Aerobic and Anaerobic decomposition of organic compounds.

Anaerobic and Aerobic decomposition of typical organic compounds:

Decomposition is the process of degradation of organic compounds which are degradable either in presence of oxygen or its absence with help of various microorganisms and form a simpler compounds like CO₂, CH_4, H₂O, nitrate, CH_4, etc.. There are two methods of degradation of organic compounds. They are as follow:

1. Anaerobic decomposition of typical organic compounds.
2. Aerobic decomposition of typical organic compounds.

• Anaerobic decomposition of typical organic compounds:

Anaerobic decomposition is the collection of processes by which microorganisms breakdown biodegradable materials in the absence of oxygen. An anaerobic decomposition is a partial conversion of organic substances into simpler organic compounds ( like organic acids and gases like CH4) by the microorganisms. The anaerobic decomposition is used both for domestic and industrial purposes to manage the waste and/or to produce fuels or food products. Many of fermentation used industrially to produce food and beverages also apply anaerobic decomposition of organic substrate.

The organic compounds of complex and polymeric organic molecules such as polysaccharides, polypeptides, lignins, lipids, etc. Under anaerobic condition the decomposition of these organic compounds (found in agricultural waste) like domestic solid waste, garbage waste, kitchen waste, etc takes place by the activity of both mesophilic and thermophilic microorganisms resulting in the production of CO2, S2,, alcohol, organic acids and many mal-odorous compounds (fowl smelling). Among the mesophilic flora,bacteria are more active than fungi and actinomycetes. However, the activity of both mesophiles and thermophiles is important in breakdown of organic substrates. Anaerobic decomposition of organic compounds accomplishes as follow:

1. Anaerobic decomposition begins with hydrolysis of complex and polymeric substances into simpler or monomeric units by mesophilic aerobic or facultative microorganism.
2. The monomers like sugars, aminoacids etc. are converted into different organic products like organic acids and gases.
3. This is polluted by the activity of methane gas, producing microorganisms (if present). These microorganisms are strictly anaerobic and primarily act on the organic acid to convert them to methane and carbon dioxide.

Flow chart of Anaerobic decomposition of organic compounds.

Microbiology of Anaerobic decomposition:

• Non-methanogens:

Clostridium acetobutylicum, Clostridium propionic, Eubacterium limosum, Coliforms, Syntrophomas wolfie, etc.

• Methanogens :Hydrogens oxidising methanogens:

Methanobacterium, Methanobrevibacter, Methanospirillum

Aceticlastic methanogens:

Methanosarcina

Mechanism of Aerobic and Anaerobic composting:

Composting:

Decomposition of organic wastes by microbial assimilatory/ dissimilatory activity to stable end product under aerobic and anaerobic condition is called composting. Composting involve the biological degradation of solid organic materials. As composting is solid phase biological treatment the target compoundsn that undergo composting are either solid or liquid associated with solid matrix. The end product of microbial activities on organic materials is called compost, that is rich in simpler organic compounds. Copmost is a source of nutrients to the plants, sustains soil fertility, improves physicochemical properties. For example: CEC (Cation Exchange Capacity) and biological properties of soil.

The process of composting requires accumulation of wet organic matters and waiting for the materials to breakdown by microbial activities. Raw organic materials such as crop residues, animal waste, garbages, food particles, some municipal wastes and suitable indusrial waste can be applied to composting. Organic materials are suitably prepared and various parameters are optimized so as to increase the decomposition rate by microbes. This is usually done by:

1. Size reduction, pH and temperature adjustment and moisture optimization should be done.
2. Organic or inorganic nutirents should be added.
3. Maintainence of Carbon:Nitrogen (C:N) ration
4. Addition of bulking agents like saw dust, straw, rise-hulls etc.

The organic matters serves as source of carbon and energy of microbes. The microorganisms produce enzymes which degrades these organic compounds during the growth phase there by forming compost.

Fig:A outline of Composting

Types of composting:

Composting may be divided into two categories by the nature of decomposition process:

• Aerobic Composting:

Aerobic composting is one of the most frequently used method of composting. It is widely used method for converting solid organic waste into stable,sanitary ,nuisance-free humus-like material that can be safely disposed into environment. It is the decompostion of organic matter by aerobic microorganisms. The decomposers are naturally present in soil, usually in moist and nutrient rich environment. The bacteria and fungi began the decomposition of organic compound. The metabolism of bacteria, fungi and actinomycetes create heat in compostingpile.

#Setting of composting parameters:

1. Place : A plane surface i.e. accessible and convenient,away from drainage and waterways to prevent ground water contamination.
2. Materials/ Waste : Leaves, manure, grass,clipping, yard and garden trimmings,sea weed, coffee ground and filters,eggs shells, pet and human hairs, plant matters, saw dust, fruits and vegetable scraps. Avoid meat and bones, pets or human waste, animal fat or oils.
3. Aeration / Oxygen : A minimum 5% of oxygen is required within the pore spaces in media.
4. Moisture : 43-46%
5. Temperature :43-65 *C (may rise above 70*C)
6. Carbon- Nitrogen ratio: The ideal ration of Carbon to Nitrogen (C:N) IS 30:1 measured on dry weight bases. High carbon-rich materials include dry leaves, straw, paper, saw dust etc.and high nitrogen rich material include kitchen waste and green plant parts. Most of the organic materials do not contain this ideal ration. Hence, it may be necessary to balance the ration by mixing different substances.
7. Particle size, porosity, structure and texture: Ideal particle 92-3inches), in some cases the materials may be too dense to permit adequate air flow or may be too moist. In such cases a bulking agent (straw,dry, leaves, saw dustetc.) is used or added.
8. Time : Compost can be harvested when it is nutrient deep rich, dark, brown in colour and has an earthy smell. Time consumed for it is 1-24 months.
9. pH : 6.5- 8.5

# Mechanism of Aerobic Composting (with respect to temperature change):

Composting is a complex process involving participitation of several bacteria, actinomycetes and fungi. The microbial activity brings about decomposition of organic materials and generate heat. The composting process proceeds as below:

1. Mesophilic Stage : At this stage fungi and bacteria are active and temperature increases from amnient to about 40*C , as a result heat produced during metabolism of bacteria and fungi. The rise in temperature continues to a point at which their own activities are suppressed.
2. Thermophilic Stage : As the composting proceeds temeprature rises from 40-70 *C. At this stage, thermophilic microbes are active. The rate of degradation of organic waste is very high and maximum degradation of organic matter occurs. The peak heating phase is important for quality of compost as it kills pathogens and weeds.
3. Cooling Stage : The microbial degradative activity slows down and the temperature decreases subsquently to mesophilic range. The mesophilic range.The thermophiles are replaced by mesophiles. This stage is characterized by formation of water, pH,neutralization and humus formation. The Carbon:Nitrogen ratio decreases and coarse of organic materials are further degraded. The degraded material become dark brown to black. The particles become consistent, small and soil like in texture.
4. Curing Stage :The active composting is followed by active curing state (last stage) at which the pile doesnot get heated anymore. This stage is very important as it remove pathogens and phytotoxins, organic acids etc.and provide safety against the risk of using immature compost.

# Microbiology of Composting:

# Methods of Aerobic composting:

1. Aerated static pile: The organic materials to be composed are placed over a grid of aeration of exhaust pipe. Air supplied through blowers for efficient aeration. A large of compost is kept over the top of aerated piles to retain heat, moisture loss,prevent flies and as a filter against odour. The pile is not turned and takes 3-5 week for composting. The best material is of wood chips, chopped straw etc. The piles are usually 5-8 feet high.
2. Windrow system :Windrows are the type of static piles of the composting materials arrange in a row, the piles are periodically turned and mixed usually at weekly interval. It is associated usually at weekly interval. It is associated with release of offensive unpleasant odours. The windrows may be opened or covered and its time duration is of 3-4 weeks.
3. In vessel system :In-vessel system composting is carried out in closed bioreactors. This is an advanced method designed to minimize the time of composting by optimizing different parameters and to control the release of offensive odour and its time duration is 1month.

• Anaerobic composting:

Anaerobic composting is slow decaying of organic matter through fermentative process. The composting materials arenot heated like aerobic process and microorganism are thrive in a low oxygen environment and are usually bacteria.

Anaerobic composting is the process of putrificative breakdown of organic matter through fermentation. The final product is composed of many organiccompounds like acids, alcohols etc. Many gases are produced which release unpleasant odour.

# Parameters of Anaerobic composting:

1. Time : 3 months to 1 year
2. Temperature: upto 55*C
3. Moisture : 50% by volume of waters.
4. Methods :

• Stacks (piles / heap method)
• Submergedcomposting: In this method materials are kept submerged to prevent release of unwanted gases and 80% or more moisture content trpped offensive gases.
• Container composting.
• Pit method.
• DRANCO method.

# Advantages of Composting:

1. The composting method of decompostion of organic compounds which is ecofriendly and less expensive.
2. It is re-usuable and recycled process.
3. We can increase the soil fertility as compostis biologically active and withits supplying range of microorganism their will be enhancement of soil and crops.
4. Their will be decreasement in execessive use of chemical chemical fertilizers.
5. The organic matters in compost functions like sponge which enable soil to retain nutrients and moisture in root zone.
6. The process of composting helps to improve soil structure allowing effective drainage, extensive root-growth and soil aggregate stabilization, so that soil is less subjected to wind, water and erosion.
7. Its plays vital role in waste management and reduction in pollutions and pollutants from environment.

#Disadvantages of Composting:

1. It can be time consuming process, produce bad odour and diffiiculty in its proper management.
2. The productis bulky and weighty which may cost high transportation charges.
3. The nutrient composition of compost may vary in comparision to chemical fertilizers.

Refrence:

1. Alexander, M. Introduction to Soil Microbiology. Academic Press, 1961..
2. Rangaswami, G and Bagyaraja PT. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.