

Process Parameters Affecting The Biogas Production:-

All kinds of organic waste such as kitchen waste and garden waste, cattle dung and sewage etc. can be used in a biogas plant. The efficiency of the biogas production  is affected by the following factors:

1). Amount of organic material
2). Digestibility of the material
3). A combination of microbial and engineering factors such as

a). Organic Loading rates:- It is a measure of how much organic material is fed to each cubic meter of reactor volume during one day (i.e. Kg of VS/m³ or COD/m³). The biogas is formed from the anaerobic degradation of volatile solids (VS). The volatile solids fed per Kg in the reactor will normally produce maximum biogas in the range of 0.25 - 0.70 m³ depending on the operating conditions of reactor.

According, the loading rate should be in the range of  1-1.5 Kg volatile solids/m³ digester/day which is highly dependent on the reactor design. Volatile Solids (VS) is measured as the weight of solids that is combustible "volatilized" at a temperature of 550 C. Approximately 50-70% of the VS can be converted to biogas depending on the design of the digesters.

b). pH value:-  The pH affects severely the biogas and methane production rate which decreases with high and low pH values. The pH value should be between 7 and 7.4 and the dilution for water and fresh dung ratio 1:1 by weight. The maximum percentage of methane can be produced in the optimum range of pH values 6.8 - 7.2. The methane forming bacteria can survive up to 4.5 pH. Therefore, the water is normally used for buffering solution and to be used in sufficient quantities for anaerobic digestion/ fermentation process.

The control of decrease in pH of an anaerobic reactor may be done by stopping the feeding and increase the buffering capacity eg. through adding some chemicals as  calcium carbonate, sodium bicarbonate or sodium hydroxide

c). Alkalinity:- It is mainly a measure of its acid neutralizing capacity. Carbonate, bicarbonate and hydroxide ions are normally used for as neutralizing agents.

d). Temperature:- The biogas production is highly dependent on operating temperature of anaerobic digester. Three ranges of temperature and their respective retention times are given:-

            Temperature Range                                                 Retention Time

•  Psychrophillic                                                   0 to 20 C & 100 -120 days
•  Mesophillic                                                        20 to 42 C & 30 - 60 days
•  Thermophillic                                                   42 to 75 C  & 3 - 20 days

Methanogenesis is also possible under psycrophillic condition (temperature below 20 C) but occurs at lower rates. The quantity and quality of biogas production are different in different temperature ranges. Optimum gas production is also found at 33 C and 52 C in Mesopillic and Thermophillic ranges.

e). Carbon to Nitrogen Ratio:- 

Microorganisms need both nitrogen and carbon for assimilation into their cell structures. The C/N ratio varies from a feedstock to another. It is mentioned as an parameter, affecting the biogas production.

Temperature Effect on Gas Production

f). Nutrients and Trace Elements:-

Microorganisms require the macro and micro nutrients as trace elements such as phosphorous, nitrogen, sulfur, calcium, potassium, iron, nickel, cobalt, zinc and copper. These are essential required for optimum activity of the micro-organisms involved in anaerobic digestion. The most important nutrients are nitrogen and phosphorus and it has been suggested that the C: N: P ratio should be kept at a minimum of 100:28

g). Hydraulic Retention Time (HRT)

I with operating temperature of the anaerobic digester. It may be defined as the average time a volume element of the liquid medium resides inside the reactor. A better production of biogas found at an increase in HRT, if all other parameters kept constant.

Anaerobic digestion can be performed with a relatively short HRT i.e. "High rate"
systems or with long HRT i.e. "Low rate" systems are normally used to digest slurries and solid wastes while high rate systems are usually used for treatment of waste water.

Feedstock Characteristic 

h). Toxicity:- 

Methanogens are most sensitive to any kind of toxicity in comparison to other microorganisms in anaerobic degradation. The limit concentrations (mg/l) for various inhibitors.

                                                 Substance                                mg / l

                                                 Copper               →                  10-250

                                                 Calcium             →                   8000

                                                 Sodium               →                  8000

                                                 Magnesium        →                  3000

                                                 Nickel                 →                  100-1000

                                                 Zinc                    →                  350-1000

                                                 Sulphur              →                  200

i). Degree of Mixing

Mixing is a control process to keep uniform the pH and other environmental conditions of slurry in the digester. It distributes the buffering agents throughout the reactor volume and prevents localized build up of high concentrations of intermediate metabolic products, which may inhibit methanogenic activities.

BENEFITS:-

The cooking in rural area is still largely depends on the use of traditional cook stoves (Chulla's). They are burning dung cake, fire-wood and agricultural waste in addition to kerosene upto some extent. The installation of bio-gas plant would directly replace the use of above tree and in saving them, the following gains would be made:-

•  Reduction in pollution due to burning of dung and other other biomass materials.
• The rural people would not be dependent on wood is used for cooking. The deforestation and ecological imbalances can be reduced.
• In rural areas instead of kerosene the biogas can be used for lighting. This would reduce the dependence on fossil oil directly and in saving foreign exchange.
• The most important benefit would be in keeping the clean inhabitation and environment. The human beings can be saved form bacterial infections and other insects.
• The combustion of biogas produces carbon dioxide, a greenhouse gas. The carbon in biogas comes from plant matter that fixed this carbon from atmospheric. Thus, Biogas production is carbon neutral and does not add to greenhouse gas emissions.

Video Regarding:- Working of Biogas Plant

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GAURAV SARASWAT