Composting | Environmental Engineering - Civil Engineering (CE) PDF Download

COMPOSTING

• Composting is the biological reclamation of organic materials by natural decomposition process. Examples: decay of fallen leaves in forests, decay of wood in a stand and animal carcasses decaying in a preserve. These natural processes in nature return organic material to the ecosystem.  
• Composting of agricultural waste and municipal solid waste has a long history and is commonly employed to recycle organic matter back into the soil to maintain soil fertility . Composting is seen as an environmentally acceptable method of waste treatment .  
• It is an aerobic, biological process which uses naturally occurring microorganisms to convert biodegradable organic matter into a humus-like product. The process destroys pathogens, converts N from unstable ammonia to stable organic forms, reduces the volume of waste and improves the nature of the waste. 
• Composting is a successful strategy for sustainable recycling of organic wastes. It is an ecological alternative to mass burning and land-filling of MSW. 
• Composting reduces the volume of waste to dispose to landfill and incineration and it recovers the useful organic matter for use as soil amendment. By contrast, odors, noise, vermin nuisance, bioaerosol (organic dust containing bacterial or fungal spores) generation and emissions, emission of volatile organic compounds (VOCs), and potential pathway from use on land for contaminants to enter food chain, are the disadvantages of composting . 
• Composting is one element of an integrated solid waste management strategy that can be applied to mixed municipal solid waste (MSW) or to separately collected leaves, yard wastes, and food wastes. The three basic functions of composting are (1) preparation, (2) decomposition, and (3) post-processing . 

PROCESS OF COMPOSTING

• Compost results in a physical breakdown of organic matter layered with small amounts of soil by a process known as aerobic disintegration.  
• Structure of the matter is broken down by bacteria and fungi of decay until it is part of the soil mass. For example, a piece of newspaper would, under ideal conditions, become a part of the humus in the soil within two to four weeks. A tin can biodegrades in about 100 years and an aluminum can in about 500 years.
• During composting, heat is generated because of interaction of organic material interaction with moisture, air, bacteria and fungi.  

Phases of Composting

The composting process can be divided into three phases determined by temperature and heat output. a) During the first phase, the initial 24-48 hours, temperatures gradually rise to 40-50 ^oC. During this time, sugars and other easily biodegradable substances are metabolized mostly by bacteria and fungi. b) During the second phase, which may occur over extended periods of time, temperatures between 40 and 65 ^oC prevails. 2Cellulose and other more difficult substances to biodegrade are destroyed at that time. Lignins, the darker, woody components in plant tissues, break down even more slowly. During this high temperature phase, plant pathogens, weed seeds and biocontrol agents (excepting Bacillus spp.) are killed by the heat. Turning compost piles ensures uniformly high temperatures and helps produce a homogeneous product. c) The third stage is the curing phase when the concentrations of materials that readily decompose decrease. The rates of decomposition, heat output and temperature decline during this phase. A micro-flora, similar to that found in soil, now colonizes the compost. Mature compost has a dark color, consists largely of lignins, humus and biomass and has a distinctive soil or "earthy" odor. This odor is attributed to the soil microflora present in the compost .

Optimum Conditions for Composting 

Food:- organic waste containing water (moisture content between 30-80%) & added nutrients (Nitrogen, Phosphorous, Sulfur) present organic matter content in waste serves as a source of carbon, nutrients & energy for the metabolic reactions during bioremediation process. 

Micronutrients in addition to N, P & S many other micronutrients are needed to a lower concentration such as K, Ca, Mg, Fe, Ni & others. 

Oxygen if required (aerobic types): 2-5 kg of oxygen per kg of organic compound to be converted. 

Moderate pH: between 6-9, neither too acidic nor too alkaline. 

Moderate Temperatures: 50^o to 100^o F. 

Enzymes: Chemical catalysts to break waste materials into smaller pieces. 

Small Scale Composting A households’ compost pile includes such things as leaves, plant refuse, vegetables parings, weeds, wood shavings, lawn clippings, and non-greasy food wastes. Commercial nitrogen fertilizer are also included in the pile to expedite the decay process and ground limestone is added to balance the pH. The pile is kept moist and periodically is turned to aerate the mass and mix the materials for better decomposition.  Household compost is usually used as a soil conditioner. It helps aggregate soil particles, adds some nutrients, and increases water holding capacity.  

Commercial Compost
 Composting plants established globally and, in India also, have met with little success. The process of composting on a large scale differs from household composting. It is important to control the methane gas that develops during decomposition (as in a landfill) and prevent leaching. Yet, there are success stories.  The ‘mass sorting processes’ of  most commercial composting provides a rough mix of grades of paper, wood, fiber, food scraps and miscellaneous other materials. The irregularity of the materials going into the compost process suggests that what comes out is also irregular. This irregularity of material is reflected in particle size, purity of the compost, and usability of the end product .

The downfall of most commercial compost facilities is the lack of markets for the end product. There appears to be consistent discrepancy between the quality and the perceived value of the compost.  Co-composting (mixing wastes with sludge from sewage treatment facilities) provides a high-quality soil additive but this product cannot be used on vegetable gardens and tuber, root or leafy crops. Use of co-compost in other fields is acceptable if the compost is monitored for heavy metal content. The irregular quality of the mass-sort compost makes it a difficult product to market. Greater effort is needed to create a sustainable quality and quantity of product and market match .

HUMAN HEALTH RISKS DUE TO COMPOSTING 

MSW contains a number of chemical and biological agents, hence it contains a lot of harmful substances. These contaminants may expose different populations to health hazards, ranging from the composting plant workers to the consumers of vegetable products grown in soils treated with compost. Health risks are due to occupational exposure to organic dusts, bioaerosols and microorganisms in MSW composting plants. Potential health risks are due to volatile organic compounds (VOCs) released during composting . 8With respect to the health risks of compost, three are three main exposure routes for the population: a) ingestion of soils treated with compost, b) contamination through the food chain by consumption of products cultivated in soils where compost has been applied, and c) dispersion of atmospheric dust of compost that transports microorganisms and toxicants susceptible of being inhaled . 

ISSUES OF GHG EMISSIONS DUE TO COMPOSTING 

Green house gas (GHG) emissions due to composting are often neglected. Aerobic decomposition from well managed composting results in the emission of CO₂ and H₂O. Due to the heterogeneous nature of a compost pile, some CH₄ may form in anaerobic pockets within the pile . However, studies have shown that the majority of this CH₄ emission oxidizes to CO₂ in aerobic pockets and near the surface of the compost pile, making CH₄ emission negligible . However, many investigators have reported considerable CH₄ emission even in well managed systems. This happens due to various variables controlling the nature of the compost piles .  There is other side of the coin also, the production of compost helps mitigate GHG emissions in following ways : 

1. Decreasing the need of chemical fertilizers and pesticides; thereby reducing GHG emissions from the use of fossil fuel associated with their production and application . 

2. Allowing for more rapid growth in plants, thereby increasing carbon uptake and storage within the plant . This is a form of carbon sequestration which removes CO₂ from the atmosphere. 

3. Sequestering carbon in soil that has receives the compost . It is estimated that approximately 50 kg carbon (183 kg CO₂) gets sequestered per ton of wet compost . This figure is however specific to the US, and to a particular soil type . 

4. Improving tillage and workability of soil (thereby reducing emissions from fossil fuel that would otherwise be used to work the soil) . 

THE FUTURE FOR COMPOSTING

Composting of selected organic materials can be a valuable component of an integrated waste management system. It is a process as natural as nature and as technologically advanced as recycling . Composting will be certainly important in the future. Generally, conditions in India are very conducive for composting in terms of waste composition and weather conditions. However, composting has never flourished as an option for refuse treatment and disposal. Most local authorities feel, based on local experience, that the running costs of composting plants are excessive and unjustifiable.