25 Composting

Dr. Yogalakshmi K. N

 

Objectives:

1.      To familiarize the definition of composting

2.      To gain knowledge on the phases through which a good quality compost is prepared

3.      To understand the steps involved in the preparation of compost

4.      To gain knowledge about the optimal conditions required for good composting

5.      To familiarize the different composting technologies and the various technical options available for composting in Rural Areas of India

6.      Finally, to study the properties of a good compost

Composting

Composting is an aerobic process that involves biological degradation of organic wastes. It is nature’s way of recycling decomposed organic materials into a rich soil known as compost. It is one among the popular biological conversion route of organic fraction of solid waste. The organic wastes are biodegradable and includes garden, vegetable, fruit and food wastes collected from households, gardens, parks and other civic amenities sites. Composting is a relatively fast biodegradation process that removes a large part of the organic biodegradable wastes. It takes about 4-6 weeks to completely degrade the waste into a stabilized product. The by-products of this process are carbon dioxide and water. By composting, the nutrients in the organic waste is returned back into the soil so as to continue the cycle of life. Finished compost looks like soil and is dark brown in colour, crumbly and smells like a forest floor. High-quality compost is devoid of weed seeds and organisms that may be pathogenic to humans, animals, or plants. Cured compost is also relatively stable and resistant to further decomposition by microorganisms.

Composting is practiced on a small scale at households and on a large scale at central composting facilities. The organic wastes are collected, separated, composted and the resulting stabilized or biodegraded product obtained are known to be beneficial. They improve the soil structure especially in clay soils. Compost contain a lot of nutrients and hence are used as fertilizers to improve soil fertility.

They are also used to retain moisture of the soil. Compost is used as a mulch, a layer of material applied to the surface of the soil to conserve moisture, to improve fertility, to reduce weed growth and to enhance visual appearance of the soil. Due to these properties it has been widely used in land restoration and landscaping.

Chemical reaction involved in the process

The overall composting process can be explained as follows:

Types of composting

Composting is of two types: active and passive composting. Composting taking place in a natural way is called as passive composting. It is a slow process and takes around 6 months to degrade the waste. Unlike passive composting, active composting is a fast process where the natural process of composting is accelerated by maintaining or performing the composting process under proper conditions of air, moisture, temperature and carbon to nitrogen ratio. Regular turning off and maintenance of 40-60% moisture will fasten up the composting process and complete them within 3 months.

Benefits of composting

The composting process is an environmentally friendly means of recycling organic materials and not a means of waste disposal. Composting removes a large part of biodegradable wastes from the waste stream. Due to composting practice, the biodegradation process operating in the landfills will be reduced resulting in less generation of landfill gases such as carbon dioxide and methane the so called greenhouse gases. Composting is an efficient and eco-friendly way of solid waste management compared to waste landfill and incineration. Compost, because of its high organic matter content, makes a valuable soil amendment and is used to provide nutrients for plants. When mixed with soil, compost promotes a proper balance between air and water in the resulting mixture, helps reduce soil erosion and serves as a slow-release fertilizer.

Composting process

The process of composting municipal solid waste involves a number of stages as depicted in Figure 1.

Collection, segregation and storage of solid waste:

This is the initial stage of the composting process and it involves collection of wastes from different sources followed by source based segregation wherein segregation of biodegradable fraction is made through kerbside collection or ‘bring’ schemes. Mixed municipal solid waste would require separation of component waste on a scale of material recovery facility to remove inert materials such as glass, ferrous and non-ferrous metals etc. The input biodegradable waste is stored prior to shredding and pulverization.

Figure 1: Composting of solid waste

Shredding and homogenization

Source segregated organic wastes collected from households, kerbside or through ‘bring’ schemes would require a greater degree of pre-processing to remove contaminants and poorly segregated wastes. Pre-processing involves shredding and pulverization. This process reduces the size of waste and produces a more homogenized product for composting. The size of the shredded and pulverized fraction ranges between 1 and 10 cm2 depending on the type of waste.

Stages of composting

The composting process is aerobic and relies on abundant supply of oxygen. Regular aeration in required to maintain aerobic conditions. The composting process is further classified into three stages

i) High rate composting stage: This is the first phase of composting. This stage involves increasing temperatures and a high rate of microbial activity. Simple carbohydrates and proteins are readily degraded biologically by mesophilic microorganisms followed by thermo-tolerant and thermophilic microorganism as the temperature rises above 45⁰C during the process of degradation. The mesophilic stage lasts for a couple of days after which the temperature of the pile increases upto 52 ⁰C.

ii) Thermophilic or Stabilisation stage: The second stage involves biodegradation of waste by thermophilic microorganisms. It is an exothermic process as the temperature in the pile can reach up to 70⁰C. Such high heating lasts only a few days. It remains localized in the upper portion of the pile where the fresh material is being added. In this stage, weed seeds and pathogenic microorganisms are thermally destroyed as the temperature condition is high. Even the humic acid production takes place at this temperature. Humic acid helps plant to assimilate nutrients. Higher the temperature faster will be the composting

iii) Maturation stage – Cooling and curing: This stage is characterized by lower temperatures. During this phase, the microorganisms that were replaced by the thermophiles, regain in the pile and help in digesting the coarser and more resistant organic particles. Fungi and macroorganisms that break the coarser elements down into humus also invade and move back into the piles. Curing period is an important phase in composting process because immature compost may be harmful to plants. Uncured compost can produce phytotoxins as they rob the soil of oxygen and nitrogen, and can contain high levels of organic acids. This phase may take several weeks for completion.

Sieving and product grading

The final stages of the composting processes involves sieving and grading to remove un-composed materials and contaminants such as glass, plastics and metals and then size reduction and screening. The end product of the compost is marketed as organic manure as it has properties of improving soil fertility.

Biological processes involved during composting

The organic waste contains simple carbon compounds such as soluble sugars and organic acids that are easily metabolized by heterotrophic and heterogeneous microorganisms such as bacteria, fungi, actinomycetes and larger organisms such as insects and earthworms. They break down organic matter and produce carbon dioxide, water, heat and humus. The end product of humus is compost. The microorganisms consume some of the carbon to form new microbial cells, as they increase their population resulting in increased microbial activity. The high metabolic activity and exothermic processes increases the temperature of the compost heap. Due to the low thermal conductivity of the compost heap the heat generated cannot be dissipated and so the temperature increases further. This increase in temperature allows only the thermophilic microorganism to be active. The other substances such as cellulose, starch, pectin, lignin etc are degraded later by fungi and actinomycete; the decrease in temperature, moisture and pH increases the activity of fungi and actinomycete microorganism. Cellulose decomposition is intense throughout the process and occurs mostly through degradation by eumycete microorganisms. Lignin degradation is restricted to a group of fungi called basidiomycete. The nitrogen content present in the organic waste is decreased through ammonia formation and volatilisation during composting process. The loss of carbon dioxide and water from the carbon and hydrogen containing constituents in the waste results in the overall decrease in C: N ratio. In addition, during the later stages of composting, the nitrogen content slightly increases by nitrogen fixation by the microorganisms.

Factors influencing composting

Oxygen content: A minimum of 18% of oxygen supply to maintain aerobic condition in the compost is recommended. However, it should not go beyond 6%.

Temperature: Maximum microorganism activity is observed at temperatures ranging from 30-35⁰C

Moisture content: Biodegradation is efficient when there is a minimum of 40% moisture but at moisture lesser than 40% biodegradation is significantly reduced. Likewise, higher moisture contents are to be avoided as they occupy intraparticle spaces creating anaerobic conditions.

pH: Optimal composting of the waste is achieved at pH range of 5-5.8. Bacteria prefers a neutral pH whereas fungi prefer an acidic environment.

Table 1 Optimal condition for composting

Carbon/Nitrogen ratio: The optimal C:N ratio in the starting waste material is about 25. Higher values result in slow rate of decomposition and lower ratio result in the loss of nitrogen. The organic fraction of MSW has a C:N ratio between 26 and 45.

Size range: Shredding of the waste material increases its surface area and results in enhanced rates of composting.

Particle size and shape, porosity of the pile is also important. Pile size is also important. Small piles maintain higher internal oxygen concentrations than large piles. However, they will not retain heat if the pile is too small. Large piles retain higher temperatures than small piles. Aeration becomes insufficient when the pile size is too large.

Methods of aeration

Composting involves biodegradation of sample under aerobic condition and hence requires surplus aeration. There are different types of aeration systems such as Windrow system, Forced aeration system, In-vessel system and vermicomposting.

Windrow system: In this system, the biodegradable wastes are piled into heaps of about 2 m high, 3-4 m in width and 50 m in length. The waste is turned periodically by mechanical turning and the turning rates vary from one turn per day in the early stages of composting to one turn for every five days towards the end of the process. The turning process introduces fresh air, releases trapped heat, moisture and stale air. The windrow systems are placed on a gravel bed to aid the collection of leachate that may be formed. Windrows are arranged in rows and mechanical turning vehicle moves up and down turning the waste thereby aerating the pile.

Forced aeration system: In this system, air is blown or sucked using a fan through the pile of undisturbed waste that is located on an aeration block. Air is distributed through a perforated pipe covered with a porous base material that acts as a filter. Forced aeration systems have pile heights of 2-3 m or 2-6 m in width and up to 30 m in length. Air is passed through the pile continuously or periodically. If the air is drawn down the pile, the odours from the compost are contained in the system requiring control and treatment. But if the air is blown up through the pile heat is transferred from the inner pile to the outer regions.

In- vessel system: This system includes containers, silos or towers, enclosed halls, tunnels, rotating drums or reactor tanks. They allow control of moisture, temperature, aeration and waste mixing rates.

Container type in-vessel systems are employed for small scale wastes from food processing and catering industries that operate in batches.

Silos or tower systems are vertical units operating on a continuous basis. Biodegradable wastes are fed from top of the silo and composting takes place as the wastes moves down the tower. After several days, the compost is collected at the bottom of the tower.

In the enclosed halls, composting is carried out in large areas inside a purpose-built composting building.

Tunnel composting systems are large scale systems that involves some form of mechanical agitation and operate continuously or in batches.

In rotating drum system, wastes are placed in long rotating drum of length 50 m and diameter 3-4 m, stirred and tumbled with forced aeration. Waste stays in drums for several days till a maturation stage is reached which indicates the completion of the composting process.

Reactor tank composting units are sophisticated systems. Here, the waste is stirred by series of augers that are perforated and allows air to be blown into the waste pile. The augers are located on the stirring arm.

Technical Options for Solid and Liquid Waste Management in Rural Areas

The Ministry of Drinking Water and Sanitation, Government. of India has suggested several technological options for the management of solid wastes in rural areas. They include

I. Pile Method of composting

II. NADEP Method

III.  Bangalore method

IV.Indore method

V.  Coimbatore Method

VI.Vermicomposting

VII. Thermophilic Composting VIII. Biogas Technology

Few of the composting options are discussed below.

NADEP method: The NADEP method of organic composting was developed by a Gandhian follower Sh.N.D. Pandharipande (Narayan Deotao Pandharipande) of Maharastra in 2008. This method was applied for degrading organic materials such as dead plant material such as crop residues, weeds, forest litter, cattle dung and kitchen waste. It is a simple method. The composting pits consists of simple, rectangular brick tank with enough spaces between the bricks to facilitate aeration. The inner size of the tank is 3m x 1.8 m or 3.6 m x 1.5m with 25 cm thick perforated brick wall covered with mud or cement to a height of 0.9 m above ground. The floor of the tank is laid with bricks. The passage of air is facilitated through the above ground perforated structure. the waste to be composted in this pit is layered inside the tank. Agro-waste forms the base of the first layer followed by cow dung and soil. They are used in the ratio of 45:5:50 by weight (i.e) Each layer can be of about 45 kg vegetable matters, 5 kg of dung mixed in 70 lts of water and 50 kg of soil. A total of thirty layers are filled in the pit for composting. A schematic of the layering is depicted in Figure 2. After 15-30 days of filling, the waste layer inside the tank gets compacted to 2 ft. once again the tank is refilled with 2-3 layers and left undisturbed for about 3 months. The waste is degraded with the help microorganisms. During the process of composting, twice a week about 22-50 l of water is sprinkled to the tank is fully loaded.

Figure 2 NADEP method of composting

Bangalore method: This method of composting was developed by Acharya (1939). This process was developed to compost town refuse and night soil. This process is carried out in pits and is anaerobic in nature. The Bangalore method of composting is also called as Hot Fermentation Mechanism of composting. The waste is stabilized in pits under anaerobic condition where organic wastes and animal dung is alternated in pits. This method is used in low rainfall areas as the pits conserve moisture. The pits are 4 feet wide and 2 feet deep. The waste inside the pit is loaded as per NADEP method except for water addition. Water is added only if it is necessary. After the waste loading, the pit is covered with a final layer of soil to prevent entry of water, minimize flies etc. The waste is allowed to decompose for 4 to 6 months. A large land area is required to follow bangalore method of composting. Also this method creates odour problems.

Indore method: This process was developed by Howard and Wad in 1931 at Indore, Madhya Pradesh.

It is an aerobic method that composts waste materials such as plant residues, animal wastes, weeds, street refuse and other organic wastes. The waste materials are shredded and spread in pits or heaps to a height of 10-15 cm. Heaps can be of dimensions 1.0 m wide, I.0 m deep and of convenient length. They are moistened with cow dung. The composting is activated or fastened by turning the heaps three to four times. The frequency of the turning is as follows

o   The first turn is manually given 4 to 7 days after filling.

o   The second turn is given after 5 to 10 more days.

o   Third turn is also given after 5-10 days.

o   Further turning is normally not required and the compost is ready in 4-5 weeks. No odourous gases are not generated in this method. It is an environment friendly method.

Summary

To summarize, at the end of this module we have

• Defined composting
• Studied the properties of good compost
• Understood the various and steps involved in composting process Familiarized composting technologies
• Studied about the composting technologies followed in rural areas of India

Finally discussed about the factors influencing compost formation

References

• Williams, Paul T. (2013) Waste treatment and disposal, John Wiley Publishers.