Solid waste management- sources- composting techniques

Sources

Raw organic materials such as crop residues, animal wastes, food garbage, some municipal wastes and suitable industrial wastes, enhance their suitability for application to the soil as a fertilizing resource, after having undergone composting.

Composting techniques

There are two types of composting that are aerobic and anaerobic techniques. In anaerobic composting, decomposition occurs where oxygen (O) is absent or in limited supply. Under this method, anaerobic micro-organisms dominate and develop intermediate compounds including methane, organic acids, hydrogen sulphide and other substances. In the absence of O, these compounds accumulate and are not metabolized further. Many of these compounds have strong odours and some present phytotoxicity. As anaerobic composting is a low-temperature process, it leaves weed seeds and pathogens intact. Moreover, the process usually takes longer than aerobic composting. These drawbacks often offset the merits of this process, viz. little work involved and fewer nutrients lost during the process.

The aerobic composting process

The aerobic composting process starts with the formation of the pile. The temperature rises rapidly to 70-80 °C within the first couple of days. First, mesophilic organisms (optimum growth temperature range = 20-45 °C) multiply rapidly on the readily available sugars and amino acids. They generate heat by their own metabolism and raise the temperature to a point where their own activities become suppressed. Then a few thermophilic fungi and several thermophilic bacteria (optimum growth temperature range = 50-70 °C or more) continue the process, raising the temperature of the material to 65 °C or higher. This peak heating phase is important for the quality of the compost as the heat kills pathogens and weed seeds.

Factors affecting aerobic composting

Aeration, moisture, nutrients, Temperature, Lignin content, Polyphenols, pH value

Techniques for effective aerobic composting

Improved aeration

In order to obtain the end product of uniform quality, the whole of the pile should receive a sufficient amount of O so that aerobic micro-organisms flourish uniformly. The methodologies deliberated in this publication made use of the techniques as presented below.

Turning

Once the pile is formed and decomposition starts, the only technique for improving aeration is turning. Frequency of turning is crucial for composting time. While the Indian Bangalore method  requires six to eight months to mature, the Indian Coimbatore method  (turning once) reduces the time to four months, and the Chinese rural composting pit method (turning three times) reduces the time to three months. An extreme example is the Berkley rapid composting method, which employs daily turning to complete the process in two weeks. In some cases, turning not only distributes air throughout the pile, it also prevents overheating as it kills all the microbes in the pile and terminates decomposition. However, turning too frequently might result in a lower temperature.

Inoculation

Inoculum organisms utilized for composting are mainly fungi such as Trichoderma sp.  and Pleurotus sp. (composting Coir Pith and composting weeds).also features 'effective micro-organisms' (EMs) (EM-based quick compost production process.

Supplemental nutrition

The techniques mentioned above often need to be complemented by the provision of nutrients. One of the most common practices is to add inorganic fertilizers, particularly N, in order to modify a high C:N ratio. Similarly, P is sometimes applied as the C:P ratio of the material mix is also considered important (the ratio should be between 75:1 and 150:1). When micro-organisms are inoculated, they require sugar and amino acids in order to boost their initial activities; molasses is often added for this purpose.

Shredding

Downsizing, or chopping up the materials, is a sound and widely-practised technique. It increases the surface area available for microbial action and provides better aeration. This technique is particularly effective and necessary for harder materials such as wood.

Other measures

Lime is applied to weaken the lignin structure of the plant materials and enhance the microbial population. However, in some cases, liming is not recommended as the pile may become too alkaline, resulting in significant N loss.

1.     Small scale composting

Traditional methods

Anaerobic composting

Indian Bangalore method

This method of composting was developed at Bangalore in India in 1939. It is recommended where night soil and refuse are used for preparing the compost. The method overcomes many of the disadvantages of the Indore method such as the problem of heap protection from adverse weather, nutrient losses from high winds and strong sun, frequent turning requirements, and fly nuisance. However, the time required for the production of finished compost is much longer. The method is suitable for areas with scanty rainfall.

Pit preparation

Trenches or pits about 1 m deep are dug; the breadth and length of the trenches can vary according to the availability of land and the type of material to be composted. Site selection is as per the Indore method. The trenches should have sloping walls and a floor with a 90-cm slope to prevent waterlogging.

Filling the pit

Organic residues and night soil are put in alternate layers. After filling, the pit is covered with a layer of refuse of 15-20 cm. The materials are allowed to remain in the pit without turning and watering for three months. During this period, the material settles owing to reduction in biomass volume. Additional night soil and refuse are placed on top in alternate layers and plastered or covered with mud or earth to prevent loss of moisture and breeding of flies. After the initial aerobic composting (about eight to ten days), the material undergoes anaerobic decomposition at a very slow rate. It takes about six to eight months to obtain the finished product.

3. Large-scale composting

Wind-row composting

Turned wind-rows

Wind-row composting consists of placing the mixture of raw materials in long narrow piles called wind-rows  that are agitated or turned on a regular basis. The turning operation mixes the composting materials and enhances passive aeration. Typically, the wind-rows are from 90 cm high for dense materials such as manures to 360 cm high for light, voluminous materials such as leaves. They vary in width from 300 to 600 cm. The equipment used for turning determines the size, shape and spacing of the wind-rows. Bucket loaders with a long reach can build high wind-rows. Turning machines produce low, wide wind-rows.

Vermicomposting

Earthworms used are  Eisenia fetida­- and Eudrillus euginae